Phobos

• There is good imagery at http://mars.jpl.nasa.gov/mro/gallery/press/20080409a.html . As the commentary states, there are obvious crater chains on Phobos. It is suggested that these roughly straight line chains may have been caused when debris from an impact on Mars later hit Phobos.
• Here are suggestions from outside NASA. Phobos may be composed to a considerable extent of volatile solids. Stress concentration along a line caused some flow of material along a line causing that material to be preferentially exposed to the vacuum of space. In the course of a billion and more years enough material along that line sublimed away to cause the collapse of overlying material, leaving craters on the surface. This linear irregularity caused neighboring linear stress concentrations resulting in further subsidence and other linear crater chains. Alternatively Phobos could be an alien interstellar spaceship with rows of gun ports and sensors which is only disguised to look like a celestial object.