Download Mechanisms for lithospheric heat transport on Venus Implications for

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. Btt, PAGES 9236-9246,NOVEMBER t0, 1982
Mechanismsfor LithosphericHeat 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,
BrownUniversity,Providence,RhodeIsland02912
The tectonicand volcaniccharacteristicsof the surfaceof Venus are poorly known, but thesecharacteristicsmust be closelyrelated to the mechanismby which Venus rids itself of internal heat. On the other
solidplanets
andsatellites
of thesolarsystem,
lithospheric
heat[ransport
is dominated
by 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 eachmechanismfor the interpretationof Venussurfacefeatures.Despiteclaimsmade
to the contrary in the literature,plate recyclingon Venus cannot be excludedon the basisof either
theoreticalargumentsor presentobservationson topographyand radar backscatter.Landformsresulting
from plate convergenceand divergenceon Venus would differ substantiallyfrom those on the earth
becauseof the high surfacetemperatureand the absenceof oceanson Venus,the lack of free or hydrated
water in subductedmaterial,the possibilitythat subductionwould more commonlybe accompaniedby
lithosphericdelamination,and the rapid spreadingratesthat would be requiredif plate recyclingremoves
a significantfractionof the internalheat.If plate recyclingoccurson Venus,the rolling plainsand lowlands
provinceswouldbe approximateanalogsto terrestrialoceanbasinsin termsof age,igneousrock type,and
formativeprocess;highlandson Venuswould be roughlyanalogousto terrestrialcontinents.The hypothesisthat lithosphericconductiondominatesshallowheat transferon Venusleadsto 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 Venusat an averagedepth of
about 40 km. Unlessthe mantleconvectiveplanformcan maintainlithosphericregionsof persistentlylow
heat flow or unlessthe presentatmosphericgreenhouseon Venus is geologicallyrecent,then such a
lithosphericthicknessleadsto the conclusionthat the topographicfeaturescontributingto the 13 km of
relief on the planet must be geologicallyyoung. The hypothesisthat hot spot volcanismdominates
lithosphericheat transferon Venusleadsto the predictionthat the surfacemustbe coveredwith numerous
activevolcanicsources.In particular,if a typicalVenushot spothas a volcanicflux equal to the average
fluxfor theHawaiianhot spotfor thelast40 m.y.,then10'• suchhot spotsare necessary
to removethe
Venusinternalheat by volcanism.Sucha numberwould produceenoughvolcanicmaterialto resurfacethe
entire planet to a depth of 1 km every 2 m.y.; few areas of the planet would escaperesurfacingfor
geologicallylong periodsof time. We find that noneof the mechanisms
for lithosphericheat transferon
Venus can be excludedas unimportant at present;it is likely that, as on earth, a combinationof
mechanisms
operateson Venus.The strongestconclusionto emergefrom this evaluationis that most of
the major topographicfeaturesand probablymany of the surfacegeologicalunitson Venusare youngby
comparisonwith the surfacesof the smallerterrestrialplanets.
INTRODUCTION
mon, 1981]. For Jupiter'smoon Io, eruptions at individual
volcaniccenters,or 'hot spots,'dominatethe lithosphericheat
flux [Matson et al., 1981; Reynoldset al., 1980; O'Reilly and
Davies,1981] and resultin geologicallyrapid rates of global
resurfacing[Johnsonet al., 1979].
For Venus, the availableradar imaging and altimetry data
examplesof bodiesin which one of three distinct mechanisms
hasdominatedheatloss(Figure1),and the resultinggeological [Goldsteinet al., 1976,1978;Campbellet al., 1976,1979;Camphistoriesfor thesebodiesdiffer 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 processes
of creation,cooling,and tectonic and volcanic featuresdiagnosticof one or more of
subductionof oceaniclithosphere[Sclateret al., 1980].For the thesemechanismsof lithosphericheat transferto be discerned
smaller terrestrialbodies,includingMars, Mercury, and the unequivocally.As a result,debateshaveensuedoversuchissues
earth'smoon, heat transporthas occurredprincipallyby con- as whetherthe surfaceof Venusdisplaysfeaturesindicativeof
ductionthrougha globallycontinuouslithosphericshell[Solo- lithosphericrecyclinganalogousto terrestrialplate tectonics
mon,1978],and thelevelsof bothtectonicand 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 andPhillips,1981;Arvidsonand Guinness,1982; Brassand Harrison, 1982; Phillipsand
Malin, 1982]. Rather than simplycontinuingsucha debate,we
Copyright1982by theAmericanGeophysicalUnion.
takein thispapera differentapproachto the coupledquestions
of lithosphericheattransportand volcanicand tectonicactivity
Paper number2B1314.
The mechanismby which a solid planet transportsheat
acrossthe outer 100 km of its interior playsa pivotal role in
determiningthe stylesand magnitudesof tectonicand volcanic
activityat theplanet'ssurface.Amongtheplanetscanbefound
on Venus.
0148-0227/82/002B-1314505.00
9236
SOLOMONAND HEAD: LITHOSPHERICHEAT TILANSPORTON VENUS
LITHOSPHERIC
CONDUCTION
'//•Moon,
Mercury
MECHANISMS
OF
/
/
•Mars
•
remainingfrom such ancient processesas planetary accretion
and core-mantledifferentiation.A recentreviewof theseaspects
of the thermal history problem is given by Solomonet al.
