An image of Deimos taken by the Mars Reconnaissance Orbiter.
|Discovered by||Asaph Hall|
|Discovery date||1877, August 12|
|Semi-major axis||23 460 km|
|Orbital period||1.262 44 d|
|Average orbital speed||1.35 km/s|
|Inclination||0.93° (to Mars' equator)
1.793° (to the local Laplace plane)
27.58° (to the ecliptic)
|Dimensions||15 × 12.2 × 10.4 km|
|Mean radius||6.2 km|
|Mass||1.48 × 1015 kg|
|Mean density||1.471 g/cm³|
|Equatorial surface gravity||0.003 9
0.000 40 g (400 µg)
|Escape velocity||5.6 m/s (20 km/h)|
|Apparent magnitude||12.4 |
Deimos (pronounced /ˈdaɪməs/ DYE-məs; also /ˈdiːməs/ DEE-məs, as in Greek Δείμος), 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 was discovered by Asaph Hall, Sr. on August 12, 1877 at about 07:48 UTC (given in contemporary sources as "August 11 14:40" Washington mean time using the old astronomical convention of beginning a day at noon, so 12 hours must be added to get the actual local mean time). Hall also discovered Phobos at the same time, after deliberately searching for Martian moons.
The names, originally spelled Phobus and Deimus, respectively, were suggested by Henry Madan (1838–1901), Science Master of Eton, from Book XV of the Iliad, where Ares (the Roman god Mars) summons Dread (Deimos) and Fear (Phobos).
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. 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.
Deimos' orbit is nearly circular and is close to Mars' equatorial plane. Mars' outer moon is possibly an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still controversial and disputed. Both Deimos and Phobos have very circular orbits which lie almost exactly in Mars' equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most likely by a combination of atmospheric drag and tidal forces, although it is not clear that sufficient time is available for this to occur for Deimos.
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 ("full moon") 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' phases, which take 1.2648 days (Deimos' synodic period) to run their course.
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.
Because Deimos’ 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°.
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.
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. Capture also 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 large planetesimal.
Only two geological features on Deimos have been given names. The craters Swift and Voltaire are named after writers who speculated on the existence of Martian moons before they were discovered.
|Crater||Named after||Coordinates||Diameter (m)|
It is named after the god Deimos in Greek mythology.
The other moon, Phobos, is the larger of the two.