Mars Climate, Now and In the Past
Theories About Mars Climate
So, maybe Mars had a thicker atmosphere and periods of climate change. There are two theories as to how this may have happened.
1.) Warm weather caused river channels & floods, which released water into the atmosphere. Rain returns water to the same ground.
2.) Floods caused water to flow (over time) from the higher ground to low ground. It was not released again, even when the weather became warm, but remained frozen into the ground .
More research is needed before either of these can be proven or disproven. Future missions to Mars will be very helpful.