Viking imaging of Phobos and Deimos: An overview of the primary mission

The Viking primary mission (June 20 to November 15, 1976) yielded approximately 50 images of Phobos and Deimos. These pictures completed the surface coverage of Phobos begun by Mariner 9 in 1971–1972 and extended the coverage of Deimos. The effective resolution of the Viking images exceeds that obtained by Mariner 9 for two main reasons: improved camera performance and closer encounter distances. The improved resolutions revealed a number of unexpected surface features such as linear chains of irregular craters and elongated grooves and striations. The Viking coverage has provided the first high-resolution color information on the surfaces of the two satellites and has extended the phase angle coverage to 125°. Additionally, a number of images of the satellites were obtained against star backgrounds to refine further the ephemerides.