Deimos is the smaller and outer of Mars’ two moons. It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is Mars II.
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 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 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.
As seen from Mars, Deimos has an angular diameter of no more than 2.5 minutes (sixty minutes make one degree) and therefore appears almost star-like to the naked eye. At its brightest (“full moon”) it is about as bright as Venus is from Earth; at the first- or third-quarter phase it is 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°.