User:Robertinventor/Modern Mars habitability

Edited version of Draft:Present_day_habitability_of_Mars removing section on Present day Mars habitability analogue environments on Earth which already exists now as a separate article.

Artist's impression of the Phoenix Lander landing on Mars.

Phoenix's atmospheric measurements of isotope ratios of carbon and oxygen gave evidence for liquid water on the surface now or in the recent geological past. Also its 2008 observations of possible droplets on its legs suggested new ways that water could be stable temporarily on Mars. These observations lead many scientists to reassess the present habitability of Mars

One of the central questions of modern Astrobiology is whether there is, or ever has been life on Mars. Mars probably had oceans in the past, and it definitely had lakes and a thicker atmosphere. Modern Mars has become cold, dry, and almost uninhabitable, yet, if life did ever arise on Mars, some hardy microbes and perhaps even multicellular life might survive there right through to the present. The only missions to search for life on Mars, the two Viking missions, returned results that were inconclusive . However the instruments were not designed to cope with the unusual conditions which Viking discovered on Mars, which may have confused the results of the experiments. Also, they didn't know enough about Mars at that time to target the regions we now think are most likely to have present day life. .

Life would meet many challenges on present day Mars. Liquid water boils at 0°C, over much of its surface. Even at the depths of the Hellas basin, any water is close to its boiling point of 10°C and will dry out quickly. Ice also evaporates into the atmosphere over geological timescales - and most of the equatorial regions are thought to be dry to depths of tens of meters. As its axial tilt varies, Mars atmosphere is sometimes thicker, and liquid water may then form on the surface - but any dormant life in the top few meters of soil would be destroyed over periods of millions of years by cosmic radiation.

However, in 2008, droplets were observed on the landing legs of Phoenix. Sadly, there was no way to analyse them, but the leading hypothesis is that they were droplets of salty water. Phoenix also made isotopic measurements which show that the Mars atmosphere has exchanged oxygen molecules with liquid on the surface in the recent geological past. This could indicate either recent episodic occurrences of liquid water (for instance after a meteorite strike) or water present every year, in contact with the atmosphere.

We now know of many seasonal changes in the surface of Mars which are only visible in high resolution photographs. Most of these are now thought to be caused by dry ice or wind effects. However, the "Recurrent slope lineae", and some of the "flow like features" form in conditions that suggest the occasional presence of small quantities of water on Mars. The evidence of flowing brines in the RSLs is strong, though it's not known if they are habitable. Curiosity has also found indirect evidence of a brine layer 15 cm below the sands that it drives over, though most scientists think that this layer is not habitable for Earth life.. Recent Mars surface simulations by Nilton Renno and his team have shown that small droplets of water can form on salt / ice interfaces for a few hours per day almost anywhere on the surface of Mars, and this may explain the Phoenix leg droplets observations.

In a separate development, research by the German aerospace company DLR in Mars simulation chambers and on the ISS show that some Earth life can survive simulated Mars surface conditions without any water at all, and photosynthesize and metabolize, slowly. It can do this using the high relative humidity of the Mars atmosphere at night. All of this work was done after the Phoenix discoveries in 2008.

Other potential habitats include lakes formed in the higher latitudes after cometary or meteorite impacts, or as a result of volcanism. Covered by ice, these may remain liquid for centuries, or up to a few thousand years for the largest impacts. The planet may also have underground trapped layers of water heated by geothermal hotspots. Also there are suggestions that Mars may have a deep hydrosphere, a liquid layer below its cryosphere, a few kilometers below the surface. Deep rock habitats on Earth are inhabited by life so if this layer exists, it may also be habitable on Mars.

The main questions are

• Do these potential habitats exist?
• Are they habitable? For instance, liquid water, if present, could be too cold, or too salty for Earth life
• Are they in fact inhabited by any forms of life? As Mars is so inhospitable, life might not be able to spread to new habitats easily. So there might be life in some of the habitats and not in others. Or life on Mars may have gone extinct, or never evolved at all, in which case none of the habitats would be inhabited.

These discoveries have renewed interest in this topic, with many astrobiologists saying that they think present day Mars may be more habitable than previously thought. The first conference on the Present Day Habitability of Mars was held in 2013 in UCLA. . The 2017 conference session on Modern Mars Habitability will run from April 24-28 in Mesa, Arizona

Viking observations - did Levin's labeled release experiment find life?

Carl Sagan with a model of the Viking Lander in Death Valley California. Viking 1 and II were the first spacecraft to search for present day life on Mars.

The Viking landers (operating on Mars from 1976-1982), are the only spacecraft so far to search directly for life on Mars. They landed in the equatorial regions of Mars. With our modern understanding of Mars, this would be a surprising location to find life, as the soil there is thought to be completely ice free to a depth of at least hundred meters, and possibly for a kilometer or more. It's not totally impossible though, as some scientists have suggested ways that life could exist even in such arid conditions, using the night time humidity of the atmosphere, and possibly in some way utilizing the frosts that form frequently in the mornings in equatorial regions..

The Viking results were intriguing, and inconclusive. There has been much debate since then between a small number of scientists who think that the Viking missions did detect life, and the majority of scientists who think that it did not.

The Viking lander had three main biological experiments, but only one of these experiments produced positive results.

• The Gas Chromatograph/Mass Spectrometer searched for organics, and found no trace of them.
• The Gas Exchange experiment searched for any gases that evolved from a sample of the Mars soil left in a nutrient solution in simulated martian atmosphere for twelve days. This experiment did detect gases, but so did the control, which repeated the experiment with a sample heated to sterilize it of any possible life. This suggests a chemical explanation.
• The labeled release experiment used nutrients tagged with ¹⁴C. It then monitored the air above the experiment for radioactive ¹⁴CO₂ gas as evidence that the nutrients had been taken up by micro-organisms. This experiment produced positive results. Also, in this case, the control experiments came out negative. Normally this would suggest a biological explanation. For this experiment the microbes don't have to grow, reproduce. They just have to metabolize the organics and produce the ¹⁴CO₂ gas in the process.

The conclusion at the time, for most scientists, was that the Labeled release experiment had to have some non biological explanation involving the unusual chemistry on Mars. One idea put forward by Albert Yen of JPL was that first carbon dioxide could react with the soil to produce superoxides in the cold dry conditions with UV radiation, which could then react with the small organics of the LR experiment to produce carbon dioxide. The other two experiments seemed to rule out any possibility of a biological explanation.

Some of the LR data remained hard to explain as chemistry and the experimenter's Principle Investigator Gilbert Levin maintained from the beginning that his experiment probably detected life. Here are some of the things that any theory has to explain, in addition to the non detection of organics by the other instruments:

• The LR response produced a lot of carbon dioxide rapidly, which some criticized as "too much too soon" for the levels of life expected there.
• A second injection of nutrient actually lead to a 20% decrease in the previously evolved ¹⁴CO₂
• A sample maintained at 10°C in darkness for two months at one site and three months at another had no response to the nutrient
• A sample heated to 46°C produced 60% less gas
• A sample heated to 51°C became erratic and produced 90% less gas

His comments on how this could be explained biologically are that first, the amount of ¹⁴CO₂ released is comparable to a sample from Antarctica and less than is usually released in tests on Earth. The second injection seems to have just wetted the sample and lead to absorption of ¹⁴CO₂ and his conclusion is that the life died during the experiment, which is not too surprising given that most microbes even on Earth can't be cultivated in the laboratory. The difference in effect between 46°C and 51°C he considers to be strongly suggestive of life and hard to explain chemically for such a small change. The results for the samples kept in darkness he also considers to be hard to explain without biology.

Most other scientists at the time continued to regard the experiments as inconclusive. However, work since then has suggested a possible re-evaluation of those results.

First, some have suggested that the gas chromatograph may not have been sensitive enough to detect the organics. Though other scientists have suggested that they could have detected low levels of organics....

Then in 2002, Joseph Miller, a specialist in circadian rhythms thought he spotted these in the Viking data. He was able to get hold of the original Viking raw data (using printouts kept by Levin's co-researcher Pat Straat) and on re-analysis this seemed to confirm his conclusions.

• The data, though it follows temperature changes, is smoother than you'd expect from a purely chemical reaction response.
• It is also delayed by 2 hours. From analysis of the experiment he concluded that though a 20 minute delay could be explained using variability in CO2 solubility, 2 hours seems to much of a delay to explain that way.
• There are signs of a change of rhythm after the second nutrient injection.
• In an accidental experiment, one of the samples was kept for two months in cold and darkness before it was used. This showed no daily cycle. This is quite hard to explain on basis of chemistry.

Another paper published in 2012 uses cluster analysis cluster analysis and suggested once more that they may have detected biological activity.

On the other hand, a paper published in 2013 by Quinn has refined the chemical explanations suggested for the labeled release observations, using radiation damaged perchlorates. By simulating the radiation environment on Mars, he was able to duplicate radioactive ¹⁴CO₂ emission from the sample.

In short, the findings are intriguing but there is no consensus yet on whether the correct interpretation is biological or chemical. Most scientists still favour the chemical explanation, though a few scientists have recently shown renewed interest in a possible biological explanation.

Phoenix observations

Droplets on the Phoenix legs

Until 2008, most scientists thought that there was no possibility of liquid water on Mars for any length of time in the current conditions there. However, in 2008 through to 2009, droplets were observed on the landing legs of Phoenix.

Unfortunately, it wasn't equipped to analyse them but the leading theory is that these were droplets of salty water. They were observed to grow, merge, and then disappear, presumably as a result of falling off the legs.

These may have formed on mixtures of salt and ice that were thrown up onto its legs when it landed. Experiments by Nilton Renno's team in 2014 in Mars simulation chambers show that water can form droplets readily in Mars conditions on the interface between ice and calcium perchlorate salts. The droplets can form within minutes in Mars simulation conditions. This is the easiest way they have found to explain the observations.

Phoenix isotope evidence of liquid water on the Mars surface in the recent geological past

The deck of the Phoenix lander, photographed on Mars. The mass spectrometer used to make the atmosphere isotope measurements is at bottom right. Its observations showed that liquid water on the surface of Mars has exchanged oxygen atoms chemically with the carbon dioxide in the atmosphere in the recent geological past. Though it wasn't able to distinguish between water that is present on the surface intermittently (e.g. after a meteorite impact or volcanic eruption) or continuously (e.g. as deliquescing subsurface brines).

Phoenix also made isotopic measurements of the carbon and oxygen atoms in the atmospheric CO₂ in the atmosphere. These measurements show that the oxygen has exchanged chemically with some liquid on the surface, probably water, in the recent geological past. This gives indirect but strong evidence that liquid water exists on the surface or has existed, in the very recent geological past.

In detail, first they found that the ratio of isotopes for ¹³C to ¹²C in the atmosphere is similar to Earth. Mars should be enriched in ¹³C because the lighter ¹²C is lost to space, but isn't. So this shows that the CO₂ must be continually replenished. So Mars must be geologically active at least from time to time in the recent geological past.

Then with the oxygen, their findings were the other way around. The CO₂ is enriched in ¹⁸O compared with the ¹⁶O compared with CO₂ as emitted from volcanic activity. They can make this deduction using information from meteorites from Mars, one of which was formed as recently as 160 million years ago. This shows that the oxygen in the CO₂ in the atmosphere must have reacted chemically with water on the surface in order to take up heavier oxygen-18.

This research wasn't able to determine if this liquid water is episodic (e.g. after a meteorite strike) or continuously present. However their findings suggested that the exchange with the liquid water happened primarily at temperatures near freezing, which may rule out some hypotheses, particularly hydrothermal vent systems, as the primary source for the water.

Methane plume observations by Curiosity and from Earth

This image illustrates possible ways methane might be added to Mars' atmosphere (sources) and removed from the atmosphere (sinks). NASA's Curiosity Mars rover has detected fluctuations in methane concentration in the atmosphere, implying both types of activity occur on modern Mars.

Methane was detected in the Mars atmosphere for the first time in 2004. This stimulated follow up measurements, and research into possible biological or geological origins for methane on Mars. ..

If these measurements are valid (they were confirmed by three independent teams at the time), then there must be some source continually producing methane. Methane dissociates in the atmosphere through photochemical reactions - for instance it reacts with hydroxyl ions forming water and CO₂ in the presence of sunlight. It can only survive for a few hundred years in the Mars atmosphere.

