Actually when I first looked at the image, I thought it was just what you said, a sloping blocky terrain with no sign of activity -- except for the sublimation of the snow cover...

But then another look suggested that indeed, we could see here hundreds and hundreds of active vents, all erupting simultaneously. What an amazing sight! (And given the intense coverage of the polar regions thanks to the orbit of the MGS, if such a phenomenon were to occur, it would be more likely than not that the spacecraft would capture it.)

It appears that the material being expelled is quite dry, but then, you would expect that, given temperatures and pressures. The venting also appears to be very energetic.

In addition to the dark plumes, there are also light colored plumes (or at least deposits) in some areas. This could indicate water ice or CO2 ice or both is being expelled.

The north polar region seems to show quite a few volcanic features surrounding the polar layers. I haven't seen the same around the south polar region.

The notion of transparent (CO2) slab ice providing the mechanism to focus and concentrate sunlight beneath the surface to provide the energy for these geysers or vents is interesting. Recall some of the Mariner 9 and Viking polar images seemed to show evidence of a good deal of quite thick transparent ice at the poles, even some signs of transparent blocks broken off the surface layer, but I haven't seen anything from the MGS that suggests its existence.

Do yu know of any visual evidence from MGS that could verify the existence of transparent slab ice at the poles?

Cheers, BFelix