Standardizing the nomenclature of Martian impact crater ejecta ...

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Nov 25, 2000 - •Department of Physics, University of Central Florida, Orlando. 2Office of Space Science, NASA Headquarters, Washington, D.C..
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. Ell, PAGES 26,733-26,738,NOVEMBER 25, 2000

Standardizing the nomenclature of Martian impact crater ejeeta morphologies Nadine G.Barlow •, Joseph M. Boyce 2,Francois M. Costard 3,Robert A. Craddock 4, James B. Garvin s,Susan E. H. Sakimoto s,Ruslan O.Kuzmin 6,DavidJ.Roddy 7, andLaurenceA. Soderblom 7 Abstract. The Mars CraterMorphologyConsortiumrecommends the useof a standardized nomenclature systemwhendiscussing Martianimpactcraterejectamorphologies.The system utilizesnongenetic descriptors to identifythevariousejectamorphologies seenon Mars. This systemis designedto facilitatecommunication andcollaborationbetweenresearchers.Crater morphologydatabases will be archivedthroughtheU.S. GeologicalSurveyin Flagstaff,wherea comprehensive catalogof Martian cratermorphologicinformationwill be maintained. 1. Introduction

Fresh Martian impact craters are typically surroundedby ejectastructuresthat differ in morphologyfrom the radial ejecta patternsseenaroundlunar and Mercurian craters. The Martian ejectastructuresare typically composedof one or more layersof material, commonly displayed in a lobed pattern. These structureshave been described by a number of adjectives, including fluidized, lobate, rampart, splosh, and flower. Although originally thoughtto be the result of wind erosionon the basisof Mariner 9 image analysis[McCauley, 1973;Amidson et al., 1976], Viking Orbiter imagesrevealedthat thesestructures were distributedglobally and likely the resultof emplacementby fluidizationprocesses,either from impact into and vaporization of subsurface volatiles[Cart et al., 1977; Wohletzand Sheridan, 1983] or by ejecta entrainmentby the thin Martian atmosphere [Schultz and Gault, 1979; Barnouin-Jha and Schultz, 1998]. Now as more details about the ejecta morphologies and morphometriesbecome available becauseof the Mars Global Surveyorand upcomingMars Surveyormissions,it is apparent that a standardizedsystemof nomenclatureis neededto facilitate the exchangeof data between researcherswho compile crater data for use in studies on erosional history [Craddock and Maxwell, 1990, 1993; Craddock et al., 1997; Grant and Schultz, 1990, 1993; Barlow, 1995; Hartmann and Esquerdo, 1999],

implicationsfor subsurface volatiles[Cintala and MouginisMark, 1980; Mouginis-Mark, 1981, 1987; Kuzmin et al., 1988; Costard, 1989; Barlow and Bradley, 1990; Barlow, 1994; Boyce and Roddy, 1997; Costard and Gosset, 1998; Demura and Kurita, 1998], cratermorphometriesandformation[Roddy,1977;

andFrawley,1998;Gaminet al., 1999],andthegeneralgeologic history of the planet [Soderblomet al., 1974; Tanaka, 1986; Barlow, 1988; Hartmann, 1999]. This articledescribesa systemof nomenclature recommended

by the Mars Crater MorphologyConsortiumfor use when describing Martian impact structures. The Consortium, composed of this article'sauthors,met at the U.S. Geological Surveyin Flagstaff,Arizona, in May 1998 and July 1999 to discussand developtheserecommendations.The Consortium membershave beenactivelyinvolvedin utilizing Martian impact cratersin a numberof studiesover the past20 years,and several

haveproduced catalogs of cratercharacteristics. Thesedatabases contain informationthat is often complementaryto but not containedin the other cratercatalogs. One of the goalsof the Mars CraterMorphologyConsortiumis to combinethe existing crater databasesinto one system that can be queried for informationon crater location,size, shape,preservationalstate,

ejectaand interiorstructures, and morphometric characteristics (craterdepth,rim height,centralpeakheightandwidth,central pit diameteranddepth,ejectaextentandsinuosity, etc.). Because of the variety of classificationsystems,many using different terminologyto describethe samemorphology,the Consortium agreedthat the first courseof actionwas to standardize the nomenclature for cratermorphologicfeatures.The first features to be standardized are the ejectamorphologies.

