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Phobos (moon)
From Wikipedia, the free encyclopedia
 Phobos as imaged by Mars Global Surveyor on June 1, 2003. |
|
| Discovery | |
|---|---|
| Discovered by: | Asaph Hall |
| Discovery date: | August 18, 1877 |
| Orbital characteristics | |
| Epoch J2000 | |
| Periapsis: | 9235.6 km |
| Apoapsis | 9518.8 km |
| Semi-major axis: | 9377.2 km [1] |
| Eccentricity: | 0.0151 |
| Orbital period: | 0.318 910 23 d (7 h 39.2 min) |
| Avg. orbital speed: | 2.138 km/s |
| Inclination: | 1.093° (to Mars' equator) 0.046° (to local Laplace plane) 26.04° (to the ecliptic) |
| Satellite of: | Mars |
| Physical characteristics | |
| Dimensions: | 26.8 × 21 × 18.4 km |
| Mean radius: | 11.1 km (0.0021 Earths) |
| Surface area: | ~6,100 km² (11.9 µEarths) |
| Volume: | ~5,500 km³ (5.0 nEarths) |
| Mass: | 1.07×1016 kg (1.8 nEarths) |
| Mean density: | 1.9 g/cm³ |
| Equatorial surface gravity: | 0.0084-0.0019 m/s² (8.4-1.9 mm/s²) (860-190 µg) |
| Escape velocity: | 0.011 km/s (11 m/s) |
| Rotation period: | synchronous |
| Rotation velocity at equator: | 11.0 km/h (at longest axis' tips) |
| Axial tilt: | 0° |
| Albedo: | 0.07 |
| Temperature: | ~233 K |
| Apparent magnitude: | 11.3 [2] |
Phobos (pronounced /ˈfoʊbəs/ foe'-bəs) (systematic designation: Mars I) is the larger and closer of Mars' two moons (the other being Deimos). It is named after the Greek god Phobos (which means "fear"), a son of Ares (Mars). Phobos is one of the smallest moons in the solar system, and orbits about 6000 km (3728 mi) above the surface of Mars, closer to its primary than any other planetary moon.
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Phobos was discovered by American astronomer Asaph Hall, Sr., on August 18, 1877, at the US Naval Observatory in Washington, D.C., at about 09:14 GMT (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as August 17 16:06 Washington mean time).[3][4][5] Hall also discovered Deimos, Mars' other moon.
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 summons Dread (Deimos) and Fear (Phobos).[6][7]
Phobos is a dark body that appears to be composed of carbonaceous surface materials.[8] It is similar to the D-type asteroids.[9] Phobos' density is too low to be pure rock, however, and it is known to have significant porosity.[10][11][12] These results led to the suggestion that Phobos might contain a substantial reservoir of ice, but spectral observations have ruled out this hypothesis.[13]
Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed. [14]. Recent images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith at least 100 metres thick; it is believed to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity.[15]
Phobos is highly nonspherical, with dimensions of 27 × 21.6 × 18.8 km. Because of its shape alone, the gravity on its surface varies by about 210%; the tidal forces raised by Mars more than double this variation (to about 450%) because they compensate for a little more than half of Phobos' gravity at its sub- and anti-Mars poles.[citation needed]
Phobos is heavily cratered.[16] The most prominent surface feature is Stickney crater, named after Asaph Hall's wife, Angeline Stickney Hall, Stickney being her maiden name. Like Mimas's crater Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos.[17] Many grooves and streaks also cover the oddly shaped surface. The grooves are typically less than 30 m deep, 100 to 200 m wide, and up to 20 km in length, and were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the Mars Express spacecraft, however, revealed that the grooves are not in fact radial to Stickney, but are centered on the leading apex of Phobos in its orbit (which is not far from Stickney), and must have been excavated by material ejected into space by impacts on the surface of Mars.[18] The grooves thus formed as crater chains, and all of them fade away as the trailing apex of Phobos is approached. They have been grouped into 12 or more families of varying age, presumably representing at least 12 Martian impact events.[18]
The unique Kaidun meteorite is claimed to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon.[19]
| This section needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (May 2007) |
Phobos's unusually close orbit around its parent planet produces some unusual effects.
As seen from Phobos, Mars would appear 6400 times larger and 2500 times brighter than the full Moon appears from Earth, taking up a ¼ of the width of a celestial hemisphere.
Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice a day (every 11 h 6 min). Since it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°.
As seen from Mars' equator, Phobos would be one-third the angular diameter of the full Moon as seen from Earth. Observers at higher Martian latitudes would see a smaller angular diameter because they would be significantly farther away from Phobos. Phobos' apparent size would actually vary by up to 45% as it passed overhead, due to its proximity to Mars' surface: for an equatorial observer, for example, Phobos would be about 0.14° upon rising and swell to 0.20° by the time it reaches the zenith. By comparison, the Sun would have an apparent size of about 0.35° in the Martian sky.
