Deliquescing salts taking up moisture from the Mars atmosphere: Difference between revisions
Deliquescing salts taking up moisture from the Mars atmosphere (edit)
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By Elizabeth Howell - Astrobiology Magazine (NASA) Aug 28, 2014</ref> 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.
The perchlorates may be formed by interactions of UV with the soil. In particular silicon dioxide acts as a photocatalyst to boost the production of perchlorates in experiments using Martian regolith simulants <ref>Carrier, B.L. and Kounaves, S.P., 2015. [https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015GL064290 The origins of perchlorate in the Martian soil]. Geophysical Research Letters, 42(10), pp.3739-3745.</ref>
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.
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[[File:Ice on Mars Utopia Planitia (PIA00571).jpg|frame|center|600px|alt=Ice on Mars Utopia Planitia |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.<br><br>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 high 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 is now thought that these could occur almost anywhere on Mars if the right mixtures of salts exist on the surface, even
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.<ref name=Osanasaltpillars>Osano, A., and A. F. Davila. [http://www.hou.usra.edu/meetings/lpsc2014/pdf/2919.pdf "Analysis of Photosynthetic Activity of Cyanobacteria Inhabiting Halite Evaporites of Atacama Desert, Chile."] Lunar and Planetary Institute Science Conference Abstracts. Vol. 45. 2014.</ref>
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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.
====Effect of this====
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You can also get similar eutonic mixtures of three or more different types of salts, which typically have even lower ERH than any of the mixtures of two salts. Salts on Mars could have a mixture of perchlorates, chlorates, sulfates, and chlorides and perhaps nitrates also if present, along with cations of sodium, potassium, calcium, and magnesium. So there are many possibilities to consider here.<ref name="TonerCatling2014"/><ref name="GoughChevrier2014"/>
In this way, it doesn't matter much what the actual percentages of
It works the same way with eutectics and salts. But the temperature reductions are less. The eutectic point is normally not much reduced below the value for the salt with the lowest individual eutectic. For instance However, the eutectic point for a mixture of several salts with water is normally close to the eutectic for the salt with the lowest eutectic point with water. For instance calcium chloride combined with magnesium chloride (eutectice -33.4 °C) has a eutectic of -50.8 °C and on its own, the eutectic is -50.1 °C<ref name="TonerCatling2014"/>.
===After salt mixtures take up water, they retain it after supercooling, and reduced humidity===
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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, MgSO<sub>4</sub> 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 <ref name="TonerCatling2014"/> - omitted some of the columns). The magensium and calcium perchlorates are from Marion et al, 2010<REF>Marion, G.M., Catling, D.C., Zahnle, K.J. and Claire, M.W., 2010. Modeling aqueous perchlorate chemistries with applications to Mars. Icarus, 207(2), pp.675-685</REF>
{| class="wikitable"
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|-
| NaClO<sub>4</sub> || -34.3 °C (9.2 m)|| 11.5
|-
| MgClO<sub>4</sub> || {{convert|204.55|K|C}} (3.48 m)||
|-
| CaClO<sub>4</sub> || {{convert|198.55|K|C}} (4.20 m)||
|}
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 significantly, and in the case of magnesium chloride, it permitted more supercooling to 17°C below the -33°C eutectic, instead of 13.8° below. Only with 1.65m magnesium sulfate was the amount of supercooling reduced significantly, to 6.6°C below the eutectic instead of 15.5°C.<ref name="TonerCatling2014">{{cite journal|url=http://faculty.washington.edu/dcatling/Toner2014_SupercoolSalts.pdf|last1=Toner|first1=J.D.|last2=Catling|first2=D.C.|last3=Light|first3=B.|title=The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars|journal=Icarus|volume=233|year=2014|pages=36–47|issn=0019-1035|doi=10.1016/j.icarus.2014.01.018|bibcode=2014Icar..233...36T}}</ref><ref name="GoughChevrier2014">{{cite journal|url=https://scholar.google.com/scholar?cluster=10058227747473517083&hl=en&as_sdt=0,5|last1=Gough|first1=R.V.|last2=Chevrier|first2=V.F.|last3=Tolbert|first3=M.A.|title=Formation of aqueous solutions on Mars via deliquescence of chloride–perchlorate binary mixtures|journal=Earth and Planetary Science Letters|volume=393|year=2014|pages=73–82|issn=0012-821X|doi=10.1016/j.epsl.2014.02.002|bibcode=2014E&PSL.393...73G}}</ref>
