Deimos was discovered by Asaph Hall, Sr. at the United States Naval Observatory in Washington, D.C on August 12, 1877.
Physically, it is smaller than Phobos but is smoother. For its orbit, it takes 1.2648 days to complete one phase. Unlike Phobos, it rises in the east and sets in the west. It also takes2.7 days to orbit which is longer than a day on Mars which is 24.7 hours
Origin Theory From WikipediaEdit
The origin of the Martian moons is still controversial. The main hypotheses are that they formed either by capture or by accretion. Because of the similarity to the composition of C- or D-type asteroids, one hypothesis is that the moons may be objects captured into Martian orbit from the asteroid belt, with orbits that have been circularized either by atmospheric drag or tidal forces, as capture requires dissipation of energy. The current Mars atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated due to tidal forces. The main alternative hypothesis is that the moons accreted in the present position. Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a planetesimal.