Deimos - Just the facts (A article written in 2007)
Deimos, is the smaller & outermost of Mars’ two moons
(the other being Phobos). It is named after Deimos from Greek Mythology. Its systematic
designation is Mars II. It is a Greek word meaning dread.
Phobos
& Deimos were both discovered by American astronomer Asaph Hall. The names
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) &
Fear (Phobos).
Deimos was discovered 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).
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 some dispute. Like most bodies of its size, Deimos is highly nonspherical with dimensions of 15×12×10 km.
Deimos is composed of rock rich in carbonaceous material, much like C-type asteroids & 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 two largest craters, Swift & Voltaire, each measure about 3 kilometres across (they were given these names, for both writers had supposed the existence of two moons around Mars well before the real moons were discovered).
As seen from Deimos, Mars would be 1000 times larger & 400 times brighter than the full Moon as seen from Earth, taking up a full 1/11 of the width of a celestial hemisphere.
As seen from Mars, Deimos has an angular diameter of no more than 2.5' & would therefore appear starlike 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. When Deimos passes in front of the Sun its angular diameter is only about 2.5 times the angular diameter for Venus during a transit of Venus from Earth. With a small telescope, a Martian observer could see Deimos' phases, which take 1.2648 days to run their course (Deimos' synodic period).
Orbits
of Phobos & Deimos (to scale)Unlike Phobos, which orbits so fast that it actually
rises in the west & sets in the east, Deimos rises in the east & sets
in the west. However, the 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 it takes 2.7 days between rising & setting for an equatorial observer.
Because
Deimos' orbit is relatively close to Mars & has only a very small inclination
to Mars' equator, it cannot be seen from Martian latitudes greater than 82.7°.
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