Phobos' phases, inasmuch as they could be observed from Mars, take 0.3191 days to run their course (Phobos' synodic period), a mere 13 seconds longer than Phobos' sidereal period.
To an observer on the Martian surface, Phobos would regularly transit across the Sun. It is not large enough to cover the Sun's disk, and so cannot cause a total eclipse. Several of these transits have been photographed by the Mars Rover Opportunity. During the transits, Phobos's shadow is cast on the surface of Mars, and has been photographed by several spacecraft.
Phobos' low orbit means that it will eventually be destroyed: tidal forces are lowering its orbit, currently at the rate of about 1.8 metres per century, and in 30-80 million years it will either impact the surface of Mars or (more likely) break up into a planetary ring. Given Phobos' irregular shape and assuming that it is a pile of rubble (specifically a Mohr-Coulomb body), it has been calculated that Phobos is stable with respect to tidal forces, but it is estimated that Phobos will pass the Roche Limit for a rubble pile when its orbital radius drops to about 7100 km, and will probably break up soon afterwards.[20]
Phobos and Deimos both have much in common with carbonaceous (C-type) asteroids, with spectra, albedos and densities very similar to those seen in C-type asteroids.[9] This has led to speculation that both moons could have been captured into Martian orbit from the main asteroid belt.[21] However, both moons have very circular orbits which lie almost exactly in Mars' equatorial plane. Captured moons would be expected to have eccentric orbits in random inclinations. Some evidence suggests that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal [22].
In the 1950 and 1960s, Phobos's unusual orbit and low density led to speculations that it might be a hollow artificial object.
Around 1958, Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.[23] Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light —one calculation yielded a hollow iron sphere 16 km across but less than 6 cm thick.[24]
In a February 1960 letter to the journal Astronautics,[25] Siegfried Frederick Singer, then science advisor to U.S. President Dwight D. Eisenhower, came out in support of Shklovsky's theory, going as far as to state that "[Phobos'] purpose would probably be to sweep up radiation in Mars' atmosphere, so that Martians could safely operate around their planet". A few years later, in 1963, Raymond H. Wilson Jr., Chief of Applied Mathematics at NASA, allegedly announced to the Institute of Aerospace Sciences that "Phobos might be a colossal base orbiting Mars", and that NASA itself was considering the possibility.[citation needed]
Subsequently, however, the existence of the acceleration that had caused the claims was subjected to doubt,[26] and the problem vanished by 1969.[27] The earlier studies had used an overestimated value of 5 cm/yr for the rate of altitude loss, which was later revised to 1.8 cm/yr. The secular acceleration is now attributed to tidal effects, which had not been considered in the earlier studies. The density of Phobos is now measured to be 1.9 g/cm³, which is inconsistent with a hollow shell. In addition, images obtained by the Viking probes in the 1970s clearly showed a natural object, not an artificial one.
Similar "hollow moon" and "hollow Earth" claims have been made.
Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first was Mariner 9 in 1971, followed by Viking 1 in 1977, Mars Global Surveyor in 1998 and 2003, and Mars Express in 2004 . The only dedicated Phobos probes have been the Soviet Phobos 1 and Phobos 2; the first was lost en route to Mars, and the second returned some data and images before failing prior to its detailed examination of the moon.
The Russian Space Agency is planning to launch a sample return mission to Phobos in 2009, called Phobos-Grunt. Chinese surveying equipment will be included.[28]
Astrium in the UK is also planning a sample return mission.[29]
Geological features on Phobos are named after astronomers who studied Phobos and people and places from Jonathan Swift's Gulliver's Travels.[1] The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Johannes Kepler. Several craters have been named.[2]
| Crater | Named after |
|---|---|
| Clustril | Character in Gulliver's Travels |
| D'Arrest | Heinrich Louis d'Arrest, astronomer |
| Drunlo | Character in Gulliver's Travels |
| Flimnap | Character in Gulliver's Travels |
| Gulliver | Main character of Gulliver's Travels |
| Hall | Asaph Hall, discoverer of Phobos |
| Limtoc | Character in Gulliver's Travels |
| Reldresal | Character in Gulliver's Travels |
| Roche | Édouard Roche, astronomer |
| Sharpless | Bevan Sharpless, astronomer |
| Skyresh | Character in Gulliver's Travels |
| Stickney | Angeline Stickney, wife of Asaph Hall |
| Todd | David Peck Todd, astronomer |
| Wendell | Oliver Wendell, astronomer |
In the Doom novel based on the video game "Knee-Deep in the Dead", a marine is called to Phobos because of a distress call. Throughout the story, the protagonist is teleported to Mars's other moon, Deimos.
- ^ NASA Celestia
- ^ Classic Satellites of the Solar System. Observatorio ARVAL. Retrieved on 2007-09-28.
- ^ Notes: The Satellites of Mars 181–185. The Observatory, Vol. 1, No. 6 (September 20, 1877). Retrieved on September 12, 2006.
