Planetary geology

Planetary geology, alternatively known as astrogeology or exogeology, is a planetary science discipline concerned with the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites.[1][2] Although the geo- prefix typically indicates topics of or relating to Earth, planetary geology is named as such for historical and convenience reasons; due to the types of investigations involved, it is closely linked with Earth-based geology.

Planetary geology includes such topics as determining the internal structure of the terrestrial planets, and also looks at planetary volcanism and surface processes such as impact craters, fluvial and aeolian processes. The structures of the giant planets and their moons are also examined, as is the make-up of the minor bodies of the Solar System, such as asteroids, the Kuiper Belt, and comets.

Moon-apollo17-schmitt boulder
Planetary geologist and NASA astronaut Harrison "Jack" Schmitt collecting lunar samples during the Apollo 17 mission in early-December 1972
Mars Viking 21i093
Surface of Mars as photographed by the Viking 2 lander December 9, 1977.
Huygens surface color
ESA Huygens in situ image from Titan's surface—the only image from the surface of a body farther away than Mars
Huygens surface color sr
Same image with contrast enhanced

History of planetary geology

Eugene Shoemaker is credited with bringing geologic principles to planetary mapping and creating the branch of planetary science in the early 1960s, the Astrogeology Research Program, within the United States Geological Survey. He made important contributions to the field and the study of impact craters, Selenography (study of the Moon), asteroids, and comets.[3]

Today many institutions are concerned with the study and communication of planetary sciences and planetary geology. The Visitor Center at Barringer Meteor Crater near Winslow, Arizona includes a museum of planetary geology. The Geological Society of America's Planetary Geology Division has been growing and thriving since May 1981 and has two mottos: "One planet just isn't enough!" and "“The GSA Division with the biggest field area!"

Major centers for planetary science research include the Lunar and Planetary Institute, the Applied Physics Laboratory, the Planetary Science Institute, the Jet Propulsion Laboratory, Southwest Research Institute, and Johnson Space Center. Additionally, several universities conduct extensive planetary science research, including Montana State University, Brown University, the University of Arizona, Caltech, the University of Colorado, Western Michigan University, MIT, and Washington University in St. Louis.

Features and terms

Planetary geology uses a wide variety of standardised descriptor names for features.[4] All planetary feature names recognised by the International Astronomical Union combine one of these names with a possibly unique identifying name. The conventions which decide the more precise name are dependent on which planetary body the feature is on, but the standard descriptors are in general common to all astronomical planetary bodies. Some names have a long history of historical usage, but new must be recognised by the IAU Working Group for Planetary System Nomenclature as features are mapped and described by new planetary missions.[5] This means that in some cases names may change as new imagery becomes available,[5] or in other cases widely adopted informal names changed in line with the rules.[6] The standard names are chosen to consciously avoid interpreting the underlying cause of the feature, but rather to describe only its appearance.[4]

