Phobos (moon)

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Phobos
Phobos
Enhanced-color view of Phobos obtained by Mars Reconnaissance Orbiter on March 23, 2008.
Discovery
Discovered by Asaph Hall
Discovery date August 18, 1877
Epoch J2000
Periapsis 9235.6 km
Apoapsis 9518.8 km
Semi-major axis 9377.2 km[1]
Eccentricity 0.0151
Orbital period 0.31891023 d
(7 h 39.2 min)
Average 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 × 22.4 × 18.4 km[2]
Mean radius 11.1 km[3]
(0.0021 Earths)
Surface area ~6100 km²
(11.9 µEarths)
Volume 5680 km³[4]
(5.0 nEarths)
Mass 1.072 × 1016 kg[5]
(1.8 nEarths)
Mean density 1.887 g/cm³[4]
Equatorial surface gravity 0.0084–0.0019 m/s²
(8.4-1.9 mm/s²)
(860-190 µg)
Escape velocity 11.3 m/s (40 km/h)[5]
Rotation period synchronous
Equatorial rotation velocity 11.0 km/h (at longest axis' tips)
Axial tilt 0°
Albedo 0.071[3]
Temperature ~233 K
Apparent magnitude 11.3[6]

Phobos (pronounced /ˈfoʊbəs/ FOE-bəs, or as Greek Φόβος) (systematic designation: Mars I) is the larger and closer of Mars' two small moons, the other being Deimos. It is named after the Greek god Phobos (which means "fear"), a son of Ares (Mars). A small, irregularly shaped object, Phobos orbits about 9377 km from the center of Mars, closer to its primary than any other planetary moon.

Contents

[edit] Discovery

Asaph Hall, discoverer of Phobos.
Main article: Discovery of the Martian moons

Phobos was discovered by astronomer Asaph Hall, Sr., on August 18, 1877, at the United States Naval Observatory in Washington, D.C., at about 09:14 Greenwich Mean Time (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).[7][8][9] 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).[10][11]

[edit] Physical characteristics

A mosaic of three separate images taken by Viking 1 on October 19, 1978. The large crater (mostly in darkness) on the upper left is Stickney.

Phobos is one of the least-reflective bodies in the solar system. Spectroscopically it appears to be similar to the D-type asteroids,[12] and is apparently of composition similar to carbonaceous chondrite material.[13] Phobos' density is too low to be solid rock, however, and it is known to have significant porosity.[14][15][16] These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Spectral observations indicate that the surface regolith layer lacks hydration,[17][18] but ice below the regolith is not ruled out.[19]

Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed.[20] Recent images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith at least 100 meters 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.[21]

Phobos is highly nonspherical, with dimensions of 27 × 22 × 18 km.[2] 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.[22] 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.[23] 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). Researchers believe they have been excavated by material ejected into space by impacts on the surface of Mars.[24] 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.[24]

The unique Kaidun meteorite is thought to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon.[25]

[edit] Orbital characteristics

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Orbits of Phobos and Deimos (to scale), seen from above Mars' north pole

Phobos' 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 quarter 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, in as much as they could be observed from Mars, take 0.3191 days (Phobos' synodic period) to run their course, a mere 13 seconds longer than Phobos' sidereal period.

[edit] Solar transits

Main article: Transit of Phobos from Mars
Phobos transits Sun, as seen by Mars Rover Opportunity

An observer situated on the Martian surface in a position to observe Phobos would see regular transits of the moon across the Sun. Phobos 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' shadow is cast on the surface of Mars, which has been photographed by several spacecraft.

[edit] Future destruction

Phobos' low orbit means that it will eventually be destroyed: tidal forces are lowering its orbit, currently at the rate of about 20 meters per century, and in 11 million years it will either impact the surface of Mars or (more likely) break up into a planetary ring.[26] 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 currently 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 by a little over 2000 km to about 7100 km. At this distance Phobos will likely begin to break up and form a short lived ring system around Mars. The rings themselves will then continue to spiral slowly into Mars.[27]

[edit] Origin

Phobos, with Stickney Crater to the right

The origin of the Martian moons is still controversial.[28] Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedos and densities very similar to those of C- or D-type asteroids.[12] Based on this similarity, one hypothesis is that both moons may have been captured into Martian orbit from the main asteroid belt.[29][30] Both moons have very circular orbits which lie almost exactly in Mars' equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly-eccentric orbit, and adjusting its inclination into the equatorial plane, most likely by a combination of atmospheric drag and tidal forces,[31] although it is not clear that sufficient time is available for this to occur for Deimos.[28] Capture also requires dissipation of energy. The current Mars atmosphere is too thin to capture a Phobos-sized object by atmospheric braking.[28] Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated due to tidal forces.[30] The main alternative hypothesis is that the moons accreted in the present position. Another hypothesis is 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.[32]

[edit] "Hollow Phobos" suggestions

In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.

