Fluid Outflows from Venus Impact Craters

Home , Aurelia (crater), Cleopatra (crater), Danilova (crater), Dickinson (crater), Isabella (crater), Jeanne (crater)

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. E8, PAGES 13,643-13,665 AUGUST 25, 1992

Fluid OutflowsFrom 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 regionat 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

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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). and1 b). 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

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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 impact direction,the directionof subsequentflow The simplestforms are uniformlybright, straight to slightly appearsto follow local topography. Thus the two outflows in sinuous,with lobateends (Figures 1 c and 6). The width of the Figure6 citedabove originate on the uphill/downrangeside of the outflowis oftenapproximately constant along its lengthbut may 13,646 ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS

Figure3. Magellanfull-resolution radar image of craterAurelia (20.3øN,331.8øE;diameter 31 km). Note standardelements of craterdeposits on Venus:central peak, flat floor, terracedinner walls, sharp topographic rim, hummockyinner ejecta, lobate- edgedouter continuous ejecta, dark halo on surroundingplains, and outflowsto southeast.Note missingsector of ejectato northwest. increase or decrease downstream. Some flows are narrow and al., 1991] categorizethe craterflows as channels,resulting from sinuousupstream but feed into broadfans, possibly as they cross an essentiallyerosive process. The observedoutflow structures. breaksin slope. Some larger examplesof simpleoutflows have are interpretedas excavatedregions, with depositionof the dark interiorregions and bright margins,often with bright rings transportedand eroded material only at their distal ends. This around obstaclesin the interior of the flow (Figure 11). It is model assumesthat the same processis responsiblefor crater temptingto interpretthe brightmargins as stationaryflow levees, outflows and for the various volcanic channels or rilles discovered but this interpretationmust be made carefully. At the incidence on Venusin Magellanimages. The principalitem of evidencefor anglesused by the Magellan radar system,brightness is mostlya this interpretation is the presence of eroded or streamlined functionof surfaceroughness at the centimeterscale and not of remnant "islands"in midchannel(Figures 11 and 13). Baker et topographicrelief. Thusthe brightmargins are certainlyrougher al. assumea priori that the flow materialwas a melt-rockof some than the dark interiors, but it cannot be determinedwhether they variety;they do not evaluateany debrisflow hypotheses.The are higheror lower. Most outflowstake the form of multipleflow alternativeview is thatcrater flows are depositionalfeatures, with units, which may show forms analogous to braiding or positivesurface relief [Phillips et al., 1991]. A depositionalflow anastomosing;these multiple flows are oftenextremely complex. will displaychannel-like morphology in radar imagesif it creates Morphologysimilar to braidingis apparentin the small flow to stationarylateral levees. the southeastof Aglaonice(Figures la and 12), whereasa more The presentstudy is concernedonly with thoseflows whose chaotic,perhaps anastamosing, regime is seenin theoutflow from charactercan be interpretedas depositional,although there appear the 165-kmcrater Isabella (Figure 13). to be flows createdby both mechanisms:compare Figures 11 and Althoughthe fluidized origin of crater outflow featuresis 13. Unambiguousseparation of channelsfrom positive-relief generally accepted [Phillips et al., 1991], there are two depositsrequires much higher resolution topographyor high fundamentallydistinct and mutuallyexclusive interpretations of incidenceangle radar images;such data may be gatheredin the these structures. Some workers [ Baker et al., 1991; Komatsuet extendedmission of the Magellan project. In the absenceof ASIMOWAND •VOOD:FLUID OUTFLOWS FROM VENUS IMPACT CRATERS 13,647

Figure4. CraterKhatun in TellusRegio (40øN,86øE; diameter 44 km). Magellanfull-resolution radar image. Note bright outflow to north,filling valley in extremelydisrupted terrain. unambiguousdata, the interpretationthat many of the crater to 100 m) to standabove a depositionalflow. The similarity outflowfeatures are depositionalin originis motivatedby several betweenthe "islands"in a clearly depositionallava flow from the observations: (1) the scale and gross morphology of these large volcano Sif Mons (Figure 2a) and in some crater flows depositsare distinctfrom the channelsdescribed by Baker et al. (Figure11) is extremelystrong. [1991]. (2) The lobate planimetricform of the depositsclosely resemblesdepositional features such as lava flows and debris MODEL AND ANALYSIS flows. (3) The extraordinary width of the flows requires an extremelylow cross-sectionalaspect ratio (it is not possiblefor a The rheologicalbehavior of a broadclass of geological channelto be several kilometers deep), which will make the fluidsis oftenapproximated by a plasticmodel, first describedby assumption of unconstrained surface flow more useful in Bingham[1922], which introduces a finite yield strength Sy and a modelling the outflow processthan the usual assumptionsof constantBingham viscosity rib. A Binghamplastic obeys the flow law semiellipticalor semirectangularchannels.(4) Erosive channels must have a "drainageregion" where the fluid which excavates c•=Sy + •bk (1) the channelis eventuallydeposited, whereas the crateroutflows generally lack any such associatedfeature (with the notable relatingshear stress c• to shearstrain rate •. Morecomplex and exception of Figure 13), suggestingthat the material which general flow laws might be more accurate models of real formed the outflows constitutes the observed deposit. (5) materials,e.g., a pseudoplasticmodel with a powerlaw relation Assumingthat crateroutflows are causedby crateringevents, they betweenshear stress and strainrate. Althoughpseudoplastic were most likely producedby a sudden,short-lived process, modelsare most easily fit to experimentaldata, the Bingham whereasthe time scalesfor thermal erosion by lavas require modelis often an adequateapproximation, and it is the modelof continuoushigh-volume effusion for a significantperiod in order choicefor manygeological problems because it leadsto complete to form erosionalchannels. (6) The "streamlinedislands" thought and simple models of flow processeswhich demonstrate to represent uneroded regions between anastamosingcrater reasonableagreement with field observations[Hulme, 1974] and outflow segmentsoften show clear summit pits and generally laboratorymeasurements of actual lavas [ Shaw, 1969; Moore, circularoutlines, in contrastto the genuinelystreamlined islands 1987]. The Binghammodel will be usedthroughout this paper; in otherchannels. These are interpretedas small shieldvolcanoes, viscosity,denoted simply •1, shouldbe takento meanBingham whichare extremelycommon on Venusand are high enough(50 viscosity,except where otherwise specified. 13,648 ASIMOWAND WOOD: FLUID OUTFLOWSFROM VENUS IMPACT CRATERS

Fig5. Kemble(47.7øN, 14.9øE; diameter 25 km). Magellanfull-resolution radar image. Note simple bright flow to dueeast.

Hulme[ 1974]developed a completemodel for the unconfined mustbe a regionwhere • < •s, since• approacheszero at the flow of a massof ideal Binghamplastic fluid extrudedfrom a lateralboundaries. There will be no longitudinalflow in these small sourceonto a smoothinclined plane. This model servesto regions;they will form stationarylevees bounding the channel approximatethe depositionof fissure-extrudedmaterial onto a (i.e.,the region where • > •s; thisis differentfrom the usage of sloping surface free of preexisting channels or complex "channel"by Bakeret al. [ 1991]and does not imply erosion). If topographygiven values of slope,density, gravity, effusion rate, wb is the widthof onestationary levee, Hulme [ 1974]derives the viscosity,and yield strength.The Hulmemodel can be combined relation with morphologicalmeasurements of crateroutflows to solve wb = (2) approximatelythe inverseproblem of determiningthe yield 2gpot2 strengthand viscosityof the outflowmaterial. Interestin the whereg is the accelerationdue to gravity, p is density,and {z is Bingham model for rheology is justified a priori by the theslope of theunderlying surface. Figure 14 showsa diagramof observationsthat lava flows often stopon a slopeon time scales the ideal predictedflow profileand an illustrationof the various shorterthan cooling times (rather than ponding in depressionsas width measurements used in the model. Newtonian fluids would) and that lava flows cease to spread Thus, given a measurementof the slope of the surface laterallyabsent topographic obstacles and before surface tension (assumedconstant), the width of the flow, and the width of the becomessignificant. leveesor marginsat a givenprofile across the flow, (togetherwith Hulme's model assumes an isothermal flow with a the accelerationdue to gravityand an assumeddensity), we can dynamicallyunimportant solid crust,estimates of the growth estimatethe yield strength at thatprofile: times of solid crust show that the lateral spreadingof a silicate melt flow is arrestedbefore the crustis strongenough to contain Sy= 2wbgpot 2 (3) it, and the melt shouldbe approximatelyisothermal in cross aswell asthe central(and maximum) depth of flow at thatpoint: sectionand over reasonablelongitudinal distances if coolingis restrictedto a thincrust. The approximationsmade are most valid •02 = 2wwb a2. (4) for a flow muchwider thanit is high. Integrationof the profilefunction given above gives the cross- Lateralor downhill flow occursonly when the shearstress at sectional area of the flow: the baseof the flow exceedsthe yield strength. Downhill basal shearstress at any point is proportionalto the thicknessof 2 material• abovethat point,so thereexists a criticaldepth A= 5 w (5) belowwhich no flow occurs.Along either edge of the flow, there which will allow an estimateto be madeof flow volume,given a ASlMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS 13,649