Feature Pronunciation[7] Description Designation
Albedo feature /ælˈbiːdoʊ/ An area which shows a contrast in brightness or darkness (albedo) with adjacent areas. This term is implicit. AL
Arcus, arcūs /ˈɑːrkəs/ Arc: curved feature AR
Astrum, astra /ˈæstrəm/, /ˈæstrə/ Radial-patterned features on Venus AS
Catena, catenae /kəˈtiːnə/, /kəˈtiːni/ A chain of craters e.g. Enki Catena. CA
Cavus, cavi /ˈkeɪvəs/, /ˈkeɪvaɪ/ Hollows, irregular steep-sided depressions usually in arrays or clusters CB
Chaos /ˈkeɪɒs/ A distinctive area of broken or jumbled terrain e.g. Iani Chaos. CH
Chasma, chasmata /ˈkæzmə/, /ˈkæzmətə/ Deep, elongated, steep-sided depression e.g. Eos Chasma. CM
Colles /ˈkɒliːz/ A collection of small hills or knobs. CO
Corona, coronae /kɒˈroʊnə/, /kɒˈroʊni/ An oval feature. Used only on Venus and Miranda. CR
Crater, craters /ˈkreɪtər/ A circular depression likely created by impact event. This term is implicit. AA
Dorsum, dorsa /ˈdɔːrsəm/, /ˈdɔːrsə/ Ridge, sometimes called a wrinkle ridge e.g. Dorsum Buckland. DO
Eruptive center An active volcano on Io. This term is implicit. ER
Facula, faculae /ˈfækjʊlə/, /ˈfækjʊli/ Bright spot FA
Farrum, farra /ˈfærəm/, /ˈfærə/ Pancake-like structure, or a row of such structures. Used only on Venus. FR
Flexus, flexūs /ˈflɛksəs/ Very low curvilinear ridge with a scalloped pattern FE
Fluctus, fluctūs /ˈflʌktəs/ Terrain covered by outflow of liquid. Used on Venus, Io and Titan. FL
Flumen, flumina /ˈfluːmɪn/, /ˈfluːmɪnə/ Channel on Titan that might carry liquid FM
Fossa, fossae /ˈfɒsə/, /ˈfɒsi/ Long, narrow, shallow depression FO
Fretum, freta /ˈfriːtəm/, /ˈfriːtə/ Strait of liquid connecting two larger areas of liquid. Used only on Titan. FT
Insula, insulae /ˈɪnsjuːlə/, /ˈɪnsjuːli/ Island (islands), an isolated land area (or group of such areas) surrounded by, or nearly surrounded by, a liquid area (sea or lake). Used only on Titan. IN
Labes, labes /ˈleɪbiːz/ Landslide debris. Used only on Mars. LA
Labyrinthus, labyrinthi /læbɪˈrɪnθəs/, /læbɪˈrɪnθaɪ/ Complex of intersecting valleys or ridges. LB
Lacuna, lacunae /ləˈkjuːnə/, /ləˈkjuːni/ Irregularly shaped depression having the appearance of a dry lake bed. Used only on Titan. LU
Lacus, lacūs /ˈleɪkəs/ A "lake" or small plain on Moon and Mars; on Titan, a "lake" or small, dark plain with discrete, sharp boundaries. LC
Landing site name Lunar features at or near Apollo landing sites LF
Large ringed feature Cryptic ringed features LG
Lenticula, lenticulae /lɛnˈtɪkjʊlə/, /lɛnˈtɪkjʊli/ Small dark spots on Europa LE
Linea, lineae /ˈlɪniə/, /ˈlɪnii/ Dark or bright elongate marking, may be curved or straight LI
Macula, maculae /ˈmækjʊlə/, /ˈmækjʊli/ Dark spot, may be irregular MA
Mare, maria /ˈmɑːri, -reɪ/, /ˈmɑːriə/ A "sea" or large circular plain on Moon and Mars, e.g. Mare Erythraeum; on Titan, large expanses of dark materials thought to be liquid hydrocarbons, e.g. Ligeia Mare. ME
Mensa, mensae /ˈmɛnsə/, /ˈmɛnsi/ A flat-topped prominence with cliff-like edges, i.e. a mesa. MN
Mons, montes /ˈmɒnz/, /ˈmɒntiːz/ Mons refers to a mountain. Montes refers to a mountain range. MO
Oceanus /oʊʃiːˈeɪnəs/ Very large dark area. The only feature with this designation is Oceanus Procellarum. OC
Palus, paludes /ˈpeɪləs/, /pəˈljuːdiːz/ "Swamp"; small plain. Used on the Moon and Mars. PA
Patera, paterae /ˈpætərə/, /ˈpætəri/ Irregular crater, or a complex one with scalloped edges e.g. Ah Peku Patera. Usually refers to the dish-shaped depression atop a volcano. PE
Planitia, planitiae /pləˈnɪʃə/, /pləˈnɪʃi/ Low plain e.g. Amazonis Planitia. PL
Planum, plana /ˈpleɪnəm/, /ˈpleɪnə/ A plateau or high plain e.g. Planum Boreum. PM
Plume A cryovolcanic feature on Triton. This term is currently unused. PU
Promontorium, promontoria /prɒmənˈtɔːriəm/, /prɒmənˈtɔːriə/ "Cape"; headland. Used only on the Moon. PR
Regio, regiones /ˈriːdʒioʊ/ ~ /ˈrɛdʒioʊ/, /rɛdʒiˈoʊniːz/ Large area marked by reflectivity or color distinctions from adjacent areas, or a broad geographic region RE
Reticulum, reticula /rɪˈtɪkjʊləm/, /rɪˈtɪkjʊlə/ reticular (netlike) pattern on Venus RT
Rima, rimae /ˈraɪmə/, /ˈraɪmi/ Fissure. Used only on the Moon. RI
Rupes, rupes /ˈruːpiːz/ Scarp RU
Satellite feature A feature that shares the name of an associated feature, for example Hertzsprung D. SF
Scopulus, scopuli /ˈskɒpjʊlə/, /ˈskɒpjʊlaɪ/ Lobate or irregular scarp SC
Serpens, serpentes /ˈsɜːrpɛnz/, /sərˈpɛntiːz/ Sinuous feature with segments of positive and negative relief along its length SE
Sinus /ˈsaɪnəs/ "Bay"; small plain on Moon or Mars, e.g. Sinus Meridiani; On Titan, bay within bodies of liquid. SI
Sulcus, sulci /ˈsʌlkəs/, /ˈsʌlsaɪ/ Subparallel furrows and ridges SU
Terra, terrae /ˈtɛrə/, /ˈtɛri/ Extensive land mass e.g. Arabia Terra, Aphrodite Terra. TA
Tessera, tesserae /ˈtɛsərə/, /ˈtɛsəri/ An area of tile-like, polygonal terrain. This term is used only on Venus. TE
Tholus, tholi /ˈθoʊləs/, /ˈθoʊlaɪ/ Small domical mountain or hill e.g. Hecates Tholus. TH
Undae /ˈʌndi/ A field of dunes. Used on Venus, Mars and Titan. UN
Vallis, valles /ˈvælɪs/, /ˈvæliːz/ A valley e.g. Valles Marineris. VA
Vastitas, vastitates /ˈvæstɪtəs/, /væstɪˈteɪtiːz/ An extensive plain. The only feature with this designation is Vastitas Borealis. VS
Virga, virgae /ˈvɜːrɡə/, /ˈvɜːrdʒi/ A streak or stripe of color. This term is currently used only on Titan. VI

