Deimos (moon)
Deimos (
IPA or ; Greek
Î"είμος: "Dread"), is the smaller and outermost of
Mars' two
moons, named after
Deimos from
Greek Mythology. It is also known as
Mars II.
Phobos and 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) and 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) [
1].
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
carbon-rich rock, much like
C-type (
carbonaceous chondrite)
asteroids, and ice. 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 and Voltaire, measure about 3 kilometres across.
As seen from Deimos, Mars would be 1000 times larger and 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' and 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 and Deimos (to scale) |
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 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 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°.
*In part 3 chapter 3 (the "Voyage to Laputa") of
Jonathan Swift's famous satire
Gulliver's Travels, a fictional work written in
1726, the astronomers of Laputa are described as having discovered two satellites of Mars. (See
Phobos for a fuller discussion).
*In the
comic Sailor Moon, Deimos is the name of one of two crows belonging to
Sailor Mars.
*
Kim Stanley Robinson's
Green Mars (
1993) includes a detailed description of a manned landing on Deimos.
* The second episode of the
Computer and video game Doom takes place in a
UAC base on Deimos. Before the beginning of the game, Deimos disappears from martian orbit, and is reached by the unnamed Marine after he steps into a large teleporter on Phobos. Eventually, it is discovered that Deimos is floating above
Hell itself.
* The
UESC Marathon of the
Marathon computer game series is a hollowed-out Deimos.
* A hollowed-out Deimos is also featured in the
Zone of the Enders saga of games and
anime. In the series - notably the
Idolo OVA - the moon serves as a
linear catapult facility for launching vehicles in Mars orbit to Earth, and possibly to the Jupiter colonies as well.
* Robert S. Richardson, "If You Were on Mars",
Astronomical Society of the Pacific Leaflets, 4 (1943) 214 (1943ASPL....4..214R) (Leaflet No. 178, December 1943)
* Contemporary accounts of the discovery of Phobos and Deimos:
**
The Observatory, 1 (1877) 181**
Astronomische Nachrichten, 91 (1878) 11/12**
Monthly Notices of the Royal Astronomical Society, 38 (1878) 205**
Astronomische Nachrichten, 92 (1878) 47/48*
Phobos, the other moon of
Mars*
List of features on Phobos and Deimos*
Transit of Deimos from Mars*
Deimos rotation movie*
Animation of Deimos