There are three main hypotheses for sources for the methane

• 'Life in the form of methanogens (methane producing bacteria). These are autotrophs which require little more than hydrogen and carbon dioxide to metabolize. For the hydrogen source they could use a geothermal source of hydrogen, possibly due to volcanic or hydrothermal activity, or they could use the reaction of basalt and water. Methanogens have been found to be able to grow in Mars soil simulant in these conditions of water, CO₂ and hydrogen., and to be able to withstand the Martian freeze / thaw cycles.
• Subsurface rocks such as olivine chemically reacting with water in presence of geothermal heat in the process known as serpentization.
• Ancient underground reservoirs, or methane trapped in ice as clathrates (with the methane originally created by either of the other two methods)

The original remote observations from Earth needed confirmation by close up inspection on Mars. When Curiosity first landed, no methane was detected to the limits of its sensitivity (implying none is present at levels of the order of parts per billion).

However around eight months later, in November 2013, Curiosity detected Methane spikes up to 9 ppb. These spikes were observed in only one measurement (the measurements were taken roughly every month) and then dropped down to 0.7 ppb again. This happened again in early 2014.

This suggests a localized source to the researchers, since there is no mechanism known that could boost the global atmospheric levels of methane so quickly for such a short time. The leading hypothesis therefore is that a plume of methane gas escaped from some location not far from Curiosity and drifted over the rover, where it detected it.

However the nature of that source is currently unknown. It could as easily be due to inorganic sources as due to life.

The ExoMars Trace Gas Orbiter may help to answer this question, as it will be able to detect trace gases such as methane in the Mars atmosphere using techniques that are about a thousand times more sensitive than any previous measurements. It is due for launch in 2016 (it is part of the same mission that will land the first ExoMars static lander technology demo prior to the main 2018 rover mission).

Once it does these measurements, then the hope is that the results would have the resolution necessary to pinpoint the geographical locations of the sources on the ground. This could then be used to target rovers for later surface missions.

One way to distinguish between biogenic and abiogenic sources of methane might be to measure the carbon-12 to carbon-14 ratio. Methanogens produce a gas which is much richer in the lighter carbon-12 than the products of serpentization.

Dry Gullies

The dry gullies on Mars were first thought by many scientists to be formed by activity of water. Nowadays, it is thought that recent gullies are formed by dry ice processes, but that many of the older dry ice gullies result from the action of water.

The dry ice hypothesis for recent gullies was confirmed, reasonably conclusively, when new sections of gullies were seen to form at temperatures too low for water activity.

The hypothesis that many older gullies (but still geologically recent) were formed by action of water got strong support in January 2015. This research, while continuing to support the conclusion that the new features are formed by CO₂ processes at present, suggests that the older gullies may well have been formed by floods of melt water associated with glacial melting of glaciers that form when the Mars axis tilts beyond 30 degrees. This could have happened within the last two million years (between 400,000 and two million years ago).

Sharp-featured recent gullies (blue arrows) and older degraded gullies (gold) in the same location on Mars. These suggest cyclical climate change within the last two million years

Then in results first released in August 2016, scientists reported that they found no evidence of polysillicates (clays) in the gullies except in case where the gullies cut through clay deposits. This strongly suggests that they were not formed from water. . The Mars Opportunity rover is going to study a Martian gully close up starting in 2017, which may help resolve the question of how it formed. Meanwhile the original idea that these gullies could have formed and maybe still be forming as a result of outflows of liquid water has come to seem increasingly unlikely.

Warm Seasonal flows (Recurrent Slope Lineae)

Many dark streaks form seasonally on Mars. Most of these are thought to be due to dry ice and wind effects. This image shows an example, probably the result of avalanche slides and not thought to have anything to do with water:

Slope Streaks in Acheron Fossae on Mars - these streaks are thought to be possibly due to avalanches of dark sand flowing down the slope

However a few of the streaks form in conditions that rule out all the usual mechanisms. These are the Warm Seasonal Flows, also known as Recurrent Slope Lineae.

• They form on sun facing slopes in the summer when the local temperatures rise above 0C so far too warm for dry ice.
• They are not correlated at all with the winds and dust storms.
• They are also remarkably narrow and consistent in width through the length of the streak, when compared to a typical avalanche scar.
• They develop seasonally over many weeks, gradually extending down the slopes through summer - and then fade away in autumn

Warm Season Flows on Slope in Horowitz Crater (animated)

The leading hypotheses for these is that they are correlated in some way with the seasonal presence of liquid water - probably salty brines.

Dark Flows in Newton Crater Extending During Summer (animated)

Warm Season Flows on Slope in Newton Crater (animated)

The dark streaks resemble damp patches, but spectral measurements from orbit don't detect water. One suggestion is that the water re-arranges the sand grains so causing a darkening, for instance by removing fine dust from the surface. The images were all taken in the afternoons, so it's also possible that the water flows in the early morning and that this water has evaporated when the Mars Reconnaissance Orbiter is able to take the images and do spectroscopic imaging. The streaks are also much narrower than the resolution of the spectroscopic imaging from orbit, so water could be missed for that reason also.

Slopes with the streaks are enriched in the more oxidized ferrous and ferric oxides compared with other similar slopes without the streaks, which could be the result of water. The strength of the spectral signatures of the ferrous and ferric oxides also varies according to the season like the streaks themselves. The leading hypothesis for these streaks is that they are caused by water, kept liquid by salts which reduce the freezing point of the water.

Most of them occur at higher lattitudes, but in 2013 a few were also discovered in the Valles Marineres area, surprisingly close to the equator. This research turned up 12 new sites within 25 degrees of the equator, each with hundreds, or thousands of streaks.

Since the temperatures are relatively warm throughout the year at these locations, then without a mechanism for replenishment, any subsurface ice would probably have sublimated long ago. McEwen, from the team who discovered the streaks at this new location, suggested that this may be evidence for water emerging from groundwater deep below the crust. He suggests this may have implications for searches for Martian life.

Quoting from Nature:

"The temperatures there are relatively warm throughout the year, says McEwen, and without a mechanism for replenishment, any subsurface ice would probably already have sublimated."

"He says that this suggests that water may come from groundwater deep in the crust, which could have implications for Martian life: "The subsurface is probably the best place to find present-day life if it exists at all because it is protected from the radiation and temperature extremes," he says. "Maybe some of that water occasionally leaks out onto the surface, where we could see evidence for that subsurface life."

Upper map shows elevation, lower map shows albedo, and the black squares are confirmed sites of recurrent slope lineae.

"We observe the lineae to be most active in seasons when the slopes often face the sun. Expected peak temperatures suggest that activity may not depend solely on temperature. Although the origin of the recurring slope lineae remains an open question, our observations are consistent with intermittent flow of briny water. Such an origin suggests surprisingly abundant liquid water in some near-surface equatorial regions of Mars".

They were first reported in the paper by McEwan in Science, August 5, 2011. They were already suspected as involving flowing brines back then, as all the other models available involved liquid water in some form. Finally proven pretty much conclusively to involve liquid water in some form, possibly habitable if temperatures and salinity are right - after detection of hydrated salts that change their hydration state rapidly, reported in a paper published on 28 September 2015 along with a press conference [1]. The brines were not detected directly, because the resolution of the spectrometer isn't high enough for this, and also the brines probably flow in the morning. MRO is in a slowly precessing sun-synchronous orbit inclined at 93 degrees (orbital period 1 hr 52 minutes). Each time it crosses the Mars equator on the sunny side, South to North, the time is 3:00 pm, in the local solar time on the surface, all year round. This is the worst time of day to spot brines from orbit.

Although these features are now confirmed to involve liquid water in some form, the water could be either too cold or too salty for life. At present they are treated as potentially habitable, as "Uncertain Regions, to be treated as Special Regions".

The "Special Regions" assessment says of them:

• "Although no single model currently proposed for the origin of RSL adequately explains all observations, they are currently best interpreted as being due to the seepage of water at > 250 K, with $a_{w}$ [water activity] unknown and perhaps variable. As such they meet the criteria for Uncertain Regions, to be treated as Special Regions. There are other features on Mars with characteristics similar to RSL, but their relationship to possible liquid water is much less likely"The "Special Regions" assessment says of them:

Sun warmed dust grains embedded in ice

Möhlmann originally suggested this process in 2011 as a possible way for liquid water to form on Mars, based on a mechanism that produces liquid water in similar conditions in Antarctica. As the sunlight hits the ice, it would preferentially warm up any heat absorbing dust grains trapped inside. These grains would then store heat and form water by melting some of the ice, and the water, covered by ice, would be protected from the vacuum conditions of the atmosphere.

This process could melt the ice for a few hours per day in the warmest days of summer, and melt a few mms of ice around each grain. For instance, Losiak, et al, modeled dust grains of basalt (2-200 µm in diameter) if exposed to full sunlight on the surface of the ice on the warmest days in summer, on the Northern polar ice cap, and say this about their model, in 2014: "For example, for solar constant 350 W/m2, emissivity 0.80, grain size 2 um, and thermal conductivity 0.4 W/mK melting lasts for ~300 minutes [5 hours] and result in melting of 6 mm of ice." . They developed this model as a hypothesis to explain presence of extensive deposits of gypsum in the Northern polar ice cap and the dune fields around it, and concluded that, since the atmospheric pressure there is just above the triple point, this mms thin layer of liquid water could persist for a significant period of time there around grains of basalt in the middle of the day in summer.

This process has been been observed in Antarctica. On Mars, there could be enough water to create conditions for physical, chemical, and biological processes.

Flow like features

These intriguing high lattitude features are associated with the Martian Geysers. The geysers themselves (if that is what they are) are thought to be results of dry ice turning to gas, and the dark spots and flow like features are thought to be debris from the geysers.

However, later in the year the flow like features extend further down the slopes. The details differ for the two hemispheres. In the Southern hemisphere, all current models for this part of the process involve liquid water. In the northern hemisphere then most of the models also involve water, although the northern hemisphere flow like features form at much lower surface temperatures.

This image shows the flow like features of the southern hemisphere.

Flow-like features in Dunes on Richardson Crater, Mars. They form around the dark dune spots, in the debris of the hypothesized Martian Geysers. The dark material at the end of the flows moves at between 0.1 and 1.4 m/day in late spring / summer on Mars. All current models for it favour liquid water as a cause. Either interfacial layers, or else layers of water created through the solid state greenhouse effect.

The process starts with the dark dune spots which form in early spring. Here are some examples in Richardson Crater in the Martian southern hemisphere- one of the places where the Flow Like Features (FLFs) have been observed.

These are thought to result from the Martian Geysers.

The idea is that a semi-transparent solid such as dry ice or clear ice acts like a greenhouse to warm up a layer below the surface (the "solid state greenhouse effect"). When this lower layer consists of dry ice, then it turns into gas and as the pressure builds up, eventually escapes to the surface explosively as a Martian Geyser.

The debris from these geysers form the dark spots, and the "flow like features".

Then, as local summer approaches, the flow like features start to extend down the slope. These are small features only a few tens of meters in scale, and grow at a rate of a meter or a few meters per Martian sol through the late Martian spring and summer. This is the part of the process that is thought to be due to liquid water, in nearly all the models proposed for them so far.

A different mechanism is proposed for them in the Northern and in the Southern hemispheres.

Southern hemisphere flow like features

There are two types of these flow-like features. These ones in the southern hemisphere often get confused with the rather similar looking Northern hemisphere flow-like features. Both grow rapidly in the late spring, and they resemble each other visually, but that is as far as the resemblance goes. For an overview of the research see the Dark Dune Spots section of Nilton Renno's paper.

The two Martian polar ice caps of course have very different climates and conditions, with the northern hemisphere at a lower altitude, consisting mainly of ice, and it loses all its dry ice in summer. The northern hemisphere features form at a latitude of 12.5 degrees from the pole, at surface temperatures of around -90°C, low enough for dry ice. This is far too cold to be habitable to Earth life. Their models involve either extremely cold salty brines or dry ice and sand.

Though the southern ice cap is colder than the northern one, the southern features form in Richardson crater, which is 17.4 degrees from the pole. The southern ice cap is also much smaller in its summer than the northern polar ice cap. So the two types of feature are not directly comparable, although similar in appearance.

The flow-like features in Richardson crater form at much higher surface temperatures, which rule out dry ice. All the models for the southern hemisphere features, to date, involve some form of water. They grow at a rate of around 1.4 meters per Martian sol.

Flow-like features on Dunes in Richardson Crater, Mars. - detail. This flow moves approximately 39 meters in 26 days between the last two frames in the sequence

Solid state greenhouse effect model

Möhlmann uses a solid state greenhouse effect in his model, similarly to the process that forms the geysers, but with translucent ice rather than dry ice as the solid state greenhouse layer.

