Grazing impacts on Mars - A record of lost satellites

Over 170 grazing impact craters, representing more than 5% of the total crater population of the ridged plains of Mars, can be identified on the basis of the elongate shape of the crater and the resulting pattern of ejecta deposits. These craters appear to occur along great circles, of which the more recent are in an east-west direction while the older ones are in more northerly directions. The large number and common impact directions of craters are interpreted as due to satellites whose orbits decayed with time. The locations of the projected orbital axes on the Martian surface indicate that the geographic poles of Mars were originally located at lower latitudes. The estimated combined mass of grazing impactors would form a satellite at least 225 km in diameter. These results may provide new clues as to the origin of Phobos and Deimos.