Deimos (moon)
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An image of Deimos taken by the Viking 1 orbiter.
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Discovery
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| Discovered by | Asaph Hall |
| Discovery date | August 12, 1877 |
| Semi-major axis | 23,460 km |
| Eccentricity | 0.0002 |
| Orbital period | 1.26244 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) |
| Satellite of | Mars |
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Physical characteristics
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| Dimensions | 15.0 × 12 × 10.4 km |
| Mean radius | 6.2 km[2] |
| Mass | 1.48×1015 kg[3] |
| Mean density | 1.471 g/cm³[2] |
| Equatorial surface gravity | 0.0039 m/s² (3.9 mm/s²) 0.00040 g (400 µg) |
| Escape velocity | 5.6 m/s (20 km/hr)[3] |
| Rotation period | synchronous |
| Albedo | 0.068[2] |
| Temperature | ≈233 K |
| Apparent magnitude | 12.4 [4][2] |
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.
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[edit] Discovery
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).[5][6][7][8] 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).[9]
[edit] Characteristics
Deimos, like Mars' other moon Phobos, has spectra, albedos and densities similar to that of a C or D-type asteroid. Like most bodies of its size, Deimos is highly nonspherical with dimensions of 15 × 12 × 10 km. It has a nearly circular orbit nearly in Mars' equatorial plane.
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.[10][11]
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.[citation needed]
As seen from Mars, Deimos would have an angular diameter of no more than 2.5′ and would therefore appear almost star-like to the naked eye.[12] 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 (Deimos's synodic period) to run their course.[12]
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’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°.
[edit] Solar transits
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.
[edit] 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 regolith is highly porous and has a radar estimated density of only 1.1 g/cm³.[13] The two largest craters, Swift and Voltaire, each measure about 3 kilometres across.
[edit] Origin
The origin of the Martian moons is still controversial.[10] 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,[14]. Landis has suggested that the moons may have originated from a binary asteroid that separated due to tidal forces.[11]
[edit] Named geological features
Only two geological features on Deimos have been given names. The craters Swift and Voltaire, are named after two writers (Swift and Voltaire) who predicted the existence of Martian moons before they were discovered.[1]
[edit] References
- ^ Jacobson, R.A. and Rush, B. (2006) MAR063 (2007-Jun-28). Planetary Satellite Mean Orbital Parameters. JPL/NASA. Retrieved on 2008-02-13.
- ^ a b c d Planetary Satellite Physical Parameters. JPL (Solar System Dynamics) (2006-Jul-13). Retrieved on 2008-01-29.
- ^ a b use a spherical radius of 6.23 km; volume of a sphere * density of 1.471 g/cm³ yields a mass (m=d*v) of 1.48e15 kg and an escape velocity (sqrt((2*g*m)/r)) of 5.6 m/s (20 km/hr
- ^ Classic Satellites of the Solar System. Observatorio ARVAL. Retrieved on 2007-09-28.
- ^ 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
- ^ 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)
- ^ Notes: The Satellites of Mars, The Observatory, Vol. 1, No. 6 (September 20, 1877), pp. 181–185
- ^ The Discovery of the Satellites of Mars, Monthly Notices of the Royal Astronomical Society, Vol. 38, No. 4, (February 8, 1878), pp. 205–209
- ^ Hall, A.; Names of the Satellites of Mars, Astronomische Nachrichten, Vol. 92, No. 2187 (March 14, 1878, signed February 7, 1878), p. 47/48
- ^ a b J. A. Burns, "Contradictory Clues as to the Origin of the Martian Moons," in Mars, H. H. Kieffer et al., eds., U. Arizona Press, Tucson, 1992
- ^ a b G. A. Landis, "Origin of Martian Moons from Binary Asteroid Dissociation," American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001; abstract.
- ^ a b Richardson, R. S., If You Were on Mars, Astronomical Society of the Pacific Leaflets, Vol. 4, Leaflet No. 178 (December 1943), pp. 214–221
- ^ Busch, M. W..; et al. (2007). "Arecibo Radar Observations of Phobos and Deimos". Icarus 186 (2): 581–584. doi:.
[edit] 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
[edit] External links
- Deimos Profile by NASA's Solar System Exploration
- Deimos rotation movie
- Animation of Deimos
- USGS Deimos nomenclature
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