Blue wall of an Iceberg on Jökulsárlón, Iceland. On the Earth, Blue ice like this forms as a result of air bubbles squeezed out of glacier ice. This has the right optical and thermal properties to act as a solid state greenhouse, trapping a layer of liquid water that forms 0.1 to 1 meters below the surface. In Möhlmann's model, if ice with similar optical and thermal properties forms on Mars, it could form a layer of liquid water centimeters to decimeters thick, which would form 5 - 10 cms below the surface.

In his model, first the ice forms a translucent layer - then as summer approaches, the solid state greenhouse effect raises the temperature of a layer below the surface to 0°C, so melting it. This is a process familiar on the Earth for instance in Antarctica. On Earth, in similar conditions, the surface ice remains frozen, but a layer of liquid water forms from 0.1 to 1 meters below the surface. It forms preferentially in "blue ice".

On Mars, in his model, the melting layer is 5 to 10 cms below the surface. The liquid water layer starts off millimeters thick in their model, and can develop to be centimeters thick as the season progresses. The effect of the warming is cumulative over successive sols. Once formed, the liquid layer can persist overnight. Subsurface liquid water layers like this can form with surface temperatures as low as -56°C.

If the ice covers a heat absorbing layer at the right depth, the melted layer can form more rapidly, within a single sol, and can evolve to be tens of centimeters in thickness. In their model this starts as fresh water, insulated from the surface conditions by the overlaying ice layers - and then mixes with any salts to produce salty brines which would then flow beyond the edges to form the extending dark edges of the flow like features.

Later in the year, pressure can build up and cause formation of mini water geysers which may possibly explain the "white collars" that form around the flow like features towards the end of the season - in their model this is the result of liquid water erupting in mini water geysers and then freezing as white pure water ice.

This provides:

• A way for pure water to be present on Mars, and to stay liquid under pressure, insulated from the surface conditions.
• 5 to 10 cms below the surface, trapped by the ice above it
• Depending on conditions, the liquid layer is at least centimeters in thickness, and could be tens of centimeters in thickness.
• Initially of fresh water, at around 0°C.

If salt grains are present in the ice, then this gives conditions for brines to form, which would increase the melt volume and the duration of the melting. The brines then flow down the slope and extend the dark patch formed by the debris from the Geyser, so creating the extensions of the flow like features.

They mention a couple of caveats for their model, because the surface conditions on Mars at these locations is unknown. First it requires conditions for bare and optically transparent ice fields on Mars translucent to depths of several centimeters, and it's an open question whether this can happen, but there is nothing to rule it out either. Then, the other open question is whether their assumption of low thermal conductivity of the ice, preventing escape of the heat to the surface, is valid on Mars. The process works with blue ice on Earth - but we can't say yet what forms the ice actually takes in these Martian conditions.

This solid state greenhouse effect process favours equator facing slopes. Also, somewhat paradoxically, it favours higher lattitudes, close to the poles, over lower lattitudes, because it needs conditions where surface ice can form on Mars to thicknesses of tens of centimeters. (The examples at Richardson crater are at lattitude -72°, longitude 179.4°, so only 18° from the south pole.).

There is no in situ data yet for these locations, of course, to test the hypothesis. Though some of the predictions for their model could be confirmed by satellite observations.

Interfacial liquid layers model

Another model for these southern hemisphere features involves ULI water (undercooled liquid water) which forms as a thin layer over surfaces and can melt at well below the usual melting point of ice. In Mohlmann's sandwich model, then the interfacial water layer forms on the surfaces of solar heated grains in the ice, which then flows together down the slope. Calculations of downward flow of water shows that several litres a day of water could be supplied to the seepage flows in this way.

The idea then is that this ULI water would be the water source for liquid brines which then flow down the surface to form the features.

Northern Hemisphere flow like features

Seasonal processes in the Northern polar dunes with Flow Like Features. Time differences between the images are 22 days and 12 days. The final picture shows a long feature that formed new between the two images, and its length is 60 meters so it grew at a rate of at least 5 meters per day.

These features form at a much lower temperature than the southern hemisphere flow like features, at -90°C average surface temperature on kilometers scale - though the dark features are expected to be considerably warmer, and the subsurface is also expected to be heated by the solid state greenhouse effect through surface layers of dry ice (similarly to the proposed models for the Martian Geysers.

They progress through a sequence of changes, first wind blown, and then seepage features associated with the dune spots, and then finally, dark seepage features appear all along the dune crest as in this sequence. These images show the growth of the seepage features.

The flow like features in the northern hemisphere polar ice cap form at average surface temperatures of around 150°K - 180°K, i.e. up to -90°C approximately.

The flows start as wind-blown features but then are followed by seepage features which increase at between 0.3 meters and 7 meters a day.

"They show a characteristic sequence of changes: first only wind-blown features emanate from them, while later a bright circular and elevated ring forms, and dark seepage-features start from the spots. These streaks grow with a speed between 0.3 meters per day and 7 meters per day, first only from the spots, later from all along the dune crest."

The seepage features first form at overall surface temperatures of 160°K (-110°C), as measured with the low resolution TES data. However this has a resolution of 3 km across track and only 9 km along the track of the observations. Also, much of the area is still covered in dry ice at this point, and it is opaque in the thermal infrared band so the orbital photographs measure the temperature of the surface of the dry ice rather than the small area of the dark spots and streaks.

Then, as with the model for the Martian geysers, shortwave radiation can penetrate translucent CO₂ ice layer, and heat the subsurface through the solid state greenhouse effect.

The models suggest that subsurface melt water layers, and interfacial water could form with surface temperatures as low as 180°K (-90°C). Salts in contact with them could then form liquid brines.

An alternative mechanism for the Northern hemisphere involves dry ice and sand cascading down the slope but most of the models involve liquid brines for the seepage stages of the features.

For details see the Dark Dune Spots section of Nilton Renno's paperwhich also has images of the two types of feature as they progress through the season.

Life able to take up water from the 100% night time humidity of the Mars atmosphere

Martian conditions in miniature - In the Mars simulation chamber, DLR researchers recreated the atmospheric composition and pressure, the planet's surface, the temperature cycles and the solar radiation incident on the surface. The activity of polar and alpine lichen was investigated under these conditions.

A series of experiments by DLR (German aerospace company) in Mars simulation chambers and on the ISS show that some Earth life (Lichens and strains of chrooccocidiopsis, a green algae) can survive Mars surface conditions and photosynthesize and metabolize, slowly, in absence of any water at all. They could make use of the humidity of the Mars atmosphere. Though the absolute humidity is low, the relative humidity at night reaches 100% because of the large day / night swings in atmospheric pressure and temperature.

Lichens relying on 100% night time humidity

The lichens studied in these experiments have protection from UV light due to special pigments only found in lichens, such as parietin and antioxidants such as b-carotene in epilithic lichens. This gives them enough protection to tolerate the light levels in conditions of partial shade in the simulation chambers and make use of the light to photosynthesize. Indeed UV protection pigments have been suggested as potential biomarkers to search for on Mars.

An experiment on the ISS as part of Expose-E in 2008-2009 showed that one lichen, Xanthoria elegans, retained a viability of 71% for the algae (photobiont) and 84% for the fungus (mycobiont) after 18 months in the ISS, in Mars surface simulation conditions, and the surviving cells returned to 99% photosynthetic capabilities on return to Earth. This was an experiment without the day night temperature cycles of Mars and the lichens were kept in a desiccated state so it didn't test their ability to survive in niche habitats on Mars. This greatly exceeded the post flight viability of any of the other organisms tested in the experiment.

Another study in 2014 by German aerospace DLR in a Mars simulation chamber used the lichen Pleopsidium chlorophanum. This lives in the most Mars like environmental conditions on Earth, at up to 2000 meters in Antarctica. It is able to cope with high UV, low temperatures and dryness. It's mainly found in cracks, where just a small amount of scattered light reaches it. This is probably adaptive behaviour to protect it from UV light and desiccation. It remains metabolically active in temperatures down to -20 C, and can absorb small amounts of liquid water in an environment with ice and snow.

When exposed to full UV levels in a 34 day experiment in a Mars simulation chamber at DLR, the fungus component of the lichen Pleopsidium chlorophanum died, and it wasn't clear if the algae component was still photosynthesizing.

However, when partially shaded from the UV light, as for its natural habitats in Antarctica, both fungus and algae survived, and the algae remained photosynthetically active throughout. Also new growth of the lichen was observed. Photosynthetic activity continued to increase for the duration of the experiment, showing that the lichen adapted to the Mars conditions.

This is remarkable as the fungus is an aerobe, growing in an atmosphere with no appreciable amount of oxygen and 95% CO₂. It seems that the algae provides it with enough oxygen to survive. The lichen was grown in Sulfatic Mars Regolith Simulant - igneous rock with composition similar to Mars meteorites, consisting of gabbro and olivine, to which quartz and anhydrous iron oxide hematite (the only thermodynamically stable iron oxide under present day Mars conditions) were added. It also contains gypsum and geothite, and was crushed to simulate the martian regolith. This was an ice free environment. They found that photosynthetic activity was strongly correlated with the beginning and the end of the simulated Martian day. Those are times when atmospheric water vapour could condense on the soil and be absorbed by it, and could probably also form cold brines with the salts in the simulated martian regolith. The pressure used for the experiment was 700 - 800 Pa, above the triple point of pure water at 600 Pa and consistent with the conditions measured by Curiosity in Gale crater.

The experimenters concluded that it is likely that some lichens and cyanobacteria can adapt to Mars conditions, taking advantage of the night time humidity, and that it is possible that life from early Mars could have adapted to these conditions and still survive today in microniches on the surface.

Black fungi and black yeast relying on 100% night time humidity

In another experiment, by Kristina Zakharova et al, two species of microcolonial fungi – Cryomyces antarcticus and Knufia perforans - and a species of black yeasts–Exophiala jeanselmei were found to adapt and recover metabolic activity during exposure to a simulated Mars environment for 7 days. They depended on the temporary saturation of the atmosphere with water vapour like the lichens. The fungi didn't show any signs of stress reactions (such as creating unusual new proteins).

There Cryomyces antarcticus is an extremophile fungi, one of several from Antarctic dry deserts. Knufia perforans is a fungi from hot arid environments, and Exophiala jeanselmei is a black yeast endolith closely related to human pathogens.

The experimenters concluded that these black fungi can survive in a Mars environment.

Deliquescing salts taking up moisture from the Mars atmosphere

Mars is rich in perchlorates - a discovery made by Phoenix, and later confirmed by Curiosity and by analysis of Martian meteorites on Earth. It now seems that perchlorates probably occur over much of the surface of Mars. This is of especial interest since perchlorates deliquesce more easily than chlorides and at a lower temperature, so they could, potentially, take up water from the atmosphere more readily.

It is not yet clear how they formed. Sulfates, chlorides and nitrates can be made in sufficient quantities by atmospheric processes, but this mechanism doesn't seem sufficient to explain the observed abundances of perchlorates on Mars.

Though there is little by way of water vapour in the Mars atmosphere, which is also a near vacuum - still it reaches 100% humidity at night due to the low nighttime temperatures. This effect creates the Martian morning frosts, which were observed by Viking in the extremely dry equatorial regions of Mars.

Ice on Mars Utopia Planitia. These frosts formed every morning for about 100 days a year at the Viking location. Scientists believe dust particles in the atmosphere pick up bits of solid water. That combination is not heavy enough to settle to the ground. But carbon dioxide, which makes up 95 percent of the Martian atmosphere, freezes and adheres to the particles and they become heavy enough to sink. Warmed by the Sun, the surface evaporates the carbon dioxide and returns it to the atmosphere, leaving behind the water and dust.

The ice seen in this picture, is extremely thin, perhaps no more than one-thousandth of an inch thick. These frosts form due to the 100% night time humidity, which may also make it possible for perchlorate salt mixtures to capture humidity from the atmosphere, and this process could occur almost anywhere on Mars where suitable mixtures of salts exist.

The discovery of perchlorates raises the possibility of thin layers of salty brines that could form a short way below the surface by taking moisture from the atmosphere when the atmosphere is cooler. It's now thought that these could occur almost anywhere on Mars if the right mixtures of salts exist on the surface, even possibly in the hyper-arid equatorial regions. In the process of deliquescence, the humidity is taken directly from the atmosphere. It does not require the presence of ice on or near the surface.

Some microbes on the Earth are able to survive in dry habitats without any ice or water, using only liquid obtained by deliquescence. For instance this happens in salt pillars in the hyper arid core of the Atacama desert. They can do this at a remarkably low relative humidity, presumably making use of deliquescence of the salts.

