I did some quick research through the MOC image files and didn't come up with a source for the plume (or whatever it is). There are LOTS of graben and pits and "fossae" in the general area, though. There's also this one with a black spot about half way down the image (probably a recent impact) that our buds at Malin have taken many, many pictures of. Any of them could be a source, it seems to me.

Your notion of condensed moisture being released from underground is interesting... there are no doubt lots-n-lots of caves and such riddling the Tharsis rise... little doubt, too, that moisture could collect within. But how could it get out in such a spectacular fashion? Impact? Pressure from below?

What do ya think?

(Hi Dan!)

Cheers, B R Felix