Mechanisms for Lithospheric Heat Transport on Venus Implications For

Home , Geology of Venus, Mantle (geology)

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. Btt, PAGES 9236-9246,NOVEMBER t0, 1982

Mechanismsfor Lithospheric Heat Transport on Venus' Implications for Tectonic Style and Volcanism

SEAN C. SOLOMON

Departmentof Earth and PlanetarySciences, Massachusetts Institute of Technology Cambridge,Massachusetts 02139

JAMES W. HEAD

Departmentof GeologicalSciences, Brown University,Providence, Rhode Island 02912

The tectonicand volcaniccharacteristics of the surfaceof Venus are poorly known, but thesecharacter- isticsmust be closelyrelated to the mechanismby which Venus rids itself of internal heat. On the other solidplanets and satellites of thesolar system, lithospheric heat [ransport is dominatedby oneof three mechanisms:(1) plate recycling,(2) lithosphericconduction, and (3) hot spot volcanism.We evaluateeach mechanismas a candidatefor the dominantmode of lithosphericheat transferon Venus,and we explore the implicationsof eachmechanism for the interpretationof Venussurface features. Despite claims made to the contrary in the literature,plate recyclingon Venus cannot be excludedon the basisof either theoreticalarguments or presentobservations on topographyand radar backscatter.Landforms resulting from plate convergenceand divergenceon Venus would differ substantiallyfrom those on the earth becauseof the high surfacetemperature and the absenceof oceanson Venus,the lack of free or hydrated water in subductedmaterial, the possibilitythat subductionwould more commonlybe accompaniedby lithosphericdelamination, and the rapid spreadingrates that would be requiredif plate recyclingremoves a significantfraction of the internal heat. If plate recyclingoccurs on Venus,the rolling plainsand lowlands provinceswould be approximateanalogs to terrestrialocean basins in termsof age,igneous rock type,and formativeprocess; highlands on Venuswould be roughlyanalogous to terrestrialcontinents. The hypothesisthat lithosphericconduction dominates shallow heat transferon Venusleads to the predictionthat the lithosphereis thin. If Venus has a global heat lossper massequal to that for earth, then temperatures marking the baseof the thermal lithosphereon earth would be reachedon Venus at an averagedepth of about 40 km. Unlessthe mantleconvective planform can maintainlithospheric regions of persistentlylow heat flow or unlessthe presentatmospheric greenhouse on Venus is geologicallyrecent, then such a lithosphericthickness leads to the conclusionthat the topographicfeatures contributing to the 13 km of relief on the planet must be geologicallyyoung. The hypothesisthat hot spot volcanismdominates lithosphericheat transferon Venusleads to the predictionthat the surfacemust be coveredwith numerous activevolcanic sources. In particular,if a typical Venushot spot has a volcanicflux equal to the average flux for the Hawaiianhot spotfor the last40 m.y.,then 10 '• suchhot spotsare necessaryto removethe Venusinternal heat by volcanism.Such a numberwould produceenough volcanic material to resurfacethe entire planet to a depth of 1 km every 2 m.y.; few areas of the planet would escaperesurfacing for geologicallylong periodsof time. We find that none of the mechanismsfor lithosphericheat transferon Venus can be excludedas unimportant at present; it is likely that, as on earth, a combination of mechanismsoperates on Venus.The strongestconclusion to emergefrom this evaluationis that most of the major topographicfeatures and probablymany of the surfacegeological units on Venusare youngby comparisonwith the surfacesof the smallerterrestrial planets.

INTRODUCTION mon, 1981]. For Jupiter'smoon Io, eruptions at individual The mechanismby which a solid planet transportsheat volcaniccenters, or 'hot spots,'dominate the lithosphericheat acrossthe outer 100 km of its interior playsa pivotal role in flux [Matson et al., 1981; Reynoldset al., 1980; O'Reilly and determiningthe stylesand magnitudesof tectonicand volcanic Davies,1981] and result in geologicallyrapid rates of global activityat the planet'ssurface. Among the planetscan be found resurfacing[Johnson et al., 1979]. examplesof bodiesin which one of three distinct mechanisms For Venus, the available radar imaging and altimetry data hasdominated heat loss(Figure 1), and the resultinggeological [Goldsteinet al., 1976,1978; Campbell et al., 1976,1979; Camp- historiesfor thesebodies differ profoundly.For the earth, the bell and Burns, 1980; Pettengillet al., 1980; Masurskyet al., majority of mantle-derived heat is delivered to the surface 1980] are at too coarse a horizontal resolution to allow the throughplate recycling:the processesof creation,cooling, and tectonic and volcanic featuresdiagnostic of one or more of subductionof oceaniclithosphere [Sclater et al., 1980].For the thesemechanisms of lithosphericheat transferto be discerned smaller terrestrialbodies, including Mars, Mercury, and the unequivocally.As a result,debates have ensued over suchissues earth'smoon, heat transporthas occurredprincipally by con- as whetherthe surfaceof Venusdisplays features indicative of ductionthrough a globallycontinuous lithospheric shell [Solo- lithosphericrecycling analogous to terrestrialplate tectonics mon,1978], and thelevels of both tectonicand volcanicactivity [Masursky et al., 1980; Phillips et al., 1981; Arvidson and have been much more limited than on earth [Head and Solo- Davies,1981; Head et al., 1981;Kaula and Phillips,1981; Arvid- sonand Guinness,1982; Brassand Harrison, 1982; Phillipsand Malin, 1982]. Rather than simplycontinuing such a debate,we Copyright1982 by the AmericanGeophysical Union. take in thispaper a differentapproach to the coupledquestions Paper number 2B1314. of lithosphericheat transportand volcanicand tectonicactivity 0148-0227/82/002B-1314505.00 on Venus.

9236 SOLOMONAND HEAD: LITHOSPHERICHEAT TILANSPORTON VENUS 9237

LITHOSPHERIC remaining from such ancient processesas planetary accretion CONDUCTION and core-mantledifferentiation. A recentreview of theseaspects '//•Moon,Mercury of the thermal history problem is given by Solomonet al. / •Mars [1981]. Of the currently operativeprocesses, radioactive decay MECHANISMSOF / • contributesthe vast majority of the heat generation,with minor incrementscontributed from the motion of phase boundaries (e.g.,inner core freezing)and tidal dissipation.To determinethe partitioning of global heat loss between presentlygenerated and ancient heat, it is necessaryto solvefor the earth's thermal evolution,including particularly the effectof solidstate convection on mantle heat transfer. A number of such calculations have recently been performed using one of several par- ameterizationschemes for heat transportby mantle convection [Sharpe and Peltier, 1978, 1979; Schubertet al., 1979a, b, 1980; IO Schubert,1979; Sleep, 1979; Stevensonand Turner, 1979; Tur-