Fig6. Parra(20.5øN, 78.1øE; diameter 48 km). Magellanfull-resolution radar image. Poor data on orbit through center of crater resultedfrom tape recorder errors aboard the spacecraft. Regional gradient dips to northwest.Two brightflows originate on uphill sideof crater,turn, and flow downhillto northwest. measurementof flow length.Use of thismodel to analyzecrater leveesand flow margins. Fink and Zimbelman deteminedthat outflowsintroduces the additionalassumption that the observed whereboth are observed, it is an acceptableapproximation to use final deposithas the formof the criticalprofile, i.e., the deposit combinedmargin and levee widths as input to Hulme'smodel. continues to reflect the form which the flow assumedat the time it Hulme [1974] also developsa methodfor estimatingthe ceased to move. productof volumeflow rateand Bingham viscosity, Frl, fromthe Other workershave derived different equationsfor morphologyof the flow. In orderto obtaina viscosityestimate estimating yield strength from remote observations of fromthis method, it is unfortunatelynecessary to guessthe flow morphology:Johnson [ 1970] arrivedat rate. Most work usingthis kind of modelhas chosen to adopt valuesof viscosityfrom known lavas with yield strengths similar sy= pg;0sina, (6) to thecalculated value and thereby derive an estimateof flow rate. whereasMoore et aL [ 1978] used Forthis study, in whichthe end goal is to obtainviscosity, such an pg•02 approachis not helpful. Insteadwe chooseto estimatethe flow Sy= w . (7) rateand derive a correspondingestimated viscosity. Flow rate is a Moore et al. [1978] showedthat the three methods(of Hulme very poorlyconstrained quantity, however, unless we introduce [1974],Johnson [1970], andMoore et al. [19781,respectively) basicassumptions about the nature of theprocess. If we makethe give resultsconsistent to withina factorof 2, despitethe large (perhapsunrealistic) assumption that the materialis a steadily uncertaintiesinvolved in the Bingham model itself, unknown extrudedmelt which cools as it flows, then the work of Head and preflow topography,and the difficultiesof remotemeasurement. Wilson[1986] on coolingof lava underVenus conditions allows The Hulme[1974] model is usedhere because there is no way to us to relate flow lengthand flow rate. Head and Wilson treat measure•0 for low reliefflows on Venus,and in Hulme'smodel, coolingby radiationand convection to theatmosphere, neglecting •0 is a derivedresult rather than an input. conduction to the surface. Their result is Realflows are likely to spillover their levees due to changes in theeffusion rate over time. This processhas been observed on 180•6vX F = • (8) active Hawaiian lava flows [Fink and Zimbelman,1986]. The resultingspill-over deposits create an additionalelement of the wherek is the thermal diffusivity (about 7 x 10-7m 2 s-1for most flow structure,outboard of the levees,called margins. In remote silicatemelts) and X is totalflow length.Obtaining F from(8) measurements,it is oftennot possible to distinguishbetween flow andF? I fromHulme's method, q followsimmediately. Given. our 13,650 ASIMOWAND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS

Figure7. Smallcrater temporarily named Navka-7 (16.5øN,334.4øE; diameter 6 km). Magellanfull-resolution radar image.

Figure8. Stratigraphicrelationships between ejecta and outflows. Magellan full-resolution radar images. (a) Aglaonice(Figure la) close-up,showing modification of ejectablanket by outflowsource area. Widthof field 77 km. (b) Danilova(Figure 1 b) close-up,showing no modificationof ejectaedge of outflow. Width of field 38 km. ASIMOWAND WOOD: FLUID OUTFLOWSFROM VENUS IMPACT CRATERS 13,651

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Figure9. CraterJeanne (40øN,331.4øE; diameter 20 km). A craterwith a probableoblique impact direction to NNE but outflows whichoriginate on the northwestand west sides of the crater. intentto keepan openmind concerning the nature of theflowing assumingconstant effusion over a spanof time. If the craterflow materialand its processof emplacement,however, the assumption depositsresult from impactprocesses, however, then the material that flow lengthis governedby melt coolingis very limiting. We wascertainly supplied in a singleburst on an extremelyshort time will estimateq using Head and Wilson's method but restrict scale.There is no simpleway to correctfor thisdifficulty, but it is discussionof the resultingvalue to considerationof melt rock possiblethat broad disorganizedcrater outflows suchas that at flow models. Although the value of viscosity may not be crater Stuart (Figure l d), which lack any channel-type meaningfulfor debrisflows, the effusionrate ought to be roughly morphology,may result from a suddensplay of fluid, while constantalong a flow (oncethe leveesare established).Therefore channelizedoutflows such as at Danilova (Figure lb) may the downstreamtrend in viscosity,whatever the absolutevalues, representmore protractedeffusion. In any case, the model of may representa real trendfor debrisas well asfor melts. Hulme [1974] shouldstill give a valid approximationof yield There are severalproblems to be consideredwith the use of strength, being essentially independentof flow rate. The thismodel for the problemat hand: calculatedviscosities, however, are inversely related to the 1. The Bingham plastic model may not meaningfully assumedeffusion rate, so theymay be erroneouslyhigh by many approximatethe flowing material at the strain rates which it ordersof magnitudeif the flow materialwas suppliedby a rapid experiencedin flow. This shouldnot affect the calculationof process:apparent flow emplacementtimes (the ratio of estimated yield strength,as Newtonianrheologies do not lead to leveesor volume to assumedvolumetric effusion rate) are of the order of margins,and pseudoplasticmodels should approximately mimic days,whereas actual emplacement times may have been of the Bingham behavior (levees are very slowly flowing in order of minutes. pseudoplasticmodels as opposedto truly stationary).It shouldbe 3. There may have been large energyand pressuresources noted,as well, thatmost of the rheologicalparameters collected in acting on the flows due to impact processeswhich might this paper for comparison purposes were computed using invalidate the simple model based on gravity alone [Schultz, Bingham modelsby other authorsand that there is a general 1991a ]. The impactenergy may be coupledto the flowingdebris consensus in favor of the Bingham model as a useful by means,e.g., of an elevatedtransient crater rim, in which case approximation. the slopesmeasured at the presentera would not be meaningful. 2. The modelsof Hulme [1974] for flow morphologyand of Additionalenergy sources are difficult to correctfor in the caseof Head and Wilson [1986] for cooling rates were developed melt-rockflows, but may in fact be necessaryto explainthe very 13,652 ASIMOW AND WOOD: FLUID OUTFLOWSFROM VENUS IMPACT C RATERS

Figure10. Structuralcontrol of outflows.Temporary name Fortuna 2 (34.6øN,5.5øE; diameter 6 km). Magellan full-resolution radarimage. Note local fractures or faultsin twodirections which channel and contain outflow material.

low observedslopes in the caseof debrisflows (sccbelow under projection. Altimetry has a pixel size of 4.641 km; this low Debris flow scenarios). resolutionmakes accurateslope determinations for small-scale 4. The identificationof the observeddeposits as channelsand local features difficult. Seventeen suitable flows were identified levees or margins may bc incorrect. This is a matter of in the dataavailable as of October1, 1991 (covetingroughly 80% interpretation.For singleflow eventsit is difficultto proposeany of the Venus surface; see Table 1). Flows were included in this otherinterpretation of the observedmorphology, but the observed studyonly if a clearchannel-and-margin morphology was evident depositscould result from multipleflow events:a unit composed and only if the slope could be measured. Many exampleswere of severaldiscrete flows would have a figuredifferent from that excludedbecause their complexmorphology made interpretation predictedby Hulmc's model. This would render the analysis ambiguous.The flows are referredto hereafterby the nameof the irrelevant. Some of the depositsare exceedinglycomplex and crater with which they are associated(some of thesenames are likely resultfrom multipleflow events. Wc have chosenfor this unofficial, pendingapproval by the InternationalAstronomical study, however, only the simplest and most straightforward Union nomenclaturecommittee) plus a modifier such as flow depositsor sectionsof deposits,those which appear to resultfrom directionif necessary.The widthsof apparentlevees or margins singleevents. Nevertheless,there may be spuriousinterpretations and of central flow channelswere measuredalong regularly includedin the data set. Furthermore,there is the potentialfor spacedlines perpendicularto the centralaxis of the flow; at least majorbias due to selectioneffects. Many flowsare excludedfrom five lines were measuredfor eachflow. The total lengthof the the data set becauseno leveesor marginscan be detected. It is flow was estimated,and the slopeof the flow was determined possiblethat their flow leveesare too small to be resolvedby using the altimetric elevationsof at least two points,near the Magellan imagery or that the radar viewing geometry is origin and terminusof the flow respectively.The measuredflow unfavorableor that there are no levees. There is no way to widths, margin widths, length, and slope were used to calculate distinguishamong these cases. Flows with Newtonianrheology, yield strength,central depth, and productF•I at the locationof however, should they be present, would be systematically eachmeasured line acrossthe flow. A locallycalculated estimate excluded,since they do not form leveesor margins. of flow rate F is then used to approximate•1 at each location.

METHODOLOGY Theseestimates of Sy, •0, and•1 werecombined to compute averagevalues for eachflow and were also retainedindividually The raw data for this study were syntheticaperture radar for an analysisof the changein flow propertieswith distance (SAR) mosaicsand radar altimeter topography maps from the first along the flow. These calculatedparameters allowed us to cycle of orbital data taken by the Magellanmission. The full- computeseveral additional properties of the flow, includingflow resolutionimages have a pixel size of 75 m and are in sinusoidal volume,estimated mass, and apparent emplacement time scale. ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACTCRATERS 13,653

