Today's Image of Mars shows craters that have expanded due to the high impact temperatures sublimating the surrounding icy terrain. As you can see there are craters surrounded by wider, shallower depressions. These depressions surrounding the impact craters formed when the heat from impact made the carbon dioxide ice surrounding it turn directly into water vapor.
The depressions are elongated in the direction the meteorite struck because the impact created heated material that projected itself forward, which sublimated the ice in that direction. Imagine throwing a golf ball into a bowl of ice cream. If the ball comes in at an angle some of the ice cream will be uplifted and thrown forward, on top of other pudding. When this happens at a high enough velocity the friction between the surface of Mars and the impacting meteorite heats up the terrain it touches and launches it forward. The heat was enough that it sublimated the carbon dioxide ice and created the depression that you see oriented in the direction of the impact.
Clicking on this image will take you to the original high resolution image from HiRISE. [See their original caption for the image]
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