PLATE HOT SPOT cotte et al., 1979; Daly, 1980; Davies, 1980; McKenzie and RECYCLING VOLCANISM Weiss, 1980; Stacey, 1980; Turcotte, 1980; Cook and Turcotte, 1981; McKenzie and Richter, 1981]. Thesecalculations indicate Fig. 1. Schematic ternary diagram showing the relative contri- that some fraction between 50 and 90% of the earth's heat loss butionsof each mechanismof lithosphericheat transfertoward global heat loss on solid planets and satellites.For the moon, Mercury, and is contributedby presentlyoperative heat generationprocesses, Mars, thermal conductioncarries all of the lithosphericheat flow. The with the specific value dependent on the type of par- position of the earth, on which the majority of lithospheric heat ameterizationadopted and the assumptionsmade about heat transport occursby plate recycling,is from Sclater et al. [1980] and sources,the temperature dependenceof viscosity,and radial does not include the contribution to global heat loss from crustal radioactivity. Hot spot volcanismdominates heat loss on Iv, but the zoning in the mantle. The remainderof the earth's heat lossis a relative fraction contributed by conduction is not known precisely consequenceof the secularcooling of the planet. [Reynoldset al., 1980; O'Reilly and Davies, 1981]; at the position There are several observationaland theoretical arguments shownon the diagram, the conductiveheat flow on Iv is equal to the that provide somesupport for the assumptionof equal rates of earth'saverage heat flux. heat loss per mass for Venus and earth. Several independent cosmochemical models for the formation of the terrestrial planetsyield roughly comparablebulk abundancesby massfor We pose three hypothesesfor lithosphericheat transfer on heat-generatingelements in the two bodies [see Wood et al., Venus, each based on one of the mechanisms known to domi- 1981]. The measured abundances of U, Th, and K at the nate heat flux on other planetary bodies:(1) that lithospheric Venera 8, 9, and 10 landing sites [Vinogradov et al., 1973; heat transportoccurs primarily throughplate recycling,(2) that Surkov et al., 1977; Surkov, 1977] and the measuredK20 lithosphericconduction dominatesthe heat loss, and (3) that abundancesat the Venera 13 and 14 landing sites[Barsukov, heat is transportedthrough the lithosphereprincipally by hot 1982] are generally comparable to values in terrestrial tho- spot volcanism.These three hypothesesshould be regarded as leiitesand alkali basalts.The total amountof radiogenic'•øAr end-members;clearly, combinationsof mechanismsmay also in the Venus atmosphereis within a factor of 3 of that in the serveto deliver internal heat to the planetary surface.We test earth'satmosphere [Hoffman et al., 1980-1,though whetherthe eachhypothesis against the known propertiesof Venus,and we lower amount on Venus is attributable to differences in out- explore the implications of each for the character of tectonic gassinghistory or in the bulk K abundanceis not known. and volcanicactivity at the Venus surface.We find that none of Parameterized-convectionmodels of the thermal history of the individualhypotheses can currentlybe excluded.Although Venus [Sharpeand Peltier, 1979; Turcotte et al., 1979; Solomon specificpredictions of the three hypothesesfor surfacegeologi- et al., 1981; Phillipsand Malin, 1982] indicatethat for the same cal evolution differ substantially,each hypothesisleads to the assumptionsas those usedin modelsof the thermal history of conclusionthat many, if not most,of the physiographicfeatures the earth, the evolution of the two planets has been broadly of the Venus surfaceare geologicallyyoung. similar. In particular, secularcooling of Venus contributesto the predicted presentheat loss of Venus by an amount com- VENUS HEAT FLOW parable to that for the earth. To quantify discussionsof specificmechanisms for litho- We concludethat a scaling of heat loss by mass from the sphericheat transporton Venus,it is necessaryto adopt a value earth to Venusis a reasonableworking hypothesis.The value of for the rate of global heat lossfrom the planet.Since the surface the Venusheat loss,however, even under sucha hypothesis,has heat flux on Venus has not been measured,we assumeas a basis considerableuncertainty. The earth's heat flux may have been for an estimatethat the heat lossper masson Venusis identical 25 to 50% greaterat timesof enhancedrates of seafloorspread- to that on earth. The presentheat lossfrom the solid earth is ing in the Phanerozoic[Hays and Pitman, 1973; Pitman, 1978; 4.2 x 10•3 W, for a globallyaveraged heat flow of 82 mW/m2 Turcotte and Burke, 1978; Sleep,1979; Harrison, 1980; Sprague [-Sclateret al., 1980-[.The massof Venus is 0.815 earth masses and Pollack, 1980-1,so that the terrestrial heat loss determined [-e.g.,Ash et al., 1971-[,and the surfacearea of Venus is 4.60 for the presentearth [Sclater et al., 1980] may be lessthan the x 108km 2 rPettengillet al., 1980-].The earth-scaledvalues for averageglobal heat losson time scalesof 108 years.If the global heat loss and surfaceheat flux for Venus are therefore differencein the amountof '•øArbetween the atmospheresof 3.4 x 10•3 W and74 mW/m •, respectively. Venusand earth representsa differencein the planetaryabun- The heat flux from the earth consistsof two parts: heat danceof potassium,the Venus heat losswould be lessthan the generatedby currently operativeinternal processesand heat value assumedhere. The difference,however, would probably 9238 SOLOMON AND HEAD: LITHOSPHERIC HEAT TRANSPORT ON VENUS be small.The presentcontribution of '•øK to the heat pro- geothermscannot be extrapolatedto Venus without making ductionin terrestrialmaterial with K/U = 10'• andTh/U = 3.7 internally consistentassumptions about both the global heat by weight [Wasserburget al., 1964] is 15%; thus if the abun- lossand the fraction of that heat lossdelivered by lithospheric dancesby weight of refractoryelements such as U and Th are conduction.This point has not been addressedin severaldis- comparableon Venusand earthbut the abundanceof the more cussionsof the thermal structure of the Venus interior [e.g., volatile element K is lower by a factor of 3, the overall heat Warner, 1979; McGill, 1979; Kaula and Phillips,1981]. productionper masswould be only 10% lessfor Venusthan for BecauseVenus and the earth have similar masses,bulk den- the earth. sities,and positionsin the solar system,and on the basisof the assumedsimilarities in globalheat loss,a reasonablehypothesis PLATE RECYCLING is that Venus, like the earth, losesmuch of its heat by litho- The recyclingof lithosphereinto the underlyingmantle is the spheric recycling.We shall evaluate this hypothesisand its major governorof terrestrialvolcanic and tectonicactivity and implications for the evolution of the Venus surface in this of the formationof the large-scalephysiographic features of the section. earth'ssurface. Creation of new lithosphereat mid-oceanridges The hypothesisthat plate recycling presently occurs on is the dominant volcanicprocess on the planet. Spreadingand Venushas beenchallenged in the literature on severalgrounds: cooling of the oceaniclithosphere, the thermal boundary layer (1) that topographicfeatures indicative of terrestrialplate tec- of mantle convection,produces the characteristicrelationship tonicscannot be 'seen'on Venus [Masurskyet al., 1980;Phillips betweenocean depth and seafloorage [e.g., Parsonsand Scla- et al., 1981; Arvidson and Davies, 1981; Kaula and Phillips, ter, 1977] as well as the distinctivepattern of ridge axis and 1981; Arvidsonand Guinness,1982; Schaber,1982], (2) that fracture zone physiography.Trenches, great thrust faults, and most of the Venus surface is 'ancient,' as indicated by the volcanic island arcs mark the loci of subduction of oceanic distributionof inferredimpact cratersand basins[Schaber and lithospherebeneath another oceanicplate. Island chainsand Boyce,1977; Masursky et al., 1980; Phillipset al., 1981], (3) that many oceanicaseismic ridges are productsof the motion of becauseof the high surfacetemperature on Venus, the litho- surfaceplates over mantle sitesof persistenthot spot volcanism sphereis less likely to subduct than oceaniclithosphere on [e.g., Wilson, 1965]. Passivemargins of continentsmark the earth [Anderson,1981; Phillips and Malin, 1982], and (4) that sitesof former intracontinental rifting where new oceanswere becauseof the high surfacetemperature and from the observed created.Tectonically active mountain belts form wheresubduc- characteristicsof 'ridges' on Venus, plate recyclingis a less tion occursbeneath overriding continental lithosphere,as in 'significant'process for removing heat than on earth [Kaula the Andes, or where two continental massescollide after closure and Phillips,1981]. As we discussbelow, however,none of these of an interveningocean, as in the Himalayas [e.g., Dewey and argumentsis sufficientlystrong to rule out plate recyclingon Bird, 1970]. Venus on the basisof presentevidence. Plate recyclingis also the dominant mechanismfor heat transfer across the outermost 100 km of the solid earth. The On TopographicFeatures Diagnostic of Plate Recycling most carefulanalysis of the partitioning of terrestrialheat flow The global topographyof Venus as measuredby the Pioneer among the contributiong processesis that of Sclater et el. Venus orbiter [Pettengill et al., 1980] stimulatedconsiderable [1980]. Theseworkers estimate that about 65% of the present interestin the questionof whether the nature of tectonicand heat loss of the earth is associatedwith the plate recycling volcanic activity on that planet can be discernedfrom the process,including the creation and cooling of oceaniclitho- large-scalephysiography. In particular, the statementhas been sphereand continentalorogeny associated with plate interac- made repeatedlyin the literature that Venuslacks topographic tions. Another 20% of the heat loss occurs by conduction featuresindicative of terrestrial-styleplate tectonics[Masursky throughoceanic and continentallithosphere, and 15% is gener- et al., 1980; Phillips et al., 1981; Arvidson and Davies, 1981; ated by radioactivedecay within the continentalcrust. As noted Arvidsonand Guinness,1982; Schaber,1982]. earlier, there have been extendedperiods during the Phanero- There are severalreasons why a searchfor featuresindicative zoic when the volumesof mid-oceanridges, and by inference of lithosphericrecycling on Venus may be more difficult than the global rates of plate creation,have beensubstantially grea- has been generallyappreciated. The horizontal resolutionof ter than they are at present [Hays and Pitman, 1973; Pitman, the Pioneer Venus topographic data is about 100-200 km 1978; Turcotte and Burke, 1978]. During such times both the [Pettengill et al., 1980], so that features of lesserhorizontal total rate of global heat lossand the fraction of that heat loss dimensionsthan thesevalues are not resolvable.A topographic contributed by the plate recyclingprocess were greater than map of the earth at comparableresolution would not resolve their present values [Turcotte and Burke, 1978; Sleep, 1979; such features,for instance,as many oceanictrenches and the Harrison, 1980;Sprague and Pollack, 1980]. axial valleys on slowly spreadingmid-ocean ridges [Head et Becausesuch a large fraction of the earth's heat is lost by al., 1981]. Further, oceanictopography on earth is capableof plate recycling,the typicalthermal gradients in old oceanicand approximately50% greater relative relief becauseof the pres- continentallithosphere are muchless than theywould be in the enceof the water layer. In addition,the rate of deepeningof the absenceof plate tectonics[e.g., Sclater et el., 1980]. That con- terrestrialoceanic lithosphere, or thermal boundarylayer, with duction through the continental lithospherehas contributed ageis proportionalto the differencein temperaturebetween the only modestlyto the earth'sglobal heat lossas far back as the bottom and top of the layer [Parsonsand Sclater, 1977]; on Archcanis stronglysuggested by the observationof Burke and Venus,with its higher surfacetemperature, the rate of deepen- Kidd [1978] that productsof crustalremelting are not observed ing of the surfaceof any analog to oceaniclithosphere should on a regionallyextensive basis in exposedsections of lower be correspondinglyless. Both of theseeffects would thus act to continentalcrust in the Archcanterrain of the SuperiorProv- reducethe relief of Venusanalogs to spreadingridges, trenches, ince. A clear implication of the processof lithosphericheat and submarinehot spot tracescompared with their terrestrial transport on earth is that terrestrial continentaland oceanic counterparts[Phillips et al., 1981; Head et al., 1981; Arvidson SOLOMONAND HEAD' LITHOSPHERICHEAT TRANSPORTON VENUS 9239