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Figure11. Smalls•elds su•ounded by Aglaoniceoutflow (Figure 1a), showing"bathtub rings" a high-watermark of outflow andsummit pits, indicating volcanic nature. Magellanfull-resolution radar images. Width of field 78 •. Note alsoapparent windstre•s and•ssible dunefield in noahwestcorner of frame. The width measurementsare generallyaccurate to about200 The densityof flow materialis everywhereassumed to be m where channeland levee or margin are explicitly identified; 2500kg m'3. Thisvalue is appropriatefor mostmelts but is interpolationbetween unresolved levee segmentsmay introduce undoubtedlyhigh for manydebris flow phenomena.The results errors of unknownmagnitude. The length estimatesshould be generallyscale linearly with d.ensity,however; consequently, the accurateto within about 5 km. The reportedelevation of each total rangeof reasonabledensities introduces an uncertaintyin pixel in the topographydata represents an estimateof the average yieldstrength estimates of perhapsa factorof 5. It shouldbe kept elevationof a region4.641 km on a side. The valueis obtainedby in mind, then, that orderof magnitudecomparisons are the best fitting templatesto the altimetryradar echo and is subjectto large thatshould be expectedfrom thismethod. errorsif the topographyof the targetregion is complex,i.e., if the This analysiswas performed on the 17 selectedcrater outflow actualelevations inthe 21 km 2 representedbyone pixel range far features as well as on two channelized Venus lava flows, one from the averagevalue. An attempt was made for each crater terrestrialmudflow deposit, and one terrestrialnu•e ardente outflow to identify artifactsand resolutioneffects in the altimetry deposit. The resultsare presentedin Table 3. The resultsfor databy visuallyinspecting topographic contour maps of the area; viscosityand yield strengthare alsoshown graphically in Figures somelimited qualitativecorrections were made. Elevationshave 15 and 16 alongwith viscosityand yield strengthdata from a a nominalrelative accuracy is 15 m; the absoluteaccuracy of the varietyof otherflow processes,for purposesof comparison. measurementsis unimportant[Pettengill et al.• 1991]. The error in slopeis largestfor shorterflows. SCENARIOS AND CONTROLS The observedpresent-day slopes may differ from the slopeat the time of impact due to later tectonicevents or impact related The intent of this section is to review the two classes of verticaldeformation. There is no way to correctfor suchchanges possiblescenarios for the formationof fluidizedcrater outflows in slopeif theyhave occurred, but the extremelylow valuesof the on Venus and to consider the range of predictions of each measuredslopes are highly consistentamong the 13 observed scenario. It is hoped that by collectingrheological analyses of flows, in different tectonic regimes. If the slopeshave been various flows elsewhere in the solar system we may find an modified, a few anomalously low values might result from analogywhich matches the resultsat hand. Many of the collected tectonictilting, but a systematiceffect suchas differentialuplift analysesserve the doublepurpose of controland analogy,where surroundingan impact event must be invoked to explain the other authorshave measuredrheological properties by different consistencyof the low slopes.This sourceof errorcould be tested methods(notably direct field measurementon Earth), if adequate throughmodelling of the uplift history associatedwith impact mapsand topographyare available,we may apply our analysis events. and computeindependent estimates of the measuredproperties. 13,654 ASlMOW AND WOOD: FLUID OUTFLOWSFROM VENUS IMPACT CRATERS

Figure12. Southeastflow from Aglaonice (Figure la). Magellanfull-resolution radar image. Note multiple channels with simple braidingmorphology. Width of field 78 km.

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:'4::•'..':.. ,.....-<•. '-•- .... --'-•:• .... -•-.:......

. . .

,..•'::.-., • ...... :,<½.• • - . . ."-. ..••: ...... •..;.::•;•..';-- ....::•*:• .-.

:.• " ,... ..:.,.:.-.•. .-,• .:.

.•.<:..,,...... :...::":<.....%.:,:;:.... <•",::...:.:, -.•. , •- . -• .,.:. -•.,.,:..,• .- . •...... -• ½..:..,.. ;-.. :. .. ..-:.½ ....

Figure13. Thesecond largest crater seen to dateon Venus, Isabella (30øS,201øE; diameter 175 km). Magellanfull-resolution radarimages. (a) Overview,showing dark floor with concentric l¾actures and vague domical uplift but no central peak feature and largeoutflow compex to south and southeast. Note channel and drainage delta layout of largeoutflow to southeast. (b) Close-up of largeoutflow to southeast,showing complex, anastamosing multiple channel morphology and drain-out region. Width of field 115 km. ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS 13,655

This techniquemay confirm the usefulnessof our methodology. 1. Immediately on impact, particularly for more oblique Furthermore,the availableset of measuredor estimatedproperties impacts,there may be "jetting" of high-temperaturemolten and of different flow phenomenaprovides the basic frameworkfor vaporizedmaterial in both the uprangeand downrangedirections interpretationof our results. Estimatesof yield strengthand due to developmentof regionsof extraordinarilyhigh pressureat viscosityare meaningless in themselves,except when compared to the interfacebetween impactor and target. This processis over the sameproperties as measured for knownmaterials. quickly,by the time the impactorhas penetrated half its diameter into the target,but on the orderof oneprojectile mass of projectile The Cratering Process plustarget material may be ejected(reviewed by Melosh[1988]). All the scenariosto be consideredare intendedas analogies for The minimummass of a projectilewhich can penetrateVenus's some processassociated with impact cratering on Venus. A I,i• .... i• .... I.• ,-• •.1•,• 4; ...... ;•,• ,,411 •n,,• !;tt!• fn 4n •s•f• atmospherewithout being deceleratedtoo much to make a hypervelocitycrater is of theorder of 10i2 kg,comparable tothe impacts,the variouscandidate fi•w p•csscs •c •nly relevant estimatedmass of the largest observedflows [Phillips et al., insofaras s•mcthin• simil• mightbe td•crcd by an impact.•c 1991]. The form that a depositof jetted materialon Venuswould •ssibility that the •utfi•w features•c n•t •cncticallyderived take is not known; it dependson the interactionof jets with a fr•m theimpact is n•t c•nsidcrcd.Ideally this study should reveal denseatmosphere. This subjectis not well studied,but it may be •hy the cratcdn•pr•css resultsin these•utfi•ws •n •cnus but similar to $chultz's picture of vapor cloud interactions (see n•t elsewhere(if theyhave •cn produced•n E•h, it is •ssiblc below). thatwc w•uld n•t haverecognized them duc t• •r prostration 2. The next relevantphenomenon is the expansionof a vapor •f deposits). •crc arc at leastthree sta•csin the cratcdn• cloudor fireball. The targetand impactormaterial vaporized by processwhich c•uld provides•urcc material t• the fi•ws, in the impact is produced at extremely high temperature and vapor,melt, •r s•lid f•rm: (1) jcttin•, (2) va•r cl•ud expansion, pressure,and it rapidly expands,overtaking early ejecta and and (2) cjcctacmplaccmcnt. forming a strongshock front (reviewedby Melosh [1988]). It may entrainsome solid material,and it will begin to condenseto liquid or solidparticles as it expandsand coolsadiabatically. The characterof the resultingexpansion in understoodin broadoutline for near vertical impacts. On a planet with an atmosphere,the cloudexpands until its pressureequilibrates with the atmosphere. The effectsof obliqueimpacts are lesswell known. Somerecent laboratory-scaleexperiments, however, have shownthat in low- angleimpacts the vaporcloud or fireball couldbe producedwith substantialforward momentum,and it might be containedby the high atmosphericpressure on Venus and channelledinto a ground-huggingturbidity flow or surge, rather than a classic Figure 14. Hulme [1974] model for Binghamplastic flow on smooth hemisphericalor mushroomcloud [Schultz, 1991 a]. The massof inclinedplane. Cross-sectionalview of flow, showingstationary levees of materialvaporized should be at leastseveral times the massof the width wb alongeither edge. Total flow width is w, maximumpotential projectile,and thusthe vaporcloud would containeasily enough depthat centerline is •,o. Criticaldepth for initiationof flowis gs- massto supplythe observedflows.

TABLE1. BasicData for Sample of 14Venus Impact Craters and 17 Associated Outflows Crater Name Location Diameter, Apparent FlowName b Length, Slope, Azimuth of km Impact km radd Origin Direction a Aglaonice -26.2, 339.7 63 V 180 0.001 N,SEc Andami -17.5, 26.5 28 ? 30 0.006 W Banzhao 17.2,147.0 39 N 80 0.002 NNE Danilova -26.6, 337.2 48 5E 83 0.003 N,Sc Dickinson 74.6,177.3 69 ENE 100 0.001 NNE Edgeworth 32.2, 22.8 29 SW south 20 0.003 SSE southwest 20 0.005 SW Kemble 47.7, 14.9 25 bE 40 0.003 E Lafayette 70.3,107.5 39 S 65 0.005 S Luce -26.3, 31.3 38 SSW 30 0.003 S Macdonald 30.0,121.1 20 ? 50 0.005 SW Parra 20.2, 78.1 48 5E 60 0.008 SSW,Ec Potanina 31.6, 52.4 90 ? east 50 0.003 ENE northeast 50 0.002 bE Voynich 35.4, 55.9 50 ? 100 0.002 WSW Xantippe -10.9,11.7 40 ? northwest 40 0.001 NW southwest 50 0.0005 SW a Determinedby R. R. Herrick.7, unclearor nodata; V, vertical. b If differentfrom crater name, i.e., when more than one outflow isused from a givencrater. c Thereare two outflowsfrom these craters, although only one was included in thisstudy. d Accuracyofslope depends onlength of flow. Slopes of longestflows are accurate toabout 20%, of shortest toabout 100%. 13,656 ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS

Log Viscosity(Pa s)

-3 -2 -1 0 1 2 3 4 5 6 7 I I I I I I I I I I I Parra Lafayette Andami LDucaenilova Aglaonice Macdonald Potanina East EdgeworthSouthwest Crater Outflows Kemble EdgeworthSouth Dicldnson Potanina Northeast Banzhao XantippeNorthwest XantippeSouthwest Voynich Mudslides -- _S.urpdse CanEon .De._bds Flow (J & R) --Katmai PyroclasticHow Debris Flows Mt. St. Helens Mudflow -- WrightwoodDebris Flow (Field Analyses) . _ , S9f-p.ri•Canyon Debris Flow (PDA) Mt. l•'uego Nuee Ardente Mt St. HelensPyroelastic Flows AscraeusMons (Mars) RoyalGardens, Kilauea Padcutin 19N080 #1 (Venus) OlympusMons (Mars) Alba Patera(Mars) Lava Flows Tycho(Moon) (Field Analyses) Mauna Loa (19:42) • Etna1966 MaunaLoa (1984) 19N080 #2 (Venus) Mare Imbrium (Moon) Gula Mons #1 (Venus) Gula Mons #2 (Venus)

P.hyolite Andesite Lavas Basalts (LaboratoryData) Ultramafics(incl. Komatiite) • Lunar Fe,Ti Basalts Sulfur •(Water)

-2 -1 0 1 2 3 4 5 6 7 8 9 Log Viscosity(Poise) Figure15. Resultsfor viscosityanalyses of 17 Venuscrater outflows and reference viscosity measurements from field analysesof variousdebris flows and lava flowselsewhere in the solarsystem and laboratory analyses of magmasof severalcompositions. Citationsfor valuesfrom literatureare in text. Linesrepresent estimated error ranges.

