Crater Rim

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440 oblique impact cratering experiment. Icarus 190(2):295–333 Schultz PH, Anderson JBL, Hermalyn B (2009) Origin and significance of uprange ray patterns. Lunar Planet Sci Conf 40, abstract #2496, Houston See TJJ (1910) The origin of the so-called craters on the Moon by the impact of satellites, and the relation of these satellite indentations to the obliquities of the planets. Publ Astron Soc Pac 22(130):13–20. Reference to a letter by “Mr. W€ urdemann, of Washington, D. C, many years ago, to Dr. B. A. Gould” Shoemaker EM (1960) Ballistics of the Copernican ray system. Proc Lunar Planet Explor Colloq II(2):7–21 Shoemaker EM, Hackman RJ (1962) Stratigraphic basis for a Lunar time scale. In: Kopal Z, Mikhailov ZK (eds) The Moon. IAU symposium 14. Academic, New York, pp 289–300 Tomkins HG (1908) Note on the bright rays on the Moon. BAA 18:126 Tornabene LL, Moersch JE, McSween Jr HY, McEwen AS, Piatek JL, Milam KA, Christensen PR (2006) Identification of large (2–10 km) rayed craters on Mars in THEMIS thermal infrared images: implications for possible Martian meteorite source regions. J Geophys Res 111. doi: 10.1029/2005JE002600 Webb TW (1859) Celestial objects for common telescopes. Longman, London Wilhelms DE (1970) Summary of lunar stratigraphy – telescopic observations. U. S. Geological Survey professional paper 599-F, Washington, 47 pp Wilhelms DE (1987) The geologic history of the Moon. USGS professional paper, Washington, 1348 Yang W, Ahrens TJ (1995) Impact jetting of geological materials. Icarus 116:269–274

Crater Rim Stuart Robbins1, Veronica J. Bray2 and Henrik Hargitai3 1 Southwest Research Institute, Boulder, CO, USA 2 Planetary Laboratory, University of Arizona, Tucson, AZ, USA 3 NASA Ames Research Center / NPP, Moffett Field, CA, USA

Definition The crater rim is the edge of the crater typically elevated above the original ground surface. The maximum elevation of the rim is the rim crest.

Crater Rim

Synonyms Basin rim (for basins), Rampart (Elger 1895, obsolete), Ring mountain (obsolete)

Related Terms Rimless crater, rim crest

Description The crater rim consists of autochthonous, structurally elevated bedrock (Poelchau et al. 2009) overlaid by a thick layer of overturned allochthonous (displaced) ▶ ejecta (impact) (Fig. 1). Just outside of the crater rim is the zone of the outward-sloping continuous ejecta blanket, smothering the underlying terrain. Ejecta deposits decrease in thickness outward from the crater rim. The inner rim walls are much steeper than the external slopes. Complex craters are characterized by terraces resulted from inward collapse (slumps) inside the rim, enlarging the crater and broadening the rim-to-rim diameter relative to the transient crater diameter. Multiring basins do not appear to possess a main rim that is morphologically analogous to the rims of smaller complex craters (Wieczorek and Phillips 1999). For multiring basins, the ▶ basin rim corresponding to the topographic rim (rim crest) of complex craters is the head scarp of the first of the terrace blocks which step down into the crater (Collins 2002). It is located between the outer ring outside and the (largest) peak ring inside the basin. However, the topographic rim may be buried by subsequent lava flows.

Morphology (1) Pristine single crater rim types: (1.1) Circular, unbroken, sharp, and raised rim (Fig. 2)



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Crater Rim, Fig. 1 Cross section of crater rim (Watters 2010). (hr) Rim height, (hu) stratigraphic uplift (Melosh 1989)

Crater Rim, Fig. 2 Three kilometer-diameter, 620 m-deep pristine Zumba crater, Mars, at 28.6 S, 226.9 E. Rim height 200 m above the surrounding plains. Scale bar 1 km. HiRISE PSP_003608_1510 (NASA/JPL/ University of Arizona)

(1.2) Polygonal rim (e.g., Barringer crater, Arizona) (▶ polygonal crater) (1.3) Elliptical rim (due to oblique impact) (▶ elliptical crater) (2) Modified/degraded crater rim types include: (2.1) Circular, unbroken but smooth/subdued rim

