Phobos Description
Phobos is the largest of the two moons. It has an oblong shape with a diameter of 22.2 kilometers. The surface is covered with 150-meter-long grooves that are 25 meters deep. As they radiate out from a large crater, they are believed to be side affects of impact. The moon is dark in color and composed of carbonaceous chondrite, which is similar to objects located in the outer asteroid belt.
Deimos Description
Deimos is the smallest moon in the solar system. Its diameter is only 12.6 kilometers. The moon's shape is slightly less irregular than Phobos. The relatively smooth-looking surface is due to a thick layer of dust. Phobos does not have this because its proximity to Mars causes any debris to be sucked in by the planet's gravity. Deimos is also made up of carbonaceous chondrite.
Relationship With Mars
Phobos orbits Mars every seven hours. It also has the distinction of being the closest moon to its planet in the solar system. It is estimated that in 40 to 50 million years, Phobos will either be torn apart or crash into Mars. The moon's orbit brings it closer to Mars by 1.8 centimeters a year. Currently it is 9,378 kilometers from the planet. Deimos is 24,459 kilometers from Mars. It orbits the planet every 30 hours. The dust layer of Deimos is from deposits collected from a ring located around Mars.
Discovery and Exploration History
Phobos and Deimos were discovered in 1877 by astronomer Asaph Hall, who was working at the U.S. Naval Observatory. He was the one who named them as well. The first photographs were taken in 1971 by the Mariner 9 probe. Phobos' 10-kilometer-wide crater, Stickney, was named after Hall's wife. One Soviet probe sent in 1988 took detailed photos before dying while another lost its way entirely. These probes also detected possible evidence of water but were not able to investigate further.