Perchlorates are poisonous to many lifeforms. However, perchlorates are less hazardous at the low temperatures on Mars, and some Haloarchaea are able to tolerate them in these conditions, and some of them can use them as a source of energy as well. .

These layers are predicted to lie a few cms below the surface, and are likely to be thin films or droplets or patches of liquid brine. So,they probably won't be detected from orbit, at least not directly. Confirmation may have to wait until we can send landers to suitable locations with the capabilities to detect these layers. Some of the layers may form in equatorial regions, and analysis of results from Curiosity in early 2015 has returned indirect evidence for presence of subsurface deliquescing brines in Gale Crater. .

Whether any of these layers are habitable for life will depend on the temperatures and the water activity (how salty the brines are), which in turn depends on conditions and the composition of salts, whether they are mixed with soil, atmospheric conditions, and even the detailed structure of the microhabitats.

Eutectic mixtures, e.g. of chlorides and perchlorates deliquesce at a lower relative humidity, and remain liquid at a lower temperature than either separately

The possibility of liquid forming by deliquescence is improved hugely by the process of eutectic mixtures. The name comes from the Greek "ευ" (eu = easy) and "Τήξις" (tecsis = melting)

This is the process by which when you have a mixture of two salts, for example, a mixture of chloride with perchlorate, then the mixture is able to take up water at a lower relative humidity than either of the salts separately.

A similar process also occurs with temperature in place of humidity. A mixture of salts will remain liquid at a lower temperature than either separately. This is the way that Antifreeze works, and is also the mechanism by which salt keeps roads free from ice. See also Freezing-point depression.

Technical details of how it works

The Deliquescing Relative Humidity for a mixture of salts is the humidity needed for the entire mixture to become liquid. This varies depending on the proportion of each salt in the mixture.

The relative proportions of two salts needed to remain liquid with the lowest level of humidity is known as the eutonic point.

Any mixture of two salts, even if the proportions are well away from the eutonic point, can still take up some water vapour at this lowest level of humidity. It will continue to do this until one of the salts is entirely used up to create this optimal mixture. If there is an excess of the other salt, it remains out of solution in the solid phase.

This diagram shows how it works - for a fictitious mixture A and B.

Here DRH = Deliquescing Relative Humidity, ERH = Eutonic Relative Humidity.

E(A+B) is the optimal or Eutectic mixture. And L here refers to the liquid phase. So, to the left we have a mixture of A with E(A+B) and, once it reaches the eutonic point, only part of it is liquid, and some of the salt A will remain in its solid phase. To the right, similarly, some of the salt B remains in its solid phase above the eutonic point.

So as the humidity is increased, for a given A / B mixture, first the lower horizontal line is reached, at which point some of the mixture of salts becomes liquid. This is known as the "eutonic relative humidity" - the point at which any mixture will start to take up some water vapour.

As humidity is raised further, more and more of the mixture becomes liquid. Eventually the upper, curved line is reached - and at that point, the entire mixture will be in its liquid phase.

Similarly if the axis is temperature - then as the temperature is raised, first part of the mixture will go liquid, at a temperature corresponding to the optimal mixture of the salts, and then when the upper curved line is reached, the entire mixture will be liquid.

Effect of this

Because of this eutectic mixture effect, if you add a tiny amount of perchlorates to the less deliquescent chlorides, this is enough to reduce the minimum relative humidity needed to deliquesce to the eutonic relative humidity for the mixture. This is not only lower than the deliquescence relative humidity of the chlorides, it is also lower than the deliquescence relative humidity for the perchlorates as well.

You can also get similar eutectic mixtures of three or more different types of salts. E.g. a mixture of perchlorates, chlorates, sulfates, and chlorides (or nitrates also if present) in the case of Mars, along with cations of sodium, potassium, calcium, and magnesium. So there are many possibilities to consider here.

After salt mixtures take up water, they retain it after supercooling, and reduced humidity

In addition to this, once the salt mixtures take up water, they lose it less readily, so they can stay liquid even when the humidity is then reduced again below the eutonic point (delayed efflorescence). Similarly for eutectic freezing, they can be supercooled below the temperature where they would normally freeze, and may remain liquid for some time below the eutonic point.

You get a eutectic also for freezing of a single salt, with molar concentrations. If you have a mixture of salt and water then different mixtures will freeze at different temperatures. The eutectic is the optimal mix of water and salt with the lowest freezing temperature. As you freeze a mixture, then no matter what the original concentration, some of it will remain liquid down to the freezing point of the eutectic mixture.

However as you freeze further below that temperature, you may find that the salt continues to remain liquid. The reason for this is that for a salt to come out of solution through nucleation, it has to form a new interface between the crystal surface and the liquid, which requires energy. Once the nucleation starts, then crystallization is rapid, but the nucleation can be delayed often for many hours.

For instance, MgSO₄ has a eutectic of -3.6°C but through supercooling can remain liquid for an extra -15.5°C below that. Here is a table of some salts likely to be found on Mars, showing the eutectic temperature for each one (with the molar concentration for the optimal eutectic concentration in brackets) and the amount of supercooling below that temperature that they found with experiments (adapted from table 2 of - omitted some of the columns).

Salt system	Eutectic (°C)	Amount of supercooling below eutectic (°C)
MgSO4	-3.6°C (1.72 m)	15.5
MgCl2	-33°C (2.84 m)	13.8
NaCl	-21.3°C (5.17 m)	6.3
NaClO4	-34.3°C (9.2 m)	11.5

As the salt / liquid solution cools in Mars simulation conditions, then the results can be complicated, because for instance MgSO4 releases heat in an exothermic reaction when it crystallizes. This keeps it liquid for longer than you'd expect. In their experiments, it remained liquid for twelve hours as it gradually cooled below the eutectic temperature before eventually it froze at 15.5 degrees below the eutectic temperature. In simulated Mars conditions you also have to take account of the effect of soil mixed in with the salts. Surprisingly, using Mars analogue soil, this does not reduce the supercooling and can in some cases permit more supercooling.

With some of the salt solutions, depending on chemical composition, then the supercooling produces a glassy state instead of crystallization, and this could help to protect supercooled microbes from damage.

Effects of micropores in salt pillars

In experimental studies of salt pillars in the Atacama desert, microbes are able to access liquid at extremely low relative humidities due to micropores in the salt structures. They do this through spontaneous capillary condensation, at relative humidities far lower than the deliquescence point of NaCl of 75%.

'The Atacama desert hosts the closest analogue of what a real, live Martian might be like', in its salt rock formations.

Micro-environmental data measured simultaneously outside and inside halite pinnacles in the Yungay region (table 2 from )

Variable	Halite exterior	Halite interior
Mean annual RH, %	34.75	54.74
Maximum annual RH, %	74.20	86.10
Minimum annual RH, %	2.90	2.20

The researchers, Wierzchos et al, did detailed studies with scanning electron microscopes. At 75% relative humidity then brine was abundant inside the salt pillars. As the humidity was reduced, even at 30% RH, the cyanobacteria aggregates shrinked due to water loss, but still there were small pockets of brine in the salt pillars.

"Endolithic communities inside halite pinnacles in the Atacama Desert take advantage of the moist conditions that are created by the halite substrate in the absence of rain, fog or dew. The tendency of the halite to condense and retain liquid water is enhanced by the presence of a nano-porous phase with a smooth surface skin, which covers large crystals and fills the larger pore spaces inside the pinnacles... Endolithic microbial communities were observed as intimately associated with this hypothetical nano-porous phase. While halite endoliths must still be adapted to stress conditions inside the pinnacles (i.e. low water activity due to high salinity), these observations show that hygroscopic salts such as halite become oasis for life in extremely dry environments, when all other survival strategies fail.

Our findings have implications for the habitability of extremely dry environments, as they suggest that salts with properties similar to halite could be the preferred habitat for life close to the dry limit on Earth and elsewhere. It is particularly tempting to speculate that the chloride-bearing evaporites recently identified on Mars may have been the last, and therefore most recently inhabited, substrate as this planet transitioned from relatively wet to extremely dry conditions"

Microbes also inhabit Gypsum deposits (CaSO₄.2H₂O), however Gypsum doesn't deliquesce. Researchers found that the regions of the desert that had microbial colonies within the gypsum correlated with regions with over 60% relative humidity for a significant part of the year. They also found that the microbes imbibed water whenever the humidity increased above 60% and gradually became desiccated when it was below that figure.

Implications of these effects

The combination of all these effects means that mixtures of salts, including perchlorates in the mixture, can be liquid at lower temperatures than any of the salts separately, and also take up water from the atmosphere at lower relative humidity, and once liquid, can remain liquid for longer than you would predict if you didn't take account of these effects. And if there are micropores in the salt deposits, any life within them could also take advantage of an internal relative humidity higher than the external humidity of the atmosphere.

On Mars the relative humidity of the atmosphere goes through extremes. It reaches 100% humidity every night in the extreme cold, even in equatorial regions. In the daytime the relative humidity becomes much less, approaching 0%, and any exposed salts would lose their liquid.

The surface temperatures of the top few cms also change enormously from day to night (more stable but lower temperatures are encountered deeper below the surface) and over the entire surface of Mars, temperatures are tens of degrees below freezing every night.

But because of these other effects these liquid layers, may resist efflorescence and remain liquid longer than you'd expect as the air dries out in the daytime, and also stay liquid longer than you'd expect through supercooling as the temperatures plummet at night.

The result is that you could have layers of liquid, on Mars, quite some way below the surface 1 or 2 cms where liquid water in its pure state can form.

So this discovery of perchlorates on Mars has major implications for presence of liquid, and so habitability.

Challenges for life in these liquid layers of deliquescing salts

Given the presence of salts, and including perchlorates, widespread over Mars, it would seem that these liquid layers must surely exist, though not yet directly confirmed by observation.

However some of these liquid layers may be too cold for life (some are liquid at temperatures as low as -90C or lower), or too salty (not enough "water activity). The main focus of research here for habitability is to find out whether there are mixtures of salts that can deliquesce on Mars at the right temperature range and with sufficient water activity for life to be able to take advantage of the liquid. The consensus so far is that though many of these would be too cold, or too salty for life, it seems possible that some of these, in optimal conditions, with the right mixture of salts and at the right depth below the surface, may also be habitable for suitable haloarchaea. The lifeforms would need to be perchlorate tolerant, and ideally, able to use it as a source of energy as well..

The conditions for these liquid layers to form may include regions where there is no ice present on the surface such as the arid equatorial regions of Mars..

Curiosity observations - indirect evidence of deliquescing salts in equatorial regions

Researchers using data from Curiosity in April 2015 have found indirect evidence that liquid brines form through deliquescence of perchlorates in equatorial regions, at various times, both at the surface, and down to depths up to 15 cms below the surface. When it leaves sandy areas, the humidity increases, suggesting that the sand takes up water vapour.

At night, the water activity is high enough for life, but it is too cold, and in the day time it is warm enough but too dry. The authors concluded that the conditions in the Curiosity region were probably beyond the habitability range for replication and metabolism of known terrestrial micro-organisms.

Advancing sand dunes bioreactor

The idea behind this proposal is that the constantly moving sand dunes of Mars may be able to create a potential environment for life. Raw materials can be replenished, and the chemical disequilibrium needed for life maintained through churning of the sand by the winds.

Advancing Dune in Nili Patera, Mars. Back-and-forth blinking of this two-image animation shows movement of a sand dune on Mars. This discovery shows that entire dunes as thick as 200 feet (61 meters) are moving as coherent units across the Martian landscape. The sand dunes move at about the same flux (volume per time) dunes in Antarctica. This was unexpected because of the thin air and the winds which are weaker than Earth winds. It may be due to "saltation" - ballistic movement of sand grains which travel further in the weaker Mars gravity.

The lee fronts of the dunes in this region move on average 0.5 meters per years (though the selection may be biased here as they only measured dunes with clear lee edges to measure) and the ripples move on average 0.1 meters per year. .

The idea of the advancing sand dunes bioreactor is that this movement of the sand dunes could "mix oxidants, reductants, water, nutrients, and possibly organic carbon in what could be considered bioreactors"

The sources of carbon would come from space - it's supplied at a steady rate of 5 nanograms per square meter per sol from micrometeorites. At the equator it has a mean lifetime of 300 years - but lasts longer if buried.

On the leeward side of transgressing dunes, then the sand can be buried at the rate of centimeters per year. Since the UV light only penetrates the top centimeter of the soil, then the interplanetary carbon would be buried, beyond reach of UV, within a year.

