The existence of little green men on Mars has pretty much been ruled out – but could any of the planets or moons in our solar system support some simpler form of life?
Mars is still a prime candidate for microbial life. Mars Express and the NASA rovers have found evidence that liquid water still exists near or on the surface, and detected methane in the atmosphere that may be biogenic. Even if the near-surface water is only occasionally present, microbes may exist deeper in the crust, underneath the permafrost layer. Meanwhile, some moons of the outer planets appear to have oceans of liquid water underneath their icy crusts. On Europa, Ganymede, Callisto, Titan, Enceladus and Triton, life may survive on the nutrients pumped out by seafloor vents, as it does around “black smokers” on Earth. Titan could conceivably even have surface life, although the extreme cold makes that unlikely.
