Adv. Space Ret. Vol. 10, No. 1. (1)183—(1)186, 1990. 0273—1177/90 $0.00 + .50Printed in GreatBritain. All rights reserved. Copyright© 1989 COSPAR

THE GEOLOGIC EVOLUTION OFGANYMEDE AND ITS IMPLICATIONS FORTHE ORIGIN OF THE GANYMEDE-CALLISTO “DICHOTOMY”

Scott L. Murchie

Departmentof Geological Sciences,Brown University, Providence,RI 02912, U.S.A.

ABSTRACT

Old dark terrains on both Ganymedeand Callisto were affected by similar endogenicprocesses,volcanic and (onGanymede) extensional tectonic modification of multiringed impact structures. The two bodies’ evolutionssubsequentlydivergedwhen younger grooved and light terrains formed only on Ganyrnede. On the basis of themorphologyand geometryof Ganymede’stectonic features,it is inferred that this divergenceoccurredas the interiorof that satellitewarmedfrom an initially cold, undifferentiatedstate. At least two paradigmsof the origin of thedivergenceare Consistentwith the interpretationsand inferencessummarized herein: Ganymedepossessedan“accretionaltrigger” which Callisto did not, or Ganymede’sstyle of mantle convectionchangedradically as thesatellitewarmedbut Callisto’s style of convectiondid not.

INTRODUCTION

One importantgeologicquestionconcerningtheGalilean satellitesystemis the reasonfor thedistinctappearancesofthe surfacesof Ganymedeand Callisto. The two satellitesare similar in density and radius,althoughGanymedeisslightly larger and denser,and both possesslarge areasof heavily cratered “dark terrain.” However, unlikeCallisto, half of Ganymede’ssurface consistsof younger “grooved terrain” typically resurfacedby higher-albedo“light material” /1,2/ that is old enoughto be significantly cratered. This difference in the satellites’ geology iscommonly known as the “Ganymede-Callistodichotomy” /e.g. 3/. Most investigationsof the origin of the dichotomyhaveinvolved theoreticalmodelsof the origin of light groovedterrain/4, .5,6/ or observationalconstraint,son its origin/3/. In this paper,inferenceson the origin of the dichotomyare drawn from analysisof the geologyof the satellites’dark terrainand from a comparisonof it to the geologyof Ganymede’sgroovedterrain.

THE GEOLOGYOF GROOVED TERRAIN ON GANYMEDE

Groovedterrainconsistsof a global network of linear troughs or “grooves” averagingabout4 km in width. Thegrooves occur in close associationwith light resurfacingmaterial thought to have been emplacedvolcanically,burying older dark terrain to a depthof one to severalkilometers /1,2,7,8,9,10,11/. Four morphologic types ofgroovedterrain have been recognized/11,12/: (a) throughgoingsingle groovesor bandsof grooves (“groove lanes”)that outline polygonal blocks of lithosphere;(b) domains of regularly spaced,parallel grooves occurring in thepolygonal blocks (“grooved polygons”); (c) domainsof orthogonalgroove setsalsooccurring in the polygonal blocks(“reticulateterrain”); and (d) groovedterrainpossessingcomplexly intersectinggrooves(“complex groovedterrain”).Using observationsof structural and stratigraphicrelationsas well as crater-densitymeasurements,Murchie eta!./11/ defineda general three.stagesequenceof events that appearsto be applicableto groovedterrain formationacrosswide areas:first, dissectionof dark terrain by throughgoingfaults and grooves,accompaniedby reticulateterrainformation in the interveninglithosphericblocks; second,light-material volcanic resurfacingand grooving ofmanyof the interveningblocks, forming groovedpolygons;andthird, repeateddeformationand volcanicresurfacingof groove lanesand fewer groovedpolygons,concentratedalong the initial fracturezonesdue to their reactivation.Groovewidth reacheda minimum duringtheformationof groovedpolygons.