[1981]. Of the currentlyoperativeprocesses,
radioactivedecay
contributesthe vastmajority of the heat generation,with minor
incrementscontributedfrom the motion of phaseboundaries
(e.g.,inner corefreezing)and tidal dissipation.To determinethe
partitioning of global heat loss betweenpresentlygenerated
and ancient heat, it is necessaryto solvefor the earth'sthermal
evolution,includingparticularlythe effectof solidstateconvection on mantle
IO
PLATE
RECYCLING
HOT SPOT
VOLCANISM
Fig. 1. Schematic ternary diagram showing the relative contributionsof eachmechanismof lithosphericheat transfertoward global
heat loss on solid planetsand satellites.For the moon, Mercury, and
Mars, thermal conductioncarriesall of the lithosphericheat flow. The
position of the earth, on which the majority of lithospheric heat
transport occursby plate recycling,is from Sclater et al. [1980] and
does not include the contribution to global heat loss from crustal
radioactivity. Hot spot volcanismdominatesheat loss on Iv, but the
relative fraction contributed by conductionis not known precisely
[Reynoldset al., 1980; O'Reilly and Davies, 1981]; at the position
shownon the diagram,the conductiveheat flow on Iv is equal to the
earth'saverageheat flux.
We pose three hypothesesfor lithosphericheat transfer on
nate heat flux on other planetary bodies:(1) that lithospheric
heat transportoccursprimarily throughplate recycling,(2) that
lithosphericconductiondominatesthe heat loss,and (3) that
heat is transportedthrough the lithosphereprincipally by hot
spot volcanism.These three hypothesesshould be regardedas
end-members;clearly, combinationsof mechanismsmay also
serveto deliver internal heat to the planetary surface.We test
eachhypothesisagainstthe known propertiesof Venus,and we
explore the implicationsof each for the character of tectonic
and volcanicactivity at the Venussurface.We find that none of
the individualhypotheses
can currentlybe excluded.Although
specificpredictionsof the three hypotheses
for surfacegeological evolution differ substantially,each hypothesisleadsto the
conclusionthat many,if not most,of the physiographicfeatures
of the Venussurfaceare geologicallyyoung.
VENUS HEAT FLOW
To quantify discussionsof specificmechanismsfor lithosphericheattransporton Venus,it is necessary
to adopta value
for the rate of globalheat lossfrom the planet.Sincethe surface
heat flux on Venus has not been measured,we assumeas a basis
for an estimatethat the heat lossper masson Venusis identical
to that on earth. The presentheat lossfrom the solid earth is
4.2 x 10•3 W, for a globallyaveraged
heatflowof 82mW/m2
[-Sclateret al., 1980-[.The massof Venus is 0.815 earth masses
[-e.g.,Ash et al., 1971-[,and the surfacearea of Venus is 4.60
x 108km2 rPettengillet al., 1980-].
The earth-scaled
valuesfor
global heat loss and surfaceheat flux for Venus are therefore
3.4 x 10•3W and74mW/m•, respectively.
The heat flux from the earth consistsof two parts: heat
generatedby currently operativeinternal processes
and heat
heat transfer. A number
of such calculations
have recently been performed using one of several parameterizationschemesfor heat transportby mantleconvection
[Sharpeand Peltier, 1978, 1979; Schubertet al., 1979a, b, 1980;
Schubert,1979; Sleep,1979; Stevensonand Turner, 1979; Turcotte et al., 1979; Daly, 1980; Davies, 1980; McKenzie and
Weiss,1980; Stacey, 1980; Turcotte, 1980; Cook and Turcotte,
1981; McKenzie and Richter, 1981]. Thesecalculationsindicate
that some fraction between 50 and 90% of the earth's heat loss
is contributedby presentlyoperativeheat generationprocesses,
with the specific value dependent on the type of parameterizationadopted and the assumptionsmade about heat
sources,the temperature dependenceof viscosity,and radial
zoning in the mantle. The remainderof the earth'sheat lossis a
consequence
of the secularcoolingof the planet.
There are several observationaland theoretical arguments
that provide somesupportfor the assumptionof equal ratesof
heat loss per massfor Venus and earth. Severalindependent
cosmochemical
Venus, each based on one of the mechanisms known to domi-
9237
models
for the formation
of the terrestrial
planetsyield roughlycomparablebulk abundancesby massfor
heat-generatingelementsin the two bodies [see Wood et al.,
1981]. The measured abundancesof U, Th, and K at the
Venera 8, 9, and 10 landing sites [Vinogradov et al., 1973;
Surkov et al., 1977; Surkov, 1977] and the measuredK20
abundancesat the Venera 13 and 14 landing sites[Barsukov,
1982] are generally comparable to values in terrestrial tho-
leiitesand alkali basalts.The total amountof radiogenic
'•øAr
in the Venus atmosphereis within a factor of 3 of that in the
earth'satmosphere[Hoffman et al., 1980-1,though whetherthe
lower amount
on Venus is attributable
to differences in out-
gassinghistoryor in the bulk K abundanceis not known.
Parameterized-convection
models of the thermal history of
Venus [Sharpeand Peltier, 1979; Turcotte et al., 1979; Solomon
et al., 1981; Phillipsand Malin, 1982] indicatethat for the same
assumptionsas thoseusedin modelsof the thermal history of
the earth, the evolution of the two planetshas been broadly
similar. In particular, secularcooling of Venus contributesto
the predictedpresentheat loss of Venus by an amount comparableto that for the earth.
We concludethat a scalingof heat loss by mass from the
earth to Venusis a reasonableworking hypothesis.The valueof
the Venusheat loss,however,evenundersucha hypothesis,has
considerableuncertainty.The earth'sheat flux may have been
25 to 50% greaterat timesof enhancedratesof seafloorspreading in the Phanerozoic[Hays and Pitman, 1973; Pitman, 1978;
TurcotteandBurke, 1978; Sleep,1979; Harrison, 1980; Sprague
and Pollack, 1980-1,so that the terrestrial heat lossdetermined
for the presentearth [Sclater et al., 1980] may be lessthan the
averageglobalheat losson time scalesof 108 years.If the
difference
in the amountof '•øArbetweenthe atmospheres
of
Venusand earth representsa differencein the planetaryabundanceof potassium,the Venus heat losswould be lessthan the
value assumedhere. The difference,however,would probably
9238
SOLOMON AND HEAD: LITHOSPHERIC HEAT TRANSPORT ON VENUS
be small.The presentcontributionof '•øK to the heat productionin terrestrialmaterialwithK/U = 10'• andTh/U = 3.7
by weight [Wasserburget al., 1964] is 15%; thus if the abundancesby weightof refractoryelementssuchas U and Th are
comparableon Venusand earthbut the abundanceof the more
volatile element K is lower by a factor of 3, the overall heat
productionper masswouldbe only 10% lessfor Venusthan for
the earth.