By planet

References

  1. ^ "What is planetary geology?". James F. Bell III (Cornell University), Bruce A. Campbell (Smithsonian Institution), Mark S. Robinson (U.S. Geological Survey). Retrieved 6 October 2015.
  2. ^ "GEOL212: Planetary Geology Fall 2015". University of Maryland Department of Geology. Retrieved 6 October 2015.
  3. ^ Chapman, Mary G. "Gene Shoemaker - Founder of Astrogeology". USGS. Retrieved 21 May 2012.
  4. ^ a b "Planetary Names: Feature Types". planetarynames.wr.usgs.gov. Retrieved 9 April 2018.
  5. ^ a b Morton, Oliver. Mapping Mars: science, imagination, and the birth of a world. Farrar, Straus, and Giroux, 2002.
  6. ^ "AAS Committee on the Status of Women" (PDF). www.aas.org. Retrieved 9 April 2018.
  7. ^ Listed pronunciations are conventional or follow the traditional English pronunciation of Latin words. However, some speakers use different (often variable) pronunciations that are closer to the Latin or Greek.

Further reading

Arachnoid (astrogeology)

In astrogeology, an arachnoid is a large geological structure resembling a spider web. They are of unknown origin, and have been found only on the surface of the planet Venus. They appear as concentric ovals surrounded by a complex network of fractures, and can span 200 kilometers. Over 90 arachnoids have been identified on Venus, so far.The arachnoid might be a strange relative to the volcano, however, it is also possible that different arachnoids are formed by other processes. One possible explanation is that an upwelling of magma from the interior of the planet pushed up on the surface, causing cracks. An alternate theory concerning their origin is that they are a precursor to coronae formation.Much of what is known about arachnoids is the result of studies performed by C.B. Dawson and L.S. Crumpler.

Arsia Chasmata

Arsia Chasmata is a steep-sided depression located northeast of Arsia Mons in the Phoenicis Lacus quadrangle on Mars, located at 7.6° S and 119.3° W. It is 97 km long and was named after an albedo name.

In planetary geology, a chasma (plural: chasmata) is a deep, elongated, steep-sided depression.

Bolide

A bolide (French via Latin from the Greek βολίς bolís, "missile") is an extremely bright meteor, especially one that explodes in the atmosphere. In astronomy, it refers to a fireball about as bright as the full moon, and it is generally considered a synonym for a fireball. In geology, a bolide is a very large impactor.

One definition describes a bolide as a fireball reaching an apparent magnitude of −14 or brighter — more than twice as bright as the full moon. Another definition describes a bolide as any generic large crater-forming impacting body whose composition (for example, whether it is a rocky or metallic asteroid, or an icy comet) is unknown.A superbolide is a bolide that reaches an apparent magnitude of −17 or brighter, which is roughly 100 times brighter than the full moon. Recent examples of superbolides include the Sutter's Mill meteorite and the Chelyabinsk meteor.