The organizationand morphologyof groovedterrain provide important clues about the terrain’s formation. Thestructureof groovedterrainpossessesorganizationboth on a global scale,expressedasa dominantgroove-lanetrend/13,14/, and on a hundreds-of-kilometersscaleas “superdomains”possessingregionally coherentgrooveorientations/14/. Parmentieret al. /1.5/ showedthat truncationrelationsof cratersagainstgroovelanes areConsistentwith thelanes having originatedas volcanically flooded, rift-like depressions. In addition, Murchie et al. /11/ summarizedevidencethat groove lane morphologyand developmentare consistentwith that expectedfor passiverifts. Theglobal occurrenceof grooves/1,2/, the interpretationthat individual groovesare extensionalfeaturesie.g. 1.5/, thegeneral lack of evidence for associatedcompressionalfeatures/16/, and the interpretation that groove lanesarepassiverifts togethersuggestthat groovedterraindeformationwasdriven by globally occurringtensionalstress.

THE GEOLOGY OF DARK TERRAIN ON GANYMEDE ANDCALLISTO

The structureof dark terrain on Ganymedeis dominatedby two hemispheric-scalesystemsof concentricallyandfewer radially arranged“furrows” 6-10 km in width, one in each the sub-Jovianand anti-Jovian hemisphere

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(1)184 S. L. Murchie

/1,2,17,18/. The systemin the anti-Jovianhemisphereis arrangedarounda faint, giant “palimpsest” /8,19/, a typeof featuregenerallyinterpretedto be a degradedor modified impact structureIe.g. 19,20/. Furrows occurring indifferent regions have significantly different crater ages,yet they predatenearlyall largecraters/2 1,22/. In somelocations,depositsof dark material emanatedfrom the furrows as volcanic fluids; in other locations,such depositsbury older furrows but arecrosscutby morphologicallysimilar younger furrows/21,23,24/. Thesegeologic relationshave beeninterpreted/21/ to resultFrom formationof concentricand radial fracturezonesby large impacts,followedby dark-materialvolcanismduring which the fracturezoneswere buried and then were reactivatedby endogenicextensionaltectonism. Two speculativemodelsfor the origin of thelower albedoof dark material than light materialhave been proposed:first, entrainmentof a larger silicate fraction in dark material; second, ablation of a largefractionof ammoniahydratefrom dark material, leavinga silicate-rich lag /25 and referencestherein].

A third furrow system, consistingof radially arrangedtroughs, extends from its “center of symmetry” near25°S,122°Wto at least 60% of the distance to the antipode. The region around the center of the system ischaracterizedby the densestoccurrenceof furrows belonging to the two other systems,a concentrationof dark,smooth volcanic materials, and the globally youngestand thickest deposits of dark material ~21,22/. Theseobservationswere comparedto severalmodelsof furrow formation /21/, and were found to be bestexplainedbyfracturing and volcanism on a circular, hemispheric-scale,thermal uplift in an environmentof global tensionalstress.

Callisto’s surfacelacks a radial furrow systemlike that occurringon Ganymede,but other aspectsof the satellites’dark terrain geology are rather similar. For example, Callisto also possessessystemsof concentric and radialfracture zonesarrangedarounddegradedpalimpsests/1,19,26/,and dark material emanatedfrom these fracturezonesas well /27/. However, Callisto’sdark volcanicdepositsare muchless extensivethanGanymede’s,and thereis no clearevidencefor extensionaltectonic reactivationof theCallistoanfracturezones. Thus the geologicevolutionof dark terrain on both satellitesappearsto have includedendogenicmodification of multiringed impact structures,which was more intenseon Ganymede. The Ganymede-Callisto“dichotomy” may be more preciselydescribedas adivergenceof the satellites’evolutionswhen light andgroovedterrainsbeganto form on Ganymede.