PLATE RECYCLING
The recyclingof lithosphereinto the underlyingmantleis the
major governorof terrestrialvolcanicand tectonicactivity and
of the formationof the large-scalephysiographicfeaturesof the
earth'ssurface.Creation of new lithosphereat mid-oceanridges
is the dominant volcanicprocesson the planet.Spreadingand
coolingof the oceaniclithosphere,the thermal boundary layer
of mantle convection,producesthe characteristicrelationship
betweenoceandepth and seafloorage [e.g., Parsonsand Sclater, 1977] as well as the distinctivepattern of ridge axis and
fracturezone physiography.Trenches,great thrust faults,and
volcanic
island arcs mark
the loci of subduction
of oceanic
lithospherebeneathanother oceanicplate. Island chainsand
many oceanicaseismicridges are productsof the motion of
surfaceplatesover mantle sitesof persistenthot spotvolcanism
[e.g., Wilson, 1965]. Passivemarginsof continentsmark the
sitesof former intracontinental rifting where new oceanswere
created.Tectonicallyactivemountainbeltsform wheresubduction occursbeneath overriding continental lithosphere,as in
the Andes,or where two continental massescollide after closure
of an interveningocean,as in the Himalayas [e.g.,Deweyand
Bird, 1970].
Plate recyclingis also the dominant mechanismfor heat
transfer across the outermost
100 km of the solid earth. The
most carefulanalysisof the partitioning of terrestrialheat flow
among the contributiong processesis that of Sclater et el.
[1980]. Theseworkersestimatethat about 65% of the present
heat loss of the earth is associatedwith the plate recycling
process,including the creation and cooling of oceaniclithosphereand continentalorogenyassociatedwith plate interactions. Another 20% of the heat loss occurs by conduction
throughoceanicand continentallithosphere,and 15% is generated by radioactivedecaywithin the continentalcrust.As noted
earlier, there have been extendedperiodsduring the Phanerozoic when the volumesof mid-oceanridges,and by inference
the globalratesof plate creation,havebeensubstantiallygreater than they are at present [Hays and Pitman, 1973; Pitman,
1978; Turcotte and Burke, 1978]. During suchtimes both the
total rate of global heat lossand the fraction of that heat loss
contributed by the plate recyclingprocesswere greater than
their presentvalues [Turcotte and Burke, 1978; Sleep, 1979;
Harrison,1980;SpragueandPollack,1980].
Becausesuch a large fraction of the earth'sheat is lost by
platerecycling,the typicalthermalgradientsin old oceanicand
continentallithosphereare muchlessthan theywouldbe in the
absenceof plate tectonics[e.g.,Sclateret el., 1980]. That conduction through the continental lithospherehas contributed
only modestlyto the earth'sglobal heat lossas far back as the
Archcanis stronglysuggested
by the observationof Burkeand
Kidd [1978] that productsof crustalremeltingare not observed
on a regionallyextensivebasisin exposedsectionsof lower
continentalcrustin the Archcanterrain of the SuperiorProvince. A clear implication of the processof lithosphericheat
transport on earth is that terrestrial continentaland oceanic
geothermscannotbe extrapolatedto Venus without making
internally consistentassumptionsabout both the global heat
lossand the fraction of that heat lossdeliveredby lithospheric
conduction.This point has not been addressedin severaldiscussionsof the thermal structureof the Venus interior [e.g.,
Warner, 1979;McGill, 1979; Kaula andPhillips,1981].
BecauseVenus and the earth have similar masses,bulk densities,and positionsin the solar system,and on the basisof the
assumedsimilaritiesin globalheat loss,a reasonablehypothesis
is that Venus, like the earth, losesmuch of its heat by lithospheric recycling.We shall evaluate this hypothesisand its
implications for the evolution of the Venus surface in this
section.
The hypothesisthat plate recycling presently occurs on
Venushasbeenchallengedin the literature on severalgrounds:
(1) that topographicfeaturesindicativeof terrestrialplate tectonicscannotbe 'seen'on Venus[Masurskyet al., 1980;Phillips
et al., 1981; Arvidson and Davies, 1981; Kaula and Phillips,
1981; Arvidsonand Guinness,1982; Schaber,1982], (2) that
most of the Venus surface is 'ancient,' as indicated by the
distributionof inferredimpact cratersand basins[Schaberand
Boyce,1977;Masurskyet al., 1980;Phillipset al., 1981],(3) that
becauseof the high surfacetemperatureon Venus, the lithosphereis less likely to subduct than oceaniclithosphereon
earth [Anderson,1981; Phillips and Malin, 1982], and (4) that
becauseof the high surfacetemperatureand from the observed
characteristicsof 'ridges' on Venus, plate recyclingis a less
'significant'processfor removing heat than on earth [Kaula
and Phillips,1981]. As we discussbelow,however,none of these
argumentsis sufficientlystrong to rule out plate recyclingon
Venuson the basisof presentevidence.
On TopographicFeaturesDiagnosticof Plate Recycling
The global topographyof Venus as measuredby the Pioneer
Venus orbiter [Pettengill et al., 1980] stimulatedconsiderable
interestin the questionof whether the nature of tectonicand
volcanic activity on that planet can be discernedfrom the
large-scalephysiography.In particular, the statementhas been
made repeatedlyin the literaturethat Venuslackstopographic
featuresindicativeof terrestrial-styleplate tectonics[Masursky
et al., 1980; Phillips et al., 1981; Arvidsonand Davies, 1981;
Arvidsonand Guinness,1982; Schaber,1982].
