Signs of past or present life on Mars is the easy answer, and that would be great and history changing, of course. However, I think it would be great to answer important simpler questions, like where is the water, is there still liquid water to be found (lots of evidence so far) or whether where were active volcanoes — those all would show that Mars is an active planet and would be very cool to be part of.
I would like to discover if Mars ever had an ocean and if not what processes happened to create the terrains we currently think might be evidence for such an ocean.
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Peter commented on :
I would like to discover if Mars ever had an ocean and if not what processes happened to create the terrains we currently think might be evidence for such an ocean.