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Now published as [[Sponges on Mars? We ask Stamenković about their oxygen-rich briny seeps model]]
 
Please don't share. For publishing to my own wiki and blog. This article is mid edit. If you spot any errors be sure to say! [[User:Robertinventor|Robertinventor]] ([[User talk:Robertinventor|talk]]) 13:24, 15 December 2018 (UTC)
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{{date|NovemberOctober 925, 20182019}}
Planetary scientist Vlada Stamenković of the [[NASA]] [[Jet Propulsion Laboratory]] and colleagues have developed a new chemical model of how oxygen dissolves in salty brines in the cold [[Mars|Martian]] conditions,. whichThey raisesfound that the possibilitytrace amounts of oxygen in its thin atmosphere get concentrated into oxygen-rich brines; enough,once they reach equilibrium with the atmosphere. Their work suggests, that the coldest brines have enough oxygen to support simple animals such as sponges. The model was published in ''{{w|Nature (journal)|Nature}}'' on October 22. ''Wikinews'' caught up with him in an email2018. interview to find out more about his team's research and their plans for the future.
[[File:Vlada Stamenković.jpg|thumb|left|Dr. Vlada Stamenković{{image|[[Jet Propulsion Laboratory|JPL]]}}]]
 
''Wikinews'' caught up with him in an email interview to find out more about his team's research and their plans for the future.
This is an expanded verson of the Wikinews article [https://en.wikinews.org/wiki/Simple_animals_could_live_in_Martian_brines:_Wikinews_interviews_planetary_scientist_Vlada_Stamenkovi%C4%87 Simple animals could live in Martian brines: Wikinews interviews Vlada Stamenković] which I collaborated on, with more background information in it (I'm the volunteer reporter who interviewed him).
[[File:Vlada Stamenković.jpg|thumb|right|Dr. Vlada Stamenković [[Jet Propulsion Laboratory|JPL]] ]]
This is an expanded verson of the Wikinews article [https://en.wikinews.org/wiki/Simple_animals_could_live_in_Martian_brines:_Wikinews_interviews_planetary_scientist_Vlada_Stamenkovi%C4%87 Simple animals could live in Martian brines: Wikinews interviews Vlada Stamenković] which I collaborated on, with more background information, and his answers to some additional questions I asked via email (I'm the volunteer reporter who interviewed him).
 
==Intro==
==Background information and intro==
 
<!-- details of atmosphere of Mars in http://science.sciencemag.org/content/341/6143/263 -->
[[File:Vlada Stamenković.jpg|thumb|left|Dr. Vlada Stamenković - planetary scientist at JPL and lead author of the paper. Wikinews interviewed him about the new Mars research via email.]]
The {{w|Atmosphere of Mars|atmosphere of Mars}} is far too thin for us to breathe,. orIt indeed,would tonot extractbe anypossible oxygen at all in ourfor lungs. Itlike hasours onto averageextract onlyoxygen aroundat 0all.6% ofThe themoisture pressurelining ofour Earth'slungs atmosphere,would andboil itat iswell mainlybelow carbonblood dioxide;temperature only 0.146% of that is oxygen. Yetin the resultthin ofatmosphere their modeling was clear.(much Throughoutlike the surface of Mars there are possibilities for microbial lifeway that dependsyou oncan't oxygen,make anda ingood polar regions, perhaps 6.5%cup of thetea Martianat surface,Everest therebase maycamp bebecause enoughthe oxygenwater forboils simpleat animalstoo suchlow asa spongestemperature).
 
The Mars atmosphere has a pressure of only 0.6% of Earth's atmosphere, on average. Also it's mainly carbon dioxide; only 0.146% of that thin atmosphere is oxygen. Yet the result of their modeling was clear. In the cold conditions on Mars these minute amounts of oxygen can get into the salty seeps of water which may be present there. What's more,there's enough oxygen in the brines anywhere on Mars to support microbes that depend on oxygen (aerobes). There's a caveat here; they don't know how long it would take to reach those levels but once it reaches equilibrium with the atmosphere then that's how much oxygen there should be in the brines.
 
Oxygen permits a more energy intensive metabolism, and many microbes and almost all complex multicellular life on Earth depend on oxygen. They found that cold water would take up much more oxygen than warm water. In the coldest salty water in the polar regions pf Mars, covering perhaps 6.5% of the Martian surface, oxygen levels may be get high enough for simple animals such as sponges. It's a similar picture for the Earth, the tropical oceans, which are warmer, have less oxygen than our polar oceans.
 
As interviewed by Wikinews:
:: '''VS''': Our work really opens up new possibilities for the Martian habitability, and that’s why it’s so exciting!
 
As previously interviewed by National Geographic (October 22 2018):
 
::'''Vlada Stamenković''': We were absolutely flabbergasted. I went back to recalculate everything like five different times to make sure it's a real thing.
 
[[File:Warm Season Flows on Slope in Newton Crater (animated-cropped).gif|thumb|Seasonal changes in recurrent slope lineae on a steep slope in Newton Crater at 41.6 degrees south latitude on Mars. These long narrow dark streaks form on sun facing slopes when surface temperatures approach the melting point of ice. They extend in spring, broaden through the summer, and fade in autumn. Some water is involved as shown by the presence of hydrated salts, though there is debate about how much is due to water and how much to other processes such as dust flows. OneThis is one of theour best current candidates for possibly habitable salty water on [[Mars]].]]
 
Some background may help before we get to the main interview with Vlada Stamenković.
 
If you want to go straight to the interview itself, it's here:
'''''(background information):''''' You might wonder why they would research into these cold salty solutions. What about fresh water? Well, fresh water is likely to be rare on present day Mars. Over most of the surface fresh water is not stable at 0&nbsp;°C. The higher pressure at the depths of the huge ancient impact crater of the {{w|Hellas Planitia|Hellas basin}}, raise the boiling point to 10&nbsp;°C, but that still means that fresh water is close to boiling point already at 0&nbsp;°C, and would evaporate rapidly after any ice melts. <!--see Making a Splash on Mars-->
* skip to [[#Interview]]
 
===Why salty water?===
However, salty brines can be liquid at well below 0&nbsp;°C. Salts, and very salty brines can also take in water from the atmosphere, so counteracting the tendency of the water to evaporate at low pressures. They are especially good at doing this at low temperatures.
 