There are severalreasonswhy a searchfor featuresindicative
of lithosphericrecyclingon Venus may be more difficultthan
has been generallyappreciated.The horizontal resolutionof
the Pioneer Venus topographicdata is about 100-200 km
[Pettengill et al., 1980], so that features of lesserhorizontal
dimensionsthan thesevaluesare not resolvable.A topographic
map of the earth at comparableresolutionwould not resolve
such features,for instance,as many oceanictrenchesand the
axial valleys on slowly spreadingmid-oceanridges[Head et
al., 1981]. Further, oceanictopographyon earth is capableof
approximately50% greater relative relief becauseof the presenceof the water layer. In addition,the rate of deepeningof the
terrestrialoceaniclithosphere,or thermal boundarylayer, with
ageis proportionalto the differencein temperaturebetweenthe
bottom and top of the layer [Parsonsand Sclater, 1977]; on
Venus,with its higher surfacetemperature,the rate of deepening of the surfaceof any analog to oceaniclithosphereshould
be correspondinglyless.Both of theseeffectswould thusact to
reducethe reliefof Venusanalogsto spreadingridges,trenches,
and submarinehot spot tracescomparedwith their terrestrial
counterparts[Phillips et al., 1981; Head et al., 1981; Arvidson
SOLOMON
AND HEAD' LITHOSPHERIC
HEAT TRANSPORT
ON VENUS
,•ndDavies,1981; Kaula and Phillips, 1981]. While this reduced
relief would still permit the identification of large-scaleplate
tectonicfeatures(suchasmid-oceanridges,mountainbelts,and
hot spottraces)on earthusingonly altimetryat PioneerVenus
resolution,at issueis whether suchfeaturescould be correctly
interpretedif no other information were available [Head et al.,
1981;ArvidsonandDavies,1981].
Any tectoniccomparisonof the topographyof Venus and
earth shouldalsobe made in recognitionof the fact that many
of the physiographiccharacteristicsof terrestrial features of
plate tectonic origin are a direct result of the presentdistribution of continentsand ocean basins,the presentrange of
ridge spreadingrates,and the presenceof surfacewater. On a
planetwith a highersurfacetemperatureand negligiblesurface
water, many of thesecharacteristics
might be expectedto differ.
For example,the orthogonal pattern of mid-oceanridge segmentsand transformfaultsis held to be a diagnosticfeatureof
terrestrialplate tectonicsthat is visiblein the topographicdata
for some oceanic regions even at Pioneer Venus resolution
[ArvidsonandDavies,1981]. On the basisof laboratoryexperiments,however,it appearsthat this pattern may be a function
of materialproperties.Oldenburgand Brune[1975] foundthat
in wax analogsof spreadingcentersystems,establishmentand
maintenanceof an orthogonal ridge-transformpattern occurred only if the resistivestressesalong transform faults are
less than the shear vtrength of the lithosphere.If, compared
with the earth, the shearstrengthat the elevatedsurfacetemperature of Venus is reducedrelative to transform-faultresistance,then Venusmight displaylithosphericrecyclingwithout
the familiar orthogonalpatternat spreadingcenters.
Convergencezones on a Venus with lithosphericrecycling
may also differ substantially from analogous zones on the
earth. Island arc volcanism,thoughincompletelyunderstood,is
thought to result from partial melting initiated by the dehydration at depth of subductedoceaniccrust [e.g.,Lofgrenet al.,
1981]. If subduction occurred on Venus without the entrainment of water or hydrousphasesin the subductedmaterial, then dehydrationat depth would not occurand the Venus
equivalent of island arc volcanismneed not be present.The
deep-seatrenchesmarking the locus of subductionof oceanic
lithosphereare known to owe their physiographiccharacteristics,suchas relative depth and width, to the thicknessof the
elasticor viscoelasticportion of the subductedplate, the velocity of convergence,
and the nature of any adjacentsourcesof
sediment [e.g., Grellet and Dubois, 1982]; these parameters
would be expectedto differ for Venus. Becauseof the high
surfacetemperatureof Venus,a bettermodelfor someconvergencezoneson that planet might be basedon the notion of
lithosphericdelaminationas proposedfor zonesof terrestrial
continentalcollision[Bird, 1978,1979;Reutteret al., 1980;Bird
and Baumgardner,1981; Fleitout and Froidevaux,1982]. According to this model, a low-strengthzone at the base of the
crustpermitsdelaminationof the lithosphere,subductionof the
mantle portion, and deformation without subductionof the
more buoyant crustalmaterial. Such a model, applied to some
subductionzones on Venus, would lead to the prediction of
regions of intensesurfacedeformationsurroundingconvergencezonesbut little recyclingof surficial or crustal material
back into the underlyingmantle.
It is worth noting that there are topographicfeatureson the
Venussurfacewhichdiffer from any seenon the smallerterrestrial planets and which resemblefeatureson earth of plate
tectonic origin. These featuresinclude linear mountain belts
9239
suchas Akna and Freyja montes,continentalsizeplateaussuch
as Lakshmi Planum,and arcuateridgesand troughssuchas in
the Ut and Vesta Rupes region on the southwestmargin of
Ishtar Terra [Masursky et al., 1980; Head et al., 1981; McGill et
al., 1981, 1982; Campbellet al., 1982; Phillipsand Malin, 1982].
We thereforeregard the tectonicinterpretation of the topographic featuresof the Venus surfaceas uncertain at the resolution of our current information.Both an improvementin
the resolution of surface features on Venus and detailed investi-
gationsof the effectsof Venussurfaceconditionson the characteristiclandformsproducedby variousgeologicalprocesses
are
neededto improveour understandingin this area.