(2.2) Circular and discontinuous rim (e.g., circumbasin mountain ranges/scars on the Moon) (2.3) Elliptical rim (▶ deformed crater (tectonized)) (2.4) Serrated rim (e.g., Victoria crater, Mars, exhibiting alcoves formed by mass wasting and aeolian erosion (Grant et al. 2008)) (Fig. 3) (2.5) Scalloped rim produced by mass wasting commonly in terraced complex craters (▶ terraced crater wall (mass wasting)) and found in large numbers on the icy moon Iapetus (Singer et al. 2012) (▶ slide) (2.6) Crenulated rim formed due to the development of significant gully alcoves (Schon and Head 2012) (Fig. 4) (▶ spur and gully) (2.7) Breached rim (▶ crater breach)

Morphometry Rim heights of lunar and Martian craters (and presumably craters on all other surfaces) are proportional to crater diameter (e.g., Robbins and Hynek 2012). The measurement of rim heights varies among researchers and varies significantly



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Crater Rim, Fig. 3 Eight hundred meter-diameter Victoria, Mars, at 2.0 S, 354.5 E, displaying a highly broken, scalloped crater rim. Alcoves form at structural weaknesses where backwasting is faster, intervening capes evolve where rocks are more resistant and/or less disrupted and are eroded more slowly, leading to oversteepening and some collapse (Grant et al. 2008). HiRISE TRA_000873_1780 (NASA/JPL/University of Arizona)

Crater Rim

across any single crater when natural variation of rim height with azimuth around the crater can change by up to 75 % (Bray et al. 2012). The craters with the least natural variation in their rim height are those that may have formed in the incredibly recent time and are the result of a tangential impact (90 to the target surface). Even Meteor Crater in Arizona, often cited as the best-preserved impact crater on Earth, has a rim height that varies by over 30 m. Several approaches to measuring rim height have been taken, including the maximum (i.e., Stepinski et al. 2009) or an overall average (i.e., Robbins and Hynek 2012). Depending upon the method, sometimes very different values will result and many researchers advise being explicit about exactly what is being measured. Apparent crater diameter is measured from rim crest to the opposing rim crest. (See also crater morphometry diagrams in ▶ simple crater, ▶ central peak crater.)

Formation During the excavation of the crater, the shock waves become regular elastic waves or seismic waves near the eventual crater rim and their

Crater Rim, Fig. 4 Crenulated northern rim of Gasa crater, Mars, at 36.0 S, 129.4 E. Scale bar 1 km. CTX: P08_004060_1440_XI_36S230W (NASA/JPL-Caltech/MSSS)



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velocity drops to that of the velocity of sound in the target rocks (French 1998, p. 18). Here, at the edge of the transient cavity, material is pushed upward and outward to form part of the crater rim. Material that is ejected out of the crater on ballistic trajectories forms the ejecta deposit, falling partly onto the elevated crater rim. The uplifted portion of the crater rim material decreases rapidly with distance such that, outside two transient crater radii from the crater center, the material above the original surface is almost all ejecta deposit (Collins et al. 2005). Ejecta forms a continuous “overturned flap” (Shoemaker 1960; Roddy et al. 1975) in which beds lie in an inverted stratigraphic order. The overturned flap in the crater rim region is thrown back over itself and its uppermost layer, thrown out last, consists of the lowest-velocity and lessshocked ejecta coming from the deepest parts of the displaced target surface (Cintala et al. 1978, p. 3824; Glass and Simonson 2013, p. 22).

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lowered by erosion even to the point when it is no longer visible, producing a flat, rimless depression where the impact crater is comprised of an inner wall and crater floor. Such craters are found, e.g., in Martian highlands (Craddock et al. 1997). Grant and Schultz (1993) define rimless craters as those where the raised rim surrounds less than 50 % of the crater. Such rimless craters may occur with preserved ejecta blankets, resulting from long-term mass wasting (Grant and Schultz 1993). (3) Flattened crater: On the icy moons Ganymede and Callisto flat, impact structures with neither rim nor cavity (▶ palimpsest) form by relaxation of topography. Viscous creep relaxation may cause ▶ softened craters in ice-rich substrates, e.g., in the polar regions of Mars. (4) Inverted rim: Crater relief may be inverted, where the originally raised crater rim becomes a ▶ ring furrow.