,•ndDavies, 1981; Kaula and Phillips, 1981]. While this reduced suchas Akna and Freyja montes,continental size plateaus such relief would still permit the identification of large-scaleplate as Lakshmi Planum, and arcuateridges and troughssuch as in tectonicfeatures (such as mid-ocean ridges, mountain belts, and the Ut and Vesta Rupes region on the southwestmargin of hot spottraces) on earth usingonly altimetryat PioneerVenus Ishtar Terra [Masursky et al., 1980; Head et al., 1981; McGill et resolution,at issueis whether suchfeatures could be correctly al., 1981, 1982; Campbellet al., 1982; Phillipsand Malin, 1982]. interpretedif no other information were available [Head et al., We thereforeregard the tectonicinterpretation of the topo- 1981;Arvidson and Davies, 1981]. graphic featuresof the Venus surfaceas uncertain at the re- Any tectoniccomparison of the topography of Venus and solution of our current information. Both an improvementin earth shouldalso be made in recognitionof the fact that many the resolution of surface features on Venus and detailed investi- of the physiographiccharacteristics of terrestrial features of gationsof the effectsof Venussurface conditions on the charac- plate tectonic origin are a direct result of the present distri- teristiclandforms produced by variousgeological processes are bution of continentsand ocean basins,the present range of neededto improveour understandingin this area. ridge spreadingrates, and the presenceof surfacewater. On a planet with a highersurface temperature and negligiblesurface On the Age of the VenusSurface water, many of thesecharacteristics might be expectedto differ. A number of approximatelycircular featureson the Venus For example,the orthogonal pattern of mid-ocean ridge seg- surface,ranging in diameter from 20 to over 1000 km, have mentsand transformfaults is held to be a diagnosticfeature of beenidentified on the basisof variationsin radar reflectivityor terrestrialplate tectonicsthat is visiblein the topographicdata topography [Goldsteinet al., 1976, 1978; Schaberand Boyce, for some oceanic regions even at Pioneer Venus resolution 1977; Campbellet al., 1979; Campbelland Burns, 1980]. The [Arvidsonand Davies,1981 ]. On the basisof laboratory experi- hypothesisthat many or all of these featuresoriginated by ments,however, it appearsthat this pattern may be a function impact has led to the inferencethat much of the surface of of material properties.Oldenburg and Brune[1975] found that Venusis geologicallyancient. In particular,if the quasi-circular in wax analogsof spreadingcenter systems, establishment and regionsof low radar backscatter200 km or greaterin diameter maintenance of an orthogonal ridge-transformpattern oc- have beencorrectly identified as impact basinsdating from the curred only if the resistivestresses along transform faults are heavy bombardment of the inner solar system,then Venus less than the shear vtrength of the lithosphere.If, compared containsmajor geologicunits with surfaceages approaching 4 with the earth, the shear strengthat the elevatedsurface tem- b.y. [Schaberand Boyce, 1977; Masursky et al., 1980]. perature of Venus is reducedrelative to transform-faultresist- Becauseof the high surfacetemperature, topographic fea- ance,then Venus might display lithosphericrecycling without tures on Venus are subjectedto viscousrelaxation at rates the familiar orthogonalpattern at spreadingcenters. much higher than for the other terrestrialplanets [e.g., Weert- Convergencezones on a Venus with lithospheric recycling man, 1979]. In particular, if the surfacetemperature of Venus may also differ substantially from analogous zones on the has been comparable to its present value for much of the earth. Island arc volcanism,though incompletelyunderstood, is planet'shistory, then a 4-b.y.-oldimpact basinon Venusshould thought to result from partial melting initiated by the dehy- have negligibletopographic relief [Solomonet al., 1982]. Large dration at depth of subductedoceanic crust [e.g., Lofgrenet al., circulardepressions on Venus,suc h as Atalanta Planitia, are 1981]. If subduction occurred on Venus without the en- substantiallyyounger than 4 b.y. and are unlikely to be impact trainment of water or hydrousphases in the subductedmateri- basins.Such topographicdepressions may owe their origin to al, then dehydrationat depth would not occur and the Venus one of the tectonicmechanisms proposed for intracontinental equivalent of island arc volcanismneed not be present.The basinson earth, i.e., cooling and subsidencefollowing litho- deep-seatrenches marking the locus of subductionof oceanic sphericextension or thermaluplift and erosion[Sleep and Snell, lithosphereare known to owe their physiographiccharacter- 1976; McKenzie, 1978]. The large quasi-circular,radar-dark istics,such as relative depth and width, to the thicknessof the featureson the Venus surfacemay still be impact basins,de- elastic or viscoelasticportion of the subductedplate, the vel- graded by essentiallycomplete relaxation of long-wavelength ocity of convergence,and the nature of any adjacentsources of topography.Such featuresmay also be older versionsof circu- sediment [e.g., Grellet and Dubois, 1982]; these parameters lar lowland regionsof Venus,however, and by implication also would be expectedto differ for Venus. Becauseof the high of tectonicorigin. surfacetemperature of Venus,a better model for someconver- The question of the age of the Venus surface should be gencezones on that planet might be based on the notion of regardedas open.There are alternativetectonic interpretations lithosphericdelamination as proposedfor zones of terrestrial of the large circular featuresvisible in radar images.Even the continentalcollision [Bird, 1978, 1979;Reutter et al., 1980;Bird population of smaller 'craters' 20-200 km in diameter may and Baumgardner,1981; Fleitout and Froidevaux,1982]. Ac- contain featuresof volcanicas well as impact origin [Saunders cording to this model, a low-strengthzone at the base of the and Malin, 1976; Campbell and Burns, 1980; McGill et al., crustpermits delamination of the lithosphere,subduction of the 1982]. These uncertaintiesdo not precludethe possibilitythat mantle portion, and deformation without subduction of the portions of the Venusiansurface are billions of years old, nor more buoyant crustal material. Such a model, applied to some do they permit the rejectionof the hypothesisthat much of the subduction zones on Venus, would lead to the prediction of Venusian surface, as with the solid surface of the earth, is regions of intense surfacedeformation surroundingconver- geologicallyyoung. gencezones but little recyclingof surficial or crustal material back into the underlyingmantle. It is worth noting that there are topographicfeatures on the On LithosphericBuoyancy on Venus Venussurface which differ from any seenon the smallerterres- A necessarycondition for subduction is that the subducted trial planets and which resemblefeatures on earth of plate material be of greaterdensity than the surroundingmantle. The tectonic origin. These featuresinclude linear mountain belts negative buoyancy of the subducted slab of lithosphere on 9240 SOLOMON AND HEAD: LITHOSPHERIC HEAT TRANSPORT ON VENUS earth is a consequenceof its relativelycooler temperature and compared with 65% on earth [Sclater et al., 1980] would, if also a result of phasechanges which occur at shallowerthan true,provide an importantcontrast between the two planets. normal depth in the cooler slab [Toks& et al., 1971; Turcotte We do not regard the 15% result of Kaula and Phillips and Schubert,1971]. The argumenthas been advancedthat a [1981], however, as necessarilyvalid. Their calculation was Venus lithospheresimilar in crustalthickness and other physi- based on an identification of candidatespreading ridges on cal propertiesto oceaniclithosphere on earth would have less Venus, a measurement of the rate of decrease in elevation with negative buoyancy becauseof the higher surfacetemperature distancefrom each segmentof the ridge, an application of and smaller lithosphericthickness on Venus [Anderson,1981; thermalboundary layer theoryto scalethe physicalparameters Phillipsand Malin, 1982]. Anderson[1981] has arguedfurther of mantle convection in the earth to those of Venus, and a that an additional factor which might make the Venus litho- conversionof the measured rates of change in elevation to spherepositively buoyant is the enhanceddepth range of the estimatesfor the heat delivered along each spreadingridge basalt stability field on Venus compared with that on earth, segment.Further elaboration of the boundary layer scaling thereby openingthe possibilityfor crustalthicknesses substan- argumentsis givenby Phillipsand Malin [1982]. tially greater than in terrestrialocean basins. The candidatesfor ridges chosen by Kaula and Phillips Thesebuoyancy calculations are sensitiveto the assumptions [1981] include Beta Regio and Aphrodite Terra. Both of these made about parameterswhich are at best poorly known for features contain large regions which, together with Ishtar Venus,including the thicknessof a crustallayer, the location of Terra, composethe highlandsprovince of Venus [Masursky et the mantle residuumremaining after extraction of partial melt al., 1980]; terrestrial continentsrather than ocean ridgesmay to producethe crust,and the partitioning of planetary heat loss provide a preferableanalog to much of the Venus highland among plate recyclingand conduction.Thus the uncertainty terrain [e.g., Head et al., 1981]. Having made this selectionof attachedto an individualresult is large, and both positiveand ridges, Kaula and Phillips then note that the Venus ridge negativevalues for net lithosphericbuoyancy are possiblere- heightsdo not show a narrow distributionabout a mode as do sultsat our presentstage of understanding[Phillips and Malin, terrestrial ocean ridges well removed from hot spots [Parsons 1982]. and $clater, 1977] and that theseridges do not form an inter- That the Venusian lithosphere is less negatively buoyant connectedglobal system. While these observationsmay be than old oceaniclithosphere on earth is likely, yet even this sufficientto rule out either Beta Regio or Aphrodite Terra as conclusionis not a strong argument against some form of Venusiananalogs to terrestrialmid-ocean ridges [eft $chaber, subduction on Venus. On the earth, oceanic lithosphere with 1982], neither observationis a compellingargument against seaflooras young as 10 m.y. is subducted[Menard, 1978], yet lithosphericrecycling on Venusin general. such lithosphereis certainly lessnegatively buoyant than un- Having posedthe hypothesisthat plate recyclingdominates subductedoceanic lithosphere 150 m.y. old. Parsons[1982], in lithosphericheat transport on Venus,we should considerthe fact, has demonstratedthat the distribution of ocean floor area expectedform of Venusian spreadingcenters. A reasonable versusage on the earth is consistentwith the hypothesisthat guide to the rate of heat loss per length of spreadingridge plate consumptionis uniformlydistributed with age exceptfor followsfrom the spreading-platemodel of oceaniclithosphere seaflooryounger than 10 m.y. old. Subductionof young sea- [Williams and yonHerzen, 1974; Kaula andPhillips, 1981]: floor may, of course,be sustainedby a finite-amplitudeinstability; i.e., 10 m.y. old oceaniclithosphere may be only margin- q, = pC•,LvAT (1) ally unstable or even stable gravitationally yet continue to wherep and Cpare the densityand specific heat of the litho- subductif attachedto a negativelybuoyant slab. Sucha finite- sphericplate, L is the lithosphericthickness, v is the average amplitudeinstability might equally well be invoked to sustain half spreadingrate, and AT is the differencein temperature subduction on Venus. Once subductionof an aging piece of betweenthe bottom and top of the lithosphere.Subduction of Venusianlithosphere is initiated, by this argument,the subduc- the lithospherebefore it has reachedthermal equilibrium will tion processmay be sustainedby thermal anomaliesand elev- reduceq, from that givenby (1). Becauseof the highersurface ated phasetransitions in the subductedslab, and the character- temperatureon Venus, AT will be lessfor Venus than for the istic age of subductedmaterial may be substantiallyyounger earth'socean basins, though boundary layer scalingarguments than that initially subducted.If, as suggestedearlier, litho- suggestthat the differencein A T betweenVenus and earth may sphericdelamination occurs on Venus,then the mantle portion be somewhat less than the differencein surfacetemperatures of the lithospherewould be negatively buoyant even if the [Kaula and Phillips, 1981; Phillips and Malin, 1982]. For a crustalportion is not. Thus a thicker crust on Venuscompared spreadingridge systemon Venus to deliver to the surfacean with terrestrial oceanbasins [Anderson, 1981] need not restrict amount of heat comparable to that delivered by ridges on subductionof the mantle portion of the lithospherewhere such earth,either the characteristicspreading rate or the total length delamination occurs. To the extent that the mantle residuum of ridges(or both) must be greater on Venus than on earth. complementaryto a basalticcrust residesin the lithosphere,in Becauseeither possibilitywould likely be accompaniedby a fact, the net lithosphericbuoyancy may be largelyinsensitive to lessercharacteristic age of subductedlithosphere on Venus crustal thickness. than on earth, the product of ridgelength and mean spreading rate would exceedthat quantity for the earth by a ratio greater On the Kaula-Phillips Argumentson Heat Removal than simplythe ratio of the valuesof A T for the two bodies. by LithosphericRecycling on Venus Theseconsiderations suggest that the mostlikely candidates Kaula and Phillips [1981] estimatedan upper bound on the for spreadingridges on Venusshould be characterizedby rapid fraction of the global heat loss on Venus contributed by plate ratesof spreadingand correspondinglysmall rates of changein tectonics.While not an argument against plate recycling on topographicheight with distancefrom the ridge axis. Phillips Venus per se, their conclusionthat the heat transported by and Malin [1982] have shownthat a fast spreadingridge, such plate tectonicsis 15% or lessof the global heat flux on Venus as the East Pacific Rise, would have only modesttopographic SOLOMONAND HEAD: LITHOSPHERICHEAT TRANSPORTON VENUS 9241