On the Age of the VenusSurface
A number of approximatelycircular featureson the Venus
surface,ranging in diameter from 20 to over 1000 km, have
beenidentifiedon the basisof variationsin radar reflectivityor
topography [Goldsteinet al., 1976, 1978; Schaberand Boyce,
1977; Campbellet al., 1979; Campbelland Burns, 1980]. The
hypothesisthat many or all of these featuresoriginated by
impact has led to the inferencethat much of the surface of
Venusis geologicallyancient.In particular,if the quasi-circular
regionsof low radar backscatter200 km or greaterin diameter
have beencorrectlyidentifiedas impact basinsdating from the
heavy bombardment of the inner solar system,then Venus
containsmajor geologicunits with surfaceagesapproaching4
b.y. [SchaberandBoyce,1977;Masurskyet al., 1980].
Becauseof the high surfacetemperature,topographicfeatures on Venus are subjectedto viscousrelaxation at rates
much higher than for the other terrestrialplanets[e.g., Weertman, 1979]. In particular, if the surfacetemperatureof Venus
has been comparable to its present value for much of the
planet'shistory,then a 4-b.y.-oldimpact basinon Venusshould
havenegligibletopographicrelief [Solomonet al., 1982]. Large
circulardepressions
on Venus,such as Atalanta Planitia, are
substantiallyyoungerthan 4 b.y. and are unlikely to be impact
basins.Such topographicdepressions
may owe their origin to
one of the tectonicmechanismsproposedfor intracontinental
basinson earth, i.e., cooling and subsidencefollowing lithosphericextensionor thermaluplift and erosion[SleepandSnell,
1976; McKenzie, 1978]. The large quasi-circular,radar-dark
featureson the Venus surfacemay still be impact basins,degraded by essentiallycompleterelaxation of long-wavelength
topography.Suchfeaturesmay also be older versionsof circular lowland regionsof Venus,however,and by implication also
of tectonicorigin.
The question of the age of the Venus surfaceshould be
regardedas open.There are alternativetectonicinterpretations
of the large circular featuresvisiblein radar images.Even the
population of smaller 'craters' 20-200 km in diameter may
contain featuresof volcanicas well as impact origin [Saunders
and Malin, 1976; Campbell and Burns, 1980; McGill et al.,
1982]. Theseuncertaintiesdo not precludethe possibilitythat
portions of the Venusiansurfaceare billions of years old, nor
do they permit the rejectionof the hypothesisthat much of the
Venusian surface, as with the solid surface of the earth, is
geologicallyyoung.
On LithosphericBuoyancyon Venus
A necessarycondition for subductionis that the subducted
material be of greaterdensitythan the surroundingmantle. The
negative buoyancy of the subductedslab of lithosphere on
9240
SOLOMON AND HEAD:
LITHOSPHERIC HEAT TRANSPORT ON VENUS
earth is a consequence
of its relativelycoolertemperatureand
also a result of phasechangeswhich occur at shallowerthan
normal depth in the cooler slab [Toks& et al., 1971; Turcotte
and Schubert,1971]. The argumenthas been advancedthat a
Venus lithospheresimilar in crustalthicknessand other physical propertiesto oceaniclithosphereon earth would have less
negativebuoyancybecauseof the higher surfacetemperature
and smaller lithosphericthicknesson Venus [Anderson,1981;
Phillipsand Malin, 1982]. Anderson[1981] has arguedfurther
that an additional factor which might make the Venus lithospherepositivelybuoyant is the enhanceddepth range of the
basalt stability field on Venus compared with that on earth,
thereby openingthe possibilityfor crustalthicknesses
substantially greaterthan in terrestrialoceanbasins.
Thesebuoyancycalculationsare sensitiveto the assumptions
made about parameterswhich are at best poorly known for
Venus,includingthe thicknessof a crustallayer, the locationof
the mantle residuumremainingafter extractionof partial melt
to producethe crust,and the partitioningof planetaryheat loss
among plate recyclingand conduction.Thus the uncertainty
attachedto an individualresultis large,and both positiveand
negativevaluesfor net lithosphericbuoyancyare possibleresultsat our presentstageof understanding[Phillipsand Malin,
1982].
That the Venusian lithosphereis less negatively buoyant
than old oceaniclithosphereon earth is likely, yet even this
conclusionis not a strong argument against some form of
subductionon Venus. On the earth, oceaniclithospherewith
seaflooras youngas 10 m.y. is subducted[Menard, 1978], yet
such lithosphereis certainly lessnegativelybuoyant than unsubductedoceaniclithosphere150 m.y. old. Parsons[1982], in
fact, has demonstratedthat the distribution of ocean floor area
versusage on the earth is consistentwith the hypothesisthat
plate consumptionis uniformlydistributedwith age exceptfor
seaflooryoungerthan 10 m.y. old. Subductionof young seafloor may, of course,be sustainedby a finite-amplitudeinstability; i.e., 10 m.y. old oceaniclithospheremay be only marginally unstable or even stable gravitationally yet continue to
subductif attachedto a negativelybuoyantslab.Sucha finiteamplitudeinstabilitymight equallywell be invoked to sustain
subductionon Venus. Once subductionof an aging piece of
Venusianlithosphereis initiated,by this argument,the subduction processmay be sustainedby thermal anomaliesand elevated phasetransitionsin the subductedslab,and the characteristic age of subductedmaterial may be substantiallyyounger
than that initially subducted.If, as suggestedearlier, lithosphericdelaminationoccurson Venus,then the mantleportion
of the lithospherewould be negativelybuoyant even if the
crustalportion is not. Thus a thickercruston Venuscompared
with terrestrial oceanbasins[Anderson,1981] need not restrict
subductionof the mantleportion of the lithospherewheresuch
delamination
occurs. To the extent that the mantle residuum
complementaryto a basalticcrustresidesin the lithosphere,in
fact,the net lithosphericbuoyancymay be largelyinsensitiveto
crustal thickness.
On the Kaula-Phillips Argumentson Heat Removal
by LithosphericRecyclingon Venus
Kaula and Phillips [1981] estimatedan upper bound on the
fraction of the global heat losson Venus contributedby plate
tectonics.While not an argument against plate recyclingon
Venus per se, their conclusionthat the heat transported by
plate tectonicsis 15% or lessof the global heat flux on Venus
compared with 65% on earth [Sclater et al., 1980] would, if
true,providean importantcontrastbetweenthe two planets.