'''''(background information):''''' You might wonder why they focus their research on salty solutions. What about fresh water?
Curiosity discovered indirect evidence of this process in the equatorial regions (through humidity measurements). It is so dry that there isn't even any ice there, yet it found that brines form during winter nights in the top 15cm of the soil through {{w|Hygroscopy#Deliquescence|deliquescence}}. They take up water from the atmosphere at night when the salts reach temperatures of around -70&nbsp;°C. This water then evaporates again as the soil warms up through the day, and the process repeats every day - night cycle. <!-- "Evidence of liquid water found on Mars" in background information -->
 
Well, fresh water is likely to be rare on present day Mars. Usually the air pressure is so low that it is not stable even at just above freezing, at 0&nbsp;°C. The Mars atmospheric is thicker at the depths of the huge ancient impact crater of the {{w|Hellas Planitia|Hellas basin}} and other low points on the surface. This raises the boiling point of fresh water to 10&nbsp;°C, but that means that even there, it is close to boiling point already as soon as it melts at 0&nbsp;°C. At that temperature, the water would evaporate away rapidly, indeed the pressure there is so low that ice also isn't stable at 0&nbsp;°C either even in the Hellas basin. <!--see Making a Splash on Mars-->
There is indirect evidence for other salty brines on Mars, perhaps more habitable than the Curiosity brines. In their paper, Stamenković et al. mention the hydrated magnesium and calcium salts associated with the Recurring Slope Lineae. These seasonal streaks form in spring on sun facing slopes, extend and broaden through the summer and fade away in autumn. The streaks themselves are not damp patches, but they may be associated with thin seeps of brine just below the surface. Later research suggests dust flows may also be involved. However the hydrated perchlorate salts observation still has to be explained, as well as the seasonal timing, not correlated with the winds. It's considered to be good evidence that there is at least an element of seasonal hydration associated with the streaks. The literature on this topic has a vigorous dialog between researchers who favour greater or lesser elements of brines in this process.
 
However, salty brines can be liquid at well below 0&nbsp;°C, for the same reason salt helps keep roads ice free. These salts also counteract the tendency of the water to evaporate at low pressures. Not only that, salts can take in water from the atmosphere too, in the process known as {{w|Hygroscopy#Deliquescence|deliquescence}}, and take up water especially easily at low temperatures.
Before these new results, scientists assumed any present day Martian life would be able to grow without oxygen. There are several possibilities for this, based on Mars simulation experiments. Martian life could include certain blue-green algae such as {{w|Chroococcidiopsis#Mars colonization|chroococcidiopsis}}, some black fungi, and some purple salt loving {{w|Haloarchaea#As exophiles|haloarchaea}} found in salt ponds and hypersaline lakes on Earth. <!--for the black fungi, Zakharova et al paper in background information-->
 
Curiosity discovered indirect evidence of deliquescence in the equatorial regions (through humidity measurements). These regions are so dry that there is no ice in the surface soil. It is also dry to considerable depth, yet the Curiosity team found that brines form during winter nights in the top 15 cm of the soil when the salts reach temperatures of around -70&nbsp;°C. The water then evaporates again as the soil warms up through the day, and the process repeats every day - night cycle. <!-- "Evidence of liquid water found on Mars" in background information -->
 
===The recurring slope lineae===
 
There is indirect evidence for other salty brines on Mars, perhaps more habitable than the Curiosity brines. In their paper, Stamenković et al. mention the hydrated magnesium and calcium salts associated with the Recurring Slope Lineae. These seasonal streaks form in spring on sun facing slopes, extend and broaden through the summer and fade away in autumn. The streaks themselves are not damp patches, but they may be associated with thin seeps of brine just below the surface.
 
Later research suggests that dust flows may also be involved. However the hydrated perchlorate salts observation still has to be explained, as well as the seasonal timing, not correlated with the winds. This is considered to be good evidence that there is at least an element of seasonal hydration associated with the streaks. The literature on this topic has a vigorous dialog between researchers who favour greater or lesser elements of brines in this process.
[[Image:Martian conditions in miniature (7494313830) (2).jpg|thumb|Experiments by DLR (German aerospace company) in Mars simulation chambers and on the ISS show that some lichens such as {{w|Pleopsidium chlorophanum}} can survive Mars surface conditions and photosynthesize and metabolize, slowly, using only the humidity of the Mars atmosphere. The algal component provides oxygen for the fungal component, giving a way for multicellular life to survive without any oxgyen on Mars]]
 
===Significance of oxygen===
 
Before Vlada Stamenković and his team's new results, scientists assumed any present day Martian life in these and other habitats would have to be able to grow without oxygen. That leaves many possibilities for Martian life include certain blue-green algae such as {{w|Chroococcidiopsis#Mars colonization|chroococcidiopsis}}, some black fungi, and some purple salt loving {{w|Haloarchaea#As exophiles|haloarchaea}} found in salt ponds and hypersaline lakes on Earth. <!--for the black fungi, Zakharova et al paper in background information-->. Experiments in Mars simulation chambers by DLR in Germany found that lichens such as {{w|Pleopsidium chlorophanum}} may also have some potential to survive in the Mars surface conditions without oxygen. They can do this because the algal component of the lichen is able to make the oxygen needed by its fungal component.
 
However oxygen rich brines would permit a more energy intensive metabolism and perhaps even true multicellular animal life such as simple sponges. Almost all complex multicellular life uses oxygen.
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::'''VS''': Our work is calling for a complete revision for how we think about the potential for life on Mars, and the work oxygen can do, implying that if life ever existed on Mars it might have been breathing oxygen<!--Scientific American-->
[[File:Halichondria panicea.jpg|thumb|Halichondria panicea or the breadcrumb sponge- Stamenković et al's paper cites research by Mills et al using this sponge which showed it can survive with only 0.002 moles per cubic meter (0.064 mg per liter). This new research suggests that these concentrations can be achieved in {{w|Supercooling|supercooled}} brines on modern Mars in polar regions.]]
'''''(background information):''''' Stamenković et al in their paper cite research from 2014 that showed that some simple sponges can survive with only 0.002 {{w|Mole (unit)|moles}}per cubic meter (0.064 mg per liter) <!-- first page of Nature paper, "Meanwhile, whereas aerobic microbial life and simple animals need O<sub>2</sub> dissolved in liquids in sufficiently large concentrations to survive, recent experiments, observations and calculations have lowered the required limits of concentrations of dissolved O<sub>2</sub> for aerobic respiration to ~10−6 mol m−3 in microorganisms and to ~2 × 10−3 mol m−3 in sponges"-->. Some microbes that need oxygen can survive with as little as a millionth of a mole per cubic meter (0.000032 mg, or 32 nanograms per liter). In their model, they found that there can be enough oxygen for microbes throughout Mars, and enough for simple sponges in oases near the poles.
 