3. Finally, the ejecta proper,consisting of solid and molten behaviorof debrisis an aggregateproperty. Hybrid models,e.g., targetand impactormaterial, is subsequentlydeposited outside the debrisflow with an interstitialliquid of silicatemelt or melt rock craterrim. Ejectamasses are muchlarger than the estimatedflow flows with a substantialunmelted debris component, must also be massesfor the rangeof cratersizes observed on Venus. Ejection considered. distances calculated for ballistic flight through Venus's If the observedoutflow deposits result from melt rock flows, a atmosphereand gravity field show that the outflow features mechanismis neededeither to generateadditional melt outsidethe cannot result dire.ctly from simple ballistic eraplacement. crater or to remove melt from the interior of the crater and Nevertheless,ejecta blankets, in additionto jets and vaporclouds, segregateit from othermaterials jetted or ejected. Schultz[ 1991a] are possiblesources for fragmented,molten, or vaporizedmaterial favors supply of material by jets or vapor clouds. The outside the crater rims. temperature,pressure, and initial melt/vapor ratio of material • ,'t Rock Flow Scenarios comprisingearly jets is not well enoughknown to predictwhether The n,ost basic division among emplacementscenarios is it will condenseto a melt in a shortenough distance to behaveas a between mqt and debris flows. Melt rock flows consist of rock or melt rock flow (but see Schultz [1992]). The condensationof a mixture of rocks (normally silicate, but more exotic classicalvapor clouds is somewhatbetter studied,but it is not compositions have been considered) above the solidus clearthat a containedbase-surge style vapor cloud should behave temperature,with or withoutembedded crystals and a solidcrust. similarlyor what fractionof it will condenseto melt ratherthan to The term "melt rock" is intended to include lava producedby solid, and on what time scale. Thus the Schultzprocess could partial melting in the Venus interior as well as impact melts, produce melt rock flows or debris-type flows and will be whichmay haveunusual composition and may be at superliquidus discussedin the next sectionas well. It is likely that melt temperaiures.Debris flows consistof solid material,either alone condensedfrom a jet or fireballwould be at superliquidus or mixed with some mobilizing fluid (gas or liquid); fluid temperaturesand in anextremely turbulent flow regime, such that ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS 13,657

Log Yield Strength(Pa)

0 1 2 3 4 I I I Parra Andami Lafayette Macdonald LuceEdgeworthSouthwest p•ae• hie m,•ov•. Crater Outflows arena vast EdgeworthSouth _.Aglaonic,e. xantiove Northwest Banzh•b Potanina Northeast Dickinson • Voynich XantippeSouthwest Mudslides SurpriseCanyon Debris Flow (A&W, Mr. Fuel•o Nu6e Ardente Chaos JumBles Sturzstrom Mr. St. Helens Debris Avalanche Debris Flows ! 1 SanJoaquin Valley DebrisFlows SurpriseCanyon Debris Flow (J&R) (Field Analyses) Mt. St. AugustinePyroclastic Flow •Mt. St. Helens Mudflow Mt St. HelensPyroclastic Flows WrightwoodDebris Flow , KatmaiPyroclastic Flow

Teide Tristan da Cunha Tycho(Moon) AscraeusMons (Mars) Royal Gardens,Kilauea Helda •Alba Patera(Mars) Lava Flows Paricutin 19N080 #1 (Venus) (Field Analyses) _ OlympusMons (Mars) •l•tna 1 Oo-sima 1951 Mauna Loa 1942 Mauna Loa 1984 Mare Imbdum (Moon) 19N080 #2 (Venus) MakaopuhiLava Lake, Kilauea

.... Mt. St. Helens Dacite Lavas(Laboratory Mount Hood Andesite Data) Columbia River Basalt , Lunar Fe,Ti Basalts

, I I I I 4 5 6 7 LogYield Strength (dn½Iff 2)

Figure16. Resultsfor yield strengthanalyses of 17 Venuscrater outflows and referenceviscosity measurements from field analysesof variousdebris flows and lava flows elsewherein tile solar system,and laboratoryanalyses of magmasof several compositions.Citations for valuesfrom literature are in text. Linesrepresent estimated error ranges. its rheologymay be quitedifferent from that observed for normal (Figurela)' thereseem to be dark channelsoriginating in the lavas. continuousejecta and feedinginto the outflowstructure. Other Bakeretal. [1991]favor a sourcein thecontinuous ejecta and deposits,however, such as nearbyDanilova (Figure lb), do not a mechanismwherein a melt fractionseparates from and drains modifythe ejectablanket but seemto underlieand predateit. outof anejecta blanket initially deposited as a mixtureof meltand This stratigraphy,which is muchmore common, strongly implies fragments.It is notknown with anycertainty how the melt and that the outflow eventsgenerally precede emplacement of ejecta. solidfractions of ejectainteract and whether the melt fractioncan This favorsmelt supplyas part of the early processes(jetting and effectivelyseparate from the solidfraction or whatthe rate of such fireball),while airfall ejectawas still in flight. separationmight be. A mixtureof meltand solid rocks is subject Postimpactvolcanism could occur throughcircumferential to several simultaneousprocesses, namely, cooling and fissuresbeyond the rim, althoughno evidencefor suchfissures is crystallizationof the melt fraction,heating and meltingof the seen. The frequencywith which crateroutflows occur, however, solidfraction, and separation of thefractions. Knowledge of the in a varietyof tectonicsettings, argues against this mechanism. In rates of these processesrequires information about the at leastone case,the largecrater Cleopatra, lava from the interior compositionsof the rocks,including volatile content, as well as escapedthrough a gap in the craterwall; this mechanismis not thesize distribution of solidfragments. The relationship between evident at most other craters,which generallyhave remarkably the flow sourceareas and the ejectadeposits ought to provide intactrims and uniformlybright, unfiooded inner terrace regions insightinto this process, but the record is difficultto interpret. [Basilevsky,1991]. Althoughpostimpact volcanism can not be Bakeret al. [1991] usetheir modelto explainthe apparent ruled out in particularcases, it is unlikely to be responsiblefor modificationof the ejectablanket by flowssuch as Aglaonice most of the observed outflows. 13,658 ASIMOWAND WOOD: FLUID OUTFLOWSFROM VENUS IMPACT CRATERS

