Today's Image of Mars shows terrain in Gale Crater slightly southwest of the Mars Science Laboratory Curiosity's August 2012 landing site. This image captures three different features: older plains, overlain with some sedimentary deposits and active sand dunes.
Scientists speculate that the sedimentary deposits are hardened mud, which has fractured over time. Hardened mud is a prime target for Mars Science Laboratory because it can help scientists to determine the acidity of the water in which the mud formed. Water with low acidity is more conducive to life as we know it, so examining some could give us some idea as to the habitability of the area at various points in the Martian past.
Study of the sand dunes has shown that they were formed by heavy winds blowing from the southwest toward the northeast. The sand dunes are still active, but moving very slowly across the surface.
Clicking on this image will take you to the original high resolution image from HiRISE. [See their caption for the image]