Chaos terrain

In astrogeology, chaos terrain (or chaotic terrain) is a planetary surface area where features such as ridges, cracks, and plains appear jumbled and enmeshed with one another. Chaos terrain is a notable feature of the planets Mars and Mercury, Jupiter's moon Europa, and the dwarf planet Pluto. In scientific nomenclature, "chaos" is used as a component of proper nouns (e.g., "Aureum Chaos" on Mars).

Chasma

In planetary geology, a chasma (plural: chasmata) is a deep, elongated, steep-sided depression. As of January 2013, the IAU has named 118 such features in the Solar System, on Venus (63), Mars (24), Saturn's satellites Mimas (6), Tethys (2), Dione (8) and Rhea (5), and Uranus's satellites Ariel (7), Titania (2) and Oberon (1). An example is Eos Chasma on Mars.

Claritas Rupes

Claritas Rupes is a scarp in the Phoenicis Lacus quadrangle of Mars, located at 26° South and 105.4° West. It is 924 km long and was named after an albedo feature at 25S, 110W. The term "Rupes" is used in planetary geology to refer to an escarpments or cliff on Mars and other planets. It is the Latin word for cliff.

Corona (planetary geology)

In planetary geology, a corona (plural: coronae) is an oval-shaped feature. Coronae appear on both the planet Venus and Uranus's moon Miranda and may be formed by upwellings of warm material below the surface.

Crater chain

A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena, plural catenae (Latin for "chain"), as specified by the International Astronomical Union's rules on planetary nomenclature.Many examples of such chains are thought to have been formed by the impact of a body that was broken up by tidal forces into a string of smaller objects following roughly the same orbit. An example of such a tidally disrupted body that was observed prior to its impact on Jupiter is Comet Shoemaker-Levy 9. During the Voyager observations of the Jupiter system, planetary scientists identified 13 crater chains on Callisto and three on Ganymede (except those formed by secondary craters). Later some of these chains turned out to be secondary or tectonic features, but some other chains were discovered. As of 1996, 8 primary chains on Callisto and 3 on Ganymede were confirmed.Other cases, such as many of those on Mars, represent chains of collapse pits associated with grabens (see, for example, the Tithoniae Catenae near Tithonium Chasma).

Crater chains seen on the Moon often radiate from larger craters, and in such cases are thought to be either caused by secondary impacts of the larger crater's ejecta or by volcanic venting activity along a rift.

Cryovolcano

A cryovolcano (sometimes informally called an ice volcano) is a type of volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma, cryolava or ice-volcanic melt, these substances are usually liquids and can form plumes, but can also be in vapour form. After eruption, cryomagma is expected to condense to a solid form when exposed to the very low surrounding temperature. Cryovolcanoes may potentially form on icy moons and other objects with abundant water past the Solar System's snow line (such as Pluto). A number of features have been identified as possible cryovolcanoes on Pluto, Titan and Ceres. In addition, although they are not known to form volcanoes, ice geysers have been observed on Enceladus and potentially Triton.

One potential energy source on some solar system bodies for melting ices and producing cryovolcanoes is tidal friction. Translucent deposits of frozen materials create a subsurface greenhouse effect that would accumulate the required heat.

Signs of past warming of the Kuiper belt object Quaoar have led scientists to speculate that it exhibited cryovolcanism in the past. Radioactive decay could provide the energy necessary for such activity, as cryovolcanoes can emit water mixed with ammonia, which would melt at 180 K (−95 °C) and create an extremely cold liquid that would flow out of the volcano.

Ejecta

Ejecta (from the Latin: "things thrown out", singular ejectum) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic materials (Tephra) that came out of a volcanic explosion and magma eruption volcanic vent, or crater, has traveled through the air or under water, and fell back on the ground surface or on the ocean floor.

Inverted relief

Inverted relief, inverted topography, or topographic inversion refers to landscape features that have reversed their elevation relative to other features. It most often occurs when low areas of a landscape become filled with lava or sediment that hardens into material that is more resistant to erosion than the material that surrounds it. Differential erosion then removes the less resistant surrounding material, leaving behind the younger resistant material, which may then appear as a ridge where previously there was a valley. Terms such as "inverted valley" or "inverted channel" are used to describe such features. Inverted relief has been observed on the surfaces of other planets as well as on Earth. For example, well-documented inverted topographies have been discovered on Mars.