INFERENCES ON THE ORIGIN OF THE DIVERGENCE

Five aspectsof the geologyof GanymedeandCallisto provide cluesto origin of the satellites’divergencein geologicevolution. First, dark terrainson both satellitesappearto have beenaffectedby similar geologic processes.Second.virtually all observedtectonic featureson Ganymedeare troughs interpretedto be of endogenicextensionalorigin.There is someevidencefor sheardeformationassociatedwith groovedterrain, but ridgesof possiblecompressionalorigin are uncommonand are restricted in occurrenceto small areasof groovedterrain /18,21/. Such pervasive,global extensionaldeformation could plausibly have been driven only by planetaryexpansion.both before thedivergencewhen furrow formation was organizedon a hemisphericscale,and after the divergencewhen groovedterrainformationwas organizedon a regional scale.

Third, GolombekandBanerdt /28/ havederiveda relationshipof the width of tectonic troughs on Ganymedeto thelithosphericthermal gradient. If the strainratesduringfurrow andgroove formationwere about 10- ~ s ‘ , thenthe 6-10-kmwidth of furrows indicatesa lithosphericthermal gradientof 2’-6” km , andthe 4-km averagewidthof younger grooves indicates a thermal gradient of 6~-15”km- . The thermal gradient peakedduring theintermediatestage of grooved terrain formation, when groove width reachedits minimum. If the lithosphericthermal gradient is assumedto be an indicator of upper-mantletemperature,then it may be inferred thatGanymede’stectonic troughs formed while the satellite underwentprolonged internal warming, reachedits peaktemperature,and then beganto cool. This is consistentwith the global dominanceof extensionaltectonics,which isexpectedto have resultedfrom warmingof an initially cold interior /29/. Global thermal expansionis a suitabledriving mechanismfor extensionaltectonics,becausesufficient stressfor deformation of prefractured ice wouldaccumulatein only 1-3 x 10~yrs /29/, comparableto or less than the Maxwell time for viscous stressreliefestimatedfrom observedtopographicrelaxationof dark terraincraters(10k .10~yrs) /21/.

Fourth, growth of a single,circular, hemispheric-scale,thermaluplift on Ganymedeis not a predictedresultof tidaldeformation or of simple global expansion,but could have resulted from convection of a warming mantle. Aconvectioncell of order 1 1 would possessone axisymmetricupwelling currentand anantipodaldescendingcurrent.Theoretical studies/30/ suggestthat a planetarybody possessingsucha cell could develop a circular region ofanomalouslywarm mantleas the interior underwentprolonged radiogenicwarming, becausethe warmestmantlematerial would becomeconcentratedin the upwelling currentasconvectionbecamemore vigorous. Radial fracturingof the lithospherearrangedarounda regionof concentratedvolcanismandtectonism,as is observedin dark terrain,is a plausible result of thermal uplift over this warm mantle combined with the accompanyingglobal thermalexpansion121/. Occurrenceof an 1 1 convectioncell in a planetarybody is thought to require the radius of anyconvectively isolated (e.g. silicate-rich) core to be �0.27 times the global radius /30/, implying a Ganymedeaninterior in which silicates and ices were � 2-3% differentiated. Such an interior would have been initially cool,consistentwith the independentlyinferred global thermalhistory.

Fifth, Callisto is smallerand less massivethanis Ganymede.If Ganymedeescapedlarge-scaledifferentiationduetoaccretionalheating,then Callistomusthave as well if thesatellitesaccretedundersimilar conditions.

SCENARIOSFOR DIVERGENCE

Ideally, paradigmsfor thedivergenceof Ganymede’sevolution from that of Callisto should accountfor or refute theinterpretationsand inferenceslisted above. Models assumingan initially warm, differentiated interior ;e.g. 6,8/cannot account for them, because they predict compressionalstressesin the lithosphereduring dark terrain

Geologic Evolutionof Ganymede (1)185

formation andbecausethey imply internalcompositionallayering inconsistentwith an l~1 convectioncell. At leasttwo paradigmscan accountfor theseinterpretationsand inferences.