A historical 101 on multicellular life on Mars may be of interest here. First, of course back in the early twentieth century there was much speculation about multicellular life there, with Lowell even speculating that intelligent Martians built the canals that he thought he could see in his telescope. By the time of early spaceflight it was already clear that the atmosphere was far too thin for terrestrial animals, but there was some hope for plant life. But the early observations from space showed a barren crater covered land and since then the idea of life on Mars has focused mainly on anaerobic microbes and photosynthetic life.
 
===Lowest oxygen concentrations===
<!-- For next para: Vera et al in the background information is an example of papers on lichens such as Pleopsidium chlorophanum for Mars and the DLR source gives an overview of their experiments into lichens and blue-green algae-->
In 2014, however, Vera et al surprised many astrobiologists with their experimental result that multicellular life could be possible on present day Mars. But only photosynthetic life able to produce its own oxygen. Some lichens, such as {{w|Pleopsidium chlorophanum}} are able to survive in close to Mars-like conditions high up on Antarctic mountain ranges, and show promise in Mars simulation chamber experiments. They can do this because the algal component is able to make the oxygen needed by its fungal component. They can also do this without needing any extra water as intermediary, in the experiments they are able to grow in partial shade, using only the night time humidity of the atmosphere itself.
 
'''''Note:''''' The paper shows oxygen concentrations in moles per cubic meter. Other sources give the concentrations in miligrams per liter. To convert moles of oxygen per cubic meter to milligrams per litre multiply by 32.
In principle you could also have microscopic (sub millimeter sized) multicellular animals in anoxic conditions. There are only a three species of such creatures known on Earth, however, and they are not candidates for life on Mars. They are three species of {{W|Loricifera#In_anoxic_environment| Loricifera}}, tiny animals about the size of a large amoeba, are able to survive without oxygen in deep extremely salty mud sediments in the Mediterranean.<!--See sources in linked Wikipedia article and: Animals thrive without oxygen at sea bottom in background sources -->
 
Some {{w|Aerobic organism|aerobic}} (oxygen using) microbes can survive with as little as a millionth of a mole per cubic meter (0.000032 mg, or 32 nanograms per liter).
This new research greatly expands the possibilities for complex life on Mars. The paper includes a map of potential brine oxygen concentrations for calcium perchlorates brines on the surface of Mars (their [https://www.nature.com/articles/s41561-018-0243-0/figures/3 figure 3a]). These would be higher at the lowest points such as the floor of the {{w|Hellas Planitia|Hellas basin}}, south of the equator, where the atmospheric pressure is highest, reaching around 1% of Earth's atmosphere and lowest of all in the mountainous tropical southern uplands. But the highest values of all would be for brines at the poles.
 
Their lowest calculated oxgygen concentrations are for the tropical southern uplands, where temperatures are high and the atmosphere is thin, and for their brine with the lowest oxygen solubilities, sodium perchlorate. They also calculated this figure using their "worst case scenario" - this means, their least optimistic assumptions. However, they give reasons for believing that their more optimistic best case calculations are close to the true situation.
 
Setting everything to the worst case in this way, they obtained a value of 2.5 millionths of a mole per cubic meter (0.0008 mg per liter).
 
Levels in the tropical regions would be higher at the lowest points such as the floor of the {{w|Hellas Planitia|Hellas basin}}, south of the equator, where the atmospheric pressure is highest, reaching around 1% of Earth's atmosphere.
 
===Highest oxygen concentrations in their maps===
 
[[File:Halichondria panicea.jpg|thumb|Halichondria panicea or the breadcrumb sponge- Stamenković et al's paper cites research by Mills et al using this sponge which showed it can survive with only 0.002 moles per cubic meter (0.064 mg per liter). This new research suggests that these concentrations can be achieved in {{w|Supercooling|supercooled}} brines on modern Mars in polar regions.]]
Since cold brines take up more oxygen, it's no surprise that they found the highest concentrations in polar regions. That's where it reaches the levels high enough for simple sponges.
[[File:PIA22546-Mars-AnnualCO2ice-N&SPoles-20180806.gif|thumb|Extents of north (left) and south (right) polar CO<sub>2</sub> ice during a Martian year. These are not photos, rather they are based on infrared data from two instruments that can study the poles even at times of complete darkness. The dry ice here reaches temperature of around -125&nbsp;°C, well below its sublimation temperature of -78.5&nbsp;°C, which gives an idea of how cold the Martian poles get in winter. In Vlada Stamenković et al's model the highest oxygen concentrations occur at temperatures down to -123 to -133&nbsp;°C.]]
 
They base this on another paper from 2014 that showed that some simple sponges can survive with only 0.002 {{w|Mole (unit)|moles}}per cubic meter (0.064 mg per liter) <!-- first page of Nature paper, "Meanwhile, whereas aerobic microbial life and simple animals need O<sub>2</sub> dissolved in liquids in sufficiently large concentrations to survive, recent experiments, observations and calculations have lowered the required limits of concentrations of dissolved O<sub>2</sub> for aerobic respiration to ~10−6 mol m−3 in microorganisms and to ~2 × 10−3 mol m−3 in sponges"-->.
Stamenković et al found that even in the worst case they could devise, oxygen levels throughout Mars would be enough for the least demanding {{w|Aerobic organism|aerobic}} (oxygen using) microbes, at around 2.5 millionths of a mole per cubic meter (0.0008 mg per liter). That's the value for the tropical southern uplands, where temperatures are high and the atmosphere is thin, and for their brine with the lowest oxygen solubilities, sodium perchlorate. They also calculated this figure using their worst case estimate (where they do the calculations on their least optimistic assumptions). However, they give reasons for believing that their more optimistic best case calculations are close to the true situation.
 
TheThey highestlooked oxygenclosely concentrationsat oftwo allbrines, occurmagnesium whenand thecalcium water is colderperchlorate, which iscommon moston easilyMars. attained inPrevious polarpapers regions.show Theythat paidstarting particularwith attention to twothese brines, magnesiumin anda calciumwarmer perchlorates,liquid state common onin Mars. In simulation experiments, these stay liquidthey ascan they arebe {{w|Supercooling|supercooled}} to temperatures as low as -123 to -133&nbsp;°C while remaining liquid, before they transitiontransform to a glassy state. TheyThis dosupercooling thisworks even when the salty brines are mixed with the martian soil of Mars (regolith). It's at these very low temperatures that they will reach the optimal oxygen concentrations can be reached.
 