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.[33] 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.[33][34] In a February 1960 letter to the journal Astronautics,[35] S. Fred Singer, then science advisor to U.S. President Dwight D. Eisenhower, came out in support of Shklovsky's theory, stating:

“ [Phobos'] purpose would probably be to sweep up radiation in Mars' atmosphere, so that Martians could safely operate around their planet.

"My conclusion there is, and here I back Shklovsky, that if the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore martian made. The big 'if' lies in the astronomical observations; they may well be in error. Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them.[35]

 ”

Subsequently, however, the existence of the acceleration that had caused the claims was called into doubt,[36] and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist.[37] Singer's critique was justified when earlier studies were later discovered to have used an overestimated value of 5 cm/yr for the rate of altitude loss, which was later revised to 1.8 cm/yr.[38] The secular acceleration is now attributed to tidal effects, which had not been considered in the earlier studies. The density of Phobos has now been directly measured by spacecraft to be 1.887 g/cm³[4], 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, and the "hollow Phobos" speculations have been relegated to the status of a historical curiosity.

[edit] Exploration

[edit] 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, Mars Express in 2004 and 2008, and Mars Reconnaissance Orbiter in 2007 and 2008. 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 documented, unusual data and images before failing prior to its detailed examination of the moon.

[edit] Future missions

The Russian Space Agency is planning to launch a sample return mission to Phobos in 2009, called Phobos-Grunt. The return capsule will include a life science experiment of The Planetary Society, called Living Interplanetary Flight Experiment, or LIFE. [39] A second contributor to this mission is China with a surveying satellite called "Yinghuo-1" that will be released in the orbit of Mars, and a soil grinding and sieving system for the scientific payload of the Phobos lander.[40][41][42]

Astrium in the UK is also planning a sample return mission.[43]

Phobos has also been proposed as an early target for a Manned mission to Mars,[44] since a landing on Phobos would be considerably less difficult (and hence, much less expensive) than a landing on the surface of Mars itself. A lander bound for Mars would need to be capable of atmospheric entry and subsequent in-situ return to orbit without any support facilities (a capacity which has never been attempted in a manned spacecraft of human design), or would require the creation of support facilities in-situ (a "colony or bust" mission), while a lander intended for Phobos could be based on equipment designed for lunar and asteroid landings.

[edit] Named geological features

color MRO view of Stickney Crater

Geological features on Phobos are named after astronomers who studied Phobos and people and places from Jonathan Swift's Gulliver's Travels.[45] The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Johannes Kepler. Several craters have been named.[46]

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

[edit] Phobos in fiction

Main article: Phobos and Deimos in fiction

In the satire Gulliver's Travels by Jonathan Swift, the inhabitants of Laputa claim to have discovered two moons of Mars. This is why names from Gulliver's Travels are used for features on the moons of Mars. Swift's classic was written over 150 years before Phobos and Deimos were discovered by Asaph Hall.

In Arthur C. Clarke's novel The Sands of Mars, Mars is terraformed when Phobos is ignited as a second sun using a meson-resonance reaction, and in the novel The Fountains of Paradise, Phobos presents an obstacle to the construction of a space elevator on Mars due to its close orbit. In the short story Hide and Seek, Phobos is the scene of a seemingly unequal encounter between an agent forced to land on the tiny moon and a military spaceship pursuing him.

James Blish's novella "Mission to the Heart Stars" begins with several astronauts examining the interior of Phobos, which turns out to be an artificial surveillance satellite placed in Mars orbit to study the Solar system by the Hegemony of Malis - an alien political collective that inhabit the star systems near the hub of the Milky Way (the 'Heart Stars' of the title).

The Mars Trilogy by Kim Stanley Robinson is the chronicling of the settlement and terraforming of the planet Mars. In this series, a weapon system built into Phobos is activated, which causes the entire moon (which at that time is a transnational military base) to decelerate in orbit and destructively aerobrake in Mars' atmosphere, utterly destroying it.

In Alfred Bester's The Stars My Destination, Phobos is the second Inner Planet colony to be attacked by the Outer Satellites (the first being Earth) when the war begins to escalate.