2. Recommendations

Martian ejecta blankets have been classified into many differentgroupsbecause of the rangeof morphologies identified from the Viking Orbiter imagery. However, in general,the morphologies canbe dividedintothreemaingroups: •Department of Physics, University of Central Florida, Orlando. 1. Layered ejecta patterns,where the ejecta blanket is 2Office of Space Science, NASAHeadquarters, Washington, D.C. 3CNRS/Laboratoire de GeologieDynamique de la Terre et des composed of one or morecompleteor partialsheetsof material Planetes,Orsay,France. surroundingthe crater, appear to have been emplacedby 4Center for EarthandPlanetary Studies, NationalAir and Space fluidizationprocesses, althoughsomestructures showevidence Museum,Smithsonian Institution,Washington,D.C. of subsequent eolian erosion. 5NASAGoddard Space FlightCenter, Greenbelt, Maryland. 2. Radial ejecta blankets,which are similar to the ejecta 6Vernadsky Institute, Russian Academy of Sciences, Moscow, Russia. 7U.S.Geological Survey, Flagstaff, Arizona. patternsaroundlunarandMercuriancraters,are believedto be emplaced by secondarymaterial ejected along ballistic

Cintala et al., 1976; Wood et al., 1978; Melosh, 1989; Gamin

Copyright2000 by the AmericanGeophysicalUnion. Papernumber2000JE001258.

trajectories. 3. Combination structuresshow both layered and radial

0148-0227/00/2000JE001258509.00

patterns. 26,733

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BARLOW ET AL.: MARTIAN IMPACT CRATER NOMENCLATURE Table 1. Correlationof New MorphologyTerminologyWith PreviousNomenclature Nomenclature

LayeredEjecta Structures Pedestal(P) Pedestal Pedestal Pedestal Mound Lump Single-layerpancake(SLEPC/SLEPS) Polar Type 6 SS Type 3 Pancake Single-layerrampart(SLERC/SLERS) Type 1 Class4 SR Type 1/flower Singlelobe Double-layerrampart(DLERC/DLERS) Composite Type 2 Class3 D Type 2/rampart Double lobe Multiple-layer rampart(MLERS) Flower Type 3 Class2/flower MR Multiple lobe Radial Morphologies Radial (SLER) Lunar Class 1/lunar Type 4 Radial

Reference

McCauley [ 1973] Amidsonet al. [ 1976] Head and Roth [1976] Johansen[1979] Johansen[ 1979] Johansen[ 1979] Mouginis-Mark[ 1979] Horner and Greeley[ 1987] Costard[1989] Barlow and Bradley [ 1990]

Mouginis-Mark[ 1979] Blasiusand Cutts [1980] Horner and Greeley[ 1987] Costard[1989] Barlowand Bradley[ 1990] Johansen[ 1979] Mouginis-Mark[ 1979] Blasiusand Cutts [1980] Horner and Greeley[ 1987] Costard[1989] Barlow and Bradley [1990] Johansen[1979] Mouginis-Mark[ 1979] Blasiusand Cutts [ 1980] Horner and Greeley[1987] Barlowand Bradley [1990]

riohansen[1979] Blasiusand Cutts [1980] Mouginis-Mark[ 1979] Barlow and Bradley [1990]

