Intriguing though the idea is, we still don’t know whether life—microbial life, that is—ever existed on Mars. But NASA’s Curiosity rover has been able to sample soils at a number of sites, and it has found signs of a freshwater lake, dry now for about three and a half billion years.
One of the things that made the discovery and the sampling possible is erosion, which has uncovered layers of rock and soil relatively recently—recent, in this case, being the last 60 to 100 million years. In areas where new soil has not been exposed, radiation would have destroyed any organic compounds that might once have existed. But where wind erosion has exposed material that was once buried several feet deep, Curiosity at least has a shot at discovering organic compounds. Scientists estimate wind erosion removes about three feet from the surface of Mars over a million years.
Sampling has already shown that at least one lake had a neutral pH and was not salty, so microbes similar to those on Earth could potentially have survived there. However, Curiosity can analyze only the chemical makeup of the soil and is not equipped to search for microbes.
Read more on Curiosity’s discoveries here and on NASA’s site here.