Take a look at the Oxia Palus area where Trouvelot crater is. Here's a Mola clickable map of the area as well as a black and white Viking clickable map.

I have to admit that at first glance, the dark color and features in your photo resemble a lava bed. Something tells me, that the 'craters' in Oxia are due to bombardment rather than volcanic extractions, so there's a very good chance that the dark layering is from meteoric carbon as Dan said earlier.

In the full resolution image at your last link, I saw other examples of the same reflected whittish features randomly scattered. They seem to share the same elevation as well as the angle of the light source. Specifying the time of day when the photo was taken would be helpful (looks like it was taken just before sunset). That entire area could be a lot lighter than what the photo shows.

Thanks, BR.

Marcia