'Convergence'Continent ' Zone' extensive by this hypothesisand should be concentrated at divergent plate boundaries.Intraplate volcanic activity may SpreadingBasin' H•ghlands ____ /Center'Ocean also be present.Volcanism at subductionzones may be minor RollingLowLands Pla•__ns._.• r at presentif no free water or hydrous phasesare subducted. Tectonic activity should be widespreadand dominated by the large-scalehorizontal motions and mutual interactions of the

Plate Recycling Hypotnes•s plates. As noted above, however, the specificphysiographic characteristicsof many tectonicfeatures at plate boundaries Fig. 2. A schematicillustration of the platerecycling hypothesis for may differ from those on earth becauseof the greater surface lithosphericheat transporton Venus.The rolling plains(0 to 2 km elevationwith respectto the planetarymodal radius)and lowlands(0 temperatureand negligiblesurface water on Venus.Theoretical to -2 kin) arc analogousto terrestrialocean basins and include models are needed for the expectedform of such featuresat spreadingcenters and convergence zones not currentlyresolvable from Venus-surface conditions. altimctry or imagingdata. The highlands(2 to 11 km elevation)arc By the plate recycling hypothesis,the rolling plains and analogousto terrestrialcontinents. lowlands provincesare analogsto terrestrial ocean basinsin terms of formativeprocess, crustal composition, and geologi- cally youthful age. The spreadingcenters are expectedto be relief at Venus conditionsand that sucha ridge on Venus could characterizedby rapid spreadingrates and modest relief in not be detectedfrom Pioneer Venus altimetry given the 200-m comparisonto earth. The Venus highlandsmay be analogsto standard error for thesedata [Pettengill et al., 1980]. By the terrestrialcontinents, though whether the analogy extends to plate recyclinghypothesis for Venus, the rolling plains and compositionor simply to crustal thicknessis uncertain. The lowlands provinces (as defined by Masursky et al. [1980]) surfaceages of highland geologicunits on Venus may be con- should be the Venusian analogs to terrestrial ocean basins. siderablygreater than the agesof units in the plains and low- Altimetric and imaging data from rolling plains and lowlands lands. As with terrestrial continents, highland lithosphere areasmust be obtainedat resolutionssuperior to thoseof data should have greater buoyancy than the lithosphere beneath currently availablein order to searchfor the predictedexten- plains and lowlandsand should thereforebe more difficult to sive systemof fast spreadingridges and thereby to provide a subduct.Mountainous terrain should,by this hypothesis,form rigoroustest of the platerecycling hypothesis for Venus. by the collision of highland blocks (Himalayan analog) or at the locusof subductionbeneath highland lithosphere(Andean Assessmentand Implicationsof the Hypothesis analog).Both highland regionsin generaland mountain belts We find no convincingargument to reject at present the in particular will be modified as they age. Becauseof the high hypothesisthat someform of plate recyclingdominates litho- surface temperature and negligible water on Venus, viscous sphericheat transferon Venus.This conclusiondoes not mean relaxation may be a more rapid processthan weatheringand that plate tectonicscan presentlybe demonstratedon Venus-or erosionfor reducinghighland relief, in contrastto earth. that we may not at some future date obtain improved information on the Venusian surface or interior that will allow the LITHOSPHERIC CONDUCTION questionto be firmly resolved.Rather, the conclusiondemands The dominant mechanismfor lithosphericheat transport on that at our presentlevel of understanding,the hypothesisthat the smallerterrestrial planets is conductionthrough the single plate recycling is important for heat flow on Venus should global lithosphericshell. That lithosphericconduction domi- continue to be rigorously tested against available geological natesthe planetary heat losson Venus has been an implicit or information. explicit elementof a number of recentdiscussions of the inter- The lithosphericrecycling hypothesis for Venus is illustrated nal and surfaceevolution of Venus [Schaberand Boyce, 1977; schematicallyin Figure 2, and some of the implicationsof the Warner, 1979; Anderson,1981; Phillips et al., 1981; Arvidson hypothesisfor the surfacecharacteristics of the planet are listed and Davies,1981]. in Table 1. As on earth, volcanic activity on Venus should be If the global heat loss on Venus is delivered to the surface