[edit] See also

[edit] References

  1. ^ NASA Celestia
  2. ^ a b "Mars: Moons: Phobos". NASA Solar System Exploration. 2003-09-30. http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos. Retrieved on 2008-08-18. 
  3. ^ a b "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2006-07-13. http://ssd.jpl.nasa.gov/?sat_phys_par. Retrieved on 2008-01-29. 
  4. ^ a b c "Mars Express closes in on the origin of Mars' larger moon". DLR. 2008-10-16. http://www.dlr.de/mars/en/desktopdefault.aspx/tabid-207/422_read-13776/. Retrieved on 2008-10-16. 
  5. ^ a b use a spherical radius of 11.1 km; volume of a sphere * density of 1.877 g/cm³ yields a mass (m=d*v) of 1.07 × 1016 kg and an escape velocity (sqrt((2*g*m)/r)) of 11.3 m/s (40 km/h)
  6. ^ "Classic Satellites of the Solar System". Observatorio ARVAL. http://www.oarval.org/ClasSaten.htm. Retrieved on 2007-09-28. 
  7. ^ "Notes: The Satellites of Mars". The Observatory 1 (6): 181–185. September 20, 1877. http://adsabs.harvard.edu//full/seri/Obs../0001//0000181.000.html. Retrieved on 2009-02-04. 
  8. ^ Hall, A. (October 17, 1877, signed September 21, 1877). Observations of the Satellites of Mars. 91. Astronomische Nachrichten. pp. 11/12–13/14. http://adsabs.harvard.edu//full/seri/AN.../0091//0000013.000.html. Retrieved on 2009-02-04. 
  9. ^ Morley, T. A. (February 1989). "A Catalogue of Ground-Based Astrometric Observations of the Martian Satellites, 1877-1982". Astronomy and Astrophysics Supplement Series (ISSN 0365-0138) 77 (2): 209–226. http://adsabs.harvard.edu//full/seri/A+AS./0077//0000220.000.html.  (Table II, p. 220: first observation of Phobos on 1877-08-18.38498)
  10. ^ Madan, H. G. (October 4, 1877, signed September 29, 1877). "Letters to the Editor: The Satellites of Mars". Nature 16 (414): 475. doi:10.1038/016475b0. http://books.google.com/books?id=fC4CAAAAYAAJ&pg=RA4-PA475&lpg=RA4-PA475. 
  11. ^ Hall, A. (March 14, 1878, signed February 7, 1878). "Names of the Satellites of Mars". Astronomische Nachrichten Vol. 92 (2187): 47–48. doi:10.1002/asna.18780920304. http://adsabs.harvard.edu//full/seri/AN.../0092//0000031.000.html. 
  12. ^ a b "New Views of Martian Moons". http://www.physorg.com/news115483748.html. 
  13. ^ Lewis, J. S. (2004). Physics and Chemistry of the Solar System. Elsevier Academic Press. pp. 425. ISBN 0-12-446744-X. 
  14. ^ "Porosity of Small Bodies and a Reassesment of Ida's Density". http://www.aas.org/publications/baas/v31n4/dps99/65.htm. "When the error bars are taken into account, only one of these, Phobos, has a porosity below 0.2..." 
  15. ^ "Close Inspection for Phobos". http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031. "It is light, with a density less than twice that of water, and orbits just 5989 km above the Martian surface." 
  16. ^ Busch, M. W.; et al. (2007). "Arecibo Radar Observations of Phobos and Deimos". Icarus 186 (2): 581–584. doi:10.1016/j.icarus.2006.11.003. 
  17. ^ Murchie, S. L., Erard, S., Langevin, Y., Britt, D. T., Bibring, J. P., and Mustard, J. F., "Disk-resolved Spectral Reflectance Properties of PHOBOS from 0.3-3.2 microns: Preliminary Integrated Results from PHOBOS 2," in Abstracts of the Lunar and Planetary Science Conference, volume 22, page 943, (1991)
  18. ^ Rivkin, A. S.; et al. (March 2002). "Near-Infrared Spectrophotometry of Phobos and Deimos". Icarus 156 (1): 64. doi:10.1006/icar.2001.6767. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Icar..156...64R&db_key=AST&data_type=HTML&format=&high=452a3fe65704274. 
  19. ^ Fanale, Fraser P., "Water regime of Phobos" (1991).
  20. ^ 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" (PDF). Planetary and Space Science, Vol. 54 (2006), pp. 844–854. http://www.astro.umd.edu/~hamilton/research/reprints/ShoHamNic06.pdf. 
  21. ^ "Forgotten Moons: Phobos and Deimos Eat Mars' Dust". http://www.space.com/scienceastronomy/forgotten_moons_010313-3.html. 
