This image shows one of many fresh impact craters spotted by the UA-led HiRISE camera, orbiting the Red Planet on board NASA’s Mars Reconnaissance Orbiter since 2006. Credit: NASA/JPL-Caltech/MSSS/UA
Scientists using images from NASA’s Mars Reconnaissance Orbiter, or MRO, have estimated that the planet is bombarded by more than 200 small asteroids or bits of comets per year forming craters at least 12.8 feet (3.9 meters) across.

Read more at: http://phys.org/news/2013-05-hirise-mars-camera-reveals-hundreds.html#jCp