The Phobos monolith is a large rock on the surface of the moon Phobos,[1] which orbits Mars. It is a boulder about 85 m (279 ft) across.[2] A monolith is a geological feature consisting of a single massive piece of rock. Monoliths also occur naturally on Earth, but it has been suggested that the Phobos monolith may be a piece of impact ejecta. The monolith is a bright object near Stickney crater, described as a "building sized" boulder, which casts a prominent shadow.[3][4] It was discovered by Efrain Palermo, who did large surveys of Martian probe imagery, and later confirmed by Lan Fleming, an imaging sub-contractor at NASA Johnson Space Center.
The general vicinity of the monolith is a proposed landing site for a Canadian Space Agency vehicle, funded by Optech and the Mars Institute, for an unmanned mission to Phobos known as PRIME (Phobos Reconnaissance and International Mars Exploration).[3] The PRIME mission would be composed of an orbiter and lander, and each would carry four instruments designed to study various aspects of Phobos' geology.[5] At present, PRIME has not been funded and does not have a projected launch date.