We do not regard the 15% result of Kaula and Phillips
[1981], however, as necessarilyvalid. Their calculation was
based on an identificationof candidatespreadingridges on
Venus, a measurement of the rate of decreasein elevation with
distancefrom each segmentof the ridge, an application of
thermalboundarylayertheoryto scalethe physicalparameters
of mantle convection in the earth to those of Venus, and a
conversionof the measuredrates of change in elevation to
estimatesfor the heat delivered along each spreadingridge
segment.Further elaboration of the boundary layer scaling
argumentsis givenby PhillipsandMalin [1982].
The candidatesfor ridges chosen by Kaula and Phillips
[1981] includeBeta Regio and Aphrodite Terra. Both of these
features contain large regions which, together with Ishtar
Terra, composethe highlandsprovinceof Venus [Masurskyet
al., 1980]; terrestrial continentsrather than oceanridgesmay
provide a preferableanalog to much of the Venus highland
terrain [e.g., Head et al., 1981]. Having made this selectionof
ridges, Kaula and Phillips then note that the Venus ridge
heightsdo not showa narrow distributionabout a mode as do
terrestrial ocean ridgeswell removedfrom hot spots [Parsons
and $clater, 1977] and that theseridgesdo not form an interconnectedglobal system. While these observationsmay be
sufficientto rule out either Beta Regio or Aphrodite Terra as
Venusiananalogsto terrestrialmid-oceanridges[eft $chaber,
1982], neither observationis a compellingargument against
lithosphericrecyclingon Venusin general.
Having posedthe hypothesisthat plate recyclingdominates
lithosphericheat transport on Venus,we shouldconsiderthe
expectedform of Venusian spreadingcenters.A reasonable
guide to the rate of heat loss per length of spreadingridge
followsfrom the spreading-platemodel of oceaniclithosphere
[Williams andyonHerzen, 1974; Kaula andPhillips,1981]:
q, = pC•,LvAT
(1)
wherep andCparethedensityandspecific
heatof thelithosphericplate, L is the lithosphericthickness,v is the average
half spreadingrate, and AT is the differencein temperature
betweenthe bottom and top of the lithosphere.Subductionof
the lithospherebeforeit has reachedthermal equilibrium will
reduceq, from that givenby (1). Becauseof the highersurface
temperatureon Venus, AT will be lessfor Venus than for the
earth'soceanbasins,thoughboundarylayer scalingarguments
suggestthat the differencein A T betweenVenusand earth may
be somewhatless than the differencein surfacetemperatures
[Kaula and Phillips, 1981; Phillips and Malin, 1982]. For a
spreadingridge systemon Venus to deliver to the surfacean
amount of heat comparable to that delivered by ridges on
earth,eitherthe characteristic
spreadingrate or the total length
of ridges(or both) must be greater on Venus than on earth.
Becauseeither possibilitywould likely be accompaniedby a
lessercharacteristicage of subductedlithosphere on Venus
than on earth, the product of ridgelengthand mean spreading
rate would exceedthat quantityfor the earth by a ratio greater
than simplythe ratio of the valuesof A T for the two bodies.
Theseconsiderations
suggestthat the mostlikely candidates
for spreadingridgeson Venusshouldbe characterizedby rapid
ratesof spreadingand correspondingly
smallratesof changein
topographicheightwith distancefrom the ridge axis.Phillips
and Malin [1982] have shownthat a fast spreadingridge, such
as the East Pacific Rise, would have only modesttopographic
SOLOMON
AND HEAD: LITHOSPHERIC
HEATTRANSPORT
ON VENUS
'Convergence'Continent
'
Zone'
Spreading
/Center
'Ocean
H•ghlands
____
Basin'
Rolling
Pla•__ns._.
•
r
LowLands
Plate
Recycling
Hypotnes•s
Fig. 2. A schematic
illustrationof theplaterecyclinghypothesis
for
lithosphericheat transporton Venus.The rolling plains(0 to 2 km
elevationwith respectto the planetarymodal radius)and lowlands(0
to -2 kin) arc analogousto terrestrialocean basinsand include
spreading
centersandconvergence
zonesnot currentlyresolvable
from
altimctryor imagingdata. The highlands(2 to 11 km elevation)arc
analogousto terrestrialcontinents.
relief at Venusconditionsand that sucha ridge on Venuscould
not be detectedfrom PioneerVenus altimetry given the 200-m
standard error for thesedata [Pettengill et al., 1980]. By the
plate recyclinghypothesisfor Venus, the rolling plains and
lowlands provinces (as defined by Masursky et al. [1980])
should be the Venusian analogs to terrestrial ocean basins.
Altimetric and imaging data from rolling plains and lowlands
areasmust be obtainedat resolutionssuperiorto thoseof data
currently availablein order to searchfor the predictedextensive systemof fast spreadingridgesand thereby to provide a
rigoroustestof the platerecyclinghypothesisfor Venus.
Assessment
and Implicationsof the Hypothesis
We find no convincingargument to reject at presentthe
hypothesisthat someform of plate recyclingdominateslithosphericheat transferon Venus.This conclusiondoesnot mean
that plate tectonicscan presentlybe demonstratedon Venus-or
that we may not at somefuture date obtain improved information
on the Venusian
surface or interior
9241
extensiveby this hypothesisand should be concentrated at
divergent plate boundaries.Intraplate volcanic activity may
also be present.Volcanismat subductionzonesmay be minor
at presentif no free water or hydrousphasesare subducted.
Tectonic activity shouldbe widespreadand dominated by the
large-scalehorizontal motions and mutual interactions of the
plates. As noted above, however, the specificphysiographic
characteristicsof many tectonicfeaturesat plate boundaries
may differ from those on earth becauseof the greater surface
temperatureand negligiblesurfacewater on Venus.Theoretical
models are neededfor the expectedform of such featuresat
Venus-surface
conditions.
