Deeper CO2
Thanks for the information on the images, Alex - there are some interesting features there... some of the craters look as if they filled up and then overflowed, eroding one side.
What would happen if a thick layer of lava was deposited over deep deposits of CO2? Might the gas released reach the surface via fissures and produce the cones? (I'm trying to think from Nick's perspective here... personally I think the cones look enough like the Icelandic ones that I agree water is the most likely volatile, but if we don't examine the alternatives, they sometimes come back and bite us...)