- ^ Hall, A. (October 17, 1877, signed September 21, 1877). Observations of the Satellites of Mars pp. 11/12–13/14. Astronomische Nachrichten, Vol. 91, No. 2161. Retrieved on September 12, 2006.
- ^ 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 Phobos on 1877-08-18.38498)
- ^ Madan, H. G. (October 4, 1877, signed September 29, 1877). Letters to the Editor: The Satellites of Mars 475. Nature, Vol. 16, No. 414. Retrieved on July 26, 2007.
- ^ Hall, A. (March 14, 1878, signed February 7, 1878). Names of the Satellites of Mars 47-48. Astronomische Nachrichten, Vol. 92, No. 2187. Retrieved on September 12, 2006.
- ^ Lewis, J. S. (2004). Physics and Chemistry of the Solar System. Elsevier Academic Press, p. 425. ISBN 0-12-446744-X.
- ^ a b New Views of Martian Moons.
- ^ Porosity of Small Bodies and a Reassesment of Ida's Density. “When the error bars are taken into account, only one of these, Phobos, has a porosity below 0.2...”
- ^ Close Inspection for Phobos. “It is light, with a density less than twice that of water, and orbits just 5989 km above the Martian surface.”
- ^ Busch, M. W.; et al.; 2007; Arecibo Radar Observations of Phobos and Deimos, Icarus, Vol. 196, pp. 581-584
- ^ Rivkin, A. S.; et al. (03 2002). "Near-Infrared Spectrophotometry of Phobos and Deimos". Icarus 156 (1): 64.
- ^ Showalter, M. R.; Hamilton, D. P.; and Nicholson, P. D.. A Deep Search for Martian Dust Rings and Inner Moons Using the Hubble Space Telescope. Planetary and Space Science, Vol. 54 (2006), pp. 844–854.
- ^ Forgotten Moons: Phobos and Deimos Eat Mars' Dust.
- ^ Phobos.
- ^ Stickney Crater-Phobos. “One of the most striking features of Phobos, aside from its irregular shape, is its giant crater Stickney. Because Phobos is only 28 by 20 kilometers (17 by 12 miles), the moon must have been nearly shattered from the force of the impact that caused the giant crater. Grooves that extend across the surface from Stickney appear to be surface fractures caused by the impact.”
- ^ a b Murray, J. B.; et al.. New Evidence on the Origin of Phobos’ Parallel Grooves from HRSC Mars Express. 37th Annual Lunar and Planetary Science Conference, March 2006.
- ^ The Kaidun Meteorite: Where Did It Come From?. “The currently available data on the lithologic composition of the Kaidun meteorite– primarily the composition of the main portion of the meteorite, corresponding to CR2 carbonaceous chondrites and the presence of clasts of deeply differentiated rock – provide weighty support for considering the meteorite’s parent body to be a carbonaceous chondrite satellite of a large differentiated planet. The only possible candidates in the modern solar system are Phobos and Deimos, the moons of Mars.”
- ^ Holsapple, K. A. (12 2001). "Equilibrium Configurations of Solid Cohesionless Bodies". Icarus 154 (2): 432-448. doi:10.1006/icar.2001.6683. Retrieved on 2007-11-12.
- ^ Close Inspection for Phobos. “One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids.”
- ^ Craddock, R. A.; (1994); The Origin of Phobos and Deimos, Abstracts of the 25th Annual Lunar and Planetary Science Conference, held in Houston, TX, 14-18 March 1994, p. 293
- ^ Shklovsky, I. S.; The Universe, Life, and Mind, Academy of Sciences USSR, Moscow, 1962
- ^ Öpik, E. J. (September 1964). Is Phobos Artificial? 281-283. Irish Astronomical Journal, Vol. 6. Retrieved on September 12, 2006.
- ^ Singer, S. F.; Astronautics, February 1960
- ^ Öpik, E. J. (March 1963, signed September 1962). News and Comments: Phobos, Nature of Acceleration p. 40. Irish Astronomical Journal, Vol. 6. Retrieved on September 12, 2006.
- ^ Singer, S. F. (1967). On the Origin of the Martian Satellites Phobos and Deimos. Seventh International Space Science Symposium held 10-18 May 1966 in Vienna, North-Holland Publishing Company.
- ^ Russia, China Could Sign Moon Exploration Pact in 2006. RIA Novosti (September 11, 2006). Retrieved on September 12, 2006.
- ^ Amos, J.; Martian Moon ’Could be Key Test’, BBC News (February 9, 2007)
- Transit of Phobos from Mars
- Shadow of Phobos on Mars
- Phobos and Deimos in fiction
- Chinese space program
- Deimos (moon)
- Phobos Profile by NASA's Solar System Exploration
- USGS Phobos nomenclature
- Asaph Hall and the Moons of Mars
- Flight around Phobos (movie)
- Animation of Phobos
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