The variousmechanisms for removingmelt from the crater [1985] and Catterrnole[1987]; Moon, Hulrne [1974], Schaber interiorlead to definitepredictions concerning melt propertiesand [1973]). Resultsof a sampleof thesestudies, including lunar effusionprocess. The Baker model (drainingof melt fraction flows and martianflows, are also shownin Figures 15 and 16. from mixed melt-solidejecta after emplacement)predicts slower Included are the resultsof four analysesof Venus lava flows effusion rates and cooler temperaturesthan the more violent performedas partof thisstudy. Two arefrom the southflank of Schultz model. The former mechanism ought to produce the large volcanicedifice Gula Mons, at 19.6øN,358.5øE and Binghamrheologies and leveed flows, whereas the latterscenarios 19.5øN,358.8øE, respectively. Two flowsare from a corona-like (jet or vapor cloud supply) are likely to result in very hot, featureat 19øN,80 øE, originatingat 18.1øN,80.5øE and 19.5øN, superliquidusmelts with no crystal content,which would thus 82.2øE. Being very small features,their slopesare somewhat behaveas Newtonianfluids with zero yield strength,unless other uncertain,and thus the resultsare accurateto abouta factor of 5 in dispersedsolids are incorporatedto a significantextent (non- yield strengthand 2 ordersof magnitudein viscosity. These Newtonian properties of any fluid appear to arise from the flows,although small, were selectedfor their clearchannel-and- presenceof dispersedsolids or crystalsin suspension). This levee morphology. No larger flows were observed with model this might be invoked to explain the majority of crater identifiablemargins. ouflows,which lack visiblelevee structures in Magellanimagery. Baker et al. [1991] and Kargel et al. [1991] introducethe The most quantitativetest available for distinguishingmelt possibilityof exotic melt compositions,including lunar Fe-Ti rock origins from debris origins and perhapsfor distinguishing Mare basalts,komatiite, liquid sulfur,and carbonatite, to explain between melt rock supply scenarios,i.e., modelling of flow their observationson the Venus channels. These workerssuggest rheology,requires reference values of rheologicproperties of lava thatthe same extremely low-viscosity (ofthe order of 10 -3 to 10 -1 and melt rock. The rheologicproperties of melts are strongly Pa s) materialsmay also be responsiblefor the extendedcrater dependenton compositionand temperature,and are difficult to outflows, which they interpret as erosionalchannel features. measureaccurately even under ideal conditions.Values observed Thereis no publisheddiscussion of any quantitativemethod used in the field vary widely amongflows from varioussources, among by Bakeret al. or Kargelet al. to arriveat the viscositiesof the multipleflows from a givensource and at differentpoints along a crater "channels." Their qualitative analysis is explicitly givenflow. Field valuesfor viscosityand yield strengthalso tend dependenton the interpretationthat crater outflows are erosional to be as much as 2 ordersof magnitudelarger than laboratory channels. valuesat the samecomposition and temperature.This impliesthe The relevance of terrestrial measurementsas rheological existenceof time-dependentfactors: continuingcrystal growth at analoguesfor Venusmelts is subjectto question.The variables constantsubliquidus temperature due to nonequilibriumkinetic affecting rheology of an effusive magma are composition, constraints,as well as thixotropicproperties such as progressive temperature,pressure, and effusion rate; the effect of the polymerizationand ordering of undisturbedmelt, will cause difference in each variable between Earth and Venus conditions viscosityand yield strengthto increasewith time [e.g., Fink and needsto be examined. Venus surfacecompositions are very Zimbelman, 1986]. For these reasons, it is generally more poorly constrained,having been measured(by gamma ray instructiveto comparethe propertiesinferred for crateroutflows spectroscopyand X ray fluorescenceexperiments carried by short- with field studies and with remote observation studies than with lived landers)only a few times. Nevertheless,there is no reason laboratoryvalues. There is a universaltendency for bothviscosity to doubtthat the compositionsof mostVenus basalts, which are and yield strengthto increaseas lava cools and therefore to thoughtto constitutethe majorityof surfacerocks, fall withinthe systematicallyincrease in field observationsalong a givenmolten broadrange of compositionsobserved on Earth. Theeffects of the flow with increasingdistance from the vent. differenttemperatures at the surfacesof Earthand Venus are fairly Both viscosityand yield strengthincrease strongly with silica easy to correctfor, as viscositiesare known over a range of content in silicate melts. Laboratory data from Murase and temperatures. Much theoreticalwork has been done in this McBirney[ 1973] andother sources compiled by Ryanand Bierins direction; Head and Wilson [1986] summarizethe differencesin [1987] are shown in Figure 15, along with computedviscosity cooling behavior that would be expectedat Venus and Earth estimatesfor the sampleof 13 Venus crater flows analyzed. surfaceconditions. One variable that has not been sufficiently Errors for crater outflow viscosities are about :el order of allowedfor, however,is pressure. The surfacepressure in the magnitude. The resultsof this comparisonwill be considered plainsof Venus is about 90 atm. This preventsdegassing of below. Field measurements of various terrestrial lava flows are volatiles unless the initial concentration of volatiles in Venus also summarizedin Figure 15; these include values measured magmasis exceptionallyhigh (2-4 wt %). The consequencesof directly with a viscometer[e.g., Shaw et al., 1968] and values thispressure effect on explosivevolcanism have been considered inferredfrom Binghamflow modelsas describedabove [Fink and at somelength [Wilson and Head, 1983],but theremay alsobe a Zimbelman, 1986; Moore, 1987; Hulme, 1974 and references strongeffect on the rheologyof effusive lavas. Sparksand therein]. The generaldiscrepancy between field and laboratory Pinkerton[1978] showedthat gas lossduring fire fountainingin measurements should be noted. terrestrial basalts may be responsiblefor the non-Newtonian Yield strengthsfor 10 terrestrialbasalt flows are summarized propertiesof basalticmagma. Specifically,gas loss causes large in Figure 16. Field data on andesites,rhyolites, and other degreesof undercooling(i.e., it causesthe temperatureof the compositionsare remarkablysparse, but laboratorydata, showing magmato decreaserelative to the liquidustemperature) by (1) viscosities2-3 ordersof magnitudehigher than laboratoryvalues adiabaticdecompression; (2) raisingthe liquidustemperature; and for basalts,suggest that field measurementsof silicic magma (3) forcingthe magmato mix with cool atmosphericgases. This viscositieswould probably be a few ordersof magnitudehigher undercoolingcauses rapid crystal growth, which dramatically than basaltic field values as well. increasesboth yield strengthand viscosity. As notedabove, the Comparisonof flow viscosities.Using techniquessimilar to presenceof dispersedsolids is the primary cause of non- that of this study, several authorshave analyzedvarious lava Newtonianrheology in any liquid. Also, crystallizationleaves an flows on otherterrestrial planets (Mars, Hulme [1976], Zimbelrnan increasinglysilica-rich liquid fraction with a higher viscosity. ASIMOW AND WOOD: FLUID OUTFLOWSFROM VENUS IMPACTCRATERS 13,659

Furthermore, this mechanism affects the entire flow, whereas liquid water, of course,is not a candidatefluid on Venus. Field simplecooling (by radiation,conduction, or convection)affects studies of debris flows (in Pierson and Costa's strict sense) in only the skinof a flow. Thuslava eruptedat Venuspressures, California using Bingham plastic rheology models have arrivedat unableto lose volatiles,will likely have highly retardedcrystal useful estimatesof yield strengthand viscosity. The observed growthand thus will maintainfor sometime a smallerviscosity yield strengths range from 100 to 6000 Pa, and apparent and a muchsmaller yield strengththan the samelava eruptedat viscositiesrange from 50 to 1000 Pa s. It is not possibleto Earthconditions. We are presentlyunable to quantifythis effect convertthese apparent viscosities to Binghamviscosities using the or evaluateits relevancefor superliquidusmelt rocks. publisheddata. It has also been observedthat values of both parameterssystematically decrease with distancealong a flow, Debris Flow Scenarios From •o,,,-,-,-to,•,•,-,• t,.,-,,,;.... [wt,,. ,,, ip?;o ..,.4 ?;..•...;.-.•- , •99•ß aj. Most preliminary work on the Venus crater outflows has This is attributednot to thixotropicproperties of the materialbut implicitly assumeda melt-rockflow origin (e.g., Baker et al. simply to sortingprocesses wherein the lowest-viscosityfraction [ 1991], but see Phillips et al. [ 1991] for a more cautious of material(generally, the finestparticles) travels the farthest. As viewpoint),but this assumptionmay be extremelylimiting. The part of the presentstudy, an analysisidentical in procedureto that possibilityof debrisflow originsneeds to be tested;it is possible appliedto Venus flows was appliedto the large 1917 Sunrise that this will justify the focus on melts a posteriori. A wide Canyondebris flow depositin the Owens Valley. The analysis variety of debris flow processeson several planets has been was donefrom mapsdrawn by Johnsonand Rodine[ 1984], who observed,many of which leave depositssuperficially similar to also performed a rheological analysis of the deposit using the crater outflows on Venus. The term "debris flow" is used here somewhatdifferent techniques (modelling the longitudinalfigure in a very general sense,to describea whole class of diverse of the toe and measuringrunup at bends in the flow). Both processes,any fluidized transportor movementof fragmented. analysesof the SurpriseCanyon flow are presentedin Figures15 solid rock, with or without interstitialfluid (e.g., atmospheric and 16. The results agree to much better than an order of gases,vaporized rock, melted rock, or fines). magnitude.This wasa water-mobilizeddebris flow; the relevance A debris flow with an interstitial fluid of molten rock is to be of the absolutevalues of theserheological parameters to Venus distinguishedfrom a melt-rockflow by (1) the relativeproportions analogsis limited. The observeddownstream trend, however, of solidand melt, with solidsbeing dominant in a debrisflow, and may be characteristicof debris flow processesin the general (2) a solid componentwhich did not crystallizefrom the melt sense. fraction. In the classification scheme of Pierson and Costa Pyroclastic like flows. Pyroclastic flows are extremely [1987], nominally intendedfor differentiationamong types of destructive,rapidly moving massesof hot volcanic gasesand debris flows and debris avalanches,the most general term is entrained fine solids, although they may also transportlarge "sediment-waterflows", which is inapproriatefor Venus, given blocks [Wilson and Head, 1981]. A subclassof pyroclasticflow the absence of water. The most relevant of Pierson and Costa's is the nure ardente,or glowingcloud, which generally consists of subclassesare "debris avalanche" and "sturzstrom", i.e., those a dense basal flow and an overlying low-density cloud. flows sharingthe following five characteristics:(1) a granular Pyroclasticflows are similar in rheologicalcharacter to waterflow regime prevails (i.e., pore fluid pressureis less than mobilizeddebris flow, but they need not containany water, and hydrostaticpressure and the overburdenis carriedby grain-to- their densityis muchlower; they may be usefullydescribed by a graincontact or collisionsrather than buoyancy);(2) sediment Bingham plastic rheology and may generatea recognizable concentrationsapproach 100%; (3) velocitiesare greaterthan 10 channel-and-leveeform deposit. Althoughpyroclastic flows are m s-1' (4) therheology is plastic;and (5) theinterstitial fluid volcanicphenomena, the term "pyroclastic-typeflow" might be consists of air and/or fines and/or water. Pierson and Costa use looselyapplied to a flow phenomenonregardless of its origin. A "debris flow" in a much more restricted sense, to refer to a map of one nure ardentedeposit at Mount Fuegoin Guatemala particularclass of sediment-waterflow with somewhatlower hasbeen published by Davies et al. [1978], and this wasanalyzed sedimentconcentration than debris avalanche,in which air is not as describedearlier. Publishedresults of some analysesof an importantintersitial fluid; this is not the usageintended here. pyroclasticflows are alsoincluded in Figures15 and 16 [Beget Piersonand Costa's classificationscheme was developedwith and Limke, 1988; Limke and Beget, 1986; Wilson and Head, only terrestrialflows in mind, and it needsto be generalized 1981]. The processof emplacementof a pyroclasticflow is somewhatfor our purposes.The (general)definition of debris extremelyattractive as a model for craterflows, from a strictly flow adoptedin this study is intendedto embracepyroclastic qualitative viewpoint' the jetting process or the vapor flows,including nures ardentes, as well asrock avalanches. cloud/fireballassociated with a large impact event on Venus, The rheologicalparameters estimated for a numberof debris interactingwith the atmosphere,might producea ground-hugging flow phenomenaare includedin Figures15 and 16 [Begetand turbiditysurge, flow, or densitycurrent, which would probably Lirnke,1988; Eppler et al., 1987;Limke and Beget,1986; Fink et resemblea pyroclasticflow in manyways. It wouldconsist of a al., 1981; Johnsonand Rodine, 1984; Wilson and Head, 1981' heterogenousensemble of solid rock fragments,melt, rock vapor, Whippleand Zimbelman, 1991 ]. Fourmajor types of debrisflow and atmosphericgases with substantialforward momentum phenomenawill be consideredbelow: (1) debrisflow in the strict [Schultz,1991 b]. We might alsoconsider that if the gasfraction of sense,i.e., water mobilized, includingmudflows [Piersonand a pyroclastic-typeflow consistedof vaporizedrock as well as Costa, 1987]; (2) pyroclastic-typeflow, withoutany implication atmosphericvolatiles, then the flow might convertdownstream of volcanicorigin [Wilsonand Head, 1981]' (3) fluidizedejecta into a melt rock flow or a melt-mobilized debris flow, as the rock [Schultz,1991 b]; and (4) sturzstromor debrisavalanche [Melosh, vapor condensedduring flow. Although the mechanicsand 1979]. rheologyof vapor cloud/solidmixtures in atmospheresare very Water-mobilizeddebris flows. This categoryis includedonly poorlyunderstood, it seemspossible to first orderthat they might because water-mobilized debris flows are common on Earth and be analogousto pyroclasticflows producedby the collapseof have beenthe subjectof severalexcellent rheological analyses; volcaniceruption columns. 13,660 ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPAC'I'CRATERS

