Steven Den Beste has an interesting rejoinder to the theory that Mars was always bone-dry with a thin atmosphere. It’s clear that the only way to explain the topological features on Mars is with the presence of liquid water. Which would mean that you’d need a lot of water rushing across the surface at high speeds for some time. It would require a very large impact or a number of smaller impacts in close sequence to kick up enough material to allow for liquid water to flow for that amount of time. As Den Beste mentions, the cratering in certain areas of Mars seem to disprove that hypothesis. If Mars was geologically inactive and cold, we wouldn’t see the kind smoothness that certain southern regions have.
Of course, the best way to prove or disprove this thesis is to go there and analyze the strata itself. However, given the way in which NASA cannot even manage a low-orbit outpost, it looks like the chances of that happening any time soon seem far too slim.