CombinationMorphologies(e.g., SLERSR) Transitional

Type 5 Diverse

Table 1 providesa listingof someof the terminologythat has beenusedto describecratersin thesebasicgroups. The Mars Crater Morphology Consortiumrecommendsthe following changesin nomenclature: First, layered ejecta patternsthat have undergonesubstantial erosionowing to eolian activity, resultingin the craterand ejecta being perchedabovethe surroundingterrain,shall be referredto as "pedestalcraters"(Pd). The ejectablanketof a pedestalcrater is typified by a sharpedge (no distal ridge) that dropsoff to the lower elevationof the surrounding terrain(Figure l a). Second,other layered ejecta patternsshall be referred to as "layeredejectacraters"(LE). 1. Layered ejectacraterssurroundedby only a singlelayer of materialshallbe called"single-layerejecta"(SLE) (Figure lb). 2. Layeredejectacraterssurroundedby two layersof material shall be called "double-layerejecta" (DLE). The inner layer of the DLE morphologyhas a smallerdiameterthan the outer layer and is usuallysuperposed on the outerlayer (Figure 1c).

riohansen[ 1979] Mouginis-Mark[ 1979] Barlow and Bradley [ 1990]

3. Layered ejecta craterssurroundedby three or more completeor partiallayersof materialshallbe called"multiplelayerejecta"(MLE) (Figure l d).

4. The single-layer,double-layer,and multiple-layer categories arefurthermodifiedby termsdescribing theshapeof the ejectaterminus.Thoselayeredejectapatterns terminated by a distalridgeor rampartshallbe modifiedby theterm"rampart" (R) (i.e., Figure l d). Hence a single-layered ejectapattern terminatedin a distalridgewouldbe calleda "single-layer ejecta rampart." Layeredejectapatternsthat terminatein a concave slopewill bemodifiedby theterm"pancake" (P) (Figure1e). 5. The rampartandpancaketermsarefurthermodifiedby the adjectives "sinuous" (S) (i.e.,Figurel d) and"circular" (C) (i.e., F•igure 1e), describing thegeneralsinuosity of theejectablanket. Ideally,thedesignation of S versusC will be basedon anactual quantitative measurement of the ejectasinuosity, suchas the lobatenessmethod describedby Barlow [1994].

Using the

lobateness system,ejectamorphologies with lobateness values

BARLOW ET AL.' MARTIAN IMPACT CRATER NOMENCLATURE

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Figure 1. Examplesof the new ejectamorphologyclassifications recommendedby the Mars Crater Morphology Consortium. Arrows point to the distinguishingejecta features. (a) A 1.5-km-diameterpedestal(Pd) crater, located at 11.4øN, 161.5øW, (Viking Orbiter image 886A09). (b) Two single-layerejecta rampart sinuous (SLERS) craters. The top crateris 18.1 km in diameterand locatedat 8.80øN,72.13øW. The bottomcrateris 18.9 km in diameterand locatedat 7.96øN, 72.16øW (Viking Orbiter image858A56). (c) A 9.6-km-diameterdoublelayer ejectarampartsinuous(DLERS) crater,locatedat 21.42øN,79.02øW(Viking Orbiter image555A24). IL identifiesthe innerejectalayer; OL denotesthe outerejectalayer. (d) A 33.9-km-diametermultiple-layerejecta rampartsinuous(MLERS) crater,locatedat 26.59øN, 38.55øW (Viking Orbiter image 827A01). IL indicatesthe inner ejectalayer, ML identifiesthe middle ejectalayer, and OL denotesthe outerejectalayer. (e) A 7.7-kmdiametersingle-layerejectapancakecircular(SLEPC) crater,locatedat 26.10øS,69.04øW (Viking Orbiter image 608A29). (f) Single-layerejectaradial (SLERd) craters. The largecrateron the left is 75.3 km in diameterand locatedat 24.36øN, 10.79øW(MDIM imageMg25n012). (g) A 107.2-km-diametersingle-layerejectarampart circularradial (SLERCRd) crater,locatedat 32.32øN,337.9øW (MDIM imageMg30n337). Rd indicatesthe radial componentof the ejectablanket,while SLER identifiesthe single-layerejectarampartportionof the blanket.