Today's Image of Mars is shows Dodo-Goldilocks, a 22x35cm trench dug by the Mars Phoenix Lander, which, at its deepest reaches a depth between 7 and 8cm. The trench was dug using Phoenix's robotic arm on Sol 18 of its mission, which was June 12 2008, and this image was taken 6 Sol later on June 19 2008.
The white material that you see is ice. The ice lumps on the bottom left of the trench sublimated (turned from solid to gas) over the course of the next four days. [See the trench a few days later]
Clicking on this image will take you to the NASA page that shows the trench before and after the ice sublimated.
|Source: NASA/JPL-Caltech/University of Arizona/Texas A&M University