Here's something new, for what it's worth... I located a more recent Themis image of the area of the crater with the "plume," and the depression in which it is located. It's much smaller scale, so interpret it as you wish. The Themis image is I14551005. I am using the image from Google Mars that includes I14551005 to show the surrounding area. I've searched the area and can find no other depressed area that matches the one in the "plume" image. The image showing the plume is MOC R18-01150, and was photographed 14 June 2004. The Themis image was photographed about nine months later...26 March 2005. The footprint also agrees closely with the index map on the MSSS web site. If I've identified the depression correctly, there appears now to be a mound or dome in the crater, and a flow of some kind breaching the wall of the southeast quarter of the depression.
On the attached photo, I've shrunk the MOC image to approximately the scale of the Themis image. Inset "A" is a crop of the original MOC photo. Inset "B" is a larger crop reduced to the Themis image scale. Since the images were shot from different orbits, in Inset "C" I've taken the liberty of adjusting the MOC image slightly to align surface features. The general outline of the depression, the strike of the highlands to the northeast, and the rim of a crater about 5km south of the depression align closely with the Themis image. It's unclear if the southern area matches. The adjustment, for what it's worth, was done using Photoshop. The only change was the keystoning effect.
If I've identified the wrong depression, I'd be delighted if someone could find the correct one. It should be within about 10km of this one.
So what is it? My original guess was a geyser-like plume. I don't think that any more. So maybe a volcanic eruption with lava flow...a trick of the light...or a misidentified area? It's open to debate again.
http://i46.photobucket.com/albums/f149/onc...e/kiki9aaaa.jpg