Astrobiologists regard Mars, Titan, Europa and Enceladus as possible habitable places in our Solar System. However, I think the best place for us to search for signs of alien life is in the rocks of Mars that are protected from the ionising radiation currently bombarding the surface. Finding signatures of ancient extraterrestrial microbial life and establishing confidence in that finding will largely depend on the quality of the preserved record. Radiation can seriously alter ancient biosignature records, especially organics, but to what extent and how remains elusive. The reality is that any chemical and physical alteration can complicate and lower our confidence in interpreting signs of ancient life. The same is true for interpreting ancient terrestrial biosignatures.
On Earth, life is ubiquitous – we find life in nearly every extreme environment. This tells us that microbes are incredibly adaptive. However, microbes don’t always flourish in extreme environments. We find more and diverse life where nutrients, food and energy sources are plentiful. On Mars, I think it’s fair to assume that if life ever existed there, it would be very adaptive too. Over the aeons, life may have learned to cope or even utilise ionising radiation by moving to shielded environments, such as the subsurface, and evolving biochemistry to repair damage to cells. In any case, the ionising radiation makes the top metre or so of surface rocks an extreme environment. If microbes adapted to living in these subsurface rocks, they might flourish if there was a way for nutrients to circulate in the rocks. Perhaps the ExoMars mission will discover clues to the modern habitability of this more protected subsurface environment and if life is there.
Answered by Jennifer Eigenbrode at NASA