 The hypothesis is that gravity pull from Mars makes the reddish regolith move over the surface, exposing relatively fresh, unweathered and bluish material from the moon, while the regolith covering it over time has been weathered due to exposure of solar radiation.Because the blue rock differs from known Martian rock, it could contradict the theory that the moon is formed from leftover planetary material after the impact of a large object.In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.