Here's the most violent event ever observed on Mars, tons of rock, dust and ice plummeting down a 700-meter cliff at 15 meters per second. It's one of four avalanches the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars orbiter observed on the north polar scarps of Mars. The white material at the top of the cliff is carbon-dioxide ice, and it's possible spring sunshine caused the ice to expand and break, sending loose dust and ice hurtling down the slope. Another possible explanation: A Marsquake. Click through for the full set of images.