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As pointed out in email
3D modelling of the early Martian Climate under a denser CO2 atmosphere: Temperatures and CO2 ice clouds delivers this doleful assessment of Mars' past:

On the basis of geological evidence, it is often stated that the early martian climate was warm enough for liquid water to flow on the surface thanks to the greenhouse effect of a thick atmosphere. We present 3D global climate simulations of the early martian climate performed assuming a faint young sun and a CO2 atmosphere with pressure between 0.1 and 7 bars. The model includes a detailed radiative transfer model using revised CO2 gas collision induced absorption properties, and a parameterisation of the CO2 ice cloud microphysical and radiative properties. A wide range of possible climates is explored by using various values of obliquities, orbital parameters, cloud microphysic parameters, atmospheric dust loading, and surface properties. Unlike on present day Mars, for pressures higher than a fraction of a bar, surface temperatures vary with altitude because of the adiabatic cooling and warming of the atmosphere when it moves vertically. In most simulations, CO2 ice clouds cover a major part of the planet but greenhouse effect does not exceed +15 K. We find that a CO2 atmosphere could not have raised the annual mean temperature above 0{\deg}C anywhere on the planet. The collapse of the atmosphere into permanent CO2 ice caps is predicted for pressures higher than 3 bar, or conversely at pressure lower than one bar if the obliquity is low enough. Summertime diurnal mean surface temperatures above 0{\deg}C (a condition which could have allowed rivers to form) are predicted for obliquity larger than 40{\deg} at high latitudes but not in locations where most valley networks are observed. In the absence of other warming mechanisms, our climate model results are thus consistent with a cold early Mars scenario in which non climatic mechanisms must occur to explain the evidence for liquid water. In a companion paper by Wordsworth et al., we simulate the hydrological cycle on such a planet.

Also posted at Dreamwidth, where there are comment count unavailable comment(s); comment here or there.

Formation of frozen CO2 clouds is also a problem for keeping the early Earth from freezing over via high CO2 concentrations.

I have to wonder if their model's output would change based on the fact there /was/ an ocean in the northern hemisphere: they have found what appears to be shorelines and it also explains why there is so little drainage activity.

Large bodies of water have a massive impact on both thermal and circulation models.

Could there have been enough salt to keep that ocean liquid even at much lower temperature? Or, are the shorelines consistent with an ocean with a thick ice cover?

EDIT: AH! Dissolved CO2 will depress the freezing point.

Edited at 2012-11-29 02:30 am (UTC)

That's /definitely/ something that needs to be added to a model...

Looking at some phase diagrams, I'm wondering if I'm right.

You can cool soda water below the freezing point of water, so it freezes when you release pressure, but I'm wondering if that works because the soda water is in a metastable state.

There's also the matter of forming CO2-water hydrates.

For a while I'd hear now and then about models of Mars in which there was never any liquid water at all (except possibly for momentary melt events), and all the supposed stream beds and such were explained as flows of something else. I was kind of half-cheering for them just because they were so weird. But I presume they're dead or nearly so.