Pyroclastic-typeflows could originate by containmentof jetted Melosh[1979], rejectingthe abilityof grainflow to generate materialor vaporcloud material. The mostuseful distinction may the observedrunout distances,proposed a new process:the be that for an obliqueimpact jets are directedboth uprangeand exceptionallyhigh mobilityof theseflows in the absenceof water downrange,whereas the fireball flow travels only downrange or air resultsfrom "acousticfluidization" (see also Schultzand [Schultz,1991a; Melosh, 1988]. Some craters,naturally, have Gault, [1975]). This is a speculativeand controversialidea: ejecta distributionssuggesting near-vertical impacts; many of confirmationor acceptanceof it awaits more research,but it thesealso have outflows, e.g., Aglaonice(Figure la). Jettingmay merits discussionhere as a possible mechanism. Acoustic occurin suchcases, but the directionwould be arbitrary. Fireball fluidization occurs when elastic pressurewaves propagated containment as worked out above would not occur for vertical throughthe solidphase by directgrain-to-grain contact randomly impacts.For obliqueimpacts (a moretypical case than vertical), relievethe overburdenpressure between adjacent grains and allow any depositresulting from jet containmentflow is likely to be them to move [Melosh, 1979]. Acoustic fluidization should seenin two oppositesectors, if it is presentat all. In fact, some operatein a vacuum as well as in an atmosphere.The most cratersdo have two flows, but they are not, in general,precisely appealingfeature of thesemassive avalanche phenomena as an oppositeeach other in point of origin. Also, two flows from analoguefor crater flows is their exceptionalability to retain differentparts of a givencrater are often very different in sizeand kineticenergy over largedistances; after gaining speed and energy appearance,suggesting two differentorigins. Furthermore,most by flowing down a steepgradient, they may continueacross a flat cratershave flows originatingin only one range of directions, region for several tens of kilometers and even climb over whichappears on the basisof ejectapatterns to be within45 ø of obstacleson the orderof a hundredmeters high. the downrangedirection. Also, the fireball involves a much It is observedthat the "friction coefficient" H/L (i.e., the ratio greatermass of materialthan the jets, andfireball deposits would of elevationdrop to horizontalrunout distance) shows a rough likely obliterateany earlier jet deposits.For thesereasons, fireball inversecorrelation with flow volume. Melosh[ 1987]explains this containmentflow appearsto be the bettercandidate for originof correlationas resultingfrom the scale-dependentability of a flow outflowsfrom oblique impacts. Neither jet containmentnor to retainacoustic energy, which is dispersedonly from the surface fireball containmentis a likely processfor outflowsfrom near- of the flow and is thereforeretained more efficientlyin larger verticalimpacts. flows. As this energyis graduallylost during flow, the avalanche Fluidizedejecta etnplacement. Ejecta fluidization is observed slowsand eventuallystops. This oughtto resultin a lava like in severaldiverse settings. Impact craters on Mars often have an increasein rheologicalparameters downstream. In orderto test ejectamorphology suggesting emplacement by flow (Figure2d). the correlation of H/L and volume for Venus crater outflows, it There are flow featuressuggesting sedimentary bedforms (dunes, shouldbe notedthat the parameterH/L is relatedto the average hummocks,radial ridges, etc.) in the ejectaof mostimpact craters slopedown which the flow travels. Slopeand volumeare both on any planet,suggesting at leastsome flow after impact,perhaps determinedin this study;the sampleof crater outflowswhose simplyas a resultof the forward momentumof ejectafragments entirevolumes were measuredare plottedalong with a numberof when they land (ballistic sedimentation;see Melosh [1988] for large debrisavalanches in Figure 17. The slopesunder Venus review). The fluid characterof Martian rampartcraters is much crateroutflows are exceptionallylow, clearly differentfrom the more dramatic. Many theoriesconsider the role of water from trendfor ordinarylarge debris avalanches elsewhere; this implies meltedground ice in the fluidizationof Martian craterejecta, but that energy due to acoustic fluidization would probably be Schultz [1991b] has shown that fine material from the Martian insufficientto transportthe flow overthe observeddistances. The regolithmay act as a fluid in the absenceof water. The fine- other avalanchesplotted, however, obtainedtheir energy only grainedejecta interact with the atmosphereto createa dry debris from gravitational potential, proportionalto the vertical fall flow. This type of ejecta flow, if present,would follow and distanceH. In the caseof Venus crateroutflows, we expectan superposeany fireball vapor-and-debrisflow (seeabove) that may additionallarge nongravitationalenergy source from the impact haveoccurred. Fluidized ejecta flow tendsto producea rampart process,which would be transferred to the flow either by or low ridge at the terminusof the flows; no suchramparts are atmosphericshock wave or seismicwave. Consequently,large observed,but this doesnot rule out their presence.The Magellan volumecrater flows may be expectedto have anomalouslysmall radaris more sensitiveto roughnessthan to topographicrelief, so frictioncoefficients. If the energysupplied by the impactshock low rampartsmight not be resolved. replacesthe early steepfall associatedwith mostlarge avalanches, Debrisavalanches. Extremely large volumeavalanches with then the long runoutdistances can be explainedin spite of the anomalouslylong runoutdistances have beenobserved on Earth, extremelylow slopesobserved. as well as in the Vailes Marinefts on Mars and adjacentto crater We canestimate the magnitudeof the energywhich the impact Tsiolkovskyon the far sideof the Moon (Figure 2c). Hsii [1975] shockwould have to supplyto the flow to providethe necessary and Melosh [1987] defined 1ong-runoutrock avalancheswith a impulsefor acousticfluidization (neglectingdissipation due to volumegreater than 10 6 m3 assturzstroms, to be distinguished atmosphere). Such a calculationmay be usedto testthe feasibility from largelandslides (which move by sliding,i.e., deformationis of impactshock mobilization of sturzstromswith the observed concentratedat the base of the moving mass) by the fluid volumesand friction coefficients. This analysiswas applied only characterof their motion (sturzstroms move by flowing, i.e., to those six craters where the entire outflow deposit was sheardeformation pervades the moving mass). Hsii, notingthat measured,as opposedto the segmentsor subunitsmeasured for exampleshave been seenon the Moon, rejectedmodels based on theother 11 outflows.Using the averageregression line shownin water or mud fluidizationand modelsof slidingon trappedair Figure 17, the difference betweenthe H/L expectedfor the cushionsin favorof a modelbased on Bagnold's [1954]concept measuredflow volume and the observedslope was obtainedfor of dispersiveflow of cohesionlessgrains, in which an interstitial eachoutflow. This wasmultiplied by thetotal length of theflows, massof fine rock debris supportsthe coarsefraction by direct leadingto AH, the apparentmissing fall height for the crater grain-to-graincollisions. outflow. AH was multipliedby the accelerationof gravityto ASlMOWAND WOOD: FLUID OUTFLOWS FROM VENUS IMPAC'r CRATERS 13,661 obtain,A • thespecific energy or energy per unit mass apparently possibleto accountfor the differencebetween the typicalslopes of suppliedto theflows to replacethe missing gravitational potential sturzstromdeposits and the crater flows using an impact energy energyas a sourcefor acousticvibrations. AE canbe compared source. withthe melting and vaporization energies to investigatewhether it is feasibleto acceleratesolid flows by this means. Finally, RESULTS givenestimated flow volumesand an assumeddensity, A E was The averageresults of the rheologicalanalysis for 17 Venus convertedto an estimateof AE, the total nongravitationalenergy crater flows are shownin Table 3. The extremerange of the required.Table 2 comparesthis energy with order-of-magnitude computedvalues of viscosityand yield strengthreflects either a estimatesof the total energy of each impact. These impact significantdiversity of processesand materialsor extremelylarge energiesare obtainedfrom a scalinglaw (craterdiameter •x errors (or both). The averageviscosity estimates are compared E(0'3ñ'05)),calibrated from craters generated by shallowlyburied with the variousreference viscosities in Figure 15. It is clearthat nuclearexplosions of knownyield (reviewed by Melosh[1988] the Venus crater outflow results as well as all the reference values and Glasstoneand Dolan [1977]). Table 2 showsAE, AE, the covermany ordersof magnitudein viscosity. Nevertheless,it is estimatedimpact energy, and the ratio of AE to estimatedimpact possibleto draw someconclusions from the analysis.Most of the energyfor the four crateroutflows considered. The specific averageviscosities of crater flows cluster between 10 2 and10 4 Pa energiesare substantially smaller than the heat needed to remelt s. The observedviscosities are generallylow comparedto field igneousrocks, about 8 x 105J/kg for a basaltinitially at 730øK observationsof most terrestrial and Martian lavas. Some basalts, [BasalticVolcanism Study Project, 1981]. Also, the estimated notablythe 1984 flows from Mauna Loa and the 1966 flows from impactenergies are 104 to 106times greater than the desired Mount Etna, matchmany of the calculatedcrater flow viscosities. nongravitationalflow energies.It thusought to be energetically There are a few flows with low viscositiesin the range of lunar basalts, but there are none requiring nonsilicate exotic

