The water argument;

Here is an oldie but goodie article about water on Mars written inJune 29, 2000

I like these parts;

**There are 5 five distinct regions where we might sometimes find surface water: in the Amazonis, Chryse and Elysium Planitia, in the Hellas Basin and the Argyre Basin. Together they comprise about 30% of the planet's surface. That's not to say that liquid water really does exist in those places, just that it could."**

**The massive Hellas impact basin in the southern hemisphere of Mars is nearly 9 kilometers deep and 2,100 kilometers across. The air pressure at the bottom of the basin is about twice the global average. **

That being 12.4 millibars.

Liquid water could exist on the surface in the Hellas Basin during certain times. On the surface it would evaporate into the dry air, but an inch or so below the surface, it could remain liquid.

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The CO2 argument

Here is one for you Nick written about a year ago, titled Support For Critical Role Of Carbon Dioxide On Mars Grows

In the article, they talk about liquid CO2 creating many of the flow features.

Noted are these two paragraphs;

They suggest that liquid in the Martian crust was heated when molten rock, or magma, rose to the surface. As the liquid was heated, it expanded, until the pressure of overlying material was too great, and an explosive eruption occurred, shattering overlying rock, and causing it to move with the magma in an erosive debris flow.

The authors believe that the fluid in the crust along this area of the rim of the Hellas basin was mainly liquid carbon dioxide. A debris flow dominated by carbon dioxide would flow faster and farther than a water-based flow, they say.