Degradation Composition Modified Crater (1) Degraded rim: The originally sharp rim becomes smooth/rounded and subdued during crater degradation, while the interior slopes become shallower. Crater diameter is increasing due to backwasting, while material from interior crater wall is deposited onto the crater floor (Craddock et al. 1997). On Meridiani Planum, Mars, as crater degradation progresses, backwasting becomes dominated by aeolian deflation instead of mass wasting. At locations of structural weaknesses, backwasting becomes locally faster, and alcoves evolve, producing serrated rim (e.g., at Victoria crater) (Fig. 3). With the rim becoming more subdued, infilling with materials from crater exterior becomes more important causing the crater to fill to the level of the surroundings (Grant et al. 2008). (2) Rimless impact crater (different from originally rimless ▶ pit): Impact crater rim may be

The rim materials consist of an overturned “flap” of the target material and of ejecta deposits, which is an assemblage of loosely consolidated fragmental material derived from beds once filling the space now occupied by the crater depression. The overturned segment can sometimes preserve the target layering, albeit in a now upside-down position (Sto¨ffler et al. 2006; Watters 2010; Melosh 1989).

Regional Variations A break in the rim height-to-diameter ratio occurs at a diameter of 21 km on the Moon and indicates significantly more collapse occurring in craters larger than this diameter. This is associated with the simple-to-complex transition in lunar crater morphology. On the icy satellite Ganymede, a similar break in the rim height trend is noted at crater diameters of 9–14 km. Prior to this crater size, lunar and Ganymede rim heights were

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similar. Above 6–14 km Ganymede crater rims become at least 50 % lower than lunar craters of the same size (Schenk 1991; Bray et al. 2012), indicating significantly more wall collapse in ice. In ▶ marine-target craters, water acts as another layer in the formation of the overturned ejecta flap and ejecta curtain (Ormo¨ et al. 2010). Marine-target craters may develop a subdued crater rim (▶ nested crater).

Significance The raised rim is a distinctive mark of impact craters. Fresh impact craters always display pronounced elevated rims. Fresh craters formed by nonimpact processes and compaction crater lack raised rims. The height of crater rims is thought to offer indirect evidence of the extent of crater wall collapse (Melosh 1989; Schenk 1991).

History of Investigation In the eighteenth- to nineteenth-century sense, large craters were defined by their prominent rim. They were regarded as ringed mountains (Ring Gebirge, Montes Annulares) by Schro¨ter (1791) (Fig. 5) and M€adler (Beer and M€adler 1838) (translated to English as “annular mountains”). Some of the basin rims were termed randburg (marginal or side mountain) (Schro¨ter

Crater Rim, Fig. 5 Eighteenth century depiction of Eratosthenes crater (Schro¨ter 1791, TXV) (ETH-Bibliothek)

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1791), but this term was also used for mountains on the lunar limb (e.g., Doerfel, Leibnitz).

Similar Landforms There may be three distinct ring types in connection with an impact crater: the crater rim (formed by ejecta and shock wave pressure), the ▶ peakring structure (formed by central peak and crater rim collapse), and the outer ring(s) of ▶ multiring basin, orientale type (formed by tectonic and/or mass wasting and/or fluidization processes).

See Also ▶ Buried Crater ▶ Crater Wall ▶ Ejecta (Impact) ▶ Eroded Crater ▶ Modified Crater ▶ Septum

References Beer W, M€adler JH (1838) Physische Beobachtungen des Mars in der Opposition von 1837 Von den Herren W. Beer und Dr. Madler. Astron Nachrichten 15:219. http:// adsabs.harvard.edu/abs/1838AN.....15..219B, Provided by the SAO/NASA Astrophysics Data System Bray VJ, Schenk PM, Melosh HJ, Morgan JV, Collins GS (2012) Ganymede crater dimensions – implications for peak and pit formation and development. Icarus 217:115–129 Cintala MJ, Head JW, Veverka J (1978) Characteristics of the cratering process on small satellites and asteroids. Lunar Planet Sci Conf 9th, A79-39253 16-91, 3803–3830, Houston Collins G (2002) Numerical modelling of large impact crater collapse. PhD thesis, University of London Collins GS, Melosh HJ, Marcus RA (2005) Earth impact effects program: a web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth. Meteorit Planet Sci 40(6):817–840 Craddock RA, Maxwell TA, Howard AD (1997) Crater morphometry and modification in the Sinus Sabaeus and Margaritifer Sinus regions of Mars. J Geophys Res 102(E6):13321–13340



