Phobos is the larger of the two moons of Mars, with a mean diameter2 of about 22 km and an orbital radius of 9,376 km. Phobos orbits faster than Mars rotates, so it rises in the west and sets in the east as viewed from the planet’s surface. The low orbit has made it a target for robotic spacecraft orbiting Mars. Images show craters along with numerous grooves (Figure M.1). Explanations proposed for their origin include tidal-stress induced fracturing and secondary impacts from larger craters, such as Stickney, on Phobos. Study of recent, better-resolution images suggests that the grooves, which are absent on the trailing end of Phobos, may be chains of secondary impacts caused by debris from impacts on Mars (Murray and Heggie, 2014), or debris from impacts on Phobos that orbited the moon before impacting it (Nayak and Asphaug, 2016). Lineations within Stickney (Figure M.2) may be from landslides in the moon’s weak gravity. Study of the thermal properties of the surface may determine whether the surface material is loose or relatively coherent, as these would show different rates of heating and cooling (Figure M.3).