0.8 composition,except the smallestand thereforemost uncertain flow at craterVoynich. Laboratorymeasurements of viscosityon lavasof differentcompositions, each of whichis shownin Figure 15 for the entire range of temperaturesbetween its solidusand 0.6 liquidus,should be usedwith greatcaution, but theyimply thatthe Venus crater outflows have a wide range of silica contentsor temperatures. These results do not conclusiviely indicate composition,but if we weight field data more heavily than 0.4 laboratorydata, we may say that a melt rock flow interpretation favorsa basalticcomposition, at normalsubliquidus temperatures. No conclusionshould be drawnfrom viscosityvalues with respect to debris flows, since the method of calculation assumed lava flow 0.2- T LM coolingbehavior. The averageyield strengthestimates for Venuscrater outflows are comparedwith the variousreference yield strengthsin Figure 16. Theseresults are subjectto muchscatter and ambiguity,but 0.0 I I Iß I' ß I ß I I I 4 6 8 10 12 14 the following observationsseem reasonable. Most of the crater LogVolume, m3 flowyield strengths fall between 102 and10 3 Pa. The_typical yield strengthsof debrisflows tend to be higher than these,but VailesMarineils, Mars (Fig. 2b) thereis muchoverlap. In particular,yield strengthsof pyroclastic Tsiolkovsky,Lunar (Fig. 2c) flows are consistent with those of all but two or three of the crater VenusCrater Outflows (Figs. la, lb, 6, and7, Table 2) outflows. The one yield strength estimate available for a sturzstromis from Chaos Jumbles[ Eppler et al., 1987], 6000 to LM Lunar and Martian best fit, P. Shaller (1991) 10,000 Pa, which is higher than nearly all of the crateroutflow T Terrestrial best fit, P. Shaller (1991) values. This does not rule out avalanchetype origin for the TLM Terrestrial, Lunar, and Martian bestfit, Melosh [1987] outflows, as fluidizing mechanismsin addition to acoustic Figure17. Coefficientof frictionH/L versusvolume for largedebris fluidizationor whatevermobilizes dry avalanchesmay havebeen avalancheson Earth, Moon, and Mars, and for six crater outflows on active. A prominentexample of suchhigh fluidity, citedby Hsii Venus(those listed in Table2). Lightregression lines are from Shaller [1975], is the Huascaran sturzstrom in Peru, which was (personalcommunication, 1991), bold line is from datacollected by exceptionallymobile due to an interstitialfluid of wet mud. This Melosh [ 1987]. maybe analogousto an avalanchefrom a Venuscrater containing

TABLE2. ComparisonofEstimated Flow Energy and Scaled Impact Energy for Six Venus Crater Outflows. CraterName AE, J/kl• fie J ImpactYield, J fie/Yield Aglaonice 2x 10 5 1019 1023 104 Banzhao 7x 104 8x 1016 2x 1022 4x 104 Danilova 105 1018 4 x 1022 4 x 10-5 Dickinson 2x 105 6x 1017 1023 5x 104 Kemble 105 1017 2x 1021 5x 10-5 Parra 9x 104 1018 3x 1022 4 x 10-5 Variablesexplained in text. 13,662 ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS

TABLE 3. Resultsof BinghamPlastic Fow Modellingfor 17 VenusCrater Outflows, Two VenusLava Flows, One Terrestrial Nu6e Ardente and One Terrestrial Debris Flow Flow Mean Width, Mean Levee Mean Central Mean Yield Mean Viscosity, TotalVolume, km 3 km Width,km Depth,m Strength,Pa Pas 17 Crater Ou#lows Aglaonice* 27.3 2.6 16 220 3600 24 Andami 4.3 1.3 13 1200 2200 1.0 Banzhao * 4_5 13 6.2 200 73 0.45 Danilova * 13.6 1,5 16 440 11000 4.5 Dickinson * 10.8 1A 6.1 78 120 1.6 EdgeworthSouth 2.9 0.9 5.7 250 200 0.17 EdgeworthSouthwest 23 0.9 10 900 550 0.28 Kemble * 3A 1.1 8.2 450 400 0.39 Lafayette 8_5 1.2 21 1200 97000 4.3 Luce 7.1 1.2 12 470 7200 1.1 Macdonald 3.1 0.9 13 1200 2200 0.95 Parra * 5.7 1.9 39 6100 100000 4.9 Potanina East 6_5 1.1 10 380 2000 0.51 Potanina Northeast 5_5 0.7 4.5 83 95 0.34 Voynich 1.1 0.4 2.0 77 0.28 0.0095 XantippeNorthwest 7.8 2.6 8.6 210 70 0.62 XantippeSouthwest 15.6 2.3 4.1 24 11 0.86 Two Venus Lava Flows 19N080 flow 1 1.9 0.7 18 3600 7000 0.36 19N080 flow 2 8.6 3.2 12 380 140 1.9 Two Earth Debris Flows Fuegonu6e ardente * 0.029 0.0067 2.1 3000 5.3 SunriseCan),on * 0.050 0.0180 3.8 6800 86 ß Completeoutflow. All othersare segments of flow withmeasurable levees. rockvapor or melt. The yield strengthsare alsosomewhat lower trendsin both viscosityand yield strengthare taken to indicate thantypical values for observedlava flows, particularly silicic and "lavalike behavior" and downward trends in both parameters Martian flows. There is reasonable agreement between the define "debrislikebehavior." Among the 17 Venus flows, there calculatedyield strengthsand thoseof 1984 Mauna Loa basalts are 10 fairly unambiguous debris flowlike trends, one (300-3000 Pa) and Mare Imbrium flows (near 400 Pa). unidentifiabletrend, and six lavaliketrends. None of the upward Laboratorydata on yield strengthvary over a wide rangeas a trendsis as strongas would be expectedfor melt, e.g., at Mauna functionof temperature;near or abovethe liquidusyield strength Loa in 1984, where Bingham viscosity increased3 ordersof is essentiallyzero, and near or below the solidusthe yield magnitudefrom ventto toe. Thereis the possibilityof significant strengthsmay be of the orderof severalmegapascals. Thus the error in the use of trends,since assumptionslike constantslope laboratoryresults are of little use becausewe can match a and density may not representthe actual situation. Thus an calculatedyield strengthto nearlyany rock type by choosingan outflow which travelled down a concave(steadily decreasing) appropriatetemperature. Data quotedin McBirneyand Murase slopewould showan erroneousupward trend in yield strengths [1984] imply temperaturesaround 1175øCto match the yield computedon the basisof constantslope. This typeof errorcould strengthfor basaltsand andesites, or around1050 øCto matchMt. be corrected by using a variable slope if higher-resolution St. Helensdacite. Tentativeconclusions from yield strengthdata, topographydata were available. The observationon downstream relyingon field analyses,are (1) sturzstromand pyroclastic-type trends lead to two conclusions:(1) debris flowlike behavior is processesare not excludedfor mostflows, although an additional confirmedfor mostof the outflows,although (2) meltrock origin fluidizationagent may be required,and (2) a melt rock origin is impliedfor six of the 17 outflows. indicatesproperties broadly similar to basalt. Some insightsmay be gained by searchingfor correlations The apparentchanges in viscosityand yield strengthalong amongvarious measured and/or calculated variables. Both Hulme Venus crater outflows from sourceto terminusmay be important [1974] and Head and Wilson [1986] publishedplots of viscosity in differentiating between melt and debris flow processes. versusyield strengthfor Binghamplastic models of silicatelavas. Assumingconstant effusion rate alongthe flow, thesetrends are The Hulme relationis for averageproperties of flows of different meaningfulwhether the actual effusionrate is known or not. silicacontents, whereas the Head and Wilson curveis for a given There is considerablenoise in all of the profiles, but Table 4 material,anhydrous tholeiite basalt,at varioustemperatures. The summarizesthe trendsapparent for all 17 outflows. An up arrow 17 craterflows, separatedinto lavalike and debrislikeflows, are represents overall increase in viscosity or yield strength shown in viscosityversus yield strengthspace along with the downstream,a down arrow representsan overalldecrease, and a Hulmeline in Figure18. It is seenthat eight of theflows plot on dash indicatesa mixed, flat, or unclear trend. or nearthe Hulme line, oneplots well belowit, andeight plot well Recall that melt and acousticallyfluidized debris are expected aboveit andthat all 17 pointsclearly define a trendparallel to the to showincreases in bothviscosity and yield strengthdownstream Hulmeline. The paralleltrend may be a resultof thefact thatthe and that debris mobilized by an interstitial fluid may show samemodel was usedby Hulme and by this study,in which decreasesin both parameters. For each crateroutflow, upward viscosityand yield strength are not fully independentparameters. ASIMOWAND WOOD:FLUID OUTFLOWSFROM VENUS IMPACT CRATERS 13,663