Additionally, if there was photosynthetic life or similar in the sand dunes, this could fix CO2 from the atmosphere as an additional source (there is of course no evidence for this yet).

As for water, then their idea is that the frost that forms in the morning in the equatorial regions would also occur below the surface (is no reason for it to be confined to the surface). Then, in presence of salts, the day / night temperature cycles could force this water to migrate downwards and form potentially habitable layers of brine a few centimeters below the surface.

They suggest for instance, a eutectic mixture of Mg(ClO₄)₂ and Ca(ClO₄)₂ brines which have eutectics of -71°C and -77°C. This is well below the lowest known temperatures for growth for terrestrial microbes, of -20°C, but growth at lower temperatures may be possible on Mars so long as liquid is present.

Ferrous iron cold be the electron donor. And ferric iron or perchlorate could be the oxidant - electron acceptor.

The main nutrients (N, P, S) and trace nutrients (Mg, Ca, K, Fe, etc.) are all readily available with exception of N. They suggest that the dunes could have reduced nitrogen produced from the atmospheric N2 catalyzed by iron oxides in presence of UV radiation.

This is of special interest as a potential habitat that is accessible by MSL and other equatorial region rovers, as it doesn't require presence of surface ice.

In summary, their conclusion is that if MSL detects organic carbon, and reduced nitrogen compounds (which it has now done) then these sand dunes could be potential microbial habitats on present day Mars:

"Advancing martian dunes mix oxidants, reductants, water, nutrients, and possibly organic carbon in what could be considered bioreactors. Thus, martian dunes function as small scale analogues of the global geological cycles that are important in maintaining Earth's habitability. On Mars, carbon can be cycled from the surface of the dune to its subsurface where it may come in contact with moisture and oxidants. Compounds oxidized at the surface of dunes by UV radiation and oxygen are buried on the lee side of dunes and mixed with reductants, carbon, and ephemeral brines. In addition, reduced compounds will be exposed at the surface on the windward side of dunes where they can be oxidized and complete the cycle. ... Additional measurements by MSL such as detecting organic carbon and reduced nitrogen compounds would support the hypothesis that moving dunes are potential microbial habitats. The absence of these compounds would indicate that the today's dunes are unlikely to be habitable."

Droplets of liquid water on salt / ice interfaces

This is the result of a research team led by Nilton Renno, professor of atmospheric, oceanic and space sciences at Michigan University. He is also project scientist for Curiosity in charge of the REMS weather station on Mars, was also a scientist on the Phoenix lander team.

He made the widely reported statement about "swimming pools for bacteria" on Mars.

In the academic paper about this research he writes:

"The results of our experiments suggest that the spheroids observed on a strut of the Phoenix lander formed on water ice splashed during landing [Smith et al., 2009; Rennó et al., 2009]. They also support the hypothesis that “soft ice” found in one of the trenches dug by Phoenix was likely frozen brine that had been formed previously by perchlorates on icy soil. Finally, our results indicate that liquid water could form on the surface during the spring where snow has been deposited on saline soils [Martínez et al., 2012; Möhlmann, 2011]. 'These results have important implications for the understanding of the habitability of Mars because liquid water is essential for life as we know it, and halophilic terrestrial bacteria can thrive in brines'"

Ice and salt are both common in the higher latitudes of Mars, so these millimeter scale micro-habitats on salt / ice boundaries may likewise be a common feature on Mars.

Shallow interfacial layers a few molecules thick

These interfacial layers occur on boundaries between ice and rock due to intermolecular forces that depress the freezing point of the water. The water flows and acts as a solvent. These layers may be used by microbes in arctic permafrost, which have been found to metabolize at temperatures as low as -20°C. Life may be possible in interfacial layers as thin as three monolayers, and the model by Stephen Jepsen et al obtained 10⁹ cells/g at -20°C, though the microbes would spend most of their time in survival mode.. Models show that interfacial water should form in some regions of Mars, for instance in Richardson crater.

Ice covered lakes that form in polar regions after large impacts

This is a possibility that was highlighted recently with the close flyby of Mars by the comet Siding Spring in 2014 C/2013 A1 Siding Spring. Before its trajectory was known in detail, there remained a small chance that it could hit Mars. Calculations showed it could create a crater of many km in diameter and perhaps a couple of km deep. If a comet like that was to hit polar regions or higher lattitudes of Mars, away from the equator, it would create a temporary lake, which life could survive in.

Models suggest that a crater 30 - 50 km in diameter formed by a comet of a few kilometers in diameter would result in an underground hydrothermal system that remains liquid for thousands of years. This happens even in cold conditions so is not limited to early Mars, so a similar impact based temporary underground hydrothermal system could be created today if there was a large enough impact like Siding Spring. The lake is kept heated by the melted rock from the initial impact in hydrothermal systems fed by underground aquifers.

Temporary lakes resulting from volcanic activity

There is evidence that volcanism formed lakes 210 million years ago on one of the flanks of Arsia Mons, relatively recent in geological terms. This may have consisted of two lakes of around 40 cubic kilometers of water, and a third one of 20 cubic kilometers of water, which probably remained liquid for hundreds, or even of the order of thousands of years.

Two views of Arsia Mons, based on Viking orbiter imagery and Mars Global Surveyor elevation data, from the south (top) and north (bottom).

Arsia Mons is the southernmost of the volcanoes of Tharsis Montes. It is depicted using a Viking image mosaic draped over MOLA topography. The topography shows the caldera structure and the massive flank breakouts that produced two major side lobes on opposite sides of the volcano. The vertical exaggeration is 10:1.

There is evidence of lakes that formed 210 million years ago on the flanks of Arsia Mons. Compared with the 4.5 billion year history of Mars, this is relatively recent. It may have had two lakes of around 40 cubic kilometers of water, and a third one of 20 cubic kilometers of water, whihch stayed liquid for centuries, possibly for millennia.

Possibility of geological hot spots in present day Mars

There is clear evidence that Mars is not yet geologically inactive

• Small scale volcanic features associated with some of the volcanoes on Mars which must have formed in the very recent geological past
• The isotopic evidence from Phoenix of release of CO2 in the recent geological past.

It seems likely that there are magma plumes at least deep underground, associated with the occasional surface volcanism on the geological timescale of millions of years. And given that there has been activity on Olympus Mons as recently as four million years ago, it seems unlikely that all activity has stopped permanently.

But so far no currently active volcanism has been observed, nor have any present day warm areas have ever been found on the surface, in extensive searches. The Mars Global Surveyor scanned most of the surface in infrared with its TES instrument. The Mars Odyssey's THEMIS, also imaged the surface in wavelengths that measure temperature.

Another way to search for volcanic activity is through searches of trace gases produced in volcanic eruptions. So far nothing has been observed from Earth but instruments are limited in their sensitivity and get only limited observing time for Mars as well. This is going to be a focus of future searches however. One of the instruments on the the 2016 ExoMars Trace Gas Orbiter is NOMAD (Nadir and Occultation for Mars Discovery), which will search for trace gases indicating current volcanic activity, as well as searching directly for organics that could result from life processes, and the methane plumes.

If these hot spots exist, they could keep water liquid through geothermal heating. The water could be trapped under overlying deposits and kept at a pressure high enough to stay liquid. They could also be a source for intermittent surface or near surface water (for instance one of the hypotheses for the RSLs is that they may be occur over geological hot spots deep below the surface that indirectly supply them with water).

Another possibility is a volcanic ice tower - a column of ice that can form around volcanic vents, for instance on Mount Erebus, Ross Island, Antarctica. These would be only a few degrees higher in temperature than the surrounding landscape so easy to miss in thermal images from orbit.

Potential for cave habitats on Mars

As well as the lava tube caves, Mars may have other caves also less visible from orbit. It has most of the same processes that form caves on the Earth, and also has processes unique to Mars that may also create caves, for instance through direct sublimation of ice or dry ice into the atmosphere. Caves are of especial interest on Mars for astrobiology, because they can give protection from some of the harsh surface conditions. If the caves are isolated from the surface, or almost isolated, they may have conditions similar to similarly isolated caves on the Earth.

In the "Workshop on Mars 2001", the main possibilities for cave formation listed are:

"(1) diversion of channel courses in underground conduits; (2) fractures of surface drainage patterns; chaotic terrain and collapsed areas in general; (4) seepage face in valley walls and/or headwaters; (5) inactive hydrothermal vents and lava tubes."

They remark that caves that formed at headwaters or where liquid seeped from the rocks may be of special interest for astrobiology, and these may be places where some ice would still be present. Of course research has moved on since 2001.

In 2014, Penelope Boston lists some of the main possible types of cave. She divides into the four main categories which she then divides into further subcategories.

1. Solutional caves (e.g. on Earth, caves in limestone and other materials that can be dissolved, either through acid, or water)
2. Melt caves (e.g. lava tubes and glacier caves)
3. Fracture caves (e.g. due to faulting)
4. Erosional caves (e.g. wind scoured caves, and coastal caves eroded by the sea)
5. Suffosional caves - a rare type of cave on the Earth, where fine particles are moved by water, leaving the larger particles behind - so the rock does not dissolve, just the fine particles are removed.

She points out a few processes that may be unique to Mars. Amongst many other ideas she suggests:

Snottites in Cueva de Villa Luz in Southern Mexico. They live off H₂S, and they create sulfuric acid which eats into the rock and enlarges the cave. The colony covers itself with a mucus like layer which protects it and helps it to create its own chemical microclimate inside. Some of the microbes involved are obligate aerobes so need a small amount of oxygen to survive, but some are capable of surviving as anaerobes and don't need oxygen at all.

1. For the solutional caves, the abundance of sulfur on Mars may make sulfuric acid caves more common than they are on Mars. There's also the possibility of liquid CO2 (which forms under pressure, at depth, e.g. in a cliff wall) forming caves.
2. For the melt caves, then the lava tubes on Mars are far larger than the ones on the Earth.
3. Mars could have sublimational caves caused by dry ice and ordinary ice subliming directly into the atmosphere.

Some cave habitats on Earth, if shielded from the surface, may be almost exact duplicates of similar habitats on Mars. For instance the Snottites in the toxic sulfur cave Cueva de Villa Luz flourish on Hydrogen Sulfide gas. Some of these species are aerobes (needing only small amounts of oxygen), and others are anaerobes and could survive anywhere on Mars where similar habitats exist. Mars has been shown to be geologically active in the recent geological past through the Phoenix isotope measurements. Although there are no currently known geological hotspots or activity is currently known, there may well be subsurface thermal systems where caves similar to the Cueva de Villa Luz could occur.

Sub surface ice sheets in the equatorial regions

If these ice sheets exist, they may provide a source of water for surface life, for instance for the Recurrent Slope Lineae in the equatorial regions on the flanks of Valles Marineres.

Changes in tilt of Mars's axis. At times it tilts so far that it has equatorial ice sheets instead of the more usual polar ice caps. The sub surface ice sheets in the equatorial regions, if they exist, may be remnants of these larger ice sheets from the past.

As the axial tilt of Mars changes, at times it tilts so far that it has equatorial ice sheets instead of polar caps.

Several lines of evidence suggest, that Mars may have remnant subsurface equatorial ice sheets today. The first evidence of this was based on radar measurements from the (MARSIS) instrument aboard the Mars Express Spacecraft in 2007. These detected subsurface deposits that had similar density and dialectric constant to a mixture with more dust and sand than the polar ice deposits, and similar in volume and extent.

Other papers have provided additional, but not yet conclusive evidence that these may indeed be deposits of ice. For instance a 2014 paper reports observations of young ring-mold craters on tropical mountain glacier deposits on the flanks of Arsia and Pavonis Mons. Ring-mold craters are distinctive features that result from impact into debris covered ice. The observations suggest presence of remnant equatorial ice, over 16 meters below the surface.

Ice in the equatorial regions would normally be lost through sublimation into the near vacuum of the Mars atmosphere, to a depth of a hundred meters or more, and this happens quite rapidly over geological timescales, over timescales of order of 100,000 years or so. So for remnant ice to survive there today, then special conditions are needed. For instance trapped ice beneath an impervious layer (capstone). Or replenished from below. This is a matter for active research with no established conclusions yet.

Hydrosphere - possible layer of liquid water several kilometers below the surface

Deep rock habitats on Earth are inhabited by life so may also be on Mars. However they need liquid water to survive, which may possibly exist below the cyrosphere.

The Mars cryosphere is the layer of permanently frozen permafrost. In higher lattitudes it starts a few cms below the surface, and may continue down for several kilometers. In equatorial regions the surface of Mars may be completely dry down to a kilometer or more, so the cryosphere starts at the base of that dry layer.

