The surface geology and geomorphology of Phobos

The martian moon Phobos is 26km×22.8km×18.2km in size, and the major landforms on its surface are craters and grooves. We analyzed the visible craters on the surface of Phobos where ~1300 craters≥200m in diameter, ~70 craters≥1km, and ~30 craters≥2km are identified; Stickney, the largest crater on Phobos, is about 8km in diameter. Most craters are undoubtedly of impact origin although some small craters may be pits formed by drainage of regolith into subsurface fractures. The presence of the observed impact crater population implies that the upper hundreds of meters to a few kilometers of Phobos are heavily fractured. Using the available digital terrain model of Phobos (the dynamic version), the 24 craters larger than 2km in diameter have been subdivided into three morphologic classes on the basis of their prominence; they are characterized by the following values of d/D ratios and maximum steepness of their inner slopes: >0.1 and >20°:9 craters; 0.05–0.1 and 10–20°:7 craters; and