Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. The two moons were discovered in 1877 by American astronomer Asaph Hall. It is named after Phobos, the Greek god of fear and panic, who is the son of Ares (Mars) and twin brother of Deimos.
Enhanced color image of Phobos, as captured by the Mars Reconnaissance Orbiter. Stickney, the largest crater on Phobos, is on the right.
Asaph Hall III, discoverer of Phobos
The Phobos monolith (right of center) as taken by the Mars Global Surveyor (MOC Image 55103, 1998).
An impact crater is a depression in the surface of a solid astronomical body formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters are typically circular, though they can be elliptical in shape or even irregular due to events such as landslides. Impact craters range in size from microscopic craters seen on lunar rocks returned by the Apollo Program to simple bowl-shaped depressions and vast, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.
500-kilometre-wide (310 mi) crater Engelier on Saturn's moon Iapetus
Recently formed (between July 2010 and May 2012) impact crater on Mars showing a pristine ray system of ejecta
50,000-year-old Meteor Crater east of Flagstaff, Arizona, U.S. on Earth
Eugene Shoemaker, pioneer impact crater researcher, here at a crystallographic microscope used to examine meteorites