Phobos, the larger Mars' two satellites, was photographed intensively by the Viking orbiter during February 1977. Viking also did infrared studies of the satellite that first was discovered by Asaph Hall with the U.S. Naval Observatory telescope in 1877. Phobos was photographed in some detail first by Mariner 9 in 1969. The surface the satellite photographed was covered with craters and exhibited elongated depressions, parallel linear striations, and chains of craters.