Deimos (pronounced /ˈdaɪməs/ DYE-məs; also /ˈdiːməs/ DEE-məs, is the smaller and outer of Mars’ two moons, the other being Phobos. It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is Mars II. Deimos is probably an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still in dispute.

Deimos was discovered by Asaph Hall, Sr. on August 12, 1877 at about 07:48 UTC Hall also discovered Phobos on the same day, after deliberately searching for Martian moons.

Deimos, like Mars' other moon Phobos, has spectra, albedos and densities similar to those of a C or D-type asteroid. Like most bodies of its size, Deimos is highly non-spherical with dimensions of 15 × 12.2 × 10.4 km. It has a nearly circular orbit nearly in Mars' equatorial plane.

Unlike Phobos, which orbits so fast that it actually rises in the west and sets in the east, Deimos rises in the east and sets in the west. However, the Sun-synodic orbital period of Deimos of about 30.4 hours exceeds the Martian solar day ("sol") of about 24.7 hours by such a small amount that 2.7 days elapse between its rising and setting for an equatorial observer.

As seen from Mars, Deimos would have an angular diameter of no more than 2.5 minutes (sixty minutes make one degree) and would therefore appear almost star-like to the naked eye. At its brightest it would be about as bright as Venus is from Earth; at the first or third quarter phase it would be about as bright as Vega. With a small telescope, a Martian observer could see Deimos's phases, which take 1.2648 days (Deimos's synodic period) to run their course.

As seen from the surface of Deimos, Mars would appear 1,000 times larger and 400 times brighter than the full Moon as seen from Earth, taking up one-eleventh of the width of a celestial hemisphere.

Because Deimos’s orbit is relatively close to Mars and has only a very small inclination to Mars’ equator, it cannot be seen from Martian latitudes greater than 82.7°.

Solar transits
Deimos regularly passes in front of the Sun as seen from Mars. Due to its small size it cannot cause a total eclipse, appearing only as a small black dot traveling across the Sun. Its angular diameter is only about 2.5 times the angular diameter of Venus during a transit of Venus from Earth. On March 4, 2004 a transit of Deimos was photographed by Mars Rover Opportunity, while on March 13, 2004 a transit was photographed by Mars Rover Spirit.

Physical characteristics
Deimos is composed of rock rich in carbonaceous material, much like C-type asteroids and carbonaceous chondrite meteorites. It is cratered, but the surface is noticeably smoother than that of Phobos, caused by the partial filling of craters with regolith. The regolith is highly porous and has a radar estimated density of only 1.1 g/cm³. The two largest craters, Swift and Voltaire, each measure about 3 kilometres across.

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[14]. Landis has suggested that the moons may have originated from a binary asteroid that separated due to tidal forces

Named geological features
The only two named geological features on Deimos are the craters, Swift and Voltaire, the two writers who speculated on the existence of Martian moons hundreds of years before they were discovered.

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