By the plate recycling hypothesis,the rolling plains and
lowlands provincesare analogsto terrestrial ocean basinsin
terms of formativeprocess,crustalcomposition,and geologically youthful age. The spreadingcentersare expectedto be
characterizedby rapid spreadingrates and modest relief in
comparisonto earth. The Venushighlandsmay be analogsto
terrestrial
continents,
though
whether
theanalogy
extends
to
compositionor simply to crustal thicknessis uncertain. The
surfaceagesof highland geologicunits on Venus may be considerablygreater than the agesof units in the plains and lowlands. As with terrestrial continents, highland lithosphere
should have greater buoyancy than the lithosphere beneath
plains and lowlandsand shouldthereforebe more difficult to
subduct.Mountainous terrain should,by this hypothesis,form
by the collision of highland blocks (Himalayan analog) or at
the locusof subductionbeneathhighland lithosphere(Andean
analog).Both highlandregionsin generaland mountain belts
in particular will be modified as they age. Becauseof the high
surface temperature and negligible water on Venus, viscous
relaxation may be a more rapid processthan weatheringand
erosionfor reducinghighlandrelief,in contrastto earth.
LITHOSPHERIC CONDUCTION
that will allow the
The dominant mechanismfor lithosphericheat transport on
questionto be firmly resolved.Rather,the conclusiondemands
that at our presentlevel of understanding,the hypothesisthat the smallerterrestrialplanetsis conductionthrough the single
plate recycling is important for heat flow on Venus should global lithosphericshell. That lithosphericconduction domicontinue 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 interThe lithosphericrecyclinghypothesisfor Venusis 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 planetare listed andDavies,1981].
If the global heat loss on Venus is deliveredto the surface
in Table 1. As on earth, volcanic activity on Venus should be
TABLE 1. Implicationsof End-Member Hypothesesfor LithosphericHeat Transporton Venus
Surface Characteristics
Plate Recycling
LithosphericConduction
Hot Spot Volcanism
Volcanic activity
extensive;activity
dominantly at
divergent boundaries
minor
extensive; active
centersnearly
Tectonic
widespread;dominated
by plate interactions
possiblyextensive
primarily vertical
features
cover surface
deformation
of
tectonics
thin lithosphere
Agesof surfaceunits
rolling plains and
lowlandsgeologically
young( < 10s years)
Nature
of mountain
belts
productsof plate
convergence
Nature of highlands
analogsto terrestrial
continents
Nature of rolling plains
and lowlands
analogs to terrestrial
ocean basins
unconstrained; ancient
impact featuresmay
be preserved
much of surface
young(<• 107 years)
anomalouslythick crust
and lithosphere
volcanic
thickened crust and
thickened
lithosphere
thinned crust and
lithosphere
constructs
volcanic
crust
'normal'-thickness
volcanic crust
9242
SOLOMON AND HEAD' LITHOSPHERIC HEAT TRANSPORT ON VENUS
entirely by lithospheric conduction, then the conductive
geothermmay be estimatedfrom the averagesurfaceheat flow
of 74 mW/m2 derivedearlier.This heat flow is very closeto
L•thospher•c
L•t hospher•c Thickening
•
•
H•ghlands
Roll•ng
Plains
Lowlands
twice the heat flux in old oceanic lithosphere near thermal
equilibrium,givenas 38 + 4 mW/m2 by Sclateret al. [1980].
Thus if the Venuslithospherehas a thermal conductivitysimilar to the averagevalue of 3.1 W/m K adoptedfor the terrestrial oceaniclithosphere[Parsonsand Sclater,1977], then the
thermal gradientin the Venus lithosphereis 24 K/km, or approximately twice that in old ocean basins. Geotherms for
oceaniclithosphereat equilibrium [Sclater et al., 1980] and for
average lithosphere on Venus, assumingthat heat transfer
occurssolelyby lithospheric
conduction,
areshownin Figure3.
A consequefice
oftheVenusgeotherms
shown
in Figure3 is
that the lithospherewould be substantiallythinner than on
earth for comparablematerial propertiesof the mantle. The
baseof the thermallithospherein oceanbasinsis 1350 + 275øC
[ParsonsandSclater,1977].A temperatureof 1350øCwouldbe
reachedat a depth of about 40 km on Venusaccordingto the
geothermsshown in Figure 3. The base of the elasticlithospherein oceanicregionsis well approximatedby the position
of the 500 + 150øC isotherm [Watts et al., 1980]. An elastic
lithosphereon Venuslimited by this sametemperature,a value
governedby the ductile flow law for dry olivine [Goetze and
Evans,1979], would be at most 10 km in averagethicknessfor a
purelyconductiveVenus.
On both of thesegrounds,it is difficult to envisionmechanismsfor supportingthe 13 km of relief on Venus[Pettengillet
al., 1980] for geologicallylong periodsof time exceptperhaps
throughsheartractionsassociatedwith mantle convectiveflow.
A likely implication,therefore,of the hypothesisthat conduction dominatesheat transferon Venusis that all surfacetopographicrelief on scalessmallerthan the characteristichorizontal scalesof mantle convectionis geologicallyyoung. Preservation of topographicrelieffor extendedperiodsof time might
occur more readily if elevated regions on Venus are
500
i000
•
5O
1500
Venus:
L•thospher•c
"-,.•o
.,.., nOnly
z•
Base of
Elastic
'
characterizedby a mantle heat flux that is lower than average.
That such regionally low values of mantle heat flux could
persistfor hundredsof millionsto billionsof yearsfor a planet
with lithosphericheat transferdominatedby conduction,however, is unlikely. Alternatively,sincethe history of the surface
temperature is uncertain [Pollack, 1971, 1979], topographic
relief may have persistedfor geologicallylong periodsif the
characteristictime for viscousrelaxation is comparable to or
lessthan the time sinceformation of the presentatmospheric
'greenhouse.'The viscousrelaxationtime is unlikely to exceeda
few hundredmillion years [Solomon et al., 1982],however,so
this possibilitywould requirea geologicallyrecent greenhouse.
