-3.47°/285.42° Southern Highlands, just south of the Syrtis volcanic plain. In fact, a good deal region here seems to be a volcanic flood plain. Check out the mounds and cratering at the top of the image, inside the larger crater. It looks to me like a surface that was once molten, and almost all the circular formations are "popped bubbles" in the volcanic flood. Significant volatile content in the lavas, eh? Water? CO2? Clathrate? Methane? Here's a segment from the USGS browse image:http://ida.wr.usgs.gov/fullres/divided/e04006/e0400653j.jpg

It is difficult to rationalize such dense cratering within a crater as the result of impact alone. But I'm sure someone would try ;-) And look on the surface of the plain: same sort of dense cratering, hills, and an apparent flood of lava. Again, many of the craters may be the result of escaping volatiles. There's even a channel from the large crater into the apparent lava flood area on the highland plain: http://www.msss.com/moc_gallery/e01_e06/full_jpg_ctx_map/E04/E0400654.jpg This indicates that some of the volcanic flood originated in the large crater and spilled onto the highland plain. I actually see very few impact craters in the MOC image close up, despite thousands of circular formations....

As for your cluster, in this segment: http://ida.wr.usgs.gov/fullres/divided/e04006/e0400653f.jpg it's a phenomenon I've seen widely on the surface (though I can't point to other examples right now, there are many such clusters.) I'd bet they are impact formations, caused by a disintegrated (cometary?) object striking the surface as several dozen discreet objects.

It's long been noted that Phobos orbits within the Roche limit of Mars and some have suggested its surface striations and chains of pits may be signs that it is breaking up under tidal stress. Seems to me that any small, barely consolidated object could break up and strike the surface of Mars forming clusters of small craters like we see here. Similar object falls occur on the Earth, but our dense atmosphere slows them to such an extent that they either burn up before striking the ground or fall to the ground too gently (!) to form craters.

And what say you?

Cheers, B R Felix