With some of the salt solutions, depending on chemical composition, especially magnesium and calcium perchlorates, then the supercooling produces a glassy state instead of crystallization, and this could help to protect supercooled microbes from damage<ref name="TonerCatling2014"/>. Toner et al also found that the magnesium and calcium perchlorate supercooled solutions are so stable that they should keep the brines liquid througout the day night cycle and it is possible they could remain supercooled from summer through to winter and then through to the next summer<ref name="TonerCatling2014"/>
===Effects of micropores in salt pillars===
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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.<ref name=Rincon>{{cite web|last1=Rincon Science editor|first1=Paul|title=Evidence of liquid water found on Mars|url=https://www.bbc.co.uk/news/science-environment-32287609|website=BBC News website|date=April 13, 2015}}</ref><ref name="Martín-TorresZorzano2015">{{cite journal|last1=Martín-Torres|first1=F. Javier|last2=Zorzano|first2=María-Paz|last3=Valentín-Serrano|first3=Patricia|last4=Harri|first4=Ari-Matti|last5=Genzer|first5=Maria|last6=Kemppinen|first6=Osku|last7=Rivera-Valentin|first7=Edgard G.|last8=Jun|first8=Insoo|last9=Wray|first9=James|last10=Bo Madsen|first10=Morten|last11=Goetz|first11=Walter|last12=McEwen|first12=Alfred S.|last13=Hardgrove|first13=Craig|last14=Renno|first14=Nilton|last15=Chevrier|first15=Vincent F.|last16=Mischna|first16=Michael|last17=Navarro-González|first17=Rafael|last18=Martínez-Frías|first18=Jesús|last19=Conrad|first19=Pamela|last20=McConnochie|first20=Tim|last21=Cockell|first21=Charles|last22=Berger|first22=Gilles|last23=R. Vasavada|first23=Ashwin|last24=Sumner|first24=Dawn|last25=Vaniman|first25=David|title=Transient liquid water and water activity at Gale crater on Mars|journal=Nature Geoscience|year=2015|issn=1752-0894|doi=10.1038/ngeo2412|volume=8|issue=5|pages=357–361|bibcode=2015NatGe...8..357M}}</ref>
==Hydrogen peroxide based life taking up water directly through deliquescence==
Joop Houtkooper and Dirk Schulze Makuch proposed in 2007 that life on Mars may be using a mixture of water and biogenically created [[hydrogen peroxide]] as its internal solvent. He gave this as a possible form of life to explain some puzzling aspects of the [[Viking lander biological experiments]]. On cooling, the eutectic of 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to [[super-cool]] rather than crystallize. It is also [[hygroscopic]], an advantage in a water-scarce environment.<ref>{{cite journal| url=http://www.cosis.net/abstracts/EPSC2007/00439/EPSC2007-J-00439.pdf|format=PDF| journal=EPSC Abstracts| volume= 2| id= EPSC2007-A-00439| date=2007| publisher=European Planetary Science Congress 2007| title=The H2O2-H2O Hypothesis: Extremophiles Adapted to Conditions on Mars?| first=Joop M.| last= Houtkooper|author2=Dirk Schulze-Makuch| bibcode=2007epsc.conf..558H| pages=558}}</ref><ref>{{cite web| url=http://www.planetary.org/blog/article/00001109/| title=Europlanet : Life's a bleach| date=2007-08-24| first=Doug| last= Ellison| publisher=Planetary.org}}</ref>. It would prefer regions with lower temperatures, and would avoid liquid water. Conditions at the poles would be optimal, but it could also survive in the equatorial regions visited by Viking<ref name=peroxide_life>{{cite journal | title = A Possible Biogenic Origin for Hydrogen Peroxide on Mars | journal = International Journal of Astrobiology | volume = 6 | issue = 2 | pages = 147 | date = 2007-05-22 | first = Joop M. | last = Houtkooper |author2=Dirk Schulze-Makuch | doi = 10.1017/S1473550407003746 | arxiv = physics/0610093 |bibcode = 2007IJAsB...6..147H }}</ref>
== See also==
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