Today's Image of Mars is a reminder that features on Mars are constantly changing due to wind erosion. The image from May 2010 shows tracks left from large and small boulders as they rolled and bounced down the slope in Eastern Isidus Planitia. When you compare that with the image of the same area in February 2012 you'll notice that all but the largest trail has been completely erased from the surface, and even that has been significantly faded.
It's important that we realize that not all features on Mars are permanent. This is in direct contrast to the Moon, where even today the footprints of the Apollo astronauts can be seen. Understanding that Mars is still a changing planet is essential when sending future missions to Mars because we need to make sure that we use more permanent features for navigation while on the Red Planet. This may seem obvious, but when you look down from orbit it's easy to forget that some features can change in just a matter of months, given the right circumstances.
Click on this image to see the original in high resolution from HiRISE. [See the original HiRISE caption for this image]