Phobos Ridge Mars, the fourth planet of the Solar System. Phobos Ridge Phobos Ridge () is a rocky ridge of sandstones and shales forming the west side of Mars Glacier in the southeast corner of Alexander Island, Antarctica. The coast in this vicinity was first seen from the air by Lincoln Ellsworth on November 23, 1935, and roughly mapped from photos obtained on that flight by W.L.G. Joerg. This ridge was first surveyed in 1949 by the Falkland Islands Dependencies Survey (FIDS) and named by the United Kingdom Antarctic Place-Names Committee (UK-APC) for its association with adjacent Mars Glacier, Phobos being the
Solar System icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury), and, in