Nick Hoffman and I are close Web buddies -- and, in fact (and to my surprise), I managed to argue him completely out of his original position that Mars during its most ancient ("Noachian") era would have had any dense CO2 atmosphere that briefly formed as the result of an impact quickly refreeze. (He had misunderstood a piece by James Kasting and Darren Williams in "Icarus".) Even before then, he repeatedly said that he reserved judgement as to whether Noachian Mars was always cold enough to rule out liquid water either on the surface or near it -- as he says, his "gas-flow" theory explains the gigantic "catastrophic runoff channels" (which formed later) much better than it explains the smaller meandering "valley networks" that formed during the Noachian.

His position now, he tells me, is that Mars MUST have had a dense CO2 atmosphere during the Noachian, because the impact that created the Hellas basin alone would have vaporized enough CO2 ice to create an atmosphere so dense that it would have been able to keep itself from refreezing through its own greenhouse effect (until it disappeared gradually for other reasons, as other scientists have said).

The real debate now is whether this early dense atmosphere -- in combination with Noachian Mars' much greater volcanic activity -- could have sustained a lot of liquid water at or near Mars' surface in the Noachian Era. But that's exactly the same central exobiological question that concerned Mars scientists before Nick's "White Mars" theory came along -- so his view (which now seems to be turning into what might be called "Blue-White Mars") doesn't change the planet's biological significance much. As for the apparent absence of surface carbonates on Mars -- admittedly the biggest stumbling block to thinking that Noachian Mars had a lot of surface liquid water -- there are several alternative theories about that (including the fact that the problem is much less if the liquid water was only NEAR the surface).