NASA orbiter spots strange secondary craters on Mars

NASA's Mars Reconnaissance Orbiter (MRO) has captured a region of Mars sprayed with strange-shaped secondary craters.
New images show the region of the red planet with secondary craters from 10-kilometre Zunil Crater to the northwest, NASA said.
Secondary craters form from rocks ejected at high speed from the primary crater, which then impact the ground at sufficiently high speed to make huge numbers of much smaller craters over a large region.
In the new images, however, the secondary crater ejecta has an unusual raised-relief appearance like bas-relief sculpture.
One idea is that the region was covered with a layer of fine-grained materials like dust or pyroclastics about 1 to 2 meters thick when the Zunil impact occurred about a million years ago.

The ejecta served to harden or otherwise protect the fine-grained layer from later erosion by the wind.