It should be noted (Table 4) that of the six Venus flows with flow and may reasonablybe a function of the slope or other lavalike downstreamtrends, four plot on Hulme's line for silicate elements of the flow environment. In other words, an avalanche melts, whereas of the 10 outflows with debrislike downstream may flow down a 30ø slopeby a differentmechanism and with a trends,seven plot well away from Hulme'sline. Keepingin mind different apparent rheology than an avalanche of the same that the viscosityestimates are not meaningfulfor debrisflows, it materialand sizeon a 10ø slope. The existenceof this correlation is reasonableto supposethat plotting well off Hulme's line is may thus be construedeither as evidenceof a major flaw in the indicativeof a nonmeltorigin or an exoticmelt composition. modelor of debrisflow originsfor mostof the modelleddeposits. Both viscosityand yield strengthshow direct correlationswith There is a set of additionalqualitative tests that shouldbe the measuredslope. This is true for the data in this studyas well applied to any model for crater flow formation. A leading as the resultsof many analysesby other authorson a variety of observationto be explainedis the azimuth of the flows, with natural flows [e.g., Johnsonand Rodine, 1984]. This result is respectto the impactdirection for apparentlyoblique impacts and somewhatunsettling at first. The propertiesof a materialought to with respectto local topography,as well as the narrowlyfocused be independentof the slopedown which the materialflows. This appearanceof some flows and the broad splay of other flows. correlationthus seems to imply a seriousfailure of the modelused Althoughat first glancethe fireball containmentmodel seemsto to obtainthe materialproperties. This objectionis probablyvalid imply a narrowflow orienteddirectly downrange, P. H. Schultz for melt rock flows: the rheologic properties of melts are (personal communication, 1991) believes that the initial essentiallyintrinsic material properties, dependent on temperature, momentumfrom the impact is likely to transportthe flow only composition,pressure, and time. Debris flows, by contrast,have one or two crater radii straight downrangefrom the point of no intrinsicmaterial rheology; their behavior develops only during impact, to a point which will eventuallybe beneaththe ejecta blanket. Beyondthis, the cloud may separateinto vaporand melt phases,and the directionof subsequentflow of the vapor phase will be controlledby local topography[Schultz, 1992]. This is consistentwith the "tangential"appearance of flows at crater Andami Danilova ('")• Carson(Figure l c) and the uphill point of origin coupledwith • 10000 Aglaon•)e(•Lucc• ' subsequentdownhill flow apparenton two outflowsat craterParra l'oa acdona!, (Figure 6) on the flank of a topographichigh associatedwith a EdgeworthS Dickinso EdgeworthSW nearbyvolcano. The freshappearance of the craterimplies that it probablypostdates the volcanoand the topographichigh and so impactedupon approximatelythe presenttopography; there is evidence of extensive lava effusion from the volcano, but it does not bury or embaythe crater. The appearanceof the flows from Parrais stronglyinconsistent, on theother hand, with the modelof Voynich Bakeret al. [1991] in which melt simplydrains from ejectaafter 0.01 emplacement. This model would predict drainage on the Yield strength (Pa) downslopeside of the crater,unless the topographicslope was reversed when the crater formed, in which case the U-tums seen in Figure18. Viscosityversus yield strengthfor 17 Venuscrater outflows. Figure 6 are difficult to explain. By contrast,the flows from Circles are debris-likeflows, squaresare lavalike flows (see Table 4). Aglaonice(Figure 8), with broadsource regions at the foot of the Line showsestimated locus of valuesfor silicatemelts of varying silica contant [Hulme, 1974]. continuousejecta blanket are moreconsistent with Baker's model.

TABLE 4. DownstreamLongitudinal Trends in Viscosityand Yield Strengthand Position With Respectto Hulme's[ 1974] Relation Between Viscosity and Yield Strength for Silicate Mammas for 17 Venus Crater Outflows Name of Flow Downstream Trends Position Relative to ...... Viscosity YieldStrength Conclusion Hulme'sTrend Aglaonice - ,l, debris abovetrend Andami 'l' ,l, ? ontrend Banzhao '1' - lava ontrend Danilova ,l, ,l, debris abovetrend Dickinson ,1, ,l, debris abovetrend EdgeworthSouth 'l' 'l' lava ontrend EdgeworthSouthwest - ,l, debris neartrend Kemble - •' lava ontrend Lafayette - • lava abovetrend Luce 'l' 'l' lava abovetrend Macdonald '1' - lava ontrend Parra ,l, ,l, debris ontrend PotaninaEast ,[, - debris abovetrend PotaninaNortheast ,l, ,[, debris abovetrend Voynich ,l, ,[, debris belowtrend XantippeNorthwest ,l, ,l, debris ontrend XantippeSouthwest ,[, ,l, debris abovetrend 13,664 ASIMOW AND WOOD: FLUID OUTFLOWS FROM VENUS IMPACT CRATERS

Thereis someevidence, in thetopographic data for th6 regions absenceon otherplanets except perhaps for Earth,whose record •s of the craterflows, of local uphill motion and surmountingof rapidlyobscured. Such an explanationis availablefor bothmelt obstacles.Although the resolutionof the topographyis very poor rock flow and debrisflow hypotheses,respectively: (1) ratesof for small-scaleanalyses, and thus this observationis uncertain, impactmelt productionare higheron Venusdue to highersurface such behavior is consistent with the pyroclastic-type and temperatureand impact velocity, and melt separationprocesses sturzstrommodels, and inconsistentwith melt rock flow models. are more efficientdue to highersurface temperature and pressure The presenceof smallshield volcanoes with 50 to 100 m relief and larger ejectaparticle sizes;and (2) interactionwith a dense which are not overtoppedby the deposit(Figure 1a), however, atmosphereis requiredto contain the vapor cloud in a ground- impliesa very low, uninflatedcharacter during flow. Inflated huggingturbidity flow. Both these explanationsare subjectto behavior would clearly be expected of nu6e ardente-style furtherexperimental and theoretical testing. pyroclasticflows, and perhaps of fireballcontainment flow aswell (at leastbefore vapor melt phaseseparation). The appearanceof Acknowledgments.The authorswish to thankBrennan Klose, thesedomes in someflows, then,favors the sturzstrom-typedebris AkihikoHasimoto, Beth Holmberg,Craig Leff, andPeter Schultz model or the melt flow models for these flows. It seems for discussionsand technicalhelp. We are indebtedto Phillip inconsistentfor sturzstromsto flow up slopes but not bury Shallerand Robert Herrick for providingunpublished data. This obstaclesof equal or lesser height, but both behaviors are work was supportedby JPL contract958593. consistentwith observationsor terrestrialexamples [Hsii, 1985]. The flow only goesup a topographicobstacle if it cannotflow around it: thus the Elm flow in Switzerland climbed the walls of a REFERENCES valley,but it envelopeda manup to the neckwithout burying (or Bagnold,R.A., The Physicsof Blown Sand and Desert Dunes,Methuen, crushing)him. London, 1954. Baker, V.R., G. Komatsu, V.C. Gulick, and T.J. Parker, Channels on CONCLUSIONS Venus: An overview, Lunar Planet. Sci., XXII, 4546, 1991. BasalticVolcanism Study Project, Basaltic Volcanismon the Terrestrial The outflow featuresassociated with impact craterejecta on Planets, Pergamon,New York, 1981. Venus are very diverse. Some are apparentlychannel-like and Basilevsky,A.T., Cleopatra crater on Venus: Happy solutionof the erosive in nature, with streamlinedislands and a large drainage volcanicvs. impactcrater controversy, Lunar Planet. Sci., XXII, 59-60, 1991. region(Figure 13). Most,however, are apparently depositional in Beget,J.E., and A.J. Limke, Two-dimensionalkinematic and rheological nature. Somedeposits are very broadand poorlyorganized, with modeling of the 1912 pyroclastic flow, Katmai, Alaska, little structureapparent in radar imagery,while othersare well Bull. Volcanol., 50, 148-160, 1988. channelized,with clearmargins or levees. The depositionalflows Bingham,E.C., Fluidi.tyand Plastici.ty,McGraw-Hill, New York, 1922. studiedherein display a variety of sourceregion and downflow Cattermole,P., Sequence,Rheological Properties, and EffusionRates of Volcanic Flows at Alba Patera, Mars, Proc. Lunar Planet. Sci. Conf. morphologies,as well as diverse relationshipsto topography. 17th,Part 2, J. Geophys.Res., 92, suppl.,E553-E560, 1987. Modelling the rheologicalproperties of the flows resultsin a Davies,D.K., M.W. Quearry,and S.B. Bonis, Glowing avalanchesfrom broadrange of yield strengthand Bingham viscosity estimates and the 1974 eruptionof the Volcano Fuego,Guatemala, Geol. Soc. Am. in downstreamtrends in theseparameters. It would probablybe Bull., 89, 369-384, 1978. misleadingto seek a single explanationfor all these deposits. Eppler, D.B., J.H. Fink, and R. Fletcher, Rheologic propertiesand kinematicsof emplacementof the ChaosJumbles rockfall avalanche, Thereis, in fact, evidencefor at leasttwo differentprocesses: LassenVolcanic National Park, California, J. Geophys.Res., 92, 3623- 1. One processfollows emplacementof the ejecta and 3633, 1987. modifiesthe continuousejecta blanket. This processis most Fink, J.H., and J.R. Zimbelman, Rheologyof the 1983 Royal Gardens likely to involvesegregation of a basalticmelt fractionfrom the basalt flows, Kilauea Volcano, Hawaii, Bull. Volcanol., 48, 87-96, 1986. ejectaitself. Debrisflow scenariosare not excluded,however, as Fink, J.H., M.C. Malin, R.E. D'AIIi, and R. Greeley, Rheological the ejectaare emplacedwith substantialmomentum and internal propertiesof mudflowsassociated with the spring1980 eruptionsof energy. The processesof ballisticsedimentation, slope failure, Mount St. Helensvolcano, Washington, Geophys. Res. Lett., 8,43-46, and acoustic fluidization may combine to initiate mass 1981. movementsand lengthy outflows. Glasstone,S., and P.J. Dolan, The Effects of Nuclear Weapons,U.S. GovernmentPrinting Office, WashingtonD.C., 1977. 2. A depositionalprocess preceding ejecta emplacement, and Head, J.W., Ill, and L. Wilson, Volcanic processesand landformson stratigraphicallyunderlying the ejectablanket, must be associated Venus: Theory, predictions,and observations,J. Geophys.Res., 91, with early stagesof the impactevent (jettingand vaporcloud), 9407-9446, 1986. whichexpel basaltic magma, fluidized solid debris, or mostlikely Hulme, G., The interpretationof lava flow morphology,Geophys. J. R. a mixtureof molten,solid, and vaporized debris, whose movement Astron. Soc, 39,361-383, 1974. 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