What proof do scientists believe that they have that life once existed on planet Mars?
There is no proof, but mars was once hospitable to life and may still be in certain places. So we know life CAN live there, we just don’t know if it ever did. There are little pieces of evidence, a meteor that may or may not contain fossilized bacteria, traces of methane in the atmosphere which is usually associated with bacterial life, but not enough to make a decision on whether or not we think anything is there yet.