ForThe their calcium perchlorates map with {{w|Supercooling|supercooling}}, the lowestoxygen concentrations arein around 25calcium millionthsperchlorate ofbrines agot molehigh perenough cubicfor metersimple (0.008 mg per liter). For thissponges brinein regions poleward of about 67.5° to the north and about 72.5° to the south,. couldAs haveyou oxygenget concentrations high enough for simple sponges. Closercloser to the poles, oxygen concentrations could go higher, approaching levels typical of sea water on Earth,reaching 0.2 moles per cubic meter (6.4 mg per liter), for calcium perchlorates. The brine that achieved the highest oxygen solubilityThis is magnesiumnot perchlorates.far Withoff this,levels oxygen concentrations could reach values as high as two moles per cubic meter (64 mg per liter<!-- atomic weight here http://ciaaw.org/oxygen.htm -->)<!--abstracttypical of paper-->sea forwater theon best case with supercoolingEarth.
 
On'''''(background information):''''' on Earth, wormsthe most andoxygen clamsyou thatcan liveget in the muddywarm sea bedswater requireis 1about mg0.28 moles per liter,cubic bottommeter feeders such as crabs and oysters 3(9 mg per liter,) andat spawning20&nbsp;°C migratorywhich fishincreasess 6 mg per liter, all well within theirto 0.234 moles per liter.<!--cubic seemeter for instance the two "Dissolved Oxygen" cites in the Background sources-->. Saturated sea water is about 9(11 mg per liter) at 200&nbsp;°C rangingbecause upcold towater 11takes mgup perthe literoxygen atmore 0&nbsp;°Creadily.<!-- see for instance the two "Dissolved Oxygen" cites in the Background sources-->.
 
On Earth, worms and clams that live in the muddy sea beds require 1 mg per liter, bottom feeders such as crabs and oysters 3 mg per liter, and spawning migratory fish 6 mg per liter, all within their 0.2 moles (6.4 mg) per liter.<!-- see for instance the two "Dissolved Oxygen" cites in the Background sources-->.
 
The brine that achieved the highest oxygen solubility is magnesium perchlorates. With this, oxygen concentrations could reach values as high as two moles per cubic meter (64 mg per liter<!-- atomic weight here http://ciaaw.org/oxygen.htm -->)<!--abstract of paper--> for the best case with supercooling.
 
This new research greatly expands the possibilities for complex life on Mars.
 
==Interview==
 
===Could Mars have creatures as active as our worms and fish?===
Wikinews asked him whether their research suggests potential for life as active as the Earth animals that can survive at similar oxygen levels.
 
With this background, Wikinews asked him whether Mars could have life as active as the Earth life that survives at similar oxygen levels
[[File:WOA09 sea-surf O2 AYool.png|thumb|Shows how the oxygen dissolved in Earth's sea surface compares with these predicted values for Mars. From the World Ocean Atlas 2009, the values for the annual mean sea surface concentrations range from below 0.2 to above 0.4 moles per cubic meter (6.4 to 12.8 mg / liter). Values on Mars could range up to 0.2 moles per cubic meter for calcim perchlorate, and up to 2 moles per cubic meter for magnesium perchlroate, in extremely cold brines at the poles. {{Image source|Plumbago}}|alt=]]
:: {{WNIQ|Wikinews}} Does your paper's value of up to 0.2 moles of oxygen per cubic meter, the same as Earth's sea water mean that there could potentially be life on Mars as active as our sea worms or even fish?
::'''VS''': Mars is such a different place than the Earth and we still need to do so much more work before we can even start to speculate.''
 
===How could life use oxygen at such low temperatures?===
'''''(background information):''''' In their model, Oxygen gets into the brines at the poles so readily because they may reach extremely cold temperatures. These are far below the usual cold limit of life. It is not a hard limit because life gets slower and slower at lower temperatures to the point where individual microbes have lifetimes of millennia. Such life is hard to study. It's almost impossible to tell whether it is a) active and able to reproduce at those temperatures or b) active and not able to reproduce, or c) intermittently sometimes active and sometimes dormant. The reproduction can't be studied using cell counts. But the usual limit cited is -20&nbsp;°C<!-- see discussion in A new analysis of Mars "Special Regions" -->. That's well above the lowest temperatures studied in the paper which go down to -133&nbsp;°C.
 
'''''(background information):''''' Earth microbes live in the slow lane at lower temperatures to the point where individual microbes have lifetimes of millennia. Such life is hard to study. It's almost impossible to tell whether it is
 
# active and able to reproduce at those temperatures
# active and not able to reproduce, or
# intermittently sometimes active and sometimes dormant.
 
Researchers can't study the reproduction of such slowly growing cells using cell counts. Perhaps it is possible at lower temperatures, but the usual limit cited is -20&nbsp;°C<!-- see discussion in A new analysis of Mars "Special Regions" -->. That's well above the lowest Martian temperatures studied in the paper of -133&nbsp;°C.
 
Is Martian life able to reproduce below these temperatures?
 
<!-- This para summarizes the Schulze-Makuch paper in the background information section -->
Dirk Schulze-Makuch has proposed that Martian life might evolve an exotic metabolism with the perchlorates of Mars taking the place of the salts inside the cells of Earth life. This would have advantages on Mars, with the brines inside their own cells acting as an anti-freeze to protect them against extreme cold. Also with theirThese salts beingare soalso very hygroscopic, theywhich maymight help themlife of this sort (if it exists) to scavenge water from the atmosphere and their surroundings.
 
With this background, Wikinews asked:
Line 80 ⟶ 126:
:: {{WNIQ}} The temperatures for the highest levels of oxygen are really low -133&nbsp;°C, so, is the idea that this oxygen would be retained when the brines warm up to more habitable temperatures during the day or seasonally? Or would the oxygen be lost as it warms up? Or - is the idea that it has to be some exotic biochemistry that works only at ultra low temperatures like Dirk Schulze-Makuch's life based on hydrogen peroxide and perchlorates internal to the cells as antifreeze?
::'''VS''': The options are both: first, cool oxygen-rich environments do not need to be habitats. They could be reservoirs packed with a necessary nutrient that can be accessed from a deeper and warmer region. Second, the major reason for limiting life at low temperature is ice nucleation, which would not occur in the type of brines that we study.
'''''(background information)''''': His first suggestion here is that the cool oxygen rich reservoirs could have warmer water come up through them from below. He doesn't say where the warm water would come from, but one possibility is from geological hot spots. Our orbiting spacecraft have not yet found any, but Olympus Mons has been active as recently as 2.5 million years ago<!-- see for instance Neukum et al in the background sources -->. If sources of warmer water could rise to the surface from below and encounter these cold oxygen-rich brines, life could make use of oxygen where the two mix.
 
