Habitable Conditions in the Martian Subsurface
But Let's Not Forget Habitable Niches in Salts, Ices, and Caves!
The recent discovery of huge amounts of water in the Valles Marineris Canyon and the presence of light carbon as a potential biomarker for life on Mars propelled me to look again at a seminal paper about a possible Martian biosphere written more than ten years ago by Eriita Jones and Charles Lineweaver from the Australian National University and Jonathan Clarke from the Australian Mars Society.
In that paper, the authors presented a comprehensive model of Martian pressure-temperature conditions and compared it with Earth. As you go deeper into the Martian subsurface, temperature increases. The amount of that increase, referred to as the geothermal gradient, is less on Mars than on Earth. Nevertheless, at a depth starting at about 6 km, the pressure and temperature conditions on Mars match those of active life on Earth. It also allows water to be liquid.
The Australian research team acknowledged that the availability of nutrients and free energy would be an additional and essential criterium to set the depth limit for life on Mars. Available living space (quantified by the porosity of the rocks), however, would not be a problem, because the Martian crust is permeable down to a depth of 85 km. Deep subsurface life would have to survive with the little nutrients available and be based on chemical redox-reactions only (no chance for photosynthesis). Even though, the authors concluded that about 3.2% of the volume of present-day Mars is potentially habitable for terrestrial-like life, which adds up to a lot of “real estate.”
We must keep in mind that even if a planet is habitable, it does not necessarily mean that it hosts life. As pointed out in a paper by Charles Cockell, there should be many planets in the Universe that are in principle habitable but are uninhabited. Mars, and any other habitable planet, would only be inhabited if life either originated on that planet or if it was transported from elsewhere. In the case of Mars, I think the proposition that life originated on Mars is reasonable given that the early Martian environment was very similar to the environmental conditions on early Earth. Alternatively, microbial life could have been transported from Earth to Mars (or vice versa) via asteroids at a time when both planets had oceans on their planetary surface.
If life was present on early Mars, it could have adapted to the cold and dry planet Mars is today. Experiments have shown that life can survive if it is in protected niches close to the Martian surface. Other studies demonstrated that microbes could meet all their water needs by taking up water vapor directly from the atmosphere, even under conditions similar to Mars. Conference attendees at a workshop with the theme of extant Martian life recognized these newly gained insights and highlighted four potential niches for life on Mars: the deep subsurface, salts, ices, and caves.
It is important to note that life would be close to the surface in the case of salts (and perhaps ices), and photosynthesis would be possible as an additional energy source. If so, we would expect more biomass and more accessible biomass, meaning that a planetary mission would have an easier time getting to these locations and probing for life. This leads me to conclude that a new life detection mission to Mars is long overdue!