Macula (planetary geology)

Macula (pl. maculae) is the Latin word for 'spot'. It is used in planetary nomenclature to refer to unusually dark areas on the surface of a planet or moon. They are seen on the icy surfaces of Pluto, Jupiter's moon Europa, Saturn's moon Titan, Neptune's moon Triton, and Pluto's moon Charon. The term was adopted for planetary nomenclature when high resolution pictures of Europa revealed unusual new surface features.

Mantle (geology)

A mantle is a layer inside a planetary body bounded below by a core and above by a crust. Mantles are made of rock or ices, and are generally the largest and most massive layer of the planetary body. Mantles are characteristic of planetary bodies that have undergone differentiation by density. All terrestrial planets (including Earth), a number of asteroids, and some planetary moons have mantles.

Mensa (geology)

In planetary geology, a mensa (pl. mensae) is a flat-topped prominence with cliff-like edges. The term is derived from the Latin word for table, and has the same root as the Spanish word for table, mesa. Mensa is used in the same manner as mesa is used in the Southwest United States.

Planetary nomenclature

Planetary nomenclature, like terrestrial nomenclature, is a system of uniquely identifying features on the surface of a planet or natural satellite so that the features can be easily located, described, and discussed. Since the invention of the telescope, astronomers have given names to the surface features they have discerned, especially on the Moon and Mars. To standardize planetary nomenclature, the International Astronomical Union (IAU) was assigned in 1919 the task of selecting official names for features on Solar System bodies.

Planetary science

Planetary science or, more rarely, planetology, is the scientific study of planets (including Earth), moons, and planetary systems (in particular those of the Solar System) and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history. It is a strongly interdisciplinary field, originally growing from astronomy and earth science, but which now incorporates many disciplines, including planetary geology (together with geochemistry and geophysics), cosmochemistry, atmospheric science, oceanography, hydrology, theoretical planetary science, glaciology, and exoplanetology. Allied disciplines include space physics, when concerned with the effects of the Sun on the bodies of the Solar System, and astrobiology.

There are interrelated observational and theoretical branches of planetary science. Observational research can involve a combination of space exploration, predominantly with robotic spacecraft missions using remote sensing, and comparative, experimental work in Earth-based laboratories. The theoretical component involves considerable computer simulation and mathematical modelling.

Planetary scientists are generally located in the astronomy and physics or Earth sciences departments of universities or research centres, though there are several purely planetary science institutes worldwide. There are several major conferences each year, and a wide range of peer-reviewed journals. Some planetary scientists work at private research centres and often initiate partnership research tasks.

Rille

Rille (German for 'groove') is typically used to describe any of the long, narrow depressions in the surface of the Moon that resemble channels. The Latin term is rima, plural rimae. Typically a rille can be up to several kilometers wide and hundreds of kilometers in length. However, the term has also been used loosely to describe similar structures on a number of planets in the Solar System, including Mars, Venus, and on a number of moons. All bear a structural resemblance to each other.

Rupes

Rupes (plural rupēs) is the Latin word for 'cliff'. It is used in planetary geology to refer to escarpments on other worlds. As of January 2013, the IAU has named 62 such features in the Solar System, on Mercury (17), Venus (7), the Moon (8), Mars (23), the asteroids Vesta (2) and Lutetia (2), and Uranus's satellites Miranda (2) and Titania (1).How rupes formed is, as of 2008, a matter of speculation. Compressional strain from the cooling of the crust of terrestrial planets and large scale displacement due to impacts are the two dominant theories.

Vallis (planetary geology)

Vallis (plural valles) is the Latin word for valley. It is used in planetary geology to name landform features on other planets.

Scientists used 'vallis for old river valleys they discovered when they sent the first probes to Mars. The Viking Orbiters caused a revolution in our ideas about water on Mars; finding huge river valleys in many areas. Space craft cameras showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Some valles on Mars (Mangala Vallis, Athabasca Vallis, Granicus Vallis, and Tinjar Valles) clearly begin at graben. On the other hand, some of the large outflow channels begin in rubble-filled low areas, called chaos or chaotic terrain. It has been suggested that massive amounts of water were trapped under pressure beneath a thick cryosphere (layer of frozen ground), then the water was suddenly released, perhaps when the cryosphere was broken by a fault.

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