I am skeptical that it is liquid water causing the surface changes on Mars. It has been speculated that some other medium could be causing these changes (ie. the outgassing of CO2). So here's the question- is it water or CO2 or some other medium?

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Here's a phase diagram for CO₂
I don't see any way to get liquid CO₂ on Mars, and gully looks to me more like a product of flowing liquid than the result of a gas outburst. If it were a gas outburst, shouldn't there be a pit, or circular debris field near the top?

I found this article but its from 2001

Liquid CO2, Not Water, Likely Created Martian Gullies

http://unisci.com/stories/20012/0402013.htm

As it says here:
It's not liquid carbon dioxide flowing in the gullies.

"What's coming out is liquid CO2 that suddenly vaporizes," Musselwhite said. "As it comes out, it expands very quickly, cools, and actually produces CO2 snow. The snow is suspended in CO2 gas that hasn't solidified yet. Together with rock debris, it forms slurry. Geologists call it a 'suspended flow.' Suspended flow acts like a liquid. It doesn't take very much liquid each time to add to gully formation."

But Malin in his interview said:
"The attributes that we see, it moved very slowly on a steep slope, which means that it was changing its properties as it was moving downslope. But it's easily diverted around very, very subtle topography and it has very long, finger-like terminations at the ends of these flows. Those are all attributes of something that has liquid water in it."

Could Co2 creat this, before it vaporizes?

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"water" is more exciting ...but "wind" is more rational

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Yeah, water's exciting, but wouldn't it be frozen at the usual Martian temperature?

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Well as much as I'd love to be proven wrong on this one, I'm going with dust slippage causing a lighter area to be exposed rather than a salt deposit from liquid water. I can see how they think it is from water, but I really want more evidence before rushing to Mars for a glass of water.

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Which changes do you mean? We see lots of changes on Mars, caused by the wind, dust devils, mass movement, impact, deposition and sublimation of water and CO2 ice.

If you mean the recent gully deposits, then the probability is quite high that they are water related. None of the other processes can generate the combination of features we see in these gullies and their deposits. These include: sinuous trunk channels, tributary and distributary channels, levees, digitate fans, terrace deposits.