User:Robertinventor/Water on Mars Habitability

Habitability assessment
Present day life on Mars could occur kilometers below the surface in the hydrosphere, or in subsurface geothermal hot spots, or it could occur on or near the surface. The permafrost layer on Mars is only a couple of centimeters below the surface. Salty brines can be liquid a few centimeters below that but not far down. Most of the proposed surface habitats are within centimeters of the surface. Any life deeper than that is likely to be dormant. Water is close to its boiling point even at the deepest points in the Hellas basin, and so cannot remain liquid for long on the surface of Mars in its present state, except when covered in ice or after a sudden release of water. The Mars atmosphere varies in pressure over geological timescales, and may have been able to host liquid water in the recent geological past.

So far, NASA has pursued a "Follow the water" strategy on Mars and has not searched for biosignatures for life there directly since Viking. The observations by Phoenix in 2008 of potential drops of liquid brines forming on its legs led to a renewed interest in the potential habitability of the surface of Mars. Nilton Renno and his team recently found a way that these droplets could form rapidly when salt and ice touch each other so may have formed when salt and ice from the surface got thrown up onto the legs during the landing. Since then, experiments have led to many suggestions for potential habitats on the surface of Mars. However, though liquid water is now confirmed to occur there in brine layers, it's not yet known whether any of the liquid water on Mars is habitable. This depends on factors such as the exact mix of salts and the local conditions on Mars.

Proposed surface habitats
This is sorted roughly according to the level of attention in the literature.


 * 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: "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.


 * 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. 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. There is strong evidence now that they are associated with liquid water, due to seasonal changes in hydrated salts.    They may also be habitable but this depends on the salinity and the temperature of the water. They are currently classified as "Uncertain regions to be treated as special regions" for purposes of planetary protection. A "special region" is a region where present day Earth life could potentially survive on the surface of Mars. See  Habitability section of Seasonal flows on warm Martian slopes for details.


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


 * 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. They probably occur over much of its surface . Perchlorates deliquesce more easily than chlorides and at a lower temperature, so they could, potentially, take up water from the atmosphere more readily. The Mars atmosphere is less than 1% of the atmospheric pressure for Earth, still it reaches 100% humidity at night due to the low nighttime temperatures. Thin layers of salty perchlorate rich brines could form a short way below the surface at night and in the early morning. Since the humidity is taken directly from the atmosphere, this does not require the presence of ice on or near the surface. Some microbes on the Earth are able to survive in this way, for instance 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, some Haloarchaea are able to tolerate them, and some 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. 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. . However the brines detected by Curiosity, though they get warm enough for Earth life, are then thought to be too salty for life, and when the water activity is high enough, they are too cold. Such cold brines may be habitable to native life if it has hydrogen peroxide or perchlorates as part of its biochemistry.  Other deliquescing salts on Mars may be more habitable for Earth life.
 * 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. . 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, a process that has been been observed in Antarctica.


 * Southern hemisphere flow like features (Not to be confused with the Northern Hemisphere flow like features which form under different conditions). These flow out of the dark deposits that form after the dry ice geysers erupt in early Spring. They grow at a rate of around 1.4 meters per Martian sol. All the models for these features, to date, involve some form of water. 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. 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, where 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. This 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. 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. This solid state greenhouse effect process favours equator facing slopes at higher lattitudes, close to the poles, over lower lattitudes, where surface ice can form to thicknesses of tens of centimeters. (The examples at Richardson crater are only 18° from the south pole. ). Another model for these southern hemisphere flow like 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.This forms on the surfaces of solar heated grains in the ice, which then flows together down the slope, and can supply several litres a day of water to the seepage flows. . This ULI water would be the water source for liquid brines which then flow down the surface to form the features.


 * 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. They 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 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.
 * 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. The water source is deliquescing salts.

Proposed subsurface habitats

 * Ice covered lakes that form in polar regions after large impacts 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 C/2013 A1 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.
 * Possibility of geological hot spots in present day Mars There is clear evidence that Mars is not yet geologically inactive This includes, small scale volcanic features associated with some of the volcanoes on Mars which must have formed in the very recent geological past . There's also 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. 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." 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. She also points out a few processes that may be unique to Mars, for instance, 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. The lava tubes on Mars are far larger than the ones on the Earth. Also 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.
 * 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 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  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. . If it 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. 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"

Dormant subsurface life
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.

Habitability factors for non-dormant surface life
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 a region where Earth life could potentially survive.

UV radiation
On UV radiation, the report finds


 * "The martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completely by < 1 mm of regolith or by other organisms."

Perchlorates
Though the superoxidizing conditions are harmful to some microbes, there are many microbes that actually metabolize perchlorates on Earth. See Perchlorates - Biology. Nowadays perchlorates on Mars are generally thought as boosting habitability. Even when Phoenix discovered perchlorates in 2008, NASA said that the perchlorates do not rule out life on Mars. 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.""

Recurrent Slope Lineae - potentially habitable
These features form on sun-facing slopes at times of the year when the local temperatures reach above the melting point for ice. The streaks grow in spring, widen in late summer and then fade away in autumn. This is hard to model in any other way except as involving liquid water in some form, though the streaks themselves are thought to be a secondary effect and not a direct indication of dampness of the regolith. 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"

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. 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.