If the Mars hydrosphere exists, it lies below the cryosphere, and is a layer where the ice is kept liquid by geothermal heating, and prevented from evaporating by the overlying layers of ice.

We don't have any evidence yet of a hydrosphere, but do have evidence of a deep subsurface cryosphere. This evidence is in the form of hydrogen / deuterium isotope ratios in Martian meteorites, which give indirect evidence that Mars must have a subsurface reservoir of water, most likely in the form of ice.

Research in 2014 into the deuterium / hydrogen isotope ratios in the water in martian meteorites gives evidence of a subsurface reservoir with a ratio in between the composition of the mantle and the composition of the water mixing with its current atmosphere. This supports the hypothesis that Mars has a deep cryosphere which may contain much of the original water from Mars."

If the hydrosphere exists, estimates in a paper from 2013 put it's depth at around 5 kilometers below the surface. Whether this layer exists or not depends on the presence or otherwise of perchlorates, and clathrates, and it also depends on the total inventory of water on Mars, so there are many unknowns in the models. They used an estimate of the total inventory of less than 500m GEL (Global Equivalent Layer), and doubled the required thickness of the cryosphere, which leaves less water available for the hydrosphere than in previous models. There may still be groundwater in places where it is perchlorate rich, and isolated pockets.

But if the global inventory of water is larger than the amount they assumed for their study, there may be ground water under much of the surface of Mars.

If this hydrosphere exists, then it may be more habitable than similar depth zones on Earth because of the lower gravity, leading to larger pore size. Possible metabolisms at this depth could use hydrogen, carbon dioxide, and possibly abiotic hydrocarbons. The carbon for biomass could come from magmatic carbon in basalts which has been detected in Martian meteorites. It could also support methanogens feeding off methane released from serpentinization, and the alteration of basalt could also be a basis for iron respiration.

Similar habitats on Earth are inhabited by microbes and even multi-cellular life. So this is a potential habitat of astrobiological interest on Mars. As well as that, if the habitat exists it is a possible reservoir that could replenish surface areas of Mars with life and permit lifeforms to transfer from one part of Mars to another subsurface - a process that is known to happen beneath arctic permafrost layers.

It's not feasible to drill down to sample it in the near future. However, liquid may be released to the surface as a result of impact fracturing and other events so making it possible to sample it via surface measurements.

One prime place to visit to search for evidence of the deep hydrosphere is McLaughlin Crater. The observations suggest it contained an ancient lake, with alteration minerals rich in Fe and Mg, and the detection of carbonates there suggests that the fluids were alkaline, and are consistent with the expected composition of fluids that emerged from the deep subsurface hydrosphere. The Nature article concludes "Lacustrine clay minerals and carbonates in McLaughlin Crater might be the best evidence for groundwater upwelling activity on Mars, and therefore should be considered a high-priority target for future exploration"

Habitability factors for life on Mars

This section is organized around the listing of the main factors limiting surface and near surface life on Mars, according to Schuerger

These are thought to be (not in order of importance):

• Extreme desiccation and scarcity of water - all life on Earth requires liquid water - or else high humidity in the air. So the main focus for the search for present day life on Mars so far starts with this assumption. There may be other possibilities for exotic life that don't use water, for instance a recent suggestion that life may be able to evolve in supercritical liquid CO2 under high pressure - a potential habitat present on both Venus and Mars. So probably we shouldn't rule out the possibility of other habitats totally.
• UV light for any life on the surface exposed to full sunlight. Because of the thin atmosphere, this is hardly filtered at all, and is a major challenge for any life exposed to the light. It is easily blocked by about 0.3 mm of surface soil, sheltered by a millimeter of dust or by other organisms, or in the shadow of a rock. Mars conditions are likely to favour lifeforms that can tolerate high levels of UV radiation, at least, if they are exposed to direct unfiltered sunlight at any point in their life cycle. This could for instance involve use of protective pigments such as melanin, parietin and usnic acid which help protect some lichens from the damaging effects of UV radiation in polar and high alpine regions.
• Low pressures (hypobaria) at 1–14 mbar
• Anoxic CO₂-enriched atmosphere. All the habitats suggested so far require anaerobes - lifeforms that don't require oxygen.
• Low temperatures. There may be some warmer locations, for instance using geothermal heating. Also, surface temperatures in equatorial regions at times reach 30C on Mars, but at these temperatures the relative humidity of the atmosphere is low and any liquid exposed to such temperatures would soon evaporate. Most of the proposed habitats require Psychrophiles - microbes that are comfortable in low temperature conditions. This is a limiting factor especially for some of brines, which may be liquid at temperatures too low for life on Earth.

Other authors also cite:

• Lack of nitrogen. All life on Earth requires nitrogen. Also there are theoretical reasons for expecting alien organic life to use nitrogen, as the weaker nitrogen based amide bonds are essential for the processes by which DNA is replicated. Mars, compared with Earth, has little nitrogen, either in the air or in the soil. Levels of nitrogen in the air are low, possibly too low for nitrogen fixation to be possible. But they can form in Martian conditions by non biological processes - either brought to Mars by meteorites (some carbonaceous chondrites are rich in nitrogen), or comets, or formed by lightning, or through atmospheric processes, or there may be ancient nitrate deposits from early Mars, amongst various possible sources.
  
  Life on Mars may be limited to locations with local abundance of nitrates. Or, it may also be able to take advantage of fixation of nitrogen in monolayers of water, a process that can happen in present day Mars conditions, and may be able to produce enough nitrates to supply a subsurface biosphere.

Schuerger also mentions:

• Cosmic radiation - this is not limiting of surface life in the short term (similar to the levels inside the ISS) but prevents it from reviving if kept dormant for periods of order of hundreds of thousands of years. Martian surface or near surface life is likely to be strongly resistant to cosmic radiation, with repair mechanisms to repair the damage.

Curiosity measured ionizing radiation levels of 76 mGy a year. This level of ionizing radiation is sterilizing for dormant life on the surface of Mars. However, it varies considerably in habitability depending on its orbital eccentricity and the tilt of its axis. If the surface life has been reanimated as recently as 450,000 years ago, which is possible, then our rovers on Mars could find dormant but still viable life at a depth of only one meter below the surface, according to an estimate in the paper that published the Curiosity ionizing radiation measurements. Modern researchers do not consider that ionizing radiation is a limiting factor in habitability assessments for present-day non-dormant surface life. The level of 76 mGy a year measured by Curiosity is similar to levels inside the ISS. In the 2014 Findings of the Second MEPAG Special Regions Science Analysis Group, their conclusion was:

"From MSL RAD measurements, ionizing radiation from GCRs at Mars is so low as to be negligible. Intermittent SPEs can increase the atmospheric ionization down to ground level and increase the total dose, but these events are sporadic and last at most a few (2–5) days. These facts are not used to distinguish Special Regions on Mars."

Here a SPE is a Solar Proton Event (solar storm) and a GCR is a Galactic Cosmic Ray. A "Special Region" is defined as a region on the Mars surface where Earth life could potentially survive.

Other conditions that apply locally, rather than globally include:

• High salinity is a factor for any life within the salty brines - many of the proposed surface habitats are salty and could only be inhabited by Halophiles - microbes that are comfortable with high levels of salinity, such as Halobacteria.
• The pH (acidity) and Eh (oxidation potential) of any available liquid water
• presence of heavy metals
• acidic conditions in some soils
• oxidizing soils created by soil chemical reactions rather than UV (e.g. by anoxic hydration of pyrite)
• UV-induced volatile oxidants (e.g. O₂, O, H₂O₂, NO_x, O₃).
• Perchlorates. At high temperatures perchlorates are extremely oxidising and dangerous to life. But at the low temperatures of the Mars surface, then they are not so damaging and could actually be a benefit for microbes as an energy source. . For a modern view on them, Cassie Conley, planetary protection officer for NASA is quoted in the New York times as saying:

"The salts known as perchlorates that lower the freezing temperature of water at the R.S.L.s, keeping it liquid, can be consumed by some Earth microbes. “The environment on Mars potentially is basically one giant dinner plate for Earth organisms,” Dr. Conley said."

Lowest temperature for life on Mars

Based on the capabilities of Earth microbes, the usually cited lowest temperature for life is -20°C. However, there is indirect evidence of continuing bacterial activity in glaciers down to -40°C, at a very low metabolic rate of ten turnovers of cellular carbon per billion years.

There can be some activity at even lower temperatures. In an experiment to test incorporation of the amino acid Leucine, Karen Junge et all used two controls at -80°C and -196°C, well below the eutectic freezing point of salt, and to their surprise, they found that the Colwellia psychrerythraea strain 34H was able to continue to incorporate low levels of Leucine right down to -196°C. They hypothesize that the Leucine enters the cell boundaries at higher temperatures in the first few seconds of the experiment, then gets incorporated into the cell at lower temperatures (it doesn't get incorporated right away as they proved through zero time controls).

Price et al did a review of the literature to date, in 2004, and came to the conclusion that there is no evidence of a fixed lowest temperature limit to metabolism, in the presence of impurities and thin films of water to supply liquid to microbes.

"Our results disprove the view that the lowest temperature at which life is possible is ≈-17°C in an aqueous environment, as well as the remark that “the lowest temperature at which terrestrial and presumably martian life can function is probably near -20°C. Our data show no evidence of a threshold or cutoff in metabolic rate at temperatures down to -40°C. A cell resists freezing, due to the “structured” water in its cytoplasm. Ionic impurities prevent freezing of veins in ice and thin films in permafrost and permit transport of nutrient to and products from microbes. The absence of a threshold temperature for metabolism should encourage those interested in searches for life on cold extraterrestrial bodies such as Mars and Europa."

Lowest water activity level for life on Mars

The amount of water available for microbes to use in a salty or sugary solution is known as its water activity level which is normally expressed as the ratio of the partial vapour pressure of the water in the solution to the vapour pressure of pure water at the same temperature.

The tiny Don Juan pond in Antarctica, 100 meters by 300 meters, and 10 cm deep. This pond is about as salty as it could possibly be, with the CaCl₂ levels approaching saturation at 60% w/v. It's so salty it stays liquid all the year round, at temperatures ranging from 0°C to -40°C. It has a eutectic of -51.8°C so is believed to be liquid all the year round. The water activity level measured is an exceptionally low 0.3 - 0.6. Though the temperature range is fine for life, it may be too salty for life to reproduce there. Microbes have been found, but they could only grow in less salty conditions. It might be that microbes sometimes can grow there when the water activity level is occasionally raised through influxes of water, and then die. Or they might be washed in from the surroundings.

So far there is no evidence that microbes can actually grow there. It might be the only natural body of water on the Earth of any size without indigenous life. It's of great interest to scientists studying the water activity limits of habitability for astrobiology.

Honey has a low water activity level of 0.6. That's why honey doesn't spoil - you don't need to keep honey in a fridge, because its water activity level is so low that though microbes would find plenty to eat, and though there is plenty of water there in the honey, the water is not available to the microbes because of the low water activity level.

The fungus Xeromyces bisporus can tolerate a water activity level of 0.605 in sugar - first discovered in a 1968 study of spoilage in prunes. It can divide at these low water activity levels, so can germinate, but needs higher levels of water to create fungal spores through asexual sporulation and even more for sexual sporulation.

Until recently, it was thought that that was an isolated case, as no other microbe was known able to tolerate such levels. The usually accepted lower limit was 0.755 for halophiles. However over the last few years there have been many reports of microbes at lower activity levels than that, comparable to Xeromyces bisporus, in salt solutions. Some papers have suggested the possibility of cellular reproduction at even lower levels.

In a recent 2014 survey paper of the literature on the subject "Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life", the authors came to the conclusion that the best consensus at present is that the lowest level of water activity needed for cell division is about 0.605, and that some halophiles are able to tolerate such low levels. They remark on the difference between the situation for water activity and the situation for temperatures, where there is much better evidence of microbes able to tolerate temperatures below the usually cited -20°C.

Challenge of ionizing radiation

Radioresistant microbes are able to repair damage due to the equivalent of several hundred thousand years of Mars surface cosmic radiation within a few hours of revival from dormancy (they don't need to reproduce to do this, they repair their own DNA). For details: ionizing radiation resistance (radiodurans). This mechanism seems to be a byproduct of desiccation resistance, since the microbes have no need to tolerate high levels of cosmic radiation - shielded by the Earth's magnetosphere and atmosphere. Martian microbes are likely to have similar mechanisms, this time evolved in the presence of ionizing radiation.

