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Fluid Outflows from Venus Impact Craters

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Fluid Outflows from Venus Impact Craters JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. E8, PAGES 13,643-13,665 AUGUST 25, 1992 Fluid Outflows From Venus Impact Craters' AnalysisFrom Magellan Data PAUL D. A SIMOW1 Departmentof Earth and Planeta .rySciences, ttarvard Universi.ty,Cambridge, Massachusetts JOHN A. WOOD SmithsonianAstrophysical Observato .rv, Cambridge, Massachusetts Many impactcraters on Venushave unusualoutflow features originating in or underthe continuousejecta blanketsand continuing downhill into the surroundingterrain. Thesefeatures clearly resulted from flow of low- viscosityfluids, but the identityof thosefluids is not clear. In particular,it shouldnot be assumeda priori that the fluid is an impact melt. A numberof candidateprocesses by which impact eventsmight generatethe observedfeatures are considered,and predictionsare made concerningthe theologicalcharacter of flows producedby each mechanism. A sampleof outflowswas analyzedusing Magellan imagesand a model of unconstrainedBingham plastic flow on inclinedplanes, leading to estimatesof viscosityand yield strengthfor the flow materials. It is arguedthat at leasttwo different mechanismshave producedoutflows on Venus: an erosive,channel-forming process and a depositionalprocess. The erosivefluid is probablyan impactmelt, but the depositionalfluid may consistof fluidizedsolid debris, vaporized material, and/or melt. INTRODUCTION extremelydiverse in appearanceand may representmore than one distinctprocess and/or material. Recentlyacquired high-resolution radar images of Venusfrom the Magellan spacecraft have revealed surface features in unprecedenteddetail. In addition to new views of previously SETtING AND MORPHOLOGY OF VENUS CRATER OUTFLOW known features,a seriesof completelynew and often enigmatic FEATURES features have been discovered. Among the new phenomena Over 800 impactcraters were identifiedin imagesproduced by observed,the characterof ejecta depositsaround impact craters the Magellan missionduring its first cycle of orbital mapping, ranks as one of the most enigmatic. In additionto the expected rangingin diameterfrom 3 km to 280 km (R.R. Hertick, personal radial distribution of continuous ejecta, left by material communication,1991). The structuresbeyond the rims of Venus ballisticallyejected, there are nonradialfeatures with a flowlike or craterscan be convenientlydivided into four facies,all of which channel-like appearance that are generally too long to be are seenat craterAurelia in Figure 3: "hummockyejecta", "outer explainedby any ballistic process(Figure 1) [Phillips et al., ejecta", "dark haloes", and crater outflow features. The 1991]. These structureswill be called "crater outflow features"; hummockyejecta and outer ejecta will be groupedhereafter under this usageis meantto set them apartfrom continuous,radial, or the name"continuous ejecta"; azimuthalsectors of the continuous ballisticejecta, even when thereis evidenceof somefluidization ejecta blanket are often missing,as in Figure 3 [Phillips et al., of the main ejectablanket. The superficialresemblance of the 1991]. The edge of the continuousejecta usually has a lobate to outflow facies of Venus impact cratersto a numberof flow petal-likeshape suggestive of limited flow upon emplacement, phenomenain variousenvironments (terrestrial, Venusian, lunar, probably causedby turbulent entrainmentof very fine ejecta and Martian lava flows, as well as terrestrial, Martian, and lunar particlesby the denseVenus atmosphere [ Schultz, 1991b ]. Crater debrisflows, and Martian fluidized ejecta blankets;Figure 2), outflow features have been detected in association with at least together with their clearly nonballistic character and their 100 cratersto date. These outflows are distinguishedfrom the tendencyto extenddown topographicgradients away from their continuousor hummockyejecta blanket by their length,brightness cratersources, strongly suggests that thesestructures result from signatures,nonradial distribution, complex morphology, and/or fluid flow phenomenaof some variety. The continuousejecta sinuousplanform. Outflows are recognizedmost readily near blanketsoften display a lobateedge, suggesting some flow during cratersin plains regions,but they are observedin all geologic or after emplacement,but the crateroutflows are clearly distinct settings,from smoothplains to the highland/tesseraarea Tellus featureswith muchhigher fluidity. The presentstudy attempts to Regio (Figure 4). They may extendseveral crater radii from the confirmthis interpretation and, more importantly, to determinethe edgeof the continuousejecta, although some outflows are only a rheologyof the flow materialsand, if possible,to identify those few kilometerslong. materials. It shouldbe kept in mind that the flows observedare There are several modes of occurrence of outflow features. Some craters, such as Kemble (Figure 5), exhibit one narrow 1 Nowat Division of Geological and Planetary Sciences, California outflow originatingfrom a small region at the edge of the ejecta Instituteof Technology,Pasadena. blanket; this simple appearanceis fairly !