Basic Question â€šÃ„Ã¬ where can life live and prosper on Mars? Or can it? Part of answering this question requires us to consider the temperature structure on Mars. As near as we can tell, temperatures above 253 K / -4F (as well as aqueous liquid, shielding from UV) are required for life. Salty water can be liquid to low 200â€šÃ„Ã´s, so low temperatures are the real limit. While the average temperature on Mars is no where this high, there are many places on Mars that daily get above this limit. (The 253K limit is based on Siberian permafrost methanogens.)
The thing that really has to be brought home is how organics really can exist in briny solutions (really saly water).Â¬â€ In the laboratory, MgSO4, NaCl, MgCl2, CaCl2 Fe2(SO4)3 and other organics have happily formed. Temperature really is the limit to life as we know it.
This team looked at the temperature fluctuations at 6 Martian Lander sites: Viking 1 & 2, Mars Pathfinder, Opportunity, Spirit, and the upcoming Phoenix landing site.
They found that diurnal (day to night) temperature fluctuations on Mars are huge. At the most extreme site, the Mars Pathfinder site, temperature go from almost 270K during the day to just under 190K at night (26F to -117F!). The temperature is only above the magical 253K for about 2.5 hours at the surface. Fluctuations are less as you go below the surface, but things also donâ€šÃ„Ã´t get above 253 as easily, and even a few centimeters below the surface the never hit the critical temperature.
This means life needs to exist at the surface or just below the surface, and any microbial processes that take place must act quickly while the microbe is warm, and then the system has to shut down.
Looking at all sites, from North to South, they find Phoenix’s site is just too cold, while Opportunity and Spirit consistently get to prime temperatures every day of the year. The Opportunity site is most ideal – 253K is reached at soil depths ofÂ¬â€ 1-1.5 cm every day and the triple point of water occasionally reached. At that site, the habitability zone extends down about 2cm (for life as we know it).
While there is concern that Mars extremely thin atmosphere causes liquids to evaporate, salty liquids offer a solution â€šÃ„Ã¬ in addition to being liquid at lower temperatures, they also evaporate so slowly they donâ€šÃ„Ã´t go away during the day before the refreeze at night. This means it is possible to imagine microbes at site’s like Oppurtunity’s (and perhaps something crawled off Oppurtunity), existing protected from high UV just a centimenter or so below the surface in salty brine.
Sadly, Opportunity doesn’t have the labs necessary to look for this tantalizing possibility. ExoMars, however, will be able to look.