So I think we can just forget about the notion of flowing liquid CO2 carving these recent channels. CO2 requires a high atmospheric pressure (~5bar) to liquify (according to Nick Hoffman and phase charts I've seen), and no such pressures are available on the surface of Mars. I believe Nick has argued that some channels can be carved by a dense gasseous flow of CO2, and such channels would surprisingly simulate (or mimic) channels carved by flowing water. In this, I think he has a point. Up to a point, his gasseous flow model may serve, but there comes a point where it doesn't: for example, when we see braided channels and all those narrow dune channels.

I think the case against water on the surface of Mars has long been vastly overstated. On the other hand, the desire for some sloshy past era of oceans and such has thoroughly confused the situation.

Water behaves in a specifically Martian manner on Mars. Much of this behavior is probably very unfamiliar to us, and most of our preconceptions about it are probably off the mark.

Still puzzling here ....

Cheers, B R Felix