  22. ^ "Phobos". http://www.bbc.co.uk/science/space/solarsystem/mars/phobos.shtml. 
  23. ^ "Stickney Crater-Phobos". http://www.solarviews.com/cap/mars/phobos2.htm. "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." 
  24. ^ a b Murray, J. B.; et al.. "New Evidence on the Origin of Phobos’ Parallel Grooves from HRSC Mars Express" (PDF). 37th Annual Lunar and Planetary Science Conference, March 2006. http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2195.pdf. 
  25. ^ "The Kaidun Meteorite: Where Did It Come From?" (PDF). http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1236.pdf. "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." 
  26. ^ [0805.1454] Theoretical Formulation of the Phobos, moon of Mars, rate of altitudinal loss
  27. ^ Holsapple, K. A. (December 2001). "Equilibrium Configurations of Solid Cohesionless Bodies". Icarus 154 (2): 432–448. doi:10.1006/icar.2001.6683. http://adsabs.harvard.edu/abs/2001Icar..154..432H. Retrieved on 2007-11-12. 
  28. ^ a b c Burns, J. A. "Contradictory Clues as to the Origin of the Martian Moons," in Mars, H. H. Kieffer et al., eds., U. Arizona Press, Tucson, 1992
  29. ^ "Close Inspection for Phobos". http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031. "One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids." 
  30. ^ a b Landis, G. A. "Origin of Martian Moons from Binary Asteroid Dissociation," American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001; abstract.
  31. ^ Cazenave, A.; Dobrovolskis, A.; Lago, B. (1980). "Orbital history of the Martian satellites with inferences on their origin". Icarus 44 (3): 730–744. doi:10.1016/0019-1035(80)90140-2. 
  32. ^ 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
  33. ^ a b Shklovsky, I. S.; The Universe, Life, and Mind, Academy of Sciences USSR, Moscow, 1962
  34. ^ Öpik, E. J. (September 1964). "Is Phobos Artificial?". Irish Astronomical Journal, Vol. 6. 281-283. http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1964IrAJ....6..281.. Retrieved on 2009-02-04. 
  35. ^ a b Singer, S. F.; Astronautics, February 1960
  36. ^ Öpik, E. J. (March 1963, signed September 1962). "News and Comments: Phobos, Nature of Acceleration". Irish Astronomical Journal, Vol. 6. p. 40. http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1963IrAJ....6R..40.. Retrieved on 2009-02-04. 
  37. ^ 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. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1967mopl.conf..317S&db_key=AST&data_type=HTML&format=&high=4326fb2cf923511. 
  38. ^ "More on the Moons of Mars". Singer, S. F., Astronautics, February 1960. American Astronautical Society. Page 16
  39. ^ "Projects LIFE Experiment: Phobos". The Planetary Society. http://www.planetary.org/programs/projects/life/. 
  40. ^ "Russia, China Could Sign Moon Exploration Pact in 2006". RIA Novosti. http://en.rian.ru/world/20060911/53726392.html. 
  41. ^ "HK triumphs with out of this world invention". HK Trader. May 1, 2007. http://www.hktrader.net/200705/lead/lead-SpaceMission200705.htm. 
  42. ^ "PolyU-made space tool sets for Mars again". Hong Kong PolyU. http://www.polyu.edu.hk/cpa/polyu/hotnews/details_e.php?year=2007&news_id=1252. 
  43. ^ Amos, J.; Martian Moon ’Could be Key Test’, BBC News (February 9, 2007)
  44. ^ Landis, Geoffrey A. "Footsteps to Mars: an Incremental Approach to Mars Exploration," Journal of the British Interplanetary Society, Vol. 48, pp. 367-342 (1995); presented at Case for Mars V, Boulder CO, 26-29 May 1993; appears in From Imagination to Reality: Mars Exploration Studies, R. Zubrin, ed., AAS Science and Technology Series Volume 91 pp. 339-350 (1997). (text available as Footsteps to Mars (PDF)
  45. ^ Gazetteer of Planetary Nomenclature USGS Astrogeology Research Program, Categories
  46. ^ Gazetteer of Planetary Nomenclature USGS Astrogeology Research Program, Craters

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