Crater Wall Elger TG (1895) The Moon – a full description and map of its principal physical features. George Philip & Son, London French BM (1998) Traces of catastrophe: a handbook of shock-metamorphic effects in terrestrial meteorite impact structures. LPI Contribution No. 954, Lunar and Planetary Institute, Houston. 120 pp Glass BP, Simonson BM (2013) Distal impact ejecta layers. A record of large impacts in sedimentary deposits. Springer, Heidelberg Grant JA, Schultz PH (1993) Degradation of selected terrestrial and martian impact craters. J Geophys Res 98(E6):11,025–11,042 Grant JA, Wilson SA, Cohen BA, Golombek MP, Geissler PE, Sullivan RJ, Kirk RL, Parker TJ (2008) Degradation of Victoria crater, Mars. J Geophys Res 113: E11010. doi:10.1029/2008JE003155 Melosh HJ (1989) Impact cratering: a geological process, Oxford monographs on geology and geophysics, 11. Oxford University Press, New York Ormo¨ J, Lepinette A, Sturkell E, Lindstro¨m M, Housen KR, Holsappe KA (2010) Water resurge at marinetarget impact craters analyzed with a combination of low-velocity impact experiments and numerical simulations. GSA Spec Pap 465:81–101. doi:10.1130/ 2010.2465(06) Poelchau MH, Kenkmann T, Kring DA (2009) Rim uplift and crater shape in Meteor Crater: effects of target heterogeneities and trajectory obliquity. J Geophys Res 114:E01006. doi:10.1029/2008JE003235 Robbins SJ, Hynek BM (2012) A new global database of Mars impact craters 1 km: 1. Database creation, properties, and parameters. J Geophys Res Planet 117:E05004. doi:10.1029/2011JE003966 Roddy DJ, Boyce JM, Colton GW, Dial AL Jr (1975) Meteor Crater, Arizona, rim drilling with thickness, structural uplift, diameter, depth, volume, and mass-balance calculations. Lunar Planet Sci Conf VI:2621–2644, Houston Schenk PM (1991) Ganymede and Callisto: complex crater formation and planetary crusts. J Geophys Res 96:15635–15664 Schon SC, Head JW (2012) Gasa impact crater, Mars: very young gullies formed from impact into latitudedependent mantle and debris-covered glacier deposits? Icarus 218:459–477. doi: 10.1016/j.icarus.2012.01.002. http://adsabs.harvard.edu/abs/ 2012Icar..218..459S, Provided by the SAO/NASA Astrophysics Data System Schro¨ter JH (1791) Selenotopographische fragmente. CG Fleckeinsen, Lilenthal Shoemaker EM (1960) Penetration mechanics of high velocity meteorites, illustrated by Meteor Crater, Arizona: International Geological Congress, 21st, Copenhagen, Report, pt. 18, pp 418–434, 1960 Singer KN, McKinnon WB, Schenk PM, Moore JM (2012) Massive ice avalanches on Iapetus mobilized by friction reduction during flash heating. Nat Geosci 5:574–578

445 Stepinski TF, Mendenhall MP, Bue BD (2009) Machine cataloging of impact craters on Mars. Icarus 203:77–87. doi:10.1016/ j.icarus.2009.04.026 Sto¨ffler D, Ryder G, Ivanov BA, Artemieva NA, Cintala MJ, Grieve RAF (2006) Cratering history and lunar chronology. Rev Mineral Geochem 60:519–596 Watters WA (2010) The concave planform of transient impact craters in fractured targets. 41st Lunar Planet Sci Conf, abstract #2684, Houston Wieczorek MA, Phillips RJ (1999) Lunar multiring basins and the cratering process. Icarus 139:246–259

Crater Row ▶ Crater Chain (Impact, Primary)

Crater String ▶ Ricochet Crater

Crater Streak ▶ Wind Streak

Crater Wall Veronica J. Bray Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

Definition Crater walls are the interior sides of a crater rim.

Synonyms Circumvallation (Elger 1895, obsolete) terraced zone (if in a complex crater)

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