The Venus geotherm shown as a solid line in Figure 3 is
basedon the simplificationthat all of the heat lost by the planet
is generatedbeneaththe lithosphere.The lithosphericthermal
gradientwould be lessenedif concentrationof radioactiveheat
sourcesin the Venus crust has occurred,thereby reducingthe
heat flux from the mantle [e.g.,PhillipsandMalin, 1982].Since,
by the conductionhypothesis,plate recyclingand attendant
remelting of basalticcrust at subductionzoneswould not have
progressedon Venus to the current stageof concentrationof
heat sources in continental crust on the earth, a reasonable
T• øC
0
Lithospheric Conduction Hypothes•s
Fig. 4. A schematicillustration of the conductionhypothesisfor
lithosphericheat transport on Venus. On average,the lithosphereis
only about 40 km thick and may be readily deformedby tractions
associatedwith convectionin the underlyingasthenosphere.
Modestly
elevatedregionsin the rollingplainsmay be areasof recentlyextended
and thinnedcrustand lithosphere,
areaswhichwill subsideto lowland
elevationsduring lithosphericcoolingand thickening[e.g.,McKenzie,
1978]. The more elevatedhighlandsmay be areas of thickenedcrust
and lithosphereresultingfrom lithosphericcompression.
upper bound to the fraction of Venus heat flux contributedby
crustalradioactivityis the terrestrialvalue of about 15% [Sclater et al., 1980]. A greater concentrationof radioactive heat
sources into the crust on Venus than on the earth would also be
difficult to reconcile with the lower '•øAr abundance in the
Venus atmosphere[Hoffman et al., 1980]. With the 15% value
assumedfor the fraction of global heat loss generatedin the
-
-- L•thosphere
crust, the mantle heat flux on a Venus with the same heat loss
Earth:
•
Equ•l•br•urn
per massasthe earthwouldbe29 x 10•2 W, and the temperaturegradientin the Venuslithospherewould,as shownby the
dashedcurve in Figure 3, be only slightly modified from that
I00
150
discussed above.
I
I
Fig. 3. Average lithosphericgeothermson Venus assumingthat
conductionis the only modeof lithosphericheat transfer.The solidline
showsthe casewhenall of the heat lossfrom Venusis generatedbelow
the lithosphere;the short-dashedcurveindicatesthe casewhen 15% of
the Venusheat lossis generatedby radioactivitydistributeduniformly
in a crust 30 km thick. Also shownare the terrestrialgeothermfor an
old ocean basin in thermal equilibrium [Parsonsand Sclater, 1977;
Sclater et al., 1980] and the range of isothermsinferred to define the
baseof the elasticlithospherein oceanbasins[Watts et al., 1980].
The hypothesisthat lithosphericheat flux on Venus occurs
principally by conductioncannot be rejectedon the basisof
presentlyavailableinformation.A schematicillustrationof this
hypothesisis given in Figure 4, and a summary of the implicationsfor the surfacegeologyof Venusis givenin Table 1.The
averagelithosphericthermalgradientsare predictedto be substantially greater, and the lithosphericthicknesscorrespondingly less, than for volcanic mechanismsof heat transport,
including both plate recyclingand hot spot volcanism.The
lithosphericstrengthand resistanceto deformationshouldgenerally be lessfor this hypothesisthan for the hypothesisthat
either plate recyclingor hot spot volcanismdominateslithosphericheat transport on Venus.As a result,lithosphericand
SOLOMONAND HEAD' LITHOSPHERIC
HEAT TRANSPORTON VENUS
crustalthinning and thickeningmay occurin responseto tractions exertedon the baseof the lithosphereby mantle convective flow (seeFigure 4). The agesof surfacegeologicalunits are
not constrainedby the conductionhypothesisand may spana
greatrange.Becauseof the expectedhigh rate of viscousrelaxation, however,topographicrelief on Venus should be either
geologicallyyoungor dynamicallymaintained.Sincethe planform of mantle dynamicsis not likely to be steady on time
scalesapproachingthe ageof theplanet,it wouldbe reasonable
to concludeby this hypothesisthat none of the topographic
featuresof Venus is likely to date from the first half of the
planet'shistory.
HOT SPOT VOLCANISM
Individual volcanichot spotstransportheatfrom a planetary
interior by the ascentand coolingof magmaand by conduction
through a locally thinned lithosphere.Phillips and Malin
['1982] have proposedthat hot spot activity on Venus may
providethe dominantmechanismof lithosphericheattransfer.
The implicationsof sucha hypothesismay be quantifiedby
consideringfirst the archetypicalterrestrialhot spot,the source
of the Hawaiian-Emperor island-seamountchain ['Wilson,
1965;Mor•Ian, 1971].The rate of heatlossQvdue to volcanism
at a hot spotis givenapproximatelyby
9243
every200-km squareof the Venussurface.Whether the mantle
convection system on Venus could supply magma to this
number of volcaniccentersessentiallysimultaneouslyon geologicaltime scalesis an openquestion.
Two effectsmay increasesomewhatthe contributionof volcanismat a Hawaiian-typehot spot to the total heat flux of
Venus.The eruption temperatureof typical hot spot magmas
on Venus may be greater than on earth, due either to a lower
FeO/(MgO + FeO) ratio [Goettel et al., 1981; Phillips and
Malin, 1982] or to lower volatile abundances[McGill, 1979;
Anderson,1980] in the Venusmantle comparedwith that of the
earth.The rate of heat lossQofor a Venus'Hawaii' increasesby
0.2 x 109 W for every100K increase
in AT in equation(2),so
that even a 500 K differencein eruption temperaturebetween
Venus and earth would raise Q,by no more than 40%. A
second effect, also noted in the last section, is that concentration of radioactiveheat sourcesin the Venus crust may
reduce the heat flux from the mantