I am a novice to Mars studies - and will always be. I don't have the time to dedicate to this endeavor, though I appreciate the good information from the people who do. Your creative ideas, such as liquid CO2, make me question my own, but I still strongly favor a wet Mars, intermediate between Earth and Jupiter, not a gas giant, but something between. As you know, modest pressure will condense CO2, so it would not have to be cold for liquid C02. Perhaps the early oceans of Mars were H2O/CO2, which would be a great solvent for other gases as well.

The asteroid belly (pun) lost all its gas since there are no large bodies, so any speculation as to its original composition is just more model based speculation.

I do mean planet girdling oceans that are almost gone by the end of the Noachian. Why do you say that the Northern plains evidence limitations for Martian oceans? Could these plains be underlain by sediments such as the super thick sequence seen at Valles Marineris (and only later altered by smaller seas)? It is the evidence at Valles Marineris and the other chasms alone which makes me believe in extensive, deep oceans. Forgive me for saying this, but this is "real" evidence, not some numerical simulation or extrapolation. To me, it doesn't matter what any of these simulations claim - unless they can explain 10,000 feet of sedimentary layers. I concede that these models are very valuable, as they are the only way to estimate the early conditions of both Earth and Mars, for which no surface evidence remains.

The Martian surface is so reworked (from tectonism and volcanism), that I doubt if there is much remaining evidence (such as cratering distributions) from the early epochs. These layers may only be seen in the deepest canyons where very little is exposed.

Cheers!