:: {{WNIQ}} [follow up question] When you talked about warm water encountering the brines from below, in our interview - did you have any thoughts about where the water might come from? Geothermal hot spots?
The other possibility is that microbes can continue to function at very low temperatures in the Martian conditions. After the interview I discovered that they go into this in section ''"3.2 The lower temperature limit for life and the potential of aerobic habitats"'' in the supplementary information.
 
::'''VS''': That is possible.
From previous research, the brines he studies don't form ice crystals when cooled. Instead, they smoothly transition to a glassy state after supercooling. When microbes adapted to extremely cold conditions are cooled down, the interior doesn't freeze but transitions to a glassy state (Intracellular vitrification). This transition is driven by freezing of the external media. When living in calcium or magnesium perchlorates, with no external crystalization, they could continue to have liquid interiors enabling metabolic processes to continue right down to the supercooling limit of the brines. If the intercellular fluids can stay liquid, this will keep the viscosity low, and permit vigorous metabolic processes to continue. Even in vitrified cells, metabolic processes continue at a slow pace, but if the fluids stay liquid then fast metabolic processes are possible.
 
'''''(background information)''''': His first suggestion here is that the cool oxygen rich reservoirs could have warmer water come up through them from below. Our orbiting spacecraft have not yet found any subsurface hotspots, but they may be hard to spot from orbit. Mars counts as still geologically active as Olympus Mons has been active as recently as 2.5 million years ago<!-- see for instance Neukum et al in the background sources -->.
 
So one way to have life there could be that warmer water could rise to the surface from below, heated by the hotspots, and encounter these cold oxygen-rich brines. Then life in the warmer brines could make use of oxygen where the two mix.
 
The other possibility is that microbes can continue to function at very low temperatures in the Martian conditions. After the interview I discovered that they go into this for their paper in a section ''"3.2 The lower temperature limit for life and the potential of aerobic habitats"'' in the supplementary information.
 
When microbes able to live in extremely cold conditions are cooled down, the interior doesn't freeze but changes to a glassy state (Intracellular vitrification). So the microbes would be able to be cooled down this far without being damaged.
 
It would then depend on whether the cells can keep their intercellular fluids liquid as they cool down. If they can, this will keep the viscosity low, and permit vigorous metabolic processes to continue. If their interior changes to a glassy state then some metabolic processes do still continue but only very slowly in these vitrified cells.
 
[[File:MarsOxides.jpg|thumb|Curiosity's discovery image for the manganese-oxide minerals at a location called "Windjana,". These require abundant water and strongly oxidizing conditions to form. With the new theory these conditions may be present on Mars today, previously thought to be only possible on early Mars.]]
 
===Special minerals on Mars===
Their research also helps to explain the presence of some minerals on the Mars surface, such as manganese oxides which require conditions of water and oxygen to form. Some researchers hav suggested they formed in an early Mars atmosphere that was thick and oxygen rich (which doesn't require life; it could for instance be oxygen rich due to ionizing radiation splitting water). This new reseach shows that these minerals could also form without an oxygen rich atmosphere.
 
'''''(background information):''''' Opportunity discovered manganese oxygens on Mars. These require oxygen to form. Some researchers have suggested they formed in an early Mars atmosphere that was thick and oxygen rich (not necessarily due to life - it could be oxygen rich due to ionizing radiation splitting water).
 
Their research gives an alternative explanation for these minerals, by concentrating the minute traces of oxygen in the atmosphere into the water.
 
As previously interviewed by National Geographic (October 22):
::'''VS''': Our explanation doesn't need any special magic — it works on Mars today,
 
===More detailed models===
'''''(background information):''''' The idea that Mars had enough oxygen in the past for marine animals, billions of years ago, when the atmosphere was thicker, is not too surprising nowadays since the discovery of those manganese oxides. That it may have enough right now is what is so very surprising about this new research, given that it has such a thin atmosphere, with so little oxygen in it. The atmosphere is unbreathable, its trace amounts of oxygen can't be used by any form of terrestrial animal life, but the brines may be another story.
 
The paper is theoretical and is based on a simplified general circulation model of the Mars atmosphere - it ignores distinctions of seasons and the day / night cycle. But it takes account of topography (mountains, craters etc) and the axial tilt. They combined it with a chemical model of how oxygen would dissolve in the brines and used this to establish predicted oxygen levels in the brines at the various locations on Mars.
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[[File:Changes in tilt of Mars's axis PIA15095.jpg|thumb|Mars's axis varies in its tilt (obliquity) over timescales of hundreds of thousands to millions of years. It is currently tilted to about 25-degrees, with ice in relatively modest quantities at the poles (top left). Ice builds up near the equator at high obliquities (top right) and the poles grow larger at very low obliquities (bottom). Vlada Stamenković et al's model showed that the current situation is around optimal for oxygenated brines. Their model predicts the potential for oases with enough oxygen for simple sponges at tilts of up to around 45 degrees.]]
 
'''''(background information):''''' Their model took account of the tilt of the Mars axis, which varies much more than for Earth (our axis is stabilized by the presence of the Moon). They found that for the last five million years conditions were particularly favorable for oxygen rich brines, and that it continues like this for ten million years into the future, as far as they ran the model. For the last twenty million years, as far back as they took their modeling, oases with enough oxygen for sponges are still possible.
 
:: {{WNIQ}} What was the resolution for MarsWRF
 
::'''VS''': Both the horizontal and vertical resolution of the model are variable and selectable at run time; we use a 40-layer vertical grid (0–80 km), following a modified-sigma (terrain-following) coordinate. The lowest model layer with this vertical grid is ~75–100 m above ground level, depending on location and season. We use a horizontal resolution of 5° × 5°, which corresponds to a grid of 72 points in longitude × 36 points in latitude.
 