First, Gariytnedemay have possessedan “accretional trigger” that Callisto did not i’cf. 31/. In this scenarioGanymedeunderwent2-3% accretionaldifferentiation, forming a clean-iceuppermantle about20 km in thicknesswhosesilicateswere segregatedinto a small core. The upper mantlewas too thin to convect,so it transportedheatonly by conduction. Both satellites underwentinternal warming and global thermal expansiondue to radiogenicheating,generatingextensionaltectonicson Ganymedeas well as dark-terrain igneousactivity on both satellites,due to mobilization of a low melting-pointcompound/25/. Callisto developeda thin thermal lithosphere,and itsinternal temperatureneverreachedthe melting point of H, 0-ice; Ganymede’snon-convectinguppermantle imposeda thermal boundarylayerthicker than Callisto’s, driving thetemperatureat a tens-of-kilometersdepthto the H, 0-ice melting point. Eruptions of H, 0-rich melt resurfaced light terrain, and silicate-rich, H, 0-depletedmaterialfounderedto the core, displacingthe denseH, 0 phasesice-VII and ice-VIlI and driving a new episodeof globalexpansion. Core growth disruptedan earlier l 1 convection cell and a higher-orderpattern developed. Globalexpansionand stressesover multiple mantleconvectioncells, probablyconcentratedin the thinned lithosphereoverupwelling currents, drove regional-scalegrooved terrain deformation /cf. 32’. Both satellites reachedtheir peaktemperaturesand beganto cool, creatingcompressionin the lithospherethat haltedextensionaltectonics andclosedvolcanic eruptionconduits /cf. 33/.

Second,the divergencemay have resultedfrom the responseof H, 0-ice phasetransitionsto convection. Ongoingstudies of Convection in Ganymede/34,35/ suggestthat whole-mantleconvection may occur in a cool interior,whereastwo convectinglayers separatedby the endothermicice-Il/ice-V or ice.IIIice.VI phasetransition may occurin a warmer interior. In a paradigmwhosetheoretical validity is still being assessed,both satellitesaccretedcoldand undifferentiated. They both underwentradiogenicwarming and global thermal expansion,driving extensionaltectonics on Ganymedeand dark-terrain volcanism on both satellites, the latter by eruption of low-temperaturepartial melt. Subsequently,greaterradiogenicor tidal heatingof Ganymedeled to theonsetof layeredconvectioninthat satellite. Developmentof the resulting mid-mantlethermal boundarylayer within Ganymedecausedwarmingand expansionof its lower mantle, driving the mantle thermal profile to near the H, 0-ice melting curve. Bothextensionalgroovedterraindeformationand light-material volcanismresulted,and were concentratedover multiple,regional-scaleconvectiveupwellings. After peakinternal temperatureswere reached,global cooling andcompressionhaltedextensionaltectonicsandclosedvolcaniceruptionconduits.

Each of theseparadigmsmakes predictions for Ganymedethat can be testedusing experimentsplannedfor theGalileo mission, including radio trackingof the craft, spectrometry,and high resolution imaging. Both paradigmspredict the global gravitational moments expected for nearly undifferentiated interiors (J

2 = 1.3 x 10- ‘ forGanymedeand 1.7 x 10-i for Callisto /31/). However, the first paradigm predicts that craters Formed prior togrooved terrain would have excavated differentiated upper-mantle material from shallower depths andundifferentiated ice-silicate from greater depths. The secondparadigm predicts excavation of undifferentiatedmaterial from all depthsbeneatha thin volcanic cover. In addition, the first paradigmpredictsnegativediapirs thatoriginatedfrom a shallowdepthduring groovedterrain formation, possiblyaccompaniedby surfacedeformation;thesecondparadigmmakesno suchprediction. Testingof theseas well as otherpredictionsof the two paradigmsmayallow some resolution of whetherone or neither of the paradigmsis applicableto the origin of Ganymede’sandCallisto’s distinctivegeologicevolutions.

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35. C. Sotin, privatecommunication(1988)

36. C. Sotin, J. Head, andJ. Lunine providedhelpful reviewsof this manuscript. This work was supportedbyNASA GrantNAGW-137 andthe William F. Marlar Memorial Foundation.