Well....

Nick Hoffman is an extremely brilliant and insightful researcher. His hypothesis that Mars hosts (or once hosted) abundant supplies of frozen and liquid CO2 is compelling in its apparent obviousness --so long as ancient Mars was colder than it is now.

Most workers assert that ancient Mars was warmer. Indeed, warm enough to allow an ocean to form in the Northern Plains and seas and lakes to form widely.

Nick's insight that the surface of Mars was colder in the past than it is now is likely correct, and not solely due to the "faint young sun paradox." A colder surface implies a thinner atmosphere. Essentially no chance for liquid water to accumulate under clement conditions. Exceptions might occur subsequent to massive impact events, but Nick argues that such events would release mostly liquid CO2 not H2O. Fair enough.... but only if the planet's interior were quite cold, and only if high pressures could be widely maintained. Of this, there is no evidence whatsoever. Indeed the available evidence suggests that the ancient Martian interior was quite hot, certainly molten within tens of kilometers of the surface, and there is considerable surface evidence that there has been extensive, relatively recent widespread flood volcanism, which simply cannot occur if the planet is as stone cold as it needs to be for Nick's CO2 scenario to work. There is also an absence of evidence of a widespread competent surface seal that could withstand pressures required to liquify CO2.

Nevertheless, there's a good chance there were localized pockets of liquid CO2, and that these pockets did contribute to the outflow channels we see today. H2O was likely the major contributor, however, as the recent announcement of extensive water deposits demonstrate.

Nick argues that this water has nearly always been frozen. It is very important to him that it nearly always be solid, never liquid. And he seems to argue -- though I am not quite sure -- that the water budget of Mars is much less than generally believed. I think he will be shown to be wrong. I suspect we will find that instead of reservoirs of liquid CO2, we will find relatively near-surface reservoirs of warm liquid water. We will find that most of the CO2 budget of Mars is currently in the atmosphere and polar caps (residual amounts might be found in earlier polar deposits). And we will find relatively extensive subsurface deposits of carbonates.

Mars is far stranger than anyone has yet fully imagined, and I have little doubt that future discoveries will thrill and delight us, and scare us just a little, too.

Nick Hoffman has contributed greatly to current understanding of Mars, and I'm sure he will contribute more of his unique insight in the future.

Cheers, B R Felix