22nd August 2013 Sol 371

Mars has two moons Phobos – about 22km diameter, and Deimos which is about half that.  MastCam has recently imaged an occultation where Phobos passed in front of the more distant Deimos.

The first detailed images were retuned by Mariner 9 and Viking in the 1970s.  In the MastCam images you can see their dark black colour, that has led many people to think that they are captured carbonaceous (C-type) asteroids.  Phobos has also been the target of an unsuccesful sample return mission called Phobos-Grunt.  It might offer the chance to sample a primitive asteroid, or even a stopping off point for human exploration.

However, there is an alternative view, (Craddock, 2011, Icarus) has argued that the angular momentum of Mars is consistent with the planet having undergone  a large impact, perhaps associated with much of the northern lowlands area.  Material could have been ejected into circular orbits, smilar to those occupied by Phobos and Deimos.  Thats one of the reasons that refining the orbits of these moons with MastCam imagery is important.

The occultation also looks fantastic and shows the versatility of MastCam!