Mars isn’t a planet well-known for its natural satellites but it actually does have two small moons. The larger, Phobos, is an irregularly-shaped, heavily grooved and cratered world only about 17 miles (27 km) across at its widest. It orbits Mars so closely that it completes 3 orbits every day, and isn’t even visible from some parts of the planet. But Phobos has an even smaller companion in orbit: Deimos, which at the most 7.5 miles across is half Phobos’ size. Deimos orbits Mars much further away as well, taking about 30 hours to complete one orbit.
The images above are our best views to date of Deimos. They were captured by the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter (MRO) on Feb. 21, 2009, and have a resolution of 65 feet (20 meters) per pixel. They show about the same face of Deimos but were captured about 5 1/2 hours apart, so the lighting geometry is a bit different. The color is extended in that it reveals light in near-infrared, red, and blue-green wavelengths, but this allows for visibility of color variations in Deimos’ surface caused by age; older regions are red and brown while younger surface material is brighter and whiter.
The surface of Deimos appears smooth because it’s covered with a deep (like 300 feet deep) layer of powdered, pulverized rock—the result of billions of years of meteorite impacts.
It was long suspected that Deimos and Phobos were once asteroids, pushed toward Mars by the gravity of Jupiter and caught in orbit. But recently other researchers are suggesting their origin may be more dynamic, and that they are the remnants of a ring of material that once circled Mars, thrown into orbit by one or more massive impact events on the planet itself.
Source: HiRISE/University of Arizona