Discover

'Deimos' (IPA or ), is the smaller and outer of Mars’ two moons (the other being Phobos). It is named after Deimos, a figure representing panic in Greek Mythology. Its systematic designation is 'Mars II'.

Contents

Discovery

Orbital characteristics

Solar transits

Physical characteristics

Named geological features

References

See also

External links

Discovery

Main articles: Discovery of the Martian moons

Deimos was discovered by American astronomer 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).^[1][2][3][4] 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).^[5]

Orbital characteristics

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.
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 would have an angular diameter of no more than 2.5' and would therefore appear star-like to the naked eye.^[6] 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’s phases, which take 1.2648 days to run their course (Deimos’s synodic period).
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 it takes 2.7 days between rising and setting for an equatorial observer.
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

Main articles: Transit of Deimos from Mars

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 two largest craters, Swift and Voltaire, each measure about 3 kilometres across.

Named geological features

Only two geological features on Deimos have been given names. The craters are named after the two writers (Swift and Voltaire) who predicted the existence of Martian moons before they were discovered.[1]

References

1. Hall, A.; ''Observations of the Satellites of Mars'', Astronomische Nachrichten, Vol. 91, No. 2161 (October 17, 1877, signed September 21 ,1877) pp. 11/12–13/14
2. Morley, T. A.; ''A Catalogue of Ground-Based Astrometric Observations of the Martian Satellites, 1877-1982'', Astronomy and Astrophysics Supplement Series (ISSN 0365-0138), Vol. 77, No. 2 (February 1989), pp. 209–226 (Table II, p. 220: first observation of Deimos on 1877-08-12.32526)
3. ''Notes: The Satellites of Mars'', The Observatory, Vol. 1, No. 6 (September 20, 1877), pp. 181–185
4. ''The Discovery of the Satellites of Mars'', Monthly Notices of the Royal Astronomical Society, Vol. 38, No. 4, (February 8, 1878), pp. 205–209
5. Hall, A.; ''Names of the Satellites of Mars'', Astronomische Nachrichten, Vol. 92, No. 2187 (March 14, 1878, signed February 7, 1878), p. 47/48
6. Richardson, R. S., ''If You Were on Mars'', Astronomical Society of the Pacific Leaflets, Vol. 4, Leaflet No. 178 (December 1943), pp. 214–221

See also

★ Phobos, the other moon of Mars

★ List of features on Phobos and Deimos

★ Transit of Deimos from Mars

★ Phobos and Deimos in fiction

External links

★ Deimos Profile by NASA's Solar System Exploration

★ Deimos rotation movie

★ Animation of Deimos

★ USGS Deimos nomenclature