===Could sponges survive through times when the tilt is higher?===
 
Remarkably, as they say in the paper, present day Mars would have more oxygen available for life than early Earth had prior to 2.35 billion years ago. On Earth, photosynthesis seems to have come first, before complex multicellular life, generating the oxygen for the first animals. On Mars, with a different source for oxygen, oxygen breathers could arise before photosynthesis. They suggest in their paper that this gives broader opportunities for oxygen-breathing life on other planets.
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Wikinews asked Vlada Stamenković if he had any ideas about whether and how sponges could survive through times when the tilt was higher and less oxygen would be available:
 
:: {{WNIQ}} I notice from your <sup>[https://www.nature.com/articles/s41561-018-0243-0/figures/4 figure[#Figure 4|[4] ]]</sup> that there is enough oxygen for sponges only at tilts of about 45 degrees or less. Do you have any thoughts about how sponges could survive periods of time in the distant past when the Mars axial tilt exceeds 45 degrees, for instance, might there be subsurface oxygen rich oases in caves that recolonize the surface? Also what is the exact figure for the tilt at which oxygen levels sufficient for sponges become possible? (It looks like about 45 degrees from the figure but the paper doesn't seem to give a figure for this).
::'''VS''': 45 deg is approx. the correct degree. We were also tempted to speculate about this temporal driver but realized that we still know so little about the potential for life on Mars/principles of life that anything related to this question would be pure speculation, unfortunately.
[[File:Mars-water-droplets-phoenix-2008-bg.gif|thumb|Unfortunately, the Phoenix lander wasn't equipped to analyze droplets on its legs, which it observed in 2008-9. However, they appear to be droplets of some liquid, most likely salty water, from the way they behaved. These may be our first spacecraft observations of liquid brines on Mars. Nilton Renno's team's research in the University of Michigan's newly built Mars simulation chamber, published in 2014, was able to duplicate them in minutes when salt lies on top of ice. They suggested that such droplets may be common place on Mars today. Wikinews asked Vlada Stamenković if these droplets could be oxygen rich. He said he doesn't know yet, but it is a really good question.]]
 
===Could oxygen get into rapidly forming droplets where salt forms on ice?===
 
<!-- This para summarizes material in the "Liquid Water from Ice and Salt on Mars" article in the NASA Astrobiology magazine in the sources -->
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::'''VS''': Just like the answer above. Dynamics is still to be explored. (But this is a really good question 😉).
[[File:Mars-SubglacialWater-SouthPoleRegion(cropped).jpg|thumb|Possible 20-km wide subglacial lake close to the Martian south pole. The dark blue region in the overlayed scans, lower middle, is thought to be a radar bright echo from extremely cold brines, probably magnesium and calcium perchlorates at a depth of 1.5 km. Vlada Stamenković et al's model would suggest high solubility for oxygen for these brines too, and he suggested oxygen could get into them through {{w|Radiolysis|radiolysis}} from natural radioactivity of the rocks below.]]
 
:: {{WNIQ}} Any idea of timescale yet for the oxygen to be taken up - is this hours, days, months, years? E.g. could oxygen get into water from melting of morning frosts, and ice melting briefly in the Hellas basin or is this impossible on such a short timescale (if you know)? Similarly for the deliquescing salts that Curiosity found beneath its wheels as it drove over the sand dunes, that form overnight and dry up during the day
 
::'''VS''': This is still fully under investigation, so there’s unfortunately no info on this yet.
 
===Could oxygen get into the subglacial lakes?===
 
Wikinews also asked how their research is linked to the recent discovery of possible large subglacial lake 1.5 km below the Martian South Pole found through radar mapping.
 
:: {{WNIQ}} Some news stories coupled your research with the subglacial lakes announcement earlier this year. Could the oxygen get through ice into layers of brines such as the possible subglacial lakes at a depth of 1.5 km?
::'''VS''': There are other ways to create oxygen. {{w|Radiolysis}} of water molecules into hydrogen and oxygen can liberate oxygen in the deep and that O<sub>2</sub> could be dissolved in deep groundwater. The radiolytic power for this would come from radionuclides naturally contained in rocks, something we observe in diverse regions on Earth.
 
:: {{WNIQ}} Could radiolysis take the oxygen concentrations higher than the figures you got?
 
::'''VS''': This is TBD and would depend on the production rate and the depth where it is produced (~pressure).
 
<!-- this is covered in the article by Möhlmann in Background section and also in the overview article by Martinez and Renno in the background information section -->
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'''''(background information):''''' So, his answer here is that it could be possible by the same process, radiolysis of the ice through radioactivity in the rocks.
 
:: {{WNIQ}} Could undercooled liquid interface water take up oxygen too?
 
::'''VS''': Yes.
 
:: {{WNIQ}} Any idea of timescale yet for the oxygen to be taken up - is this hours, days, months, years? E.g. could oxygen get into water from melting of morning frosts, and ice melting briefly in the Hellas basin or is this impossible on such a short timescale (if you know)? Similarly for the deliquescing salts that Curiosity found beneath its wheels as it drove over the sand dunes, that form overnight and dry up during the day
 
::'''VS''': This is still fully under investigation, so there’s unfortunately no info on this yet.
 
===Planetary protection issues===
 
<!-- Next para based on Scientific American article-->
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''"The report highlights the need to include in situ detection of energy-starved or otherwise sparsely distributed life such as chemolithotrophic or rock-eating life. In particular, the report found that NASA should focus on research and exploration of possible life below the surface of a planet in light of recent advances that have demonstrated the breadth and diversity of life below Earth’s surface, the nature of fluids beneath the surface of Mars, and the likelihood of life-sustaining geological processes in planets and moons with subsurface oceans."''
 
===What would your new instrument TH2OR do?===
 
Vlada Stamenković is working on a new instrument TH2OR to send to Mars on some potential future mission. It would search for potentially habitable brines deep below its surface using ultra low frequency radio waves. This is a frequency far lower than that of ground penetrating radar, in the range of a fraction of a Hertz up to kilohertz. Wavelengths are measured in kilometers up to tens of thousands of kilometers or more. Wikinews asked him for more details
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:: {{WNIQ}} Does your TH2OR use TDEM like the Mars 94 mission - and will it use natural ULF sources such as solar wind, diurnal variations in ionosphere heating and lightning?
::'''VS''': The physical principle it uses is the same and this has been used for groundwater detection on the Earth for many decades; it’s Faraday’s law of induction in media that are electrically conducting (as slightly saline water is).''<br><br>''However, we will focus on creating our own signal as we do not know whether the EM fields needed for such measurements exist on Mars. However, we will also account for the possibility of already existing fields.
 
 
==Background information - history of ideas of oxygen breathing life on Mars==
 
A historical 101 on multicellular life on Mars may be of interest here. First, of course back in the early twentieth century there was much speculation about oxygen breathing multicellular life there, with Lowell even speculating that intelligent Martians built the canals that he thought he could see in his telescope.
 
By the time of the first orbital robotic space missions to Mars, it was already clear that the atmosphere was far too thin for terrestrial animals, but there was some hope for plant life. However, the early observations from space showed a barren crater covered land and since then the idea of life on Mars has focused mainly on anaerobic microbes and photosynthetic life.
 
