As I understand it, if life has ever existed on Mars it will most likely be at the microscopic level. There may possibly have been uni-cellular life-forms but probably no bigger than that - probably no fish or plants.
This is because larger lifeforms need billions of years to evolve and wouldn’t have had enough time on Mars before everything got wiped out.
Assuming then that we are looking for evidence of creatures that are no bigger than bacteria and probably died millions of years ago, how will we know if they were ever there? Bacteria don’t leave fossils, they don’t turn into oil like other organic creatures. What evidence would these tiny now-extinct critters leave that we could possibly find millions of years later?
We’ve discovered that there was once (probably) water on Mars, what would we need to find to prove that bacteria once lived there in the water?
Careful there. You’re assumming that a random process which took a while to produce multicellular organisms on earth would take the same amount of time to produce them on Mars. That’s a lot to base on only a single example of evolution in action.
Actually, bacteria can leave fossil traces, although the preservation potential is low. Bacteria, like any other organic matter, could also certainly contribute to petroleum deposits, but since any such things on Mars will most likely be in the subsurface they won’t be the first thing biologists and geologists are looking for.
The search for life on other worlds raises an interesting issue: if life there is similar to life here, are we dealing with a contamination issue or a common origin for life in both places? And if life truly rose on different worlds independently, how can we recognize what traces that other life might have left behind?
At this point, scientists will begin with the assumption that life, regardless of location, would leave behind traces already familiar to us. That would include “organic-looking” structures, either at the macroscopic or microscopic level; organic chemical traces similar to the metabolic products of Earthly life forms; and mineral deposits (most likely carbonate rocks of some kind, like limestone) that do not appear to have been laid down as a result of water transport.
It is just about possible that bacteria could survive being ejected from Earth by a major impact and end up on Mars where conditions for survival might be available, (or vice versa), so a common origin is possible (although I suppose this is just a stripped down panspermia hypothesis).
It’s only proper to be concerned about contamination. It would be a major bummer if they find bacteria on Mars, only to discover later that it came from the spaceprobes.
It’s even possible that the contrary could be true : mars grown bacteria surviving the flight from mars and flourishing on earth. But I would suspect it’s unlikely, since beside surviving the trip, these bacterias would have to end up in a spot with an environment allowing them to thrive…
There’s even people believing that life could have come in the same way from outside the solar system.
There is a genuine concern that we might accidentally seed Earth life in the places that we are exploring, through transfer from the rovers, say. The worry has not been so large for Mars, because regardless of the degree to which water may have been abundant on the surface in the past, there sure isn’t much to speak of now. In contrast, contamination is a HUGE concern in the plans for exploring Jupiter’s moon Europa, which is thought to have a deep liquid ocean beneath a shell of ice.
This page has some interesting info on “planetary protection.”
Actually, Astrobiology.com has a ton of interesting links, including the issue of how to determine if life existed elsewhere (look under the headings “Exobiology & Astropaleobiology” and “Panspermia”).
BTW, just an addition to my earlier comment about “signs of life”: It’s possible to have mineral deposits that are both inorganic and not laid down by water (we have those on Earth): it simply would draw attention until the nature of the deposit could be resolved as biological or non-biological in origin.