Ah, dear me.

If I was really playing the role of nutty proffessor, this would be the term paper and *you* would have to give *me* the answers!

And this is one of the big problems. Nobody has studied flows of this type, other than in the wildest arm-waving conceptual way. So we don't know for sure.

Some guesses we can make - when a CO2 flow runs out of energy, it would do so suddenly, and dump a patch of rocks. So, for instance, the "delta" in Gusev crater could be the collapsed part of a CO2-fuelled flow that came down Ma'adim vallis. Or it could be a conventional delta!

CO2 flows won't pond in the way that water does and settle out gently. The flow would be trapped, but its energy and volatiles would boil away upwards, and a fairly chaotic layer of sediment accumulate below. There would be no clear 'tidemark', recording the water level at fill. So if we find a nice eroded notch around the margin of a crater lake, that's a good sign of water.

Mineralogical consequences? CO2 can dissolve S and Cl compounds well, if it is compressed or liquid. It is *possible* that the S and Cl elementsin the soils of Mars are recording seepage of COI2 from depth, orf maybe they are conventional sulphates and cloride minerals from water cementation. I'm puzzled about the report of *Argon* in the soil. Was that a type of Al or something? How do you sequester a noble gas into a soil?

CO2 is really good at converting hydrated minerals to carbonates. The lack of carbonates on Mars suggests that this hasn't happened much, which suggests, in a CO2 case, a general lack of hydrated mineral precursors?

There is no doubt that something flowed into Gusev, and cut the valley. water is easier to understand, bu has problems, in my view.

let's wait patiently and see what else we find on this mission and with Opportunity...