<!-- For next para: Vera et al in the background information is an example of papers on lichens such as Pleopsidium chlorophanum for Mars and the DLR source gives an overview of their experiments into lichens and blue-green algae-->
In 2014, however, researchers for DLR, Vera et al surprised many astrobiologists with their experimental result that lichens could potentially grow on present day Mars. This is multicellular life, but only in the form of photosynthetic life able to produce its own oxygen. They took some lichens, such as {{w|Pleopsidium chlorophanum}} that were able to survive in close to Mars-like conditions high up on Antarctic mountain ranges, surviving only on the humidity in the air without any water. These did show promise in Mars simulation chamber experiments. They can do this because the algal component is able to make the oxygen needed by its fungal component. They can also do this without needing any extra water as intermediary, in the experiments they are able to grow in partial shade, using only the night time humidity of the atmosphere itself.
 
It is also possible to have microscopic multicellular animals that can manage without oxygen at all (sub millimeter sized). There are only three species of these creatures known on Earth, however, and they are not candidates for life on Mars. They are three species of {{W|Loricifera#In_anoxic_environment| Loricifera}}, tiny animals about the size of a large amoeba, are able to survive without oxygen in deep extremely salty mud sediments in the Mediterranean.<!--See sources in linked Wikipedia article and: Animals thrive without oxygen at sea bottom in background sources -->
 
==Background information - why oxygen is so significant for multicellular life==
<!--Note to reviewer - optional section on background information which I thought might help some readers.-->
<!-- summarizes Catling et al from 2014 in background section -->
An oxygen breathing organism can get far more energy from the same amount of food. Without oxygen, the {{w|Glucose|glucose}} that's the source of energy in Earth organisms can only be broken up into two {{w|Pyruvic acid|pyruvate}} molecules (each of 3 carbon atoms). This process produces two molecules of {{w|Adenosine triphosphate|ATP}}, the energy molecule that powers our cells. This is a fast reaction, but the amount of energy it produces is limited. This is the energy production method used in sprinter's "fast twitch" white muscle fibers. When Usain Bolt ran his 100 meters sprint he was relying on this chemical process. He wasn't really using much oxygen, mainly it was using up stores of glucose in his fast twitch muscles.
 
This is not much use if you want to run a marathon however. Luckily for us, if you use oxygen, glucose can also be reduced all the way to carbon dioxide, and in the process {{w|Cellular respiration#Efficiency of ATP production|30 or 32 ATP molecules}} can be produced from a single molecule of glucose. This is the type of energy production used in the "slow twitch" red muscle fibers of a {{w|Marathon|marathon runner}}, or {{w|Ultramarathon|ultramarathon}} runner. It lets them run for many miles using little by way of glucose from their food. It amounts to about 100 calories in excess over the resting metabolic calories, depending on body mass, or around 25 grams of glucose per mile, which in turn is derived from {{w|Glycogen|glycogen}} and then fat as the race progresses). In this way a large organism can sustain itself with little by way of food if it relies on oxygen.
[[File:Spinoloricus.png|thumb|This tiny animal, which never grows larger than a millimeter in size, is a species of {{w|Loricifera#In_anoxic_environment|loricifera}}, a rare multicellular anaerobe. Shown this colour because it is stained with Rose Bengal. Scale bar is 50 μm. In 2014, Carling et al. showed from general energetic considerations that should also apply to extraterrestrial carbon based life, that it is not easy for a food chain of anaerobes to sustain complex lifeforms even as large as 10 cms in diameter {{Image source|Danovaro et al.}}]]
A lifeform that can't use oxygen needs to eat more than ten times as much food to get the same amount of energy as one that can use oxygen. It is hard for it to grow large because so much food is needed just to sustain itself and for reproduction. On Earth we don't know of any {{w|Anaerobic organism|anaerobes}} (organisms that can manage without oxygen) larger than those three species of {{w|Loricifera#In_anoxic_environment|loricifera}}, which are less than a millimeter in size.
 
It is possible for microbes to slowly build large structures without breathing oxygen, as with the {{w|Stromatolite|stromatolites}}, which build up layer by layer. However, only the outermost layer, a thin biofilm, is actually alive. Lichens of course do use oxygen, produced by the algal component. Trees and other plants produce oxygen but also use it at night.
 
What about extraterrestrial life though? Could it use something else in place of oxygen? Well, perhaps, but there is some earlier research that may suggest that it couldn't get very large. In 2005, Catling et al investigated the amount of energy available to carbon based organisms with and without the use of oxygen in their metabolism. Oxygen produces the largest amount of free energy per electron transfer apart from fluorine and bromine, which are too reactive to build up in quantities useful to life. They found on general energetic principles that extraterrestrial life would still needs oxygen to sustain large complex lifeforms, from around 10 cms in size or larger. This is for any extraterrestrial food web for carbon based life.
 
They worked it out by looking at how many smaller organisms are needed to support larger ones in the food web. For aerobic life the number of organisms in a food web is inversely proportional to the mass, if you are ten times heavier then there are ten times fewer of you in the food web. These numbers are much less for {{w|Anaerobic organism|anaerobes}}.
 
According to Catling et al's modeling, if an organism is a hundred times larger (and so a million times more massive) in an aerobic food web then it's a million times less numerous (you'd have only one large organism for a million smaller ones).
 
Meanwhile an organism that's a hundred times larger in an anaerobic food web is a billion times less numerous in an anaerobic food web.
 
So for instance, if you have a 200 micron organism preying on 20 micron organisms and they in turn on 2 micron organisms then for a billion of the 2 micron organisms there will be a thousand of the 200 micron organisms in an aerobic food chain, but only one in an anaerobic food chain. By the time you get to large organisms of 10 cm scale or larger they should be almost non existent in an anaerobic food web.
 
If Catling et al are correct in their inference here, then on general energetic principles that an extraterrestrial biosphere with large carbon based animals, at least of 10 cms scale or larger, is going to need oxygen.
 
Whether or not this applies generally to all extra terrestrial life, it does seem to apply to Earth life. In anoxic environments, Earth animals have found it a challenge to get as large as 1 mm in size without oxygen. It is possible that larger creatures lived on Earth when the seas were all anoxic before the Great Oxygenation Event. There are no surviving multicellular lifeforms from those times but they would be likely to be soft tissued and hard to preserve.
 
==Technical details - guide to paper==
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The main points in their research are summarized in their [https://www.nature.com/articles/s41561-018-0243-0/figures/3 figure 3] which shows two versions of the map, with and without supercooling. The upper figure is the one with supercooling (note the colour-coding is different for the two maps). The map is for calcium perchlorates and they explain that results are comparable for magnesium perchlorates. The dotted lines in that diagram show the polar limit for sponges. The paper says that 6.5% of the surface area of Mars could have oxygen concentrations suitable for primitive sponges. The white and purple colored regions close to the poles are regions that could have oxygen solubilities similar to Earth's oceans, and the paper says that the polar regions have ''"the greatest potential to harbor near-surface fluids "'' at 0.2 moles per cubic meter of dissolved oxygen (6.4 mg / liter). The lowest concentration in their model for their best estimate with supercooling is ~2.5 × 10<sup>−5</sup> moles per cubic meter of dissolved oxygen in the tropical southern highlands (0.0008 mg per liter<!--https://www.google.com/search?q=2.5*32*10^-5-->).
 
