PHOBOS (MOON)

'Phobos' ( or [ˈfoʊ.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 a major planet than any other moon.

Contents

Discovery

Physical characteristics

Orbital characteristics

Solar transits

Death

Origin

"Hollow Phobos" claims

Exploration

Past missions

Future missions

Named geological features

References

See also

External links

Discovery

Main articles: Discovery of the Martian moons

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).^[2][3][4] 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).^[5][6]

Physical characteristics

Phobos is a dark body that appears to be composed of carbonaceous surface materials.^[7] It is similar to the C-type asteroids. Deimos Phobos' density is too low to be pure rock, however, and it is known to have significant porosity.^[8][9][10] These results led to the suggestion that Phobos might contain a substantial reservoir of ice, but spectral observations have ruled out this hypothesis.^[11]
Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed.
^[12]. 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.^[13]
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.
Phobos is heavily cratered.^[14] 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.^[15] 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 in fact independent of Stickney, and are deposits of material thrown into space by impacts on the Martian surface.^[16]
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.^[17]

Orbital characteristics

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.

Solar transits

Main articles: Transit of Phobos from Mars


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.

Death

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.^[18]

Origin

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. This has led to speculation that both moons could have been captured into Martian orbit from the main asteroid belt.^[9] 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 ^[20].

"Hollow Phobos" claims

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.^[21] 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.^[22]
In a February 1960 letter to the journal ''Astronautics'',^[23] Siegfried Frederick Singer, then science advisor to President 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.
Subsequently, however, the existence of the acceleration that had caused the claims was subjected to doubt,^[24] and the problem vanished by 1969.^[25] 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.

Exploration

Past missions

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.

Future missions

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.^[26]
Astrium in the UK is also planning a sample return mission.^[27]

Named geological features

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

References

1. NASA Celestia
2. Notes: The Satellites of Mars
3. Observations of the Satellites of Mars Hall, A.
4. 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)
5. Letters to the Editor: The Satellites of Mars Madan, H. G.
6. Names of the Satellites of Mars Hall, A.
7. Physics and Chemistry of the Solar System, , J. S., Lewis, Elsevier Academic Press, 2004, ISBN 0-12-446744-X
8. Porosity of Small Bodies and a Reassesment of Ida's Density
9. Close Inspection for Phobos
10. Busch, M. W.; ''et al.''; 2007; ''Arecibo Radar Observations of Phobos and Deimos'', Icarus, Vol. 196, pp. 581-584
11. Near-Infrared Spectrophotometry of Phobos and Deimos, , A. S., Rivkin, Icarus, 2002
12. A Deep Search for Martian Dust Rings and Inner Moons Using the Hubble Space Telescope Showalter, M. R.
13. Forgotten Moons: Phobos and Deimos Eat Mars' Dust
14. Phobos
15. Stickney Crater-Phobos
16. New Evidence on the Origin of Phobos’ Parallel Grooves from HRSC Mars Express Murray, J. B.; ''et al.''
17. The Kaidun Meteorite: Where Did It Come From?
18. Holsapple, K. A.; (2001); ''Equilibrium Configurations of Solid Cohesionless Bodies'', Icarus, Vol. 154, pp. 432–448
19. Close Inspection for Phobos
20. 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
21. Shklovsky, I. S.; ''The Universe, Life, and Mind'', Academy of Sciences USSR, Moscow, 1962
22. Is Phobos Artificial? Öpik, E. J.
23. Singer, S. F.; ''Astronautics'', February 1960
24. News and Comments: Phobos, Nature of Acceleration Öpik, E. J.
25. On the Origin of the Martian Satellites Phobos and Deimos Singer, S. F.
26. Russia, China Could Sign Moon Exploration Pact in 2006
27. Amos, J.; ''Martian Moon ’Could be Key Test’'', BBC News (February 9, 2007)

See also

★ Transit of Phobos from Mars

★ Shadow of Phobos on Mars

★ Phobos and Deimos in fiction

★ Chinese space program

External links

★ 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