At times the Mars atmosphere is thicker than it is now depending on variations in its orbital eccentricity and axial tilt. At other times it has extensive ice sheets which then melt. Research in january 2015 by Dickson et al suggests that the Mars axial tilt has varied beyond the 30 degrees to the point where it has thin glacier like ice sheets at the mid lattitudes within the 400,000 to 2 million years, and that this may have carved some of the older gully systems through melt water.

That's still challenging for life with a maximum of around 500,000 years dormancy on the surface. However the cosmic radiation only penetrates a few meters into the ground, with most of the effects shielded in the top 1.5 meters (400 grams per cm² of material, at 2.6 grams per cm³ typical regolith density) and significant shielding at a depth of half a meter. Below that depth, there could be dormant microbes that have survived for longer periods. Depending on the depth below the surface they could remain dormant for millions of years. Some microbes on Earth have lasted for many millions of years in ice and salt, and have been revived. So some of these on Mars also may still be viable today. Such microbes could also survive in caves on Mars in dormancy, or in subsurface locations kept habitable by geothermal hot spots, until times when Mars is more habitable than it is today.

If there is present day life on the Mars surface, these effects of ionizing radiation suggest that it has to

• Be replenished from the subsurface
• Or be able to reproduce in surface or near surface conditions with dormancy periods never longer than 500,000 years or so.

In the 2014 MEPAG classification of special regions, ionizing radiation was not considered limiting for classifying the "Special regions" where present day surface life might survive.

Views on the possibility of present day life on or near the surface

It is a challenge for life to survive on the surface, or the near subsurface, because of the hyper arid conditions, combined with low temperatures. Often when the temperature is high enough for cellular division, the humidity is too low and vice versa. . Also in surface conditions, it's not possible for microbes to remain in dormancy through the changes in axial tilt when the Mars atmosphere becomes thicker and more habitable (as it does from time to time).

Authors in recent publications present a variety of views on the possibility of present day life on the surface of Mars or in the near subsurface.

• Unlikely - these authors cite the inability of microbes to survive dormancy on the surface between periods when the atmosphere is thicker, due to ionizing radiation, the ephemeral nature of surface habitats, low temperatures, or low relative humidity, and the difficulty of colonization in surface conditions of high UV...

• Possible, recolonized from below, these point out the ability of micro-organisms to repair damage by ionizing radiation and capability to remain dormant for up to several million years in the deep subsurface, suggesting that these short lived surface habitats, such as the Recurring Slope Lineae, could be recolonized from the subsurface.

• Possible, open question if it proliferates on the surface these are investigating the possibility with experiments in simulated Mars conditions, theoretical models and study of the observations from Mars, and treat it as an open question for now, whether the present day surface and near sub surface is habitable. ( and many others).

• Likely Some researchers, particularly the researchers at DLR consider that their experiments have already shown a high likelihood that the surface of Mars is habitable, for some lichens and cyanobacteria, taking advantage of the night time humidity, and even in equatorial regions such as Gale crater. . See #Life able to take up water from the 100% night time humidity of the Mars atmosphere
• Already detected on the surface A small minority of authors believe that their reanalysis of the Viking Labeled Release experiments already indicates presence of life on present day Mars, see #Viking observations

There is greater agreement on deep subsurface habitats since conditions there may be similar to Earth conditions. They would be protected from UV, cosmic radiation, and the low pressure of the atmosphere, and water activity would be likely to be similar to Earth. For instance the deep hydrosphere (if it exists), or temporary lakes that form after impacts or volcanic eruptions, seem likely to be habitable, by analogy with similar habitats on Earth.

Plausible microbial metabolisms for present day Mars

One way to examine the possibility for life on Mars is to look at the Redox pathways that the life could use as a source of energy. This involves a pairing of an electron donor and an electron acceptor. For details see Electron transport chain, and Microbial metabolism.

Here is a table of some of the available donors and acceptors in Mars conditions, table from (added CO₂).

electron donors, any of:	electron acceptors, any of:
FeSO+ 2: available in Fe-rich silicates	Fe+ 3: available in numerous alteration minerals
H2: available in subsurface?	SO2− 4 available in salts
CO: available in atmosphere	O2: partial pressure too low
organics: meteoritic likely to be present at surface	NO− 3: presence or abundance unknown
organics: endogenous available in subsurface	ClO− 4: available but not shown to support life
-	CO2: in the atmosphere

A candidate metabolism would use one of the electron donors in the first column paired with one of the electron acceptors on the right as a source of energy. (The final dash on left hand side is there just because the list of electron donors is shorter than the list of electron acceptors).

See also the presentations in: Redox Potentials for Martian Life

Candidate lifeforms for Mars

This is a list of some of the proposed Mars analogue lifeforms, which may be capable of living on Mars (if the postulated liquid water habitats there exist).

Top candidates for life on Mars include

• Chroococcidiopsis - UV and radioresistant can form a single species ecosystem, and only requires CO₂, sunlight and trace elements to survive.
• Halobacteria - UV and radioresistant, photosynthetic (using a different mechanism), can form single species ecosystems, and highly salt tolerant. Some are tolerant of perchlorates and even use them as an energy source, examples include Haloferax mediterranei, Haloferax denitrificans, Haloferax gibbonsii, Haloarcula marismortui, and Haloarcula vallismortis
• Some species of Carnobacterium extracted from permafrost layers on Earth which are able to grow in Mars simulation chambers in conditions of low atmospheric pressure, low temperature and CO2 dominated atmosphere as for Mars.
• Geobacter metallireducens - it uses Fe(III) as the sole electron acceptor, and can use organic compounds, molecular hydrogen, or elemental sulfur as the electron donor.
• Alkalilimnicola ehrlichii MLHE-1 (Euryarchaeota) - able to use CO in Mars simulation conditions, in salty brine with low water potentials (−19 MPa), in temperature within range for the RSL, oxygen free with nitrate, and unaffected by magnesium perchlorate and low atmospheric pressure (10 mbar). Another candidate, Halorubrum str. BV (Proteobacteria) could use the CO with a water potential of −39.6 MPa
• black molds The microcolonial fungi, Cryomyces antarcticus (an extremophile fungi, one of several from Antarctic dry deserts) and Knufia perforans, adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.
• black yeast Exophiala jeanselmei, also adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.
• Methanogens such as Methanosarcina barkeri - only require CO₂, hydrogen and trace elements. The hydrogen could come from geothermal sources, volcanic action or action of water on basalt.
• Lichens such as Xanthoria elegans, Pleopsidium chlorophanum, and Circinaria gyrosa - some of these are able to metabolize and photosynthesize slowly in Mars simulation chambers using just the night time humidity, and have been shown to be able to survive Mars surface conditions such as the UV in Mars simulation experiments.
• Microbial life from depths of kilometers below the surface on the Earth that rely on geochemical energy sources - relying on metabolic pathways that can't be traced back to the sun at all. Some of these are multi-cellular. Examples include the microbe Desulforudis audaxviator which metabolizes reduced sulfur as the electron acceptor, and hydrogen as the electron donor, can fix nitrogen and has every pathway needed to synthesize all the amino acids
• Multicellular life from depths of kilometers below the surface such as Halicephalobus mephisto, a nematode feeding on bacteria, 0.5 mm long and up to 3.5 km deep, lives in water at 48°C, very low oxygen levels about a thousandth of the levels in oceans. Though it probably originates from the surface, carbon dating shows it has lived at those depths for between 3,000 and 10,000 years, and it's been suggested that this has implications for deep subsurface multi-cellular life on Mars.

Most of these candidates are single cell microbes (or microbial films). The closest Mars analogue habitats on Earth such as the hyper arid core of the Atacama desert are inhabited by microbes, with no multicellular life. So even if multicellular life evolved on Mars, it seems that most life on Mars is likely to be microbial.

Because of the low levels of oxygen of 0.13% in the atmosphere, and (as far as we know) in any of the proposed habitats, all the candidate lifeforms are anaerobes or able to tolerate extremely low levels of oxygen. This also makes multicellular animal life unlikely, though not impossible as there are a few anaerobic multi-cellular creatures. Some multicellular plant life such as lichens, however, may be well adapted to Martian conditions (the algae supply oxygen for the fungus). Also some multicellular life such as Halicephalobus mephisto can survive using very low levels of oxygen which may perhaps be present in some Mars habitats.

Expose R2 test of candidate lifeforms for Mars on exterior of ISS

Several lifeforms, including cyanobacteria Nostoc sp. Gloeocapsa Chroococcidiopsis sp., lichens Buelia frigida, Circinaria gyrosa, and fungi Cryomyces antarcticus, are currently being tested in the on-going year and a half Expose-R2 experiments in a small Mars simulation chambers on the exterior of the ISS (Expose R2) as part of BIOMEX (Biology and Mars Experiment).

Expose R2

Installing the Expose-R2 facility on the International Space Station. Image taken with the helmet camera of Oleg Artemyev, on 18 August 2014, EVA-39 - Image credit:Roscosmos

Some of these simulation chambers are kept with atmosphere and filters to simulate Mars conditions of UV, and in some of the chambers they are in Mars simulation soil to simulate the Mars surface. Others are exposed to vacuum, e.g. to test panspermia hypotheses.

Test samples include bacteria and biofilms, cyanobactera, archaea, green algae, lichens, fungi, bryophytes, and yeast that have been found to be especially resistant in ground experiments and previous experiments on the ISS. They also include pigments and cell wall components.

The experimenters are studying the same organisms in Mars simulation chambers on the ground. The experiment has multiple goals - to find out what species could survive transfer to another planet on a meteorite (panspermia), to find out what detectable biosignatures would remain after exposure to space and to Mars surface conditions, and to find out their ability to survive in these conditions and possible genetic changes.

Uninhabited habitats

Charles Cockell has analysed the possible trajectories for life on Mars using the idea of an "uninhabited habitat". On Earth these are exceedingly rare, but do occur sometimes. For instance, after a new lava flow, then the lava may initially be inhabitable but uninhabited.

It's possible to test the hypothesis that these habitats exist by finding environments on Mars with the elements needed for life, including an energy source and liquid water, with no active life.

So then there are three states for Mars:

1. Uninhabitable - doesn't have the conditions for life
2. Has habitats but they are all uninhabited
3. Has at least some habitats with life

As Mars evolved, initially when it first formed in the early solar system, it was too hot for life, and so was uninhabitable. Then there are various trajectories it could follow after that, starting from the early Mars. In his paper "Trajectories of Martian Habitability" he identifies six main possible trajectories. T

• "Trajectory 1. Mars is and was always uninhabitable."
• "Trajectory 2. Uninhabited Mars has hosted uninhabited habitats transiently or continuously during its history."
• "Trajectory 3. Uninhabited Mars was habitable and possessed uninhabited habitats but is now uninhabitable."
• "Trajectory 4. Mars is and was inhabited."
• "Trajectory 5. Mars was inhabited, life became extinct, but uninhabited habitats remain on Mars."
• "Trajectory 6. Mars was inhabited, life became extinct, and the planet became uninhabitable."

He also suggests other more complex trajectories. For instance that it starts with uninhabited habitats and the life evolves there at a much later date, or is seeded from Earth at a later date. Or trajectories where life on Mars becomes extinct, and then reoriginates on Mars or is transferred to Mars from Earth. Or even, a logical possibility but seems unlikely, that it was for some reason uninhabitable in the early Noachian and became habitable later.

In his paper he discusses ways that this could be tested with observations. For instance, if you find that promising environments with water in present day and past Mars lacked some fundamental requirement for all known life, or the conditions were outside the range of physical and chemical tolerances of all known organisms, then that could be evidence for trajectory 1. If you find conditions for life but no life, past or present, that's evidence for trajectory 2, and so on.

He points out that if Mars does have uninhabited habitats, these would be a useful control to investigate the role of biology in planetary scale biological processes on Earth.

Also to cover the pre-biological investigations in case that habitats are found that are habitable but with no life in them.

https://astrobiology.nasa.gov/media/medialibrary/2013/09/AB_roadmap_2008.pdf

GOAL 3—Understand how life emerges from cosmic and planetary precursors. Perform observational, experimental, and theoretical investigations to understand the general physical and chemical principles underlying the origins of life.

Characterize the exogenous and endogenous sources of matter (organic and inorganic) for potentially habitable environments in the Solar System and in other planetary and protoplanetary systems.