Techy aside here, Wikinews asked him about what seems to be a dicrepancy beween Figure 2a and Figure 3.
 
:: {{WNIQ|Wikinews}} The paper itself mentions a lower limit of 2.5 * 10-5 moles per cubic meter. But in Figure 2a it looks more like 1.<something> for the lower limit for both the magnesium and calcium suphates. Wondered which is right, or am I misunderstanding something and there is no discrepancy?
 
::'''VS''': Fig 2 is for an average pressure Pav and Temperature T and Pressure P not being correlated (at a point x on the surface P and T are correlated). Correlated (P,T) are used for Fig 3. Note that the lower limit for perchlorates in Fig 3 is larger than in Fig 2, for correlated (P,T) it is also larger for the other brines, and around lower limit of 2.5 * 10-5 moles per cubic meter.
 
For the effects of the different types of brine, see their [https://www.nature.com/articles/s41561-018-0243-0/figures/2 figure 2]2. This is the one that covers the sodium perchlorate and magnesium perchlorate figures. The brines are colour coded as in [https://www.nature.com/articles/s41561-018-0243-0/figures/1 figure 1], so sodium perchlorate is black, and Magnesium perchlorate is pink. The lowest number in the abstract of 2.5 millionths of a mole per cubic meter is for the sodium perchlorate black bar in figure 2e. The highest figure of 2 moles per cubic meter is for the magnesium perchnlorates pink bar in figure 2a.
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The paper is available to read in its entirety through the link provided on the author's website and the Nature Sharedit sharing initiative.
 
==Background information - why oxygen is so significant for multicellular life==
<!--Note to reviewer - optional section on background information which I thought might help some readers.-->
<!-- summarizes Catling et al from 2014 in background section -->
There is much more energy available to an organism from the same amount of food with oxygen. Without oxygen, the {{w|Glucose|glucose}} that's the source of energy in Earth organisms can only be broken up into two {{w|Pyruvic acid|pyruvate}} molecules (each of 3 carbon atoms). This process produces two molecules of {{w|Adenosine triphosphate|ATP}}, the energy molecule that powers our cells. This works quickly but the amount of energy is limited; it's the energy production method used in sprinter's "fast twitch" white muscle fibers.
 
With oxygen, glucose can be reduced all the way to carbon dioxide, and in the process {{w|Cellular respiration#Efficiency of ATP production|30 or 32 ATP molecules}} can be produced from a single molecule of glucose. This is the type of energy production used in the "slow twitch" red muscle fibers of a {{w|Marathon|marathon runner}}, or {{w|Ultramarathon|ultramarathon}} runner. It lets them run for many miles using little by way of glucose from their food (about 100 calories of excess calories over the resting metabolic calories, depending on body mass, or around 25 grams of glucose per mile, which in turn is derived from {{w|Glycogen|glycogen}} and then fat as the race progresses). In this way a large organism can sustain itself with little by way of food if it relies on oxygen.
[[File:Spinoloricus.png|thumb|This tiny animal, which never grows larger than a millimeter in size, is a species of {{w|Loricifera#In_anoxic_environment|loricifera}}, a rare multicellular anaerobe. Shown this colour because it is stained with Rose Bengal. Scale bar is 50 μm. In 2014, Carling et al. showed from general energetic considerations that should also apply to extraterrestrial carbon based life, that it is not easy for a food chain of anaerobes to sustain complex lifeforms even as large as 10 cms in diameter {{Image source|Danovaro et al.}}]]
Without oxygen a lifeform would need to eat more than ten times as much food to get the same amount of energy. It is hard for it to grow large because so much food is needed just to sustain itself and for reproduction. On Earth we don't know of any {{w|Anaerobic organism|anaerobes}} (organisms that can manage without oxygen) larger than those three species of {{w|Loricifera#In_anoxic_environment|loricifera}}, which are less than a millimeter in size. Anaerobes can buld large structures, as with the {{w|Stromatolite|stromatolites}}, which build up layer by layer but only the outermost layer, a thin biofilm, is actually alive.
 
What about extraterrestrial life though? Could it use something else in place of oxygen? Well, perhaps, but there is some earlier research that may suggest that it couldn't get very large. In 2005, Catling et al investigated the amount of energy available to carbon based organisms if with and without the use of oxygen in their metabolism. Oxygen produces the largest amount of free energy per electron transfer apart from fluorine and bromine, which are too reactive to build up in quantities useful to life. They found on general energetic principles that it still needs oxygen to sustain large complex lifeforms, from around 10 cms in size or larger. This is for any extraterrestrial food web for carbon based life.
 
They worked it out by looking at how many smaller organisms are needed to support larger ones in the food web. For aerobic life the number of organisms in a food web is inversely proportional to the mass, if you are ten times heavier then there are ten times fewer of you in the food web. These numbers are much less for {{w|Anaerobic organism|anaerobes}}.
 
According to Catling et al's modeling, if an organism is a hundred times larger (and so a million times more massive), then organisms of that size are a million times less numerous in an aerobic food web, but the same size of organism is a billion times less numerous in an anaerobic food web. So for instance, if you have a 200 micron organism preying on 20 micron organisms and they in turn on 2 micron organisms then there will be only a thousandth of the aerobic numbers in an anaerobic food chain. By the time you get to large organisms of 10 cm scale or larger they should be almost non existent in an anaerobic food web.
 
So, if Catling et al are correct in their inference here, then on general energetic principles that an extraterrestrial biosphere with large carbon based animals, at least of 10 cms scale or larger, is going to need oxygen. It seems to apply to Earth anyway. In anoxic environments, Earth animals have found it a challenge to get as large as 1 mm in size without oxygen, though it is possible that larger creatures lived on Earth when the seas were all anoxic.
 
== Sources ==
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|date = October 22, 2018
}}
*:{{anchor|Figure 3}}[https://www.nature.com/articles/s41561-018-0243-0/figures/3 Figure 3].
*:{{anchor|Figure 4}}[https://www.nature.com/articles/s41561-018-0243-0/figures/4 Figure 4].
*{{source
|url = https://static-content.springer.com/esm/art%3A10.1038%2Fs41561-018-0243-0/MediaObjects/41561_2018_243_MOESM1_ESM.pdf
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