This is a scanning electron microscope (SEM) image of a series of partly filled pits on the surface of a mineral grain from Nakhla. Similar pits are sometimes found on Earth minerals which have been exposed to microbial attack. They are interpreted as etch features caused by organic acid generated by microbes and their biofilms. The pits in the Martian minerals are partly filled with debris, possibly the clay-like remains of organic microbes.
Photo Credit: NASA/David McKay