Search for a second genesis of life on Mars

The search for life on Mars is of special interest for the search for a second example of life, which can help us to discover which of the many common shared features of the biochemistry of Earth organisms are essential for life, and which are accidents of evolution. Chris McKay puts it like this in his 2010 article "An origin of life on Mars.":

"The search for a second example of life is a key goal for astrobiology. All life on Earth shares common biochemistry and descends from a common ancestor. This prevents us from understanding which aspects of biochemistry and genetics are essential features of life and which are merely particular to the evolutionary history of life on this planet. To develop a more general understanding of life, we need more than one example. Hence, we hope that Mars may have been the site of an independent origin of life."

Until recently, it was assumed that any life on Mars would necessarily be a second genesis. But it is now understood that life could be transferred between planets on meteorites, so it is possible that life on Mars, if it exists, could be related to Earth life, or some of the life could be related to Earth life.

In order to decide whether the life is a second genesis or not, it's not enough to examine fossils. For one thing, microbes often don't form easily recognized convincing fossils, so the fossils may be hard to recognize, and rare in occurrence. But as well as that, fossils don't tell us what the chemical basis is for the life.

It's necessary to be able to study organics, and it preferably, viable cells. If life on Mars had same chirality, genetic code, choice of amino acids, lipids and so on, that would be evidence of a shared ancestry. If any of those differ, then it is likely to represent a second genesis.

Writing in 2010, Chris McKay says

"Possible targets include: (1) Life in the surface soil, (2) Life in subsurface liquid water, (3) Organisms, probably dead, but preserved in ancient salt or mineral deposits, and (4) Organisms, dead or alive, preserved in ancient ice."

Organics are common throughout the outer solar system, including meteorites, and comets. So when organics are found on Mars, the first thing to be decided is whether or not it is biological in origin. If it is related to Earth life, and sufficiently well preserved, this can be detected though search for DNA, RNA, ATP and other key molecules associated with life on Earth. But if it is not related to Earth life, then it may be harder to decide whether it is the result of biological processes.

One way to detect alien biology may be through the "Lego principle". This is the idea that chemicals used by life may be recognized because they use a wide range of chemicals with similar chemical structure, and chemicals very similar to each other (e.g. only differing in chirality) may have widely different concentrations. This is something that could be recognized even if the life has a different chemical basis from Earth life.

However, over time, the pattern degenerates as chemical bonds break and reform, especially in warmer conditions. So ideally we need to find life that is either alive, or has been preserved in cold conditions since it was deposited.

In his 2010 article, Chris McKay suggests targeting possibly still viable organisms preserved in ancient subsurface ice. This is also the main target for his proposed mission Icebreaker Life.

Even a null result in search for life on Mars would be of astrobiological significance. For instance it might tell us that the origins of life depend on particular conditions not present on Mars. For instance that it depends on a particular energy source, or material or on abundance of some particular nutrient (e.g. nitrogen).

Planetary protection issues

Search for present day life on Mars requires more stringent planetary protection than the search for past life. For instance, if Curiosity were to discover traces of liquid water on Mars, some micro habitat in conditions that make it potentially habitable to Earth life, it would not be able to approach it to measure it to search for present day life as it is not sufficiently sterilized for this task.

Regions of Mars that may be habitable for present day life are classified as "Special regions" and any parts of a spacecraft that touch such regions have to be sterilized to Viking levels of sterilization or better. So far no modern spacecraft have yet been sterilized to these levels. It is a major challenge as the heat treatment used for Viking would destroy many modern instruments. However low vapour hydrogen peroxide sterilization may be able to take the place of heat treatment - it is already approved for spacecraft use. As well as that there are emerging new ideas for sterilization that may be more effective with less damage to the spacecraft, such as use of ionized gas in vacuum conditions.

Follow the nitrogen

The best way to search for early life, as far as we can tell at present, is to search for organics. And the organics is easily confused with organics from non life processes and from space.

One of the main conclusions of Bada et al's white paper was that we should look for organics with nitrogen on Mars. Nitrogenous organics are likely to be rare because there are few sources of nitrogen on Mars.

This is important because nitrogen bonds are easily broken and are central to biology as we know it. So even if life on Mars is very different from Earth life, perhaps using different amino acids for instance with a different backbone from DNA, still it is likely to use nitrogen if it resembles Earth life.

Once we find these compounds, that's not enough as you also get nitrogenous organics from comets and meteorites and natural processes. We then need to search for biosignatures.

We also need to be able to drill below the surface (as ExoMars will be able to do) to the maximum depth possible. That's because our best chance of finding evidence of past life is to drill down below the surface layers damaged by ionizing radiation, ideally to ten meters depth or more (though the two meters depth of ExoMars is a good start here). Their main points are:

• Need for increasing mobility, and precision landing, supported by orbital observations, to access the many and varied habitable environments including subsurface, layered sediments, gullies and ice sheets.
• The "follow the water" strategy should now be followed by a "follow the nitrogen" phase combined with a search for biosignatures.
• The biosignature search can use exquisitely sensitive in situ electrophoresis techniques to identify and characterize and find the chirality of amines, nucleobases, polycyclics and other essential organic molecules.
• This search should include drilling to the greatest depth possible for the best chance of success for detecting biosignatures of past life on Mars
• They recommend that we should do a sample return only after we either identify biosignatures on Mars, or have exhausted all other possibilities by in situ research

Curiosity's observations of nitrous oxides, probably result of breakdown of nitrates

Curiosity has detected evidence of nitrates in both scooped wind drifted sand and samples drilled from sedimentary rocks. The results support 110–300 ppm of nitrate in the wind drifted samples, and 330–1,100 ppm nitrate in the mudstone deposits. The authors suggest that it is likely to be the result of fixation during meteorite impact or lightning associated with volcanoes in early Mars.

Curiosity's observation of complex organic compounds

Results from the Curiosity SAM instrument presented in March 2015 show presence of what may be a fatty acid molecule. Also confirm presence of chlorobenzene. Neither of these are biosignatures, for instance organisms use fatty acids to build cell membranes, but they can also have inorganic origins. But they show that complex organics can survive on the surface of Mars, so increasing the chance of later detecting microbial life on the surface if it is there.

Planned and proposed missions to search for present day life on Mars

Past missions

Viking 1 and 2 are the only successful missions to Mars to date designed to search for present day life.

The failed British mission, Beagle 2, had the search for present day life as an objective as well as past life.

Present missions

Curiosity (rover) and Opportunity (rover) are currently searching for habitable conditions with the main focus on past habitability. However they are not equipped to detect biosignatures of life, either past or present, and also were sent to sites selected with past rather than present day life as the main target.

Curiosity does have some capabilities that could be of interest for life detection. It can detect isotope ratios in organics or in the methane plumes suggestive of life which could give indirect evidence of life processes as life preferentially incorporates lighter isotopes.

Also Curiosity has one experiment that can potentially be used to detect chirality if it finds a potentially interesting sample to test. It has a Chirasildex column which can be used to separate out entantiomers of astrobiological significance.

Future missions

The ExoMars Trace Gas Orbiter will help with the search for trace levels of organics in the atmosphere, with sensitivity up to a thousand times greater than previous missions. Detection sensitivities are at levels of 100 parts per trillion, improved to 10 parts per trillion or better by averaging spectra which could be taken at several spectra per second . This would lead to global mapping of distribution of methane and other organics in the atmosphere which could help to pinpoint sources on the surface.

ExoMars is also designed to search for both present day and past life. It will have capabilities to test for biosignatures "in situ" on Mars. It's most interesting innovation is its capability to drill to depths of up to 2 meters which is of special interest for the search for past life. However its target regions have been selected with the search for past life as its prime objective, so it will only discover present day life if it is widespread on Mars. Its primary candidate landing site is Oxia Planum which is of interest for its multiple layers of clays which may preserve evidence of past life on Mars.

Mars 2020 is designed for sample caching for a future sample return. The payload mass is the same as for Curiosity, so to make space for the cache, as well as Moxie (an experiment in producing oxygen from the Mars atmosphere), it has a reduced mass of instruments compared with Curiosity (rover). In particular, they have removed Sample Analysis at Mars.

However in other respects it will have increased capabilities including more capable cameras, and possibly a "helicopter scout" to search local terrain up to a kilometer away from the rover. Of special for exobiology, it will have two Raman spectrometers, first to fly to the planet (except for ExoMars if they get there first). One of them is on SHERLOC which will be positioned right next to the rock to be analysed, and uses a spot of ultraviolet laser light to micro-map minerals and organics on the samples on the scale of 50 microns. A Raman spectrometer gives information about arrangements of atoms such as a carbon atom double bonded to oxygen, but it can't detect specific molecules in the sample like SAM. Also, another significant advance, its SuperCam, replacement for the remote laser analysis instrument ChemCam on Curiosity, will not only be able to heat its target like ChemCam and analyse the plasma cloud that results. It will also have Raman and time-resolved fluorescence spectroscopy. These will enable it to map the distribution of organics on the surface of Mars at a distance, a significant advance over Curiosity which can only detect organics by heating it in its oven after sample collection. It also means it won't be confused by perchlorates destroying the organics on heat.

As for Curiosity, the target will be selected with past life as its prime objective and neither ExoMars nor Mars 2020 are sufficiently sterilized to approach and examine any possible habitats for present day life such as the RSLs

Proposals for missions

Icebreaker Life is a mission suggested by Chris McKay to search for past life preserved in ice on Mars, and present day life on Mars.

ExoLance is an ingenious proposal that uses ground penetrating "lances". Curiosity carries ballast in the form of two 75 Kg tungsten weights, which it discards on arrival at Mars to help with the asymmetric trim of the aeroshell, to generate a lifting vector. That is how it manages to achieve higher precision than other missions to date. See MSL - guided entry. The idea is to put impact resistant instruments into these weights, and make them into ground penetrating "missiles" able to penetrate to a depth of a meter or so, where conditions may be more favourable for preservation of life.

NASA propose to return samples to test for biosignatures and signs of life back on Earth. Their Mars 2020 mission is designed to cache the sample, for later return to Earth by some future mission.

Instruments designed to search for present day life on Mars "in situ"

Instruments designed to search directly for this life include

Rapid non destructive sampling

• Raman spectrometry - analyses scattered light emitted by a laser on the sample. Non destructive sampling able to identify organics and signatures for life.

Detection of trace levels of organics and of chirality

• Gas chromatography - this is the idea used for the MOMA (Mars Organic Molecule Analyser). Used to analyse volatiles evolved from the soil samples in small ovens. Some of the ovens are filled with a "derivatisation agent" which can transform the chemical compounds into similar ones suitable for chiral analysis. They are then ionized and analysed with the mass spectrometer.
• UREY - designed for ExoMars under auspices of NASA but never flown because the US pulled out of the project. This uses high temperature high pressure sub critical water between 100°C and 300°C at 20 MPa, or about 200 atmospheres, for several minutes. Water has similar chemical properties to organic solvents in those conditions so Urey is able to study the organics relatively unmodified.
• Astrobionibbler - similar idea to UREY, smaller, later development. Able to detect a single amino acid in a gram of soil.
• Planetary In-situ Capillary Electrophersis - separates the organics by ionic mobility in sub millimeter capillaries. "Lab on a chip" with the fluid manipulations done within the chip itself.
• LDChip, and Solid3 using a collection of 450 polyclonal antibodies to detect a wide range of organics (not specific to Earth life). . This instrument was tested in the Atacama desert and was able to detect a layer of previously undiscovered life at a depth of 2 meters below the surface in the hyper-arid core of the desert.. As the "Life Marker Chip" it was selected for ExoMars but later descoped.

Direct search for DNA

These can detect life on Mars if it is DNA based so related to Earth life. As DNA sequencers, they can sequence the entire genome of any lifeform found.

• Miniaturized DNA sequencers

Electron microscope

• Miniaturized Variable Pressure Scanning Electron Microscope (MVP-SEM)

Search for life directly by checking for metabolic reactions

These can detect life even if it doesn't use any recognized form of conventional life chemistry. But requires the life to be "cultivable" in vitro when it meets appropriate conditions for growth.

• Microbial fuel cells, test for redox reactions directly by measuring electrons and protons they liberate. Sensitive to small numbers of microbes and could detect life even if not based on carbon or any form of conventional chemistry we know of.
• Chirality version of the Viking Labeled Release. For carbon based life which produces gases such as methane or carbon dioxide when fed amino acids, but doesn't need to be DNA based life.

See also

• Life on Mars
• Present day Mars habitability analogue environments on Earth

External links

• Three days long conference on the subject in 2013 The Present-Day Habitability of Mars 2013 under the auspices of the UCLA Institute for Planets and Exoplanets - with video archived for all the talks.