•are. Many (perhaps 25%) cratershave exactlytwo outflowsassociated with them.The Copyright1992 by the AmericanGeophysical Union. two outflows from a given crater may originatefrom different PaperNumber 9ZIE00981 pointsadjacent to the ejectablanket, often nearly oppositeeach 0148-0227/92/9ZIE-00981 $05.00 other in azimuth,such as at Aglaoniceor Danilova (Figures1 a 13,643 13,644 ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS ß;-,.. ,,':'•:! ,.-*.. • ...... :.:..:.... ..;•-.•... ß '%' :.: -; :½;-"-:-:.;:-:...½..... t •..?.. • •" ....:•;•;•-';:4•'-•:....;%'•'½-/•,..;:.;.-;;::-•:7%' :.<,::;.:.?:. ;:.;:r:- ..:...:/•. ...... ....... :;-:•'.:.;'.". .• ---:½'."--•;•.;•'.;---.•... ß..•:,,:. - ;•;. .-.: -...: ....:•..... •:':...-.::. .,..-....- . .:.;::...;:::..•::•:•.:..:;; ..•. • •.... Figure 1. Full-resolutionMagellan radar imagesof impactcraters on Venus. In all Magellanradar images,north is up, and illuminationis from the west. (a) Aglaonice(26.2øS, 339.7øE; diameter 63 lcm),with large fluid outflow to north and small outflowto southeast.(b) Danilova(26.6øS, 337.2øE;diameter 48 lcm),with largeoutflow to southand small outflow to north.(c) Carson(24øS, 343øE; diameter 39 km), with two brightflows originatingto southand flowing to eastand west.(d) Stuart(30øS, 21øE; diameter67 km). and 1b). They may alsooriginate from a commonarea but flow in outflow modifying ejecta) are evident in particularcases. The oppositedirections from that point, as at Carson(Figure l c) and edgeof the continuousejecta may appearunmodified at the source Parra(Figure 6). Anothercommon morphology for outflowsis a areaof the flow, or it may showextensive modification: the lobate broadsplay, composed of multiple subflowsand coveringup to outline may be destroyedor blurred and dark linear to arcuate 180ø in azimuth, such as at crater Stuart (Figure l d). The featuressubparallel to the outflow direction may appearin the developmentof multipleflows originatingfrom one broadsource outerregions of the ejectablanket. Often, the boundarybetween areais quitecommon at smallercraters (Figure 7). the continuousballistic ejecta and the outflowdeposits cannot be The source area of the outflows appearssometimes to be distinguished(Figure ld). beneaththe ejecta blanket and sometimeswithin it (compare The azimuthdirection of outflowsis most likely determined Figures8a and 8b). The apparentstratigraphic relationship by local topographyor impact direction, the only sourcesof betweenoutflow and ejectacan be establishedin somebut not all asymmetryor preferreddirection. Where the radialdistribution of cases,and both relationships(outflow underlying ½jectaand ejecta suggestsan oblique impact, single outflows sometimes ASIMOWAND WOOD:FLUID OUTFLOWS FROM VENUS IMPACTCRATERS 13,645 1..Qkm ,,,.•:;;'.%•.;.. '" .•::i:] . .:::.....i.:•::: .•:...,•.... ....:*:f .. -.,• 3":-.'•';/.':•- '....... ; '6-:'::.;;.' . '% :.**-•,. Figure2. Examplesof depositsleft by flowprocesses. (a) Magellanradar image of lavaflow from large volcano Sif Mons,at 7øN,350øE. Width of frame154 km. (b) Vikingorbiter mosaic 14A27-32, showing large avalanche deposit from wall of Valles Marinerison Mars, extending across valley floor. Scale shown. (c) Apollo17 orbiterframe M-2608, showing large avalanche depositfrom rim of farsidecrater Tsiolkovsky. Lineated avalanche deposit is about80 long. (d) Vikingorbiter mosaic 3A07, showingimpact crater Yuty (diameter 19 km) on Mars with characteristic fluidized ejecta blanket, including outer rampart and multipleflow lobes. appear to have originated at the downrangepoint, such as at crater but then turn and flow downhill around the crater. The Aurelia (Figure 3). More often, the azimuths of single and slopesdown which the outflowstravel are, withoutexception to multiple flows differ substantiallyfrom the apparentdownrange date, very low. Slopesobtained from Magellanaltimetry are direction,such as at Jeanne(Figure 9). In caseswhere two flows somewhatuncertain (see below underMethodology), but mostare emerge from one crater, their directions may be generally virtuallyflat, of the orderof 0.5ø or lower. The detailedpath of opposite, with one flow roughly uprange and the other the flow may be controlledby preexistingsurface structures, approximatelydownrange. Although the azimuthof the pointof especiallyfaults and grabens (e.g., Figure 10). origin of the outflowsand the near-sourceflow directionmay be The morphologyof theoutflows themselves is highlyvariable. controlledby
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