TABLE 1. Implicationsof End-Member Hypothesesfor LithosphericHeat Transporton Venus

Surface Characteristics Plate Recycling LithosphericConduction Hot Spot Volcanism

Volcanic activity extensive;activity minor extensive; active dominantly at centersnearly divergent boundaries cover surface Tectonic features widespread;dominated possiblyextensive primarily vertical by plate interactions deformation of tectonics thin lithosphere Ages of surfaceunits rolling plains and unconstrained; ancient much of surface lowlandsgeologically impact featuresmay young(<• 107 years) young( < 10s years) be preserved Nature of mountain belts productsof plate anomalouslythick crust volcanic constructs convergence and lithosphere Nature of highlands analogsto terrestrial thickened crust and thickened volcanic continents lithosphere crust Nature of rolling plains analogs to terrestrial thinned crust and 'normal'-thickness and lowlands ocean basins lithosphere volcanic crust 9242 SOLOMON AND HEAD' LITHOSPHERIC HEAT TRANSPORT ON VENUS

L•t hospher•c Thickening entirely by lithospheric conduction, then the conductive L•thospher•c • • geothermmay be estimatedfrom the averagesurface heat flow H•ghlands Roll•ng Plains of 74 mW/m2 derivedearlier. This heat flow is very closeto Lowlands twice the heat flux in old oceanic lithosphere near thermal equilibrium,given as 38 + 4 mW/m2 by Sclateret al. [1980]. Thus if the Venuslithosphere has a thermal conductivitysimi- lar to the averagevalue of 3.1 W/m K adopted for the terres- Lithospheric Conduction Hypothes•s trial oceaniclithosphere [Parsons and Sclater, 1977], then the Fig. 4. A schematicillustration of the conductionhypothesis for thermal gradient in the Venus lithosphereis 24 K/km, or ap- lithosphericheat transport on Venus. On average, the lithosphereis only about 40 km thick and may be readily deformed by tractions proximately twice that in old ocean basins. Geotherms for associatedwith convectionin the underlyingasthenosphere. Modestly oceaniclithosphere at equilibrium [Sclater et al., 1980] and for elevatedregions in the rollingplains may be areasof recentlyextended average lithosphere on Venus, assuming that heat transfer and thinnedcrust and lithosphere,areas which will subsideto lowland occurssolely by lithosphericconduction, are shownin Figure3. elevationsduring lithosphericcooling and thickening[e.g., McKenzie, 1978]. The more elevatedhighlands may be areas of thickenedcrust A consequeficeofthe Venus geotherms shown in Figure3 is and lithosphereresulting from lithosphericcompression. that the lithospherewould be substantiallythinner than on earth for comparablematerial propertiesof the mantle. The baseof the thermallithosphere in oceanbasins is 1350 + 275øC characterizedby a mantle heat flux that is lower than average. [Parsonsand Sclater, 1977]. A temperatureof 1350øCwould be That such regionally low values of mantle heat flux could reachedat a depth of about 40 km on Venus accordingto the persistfor hundredsof millionsto billionsof yearsfor a planet geothermsshown in Figure 3. The base of the elastic litho- with lithosphericheat transferdominated by conduction,how- spherein oceanicregions is well approximatedby the position ever, is unlikely. Alternatively,since the history of the surface of the 500 + 150øC isotherm [Watts et al., 1980]. An elastic temperature is uncertain [Pollack, 1971, 1979], topographic lithosphereon Venuslimited by this sametemperature, a value relief may have persistedfor geologicallylong periodsif the governedby the ductile flow law for dry olivine [Goetze and characteristictime for viscousrelaxation is comparable to or Evans,1979], would be at most 10 km in averagethickness for a lessthan the time sinceformation of the presentatmospheric purely conductiveVenus. 'greenhouse.'The viscousrelaxation time is unlikely to exceed a On both of these grounds,it is difficult to envisionmecha- few hundredmillion years [Solomon et al., 1982],however, so nismsfor supportingthe 13 km of relief on Venus [Pettengillet this possibilitywould requirea geologicallyrecent greenhouse. al., 1980] for geologicallylong periodsof time exceptperhaps The Venus geotherm shown as a solid line in Figure 3 is through sheartractions associated with mantle convectiveflow. basedon the simplificationthat all of the heat lost by the planet A likely implication,therefore, of the hypothesisthat conduc- is generatedbeneath the lithosphere.The lithosphericthermal tion dominatesheat transferon Venus is that all surfacetopo- gradient would be lessenedif concentrationof radioactiveheat graphicrelief on scalessmaller than the characteristichorizon- sourcesin the Venus crust has occurred,thereby reducingthe tal scalesof mantle convectionis geologicallyyoung. Preser- heat flux from the mantle [e.g.,Phillips and Malin, 1982]. Since, vation of topographicrelief for extendedperiods of time might by the conductionhypothesis, plate recyclingand attendant occur more readily if elevated regions on Venus are remelting of basalticcrust at subductionzones would not have progressedon Venus to the current stage of concentrationof T• øC heat sources in continental crust on the earth, a reasonable 0 500 i000 1500 upper bound to the fraction of Venus heat flux contributedby • Venus:L•thospher•c crustalradioactivity is the terrestrialvalue of about 15% [Scla- ter et al., 1980]. A greater concentrationof radioactive heat z• "--,.•o.,.., nOnly sources into the crust on Venus than on the earth would also be difficult to reconcile with the lower '•øAr abundance in the Base of Elastic ' - Venus atmosphere[Hoffman et al., 1980]. With the 15% value 5O -- L•thosphere assumedfor the fraction of global heat loss generatedin the

Earth: crust, the mantle heat flux on a Venus with the same heat loss • Equ•l•br•urn per massas the earthwould be 29 x 10•2 W, and the temperature gradientin the Venuslithosphere would, as shownby the dashedcurve in Figure 3, be only slightly modified from that I00 discussed above. The hypothesisthat lithosphericheat flux on Venus occurs principally by conductioncannot be rejected on the basisof presentlyavailable information. A schematicillustration of this hypothesisis given in Figure 4, and a summary of the impli- 150 I I cationsfor the surfacegeology of Venusis givenin Table 1. The averagelithospheric thermal gradientsare predictedto be sub- Fig. 3. Average lithosphericgeotherms on Venus assumingthat conductionis the only mode of lithosphericheat transfer.The solidline stantially greater, and the lithospheric thicknesscorrespond- showsthe casewhen all of the heat lossfrom Venusis generatedbelow ingly less, than for volcanic mechanismsof heat transport, the lithosphere;the short-dashedcurve indicatesthe casewhen 15% of including both plate recyclingand hot spot volcanism.The the Venusheat lossis generatedby radioactivitydistributed uniformly lithosphericstrength and resistanceto deformationshould gen- in a crust 30 km thick. Also shownare the terrestrialgeotherm for an old ocean basin in thermal equilibrium [Parsonsand Sclater, 1977; erally be lessfor this hypothesisthan for the hypothesisthat Sclater et al., 1980] and the range of isothermsinferred to define the either plate recyclingor hot spot volcanismdominates litho- baseof the elasticlithosphere in oceanbasins [Watts et al., 1980]. sphericheat transport on Venus.As a result, lithosphericand SOLOMONAND HEAD' LITHOSPHERICHEAT TRANSPORTON VENUS 9243

crustalthinning and thickeningmay occurin responseto trac- every 200-km squareof the Venussurface. Whether the mantle tions exertedon the baseof the lithosphereby mantle convec- convection system on Venus could supply magma to this tive flow (seeFigure 4). The agesof surfacegeological units are number of volcaniccenters essentially simultaneously on geo- not constrainedby the conductionhypothesis and may span a logicaltime scalesis an openquestion. greatrange. Because of the expectedhigh rate of viscousrelax- Two effectsmay increasesomewhat the contributionof vol- ation, however,topographic relief on Venus should be either canismat a Hawaiian-type hot spot to the total heat flux of geologicallyyoung or dynamicallymaintained. Since the plan- Venus. The eruption temperatureof typical hot spot magmas form of mantle dynamicsis not likely to be steady on time on Venus may be greater than on earth, due either to a lower scalesapproaching the ageof the planet,it would be reasonable FeO/(MgO + FeO) ratio [Goettel et al., 1981; Phillips and to concludeby this hypothesisthat none of the topographic Malin, 1982] or to lower volatile abundances[McGill, 1979; featuresof Venus is likely to date from the first half of the Anderson,1980] in the Venus mantle comparedwith that of the planet'shistory. earth. The rate of heat lossQo for a Venus'Hawaii' increasesby 0.2 x 109 W for every100 K increasein AT in equation(2), so HOT SPOT VOLCANISM that even a 500 K differencein eruption temperaturebetween Individual volcanichot spotstransport heat from a planetary Venus and earth would raise Q,by no more than 40%. A interior by the ascentand coolingof magmaand by conduction second effect, also noted in the last section, is that con- through a locally thinned lithosphere.Phillips and Malin centration of radioactiveheat sourcesin the Venus crust may ['1982] have proposedthat hot spot activity on Venus may reduce the heat flux from the mantle and thus the number of providethe dominantmechanism of lithosphericheat transfer. individual hot spotsneeded to remove that heat. Adopting, as The implicationsof sucha hypothesismay be quantifiedby above,the 15% value [Sclater et al., 1980] as an upper bound consideringfirst the archetypicalterrestrial hot spot,the source to the fraction of Venus heat lossgenerated by crustal radioac- of the Hawaiian-Emperor island-seamountchain ['Wilson, tivity, the contribution of volcanismat a Hawaiian-type hot 1965;Mor•Ian, 1971]. The rate of heatloss Qv due to volcanism spotto thetotal heat loss would still be about 1 part in 10'•. at a hot spotis givenapproximately by A planetwith 10'• hot spotsas active as Hawaii would have a total volcanic flux sufficientto affect the planetary surface dV profoundlyon geologicallyshort time scales.If the global rate Qv= p(C•,AT + Amf)dt (2) of heat lossQ is contributedentirely by hot spot volcanism,the wherep andC•, are the density and specific heat, respectively, of planetaryvolcanic flux is thevolcanic material, AHœ is theheat of fusionof themagma, dV* Q ATis the differe.!•, between theeruption temperature ofthe - (3) magmaandthe•,,ii•i•i•ent surfacetemperature, anddV/dt isthe dt p(C •, AT + AHœ ) volumetricflux of magmawith time. Shaw[-1973] has estimated For Q = 34 x 10•2 W andother parameters as adopted above, that theHawaiian hot spothas produced 8.5 x 105km 3 of lava dV*/dt = 280 km3/yr.Averaged over the surfaceof Venus,this between the time of formation of the bend in the Hawaiian- flux amountsto a rate of surfaceburial of 6 x 10-7 km/yr,or Emperorchain and the present.Adopting the figure42 m.y. for an additional 1-km thickness of new volcanic material over the the age of the bend I'Dalryrnpleand Cla•tue, 1976] gives an entireplanet every2 m.y. This global burial rate is comparable averagefor dV/dt of 2 x 10-2 km3/yr,though values larger by to the estimatedlower bound on the burial rate for Io [Johnson up to a factor of 5 have beenappropriate for time.intervalsof a et al., 1979]. few million years [Shaw, 1973]. To estimatethe averagevol- Terrestrial hot spots contribute heat to the lithosphereby canic heat loss contributedby the Hawaiian hot spot for the mechanismsother than extrusivevolcanism. Lithospheric thin- last42 m.y., we assume p = 2.8g/cm 3, C•, = 1.2J/g K, AHœ= ning beneathhot spotsleads both to an enhancedconductive 400 J/g, and AT = 1300 K [Solomonet al., 1981]. Then from heat flux and to modestelevations in surfacetopography, such (2),Qo - 3.5 x 109W, or 0.008%of the earth's global heat loss. as the Hawaiian swell associatedwith the hot spot beneath The total contribution of hot spot volcanismto the overall Hawaii [Detrick and Crough,1978]. This lithosphericthinning heat lossof the earth is minor [Sclater et al., 1980]. Of all the occurson time scalesshort compared with the characteristic proposedhot spots [e.g., Morgan, 1971; Burke and Wilson, time for lithosphericheating or coolingby conduction[Detrick 1976], Hawaii has had probablythe greatestvolumetric flux of andCrough, 1978]; asthenoSPheric stoping, or in situdelami- magmain recentgeologic history. Thus whetherthe important nation and sinkingof the lower lithosphere,may be involvedin terrestrialhot spotsnumber 20 [Morgan, 1971] or closerto 100 the rapid thinningprocess [Turcotte, 1982]. The lithosphereis [Burke and Wilson, 1976], the summed contribution of vol- apparently thinned by about a factor of 2 to 3 beneath the canicallydelivered heat from suchhot spotsto globalheat flow Hawaiian hot spot [Detrick and Crough, 1978; Crough, 1978; is less than 1%. Detrick et al., 1981; Sandwell,1982; yonHerzen et al., 1982]. Ifa If a hot spot with the samemagmatic flux as Hawaii were on large number of hot spots are operative on Venus, then the Venus, the rate of heat loss due to volcanism would be some- conductivegradient would be lessenedand the effectivelitho- what lessthan on earth becauseof the higher surfacetemper- spheric thicknessincreased in those areas distant from hot ature and thereforethe lesserextent of cooling.For an eruption spotscompared with the global averagescalculated in the last temperatureand other parametersequal to thoseassumed for section. theearth, equation (2)gives Q,= 2.5 x 109 W, or 0.007%of Theseideas may be quantifiedas follows.Let • be the glo- the global Venus heat lossif the heat loss per massof Venus bally averagedheat flux, let q0 be the averageheat flux in areas and the earth are equal.Thus to removethe total heat flux from distantfrom hot spots,let f be the fractionalarea of planetary Venusby volcanismat hot spotswith ratesof volcanicactivity surfaceoccupied by hot spots,and let r/q0 be the conducted similarto that of Hawaii,a totalof 10'• suchhot spotswould be heat flux in the vicinity of a typical hot spot.The quantity r/ needed.This figure amountsto an averageof one 'Hawaii' on may be regarded as the factor by which the lithosphere is 9244 SOLOMONAND H•D: LITHOSPHERICHEAT TRANSPORT ON VENUS characteristicallythinned by someunspecified process at a hot Large Volcanic Construct PervasiveVolcanic Centers •,•' • spot.Clearly, H•ghlands Roll•ng Plains O =fnqo + (1 -f)qo + qv (4) Lowlands / / / whereq• is the contributionto the global heat flux deliveredby volcanism. If volcanicallydelivered heat is smallcompared with conduc- Hot Spot Volcanism Hypothesis ted heat at hot spotsand elsewhere(i.e., q, ,• •), thenq0 is given Fig. 5. A schematicillustration of the hot spot volcanismhypoth- by esisfor lithosphericheat transport on Venus. Volcanismcarries the majority of the heat through the lithosphere,and activevolcanic cen- o (5) ters on the surface of Venus should be widespread.The highland qo= 1--f +fil terrain consistsof large volumes of volcanic depositsand probably thickenedcrust. The surfacematerial is generallyvolcanic and geolog- Under the assumptionthat the Hawaiian hot spot is a guide to ically young. the characteristicsof possiblehot spots on Venus, we take r/•_ 2-3 [Detrick and Crough, 1978; Crough, 1978; Detrick et al., 1981; Sandwell, 1982; van Herzen et al., 1982]. For r/= 2, sideredin this paper, and the implicationsof their model in- q0 •> •/2. That is, the lithosphericthickness far from hot spots volve a combination of aspectsof the scenariosdepicted in can be increasedby a factor only as large as 2 comparedwith Figures4 and 5. the global averagederived in the last section,with the factor of 2 approachedonly for the casewhere hot spotscover most of the surfaceof Venus (f_• 1). For r/= 3, q0 •> •/3, again with CONCLUSIONS near equality only if f •_ 1. Lithosphericthicknesses approach- Without more detailed information on the Venus surface,all ing typical terrestrial valueswould be possible,but only in a few areasof very limited spatial extent. of the mechanismsfor lithospheric heat transfer considered If, in contrast, lithosphericheat transfer on Venus occurs here (lithospheric recycling, conduction, and hot-spot vol- dominantly by magma transport at individual hot spots(i.e., canism)should be regardedas potentiallyimportant for Venus. q• -• 0 in equation(3)), the averagelithospheric thickness is not Though eachof thesemechanisms has beentreated individually constrainedby global heat flow [cf. O'Reilly and Davies,1981] in this paper, weightedcombinations of mechanismsmust, of and can be considerablygreater than estimatesobtained in the course,also be considered[e.g., Phillipsand Malin, 1982]. The last section.In particular, the average lithosphericthickness possibilitythat the dominant mechanismearly in the history of can be greaterthan the 100-km depth of isostaticcompensation the planetwas different from that at present[e.g., Phillips et al., inferred from the gravity anomaliesover a number of topo- 1981] shouldbe recognizedas well. graphicallydistinct features on the planet [Phillips et al., 1979, Each of the possibleend-member models for lithospheric 1981; Reasenberget al., 1981]. Thus large topographicrelief heat transfer on Venus carries different implicationsfor the could be supportedpassively and on nearly a global basiseither detailed characteristicsof Venus landforms. The predicted by local isostaticcompensation or by regional compensation characteristics(Table 1) shouldserve as a guideto the interpret- and lithosphericstrength. ation of existingand future imaging and topographicdata on The hypothesisthat hot spot volcanism dominates litho- the Venus surface.For each of the hypotheticalmodels for heat sphericheat transferon Venus(Figure 5) cannotbe excludedon transfer,however, there is a clear implication that either many the basisof our presentknowledge of the surface.The hypoth- of the surfacegeological units or much of the surfacetopogra- esiscarries important implicationsfor the interpretatonof sur- phy is geologicallyyouthful. face physiographic features (Table 1). The Venus surface, if A common presumptionin discussionsof comparativeplan- most of the heat loss occursby volcanism,should be densely etary evolution has been that planetary sizeplays a crucial role covered with thousands of distinct centers of current or recent in controlling the internal heat budget and the volcanic and volcanic activity. Most of the physiographicfeatures would tectonicresponses to global heat loss.The comparisonof Venus have surfacesless than 10 m.y. old, though a few areasof older and earth holds a key spot in the test of this presumption terrain may have escapedrecent resurfacing. If the majority of [Phillipset al., 1981;Head and Solomon,1981]. Establishingthe the heat deliveredat hot spotsis due to lithosphericthinning, nature of the tectonicand volcanichistory of the Venussurface then hot spotswould nearly have to cover the surfaceof Venus and inferring the dominant mechanismsfor lithosphericheat for the lithosphericthickness far from hot spotsto be substan- transferon that body remain elusivegoals of the highestpri- tially greater than the globally averagedvalue. Much of the ority for our understandingof the evolution of the terrestrial surface material should, in either case, consist of volcanic de- planets,including our earth. posits. In particular, the Venus highlands would most likely have beenformed by volcanicconstruction. Tectonic activity in Acknowledgments.Discussions at the VenusTectonics Workshop the absenceof large-scalehorizontal motions should be prin- (April 20-21, 1982), sponsoredby the Lunar and Planetary Institute cipally restrictedto that associatedwith verticalmotions of the and organizedby Roger Phillips, were helpfulin sharpeningthe argu- mentspresented here. We appreciateconstructive reviews of an earlier lithosphere. draft of this manuscript by David Armbruster, Kevin Burke, Rob The Phillips and Malin [1982] model for hot spot tectonics Comer, Ken Goettel, Bill Kaula, Mike Malin, and GeorgeMcGill. We on Venus includesboth volcanic transport of heat and litho- thank Ray Arvidson,Bob Detrick, Bill Kaula, George McGill, Roger sphericthinning beneathmajor volcaniccenters, with the latter Phillips,Jerry Schaber, and Don Turcotte for preprintsof papersprior to publication,and Dorothy Frank and Jan Nattier-Barbara for their processmaking a greatercontribution to the global heat loss. assistancein the preparation of this manuscript.This researchwas This model may be viewedas a combinationof the mechanisms supported by NASA grants NSG-7081 and NSG-7297 at MIT and of lithosphericconduction and hot spot volcanism as con- grantsNGR-40-002-088 and NGR-40-002-116 at BrownUniversity. SOLOMON AND HEAD: LITHOSPHERIC HEAT TRANSPORT ON VENUS 9245

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