I guess Martian Climatologists didn’t appreciate the Gravity of the situation.
A part of the puzzle solution, no doubt. To hold an atmosphere the gasses have to be a certain ratio. while water is flowing and raining down on Mars, all that is needed to upset the balance is a meteoric rain which would heat the atmosphere sufficiently to evaporate water into clouds no longer heavy enough for the gravity of the planet to hold the atmosphere. Half of Mars is pocked with craters, while another half is relatively scoured. IF Mars was a moon of a much larger watery planet which disintegrated forming the asteroid belt, then the catastrophes can explain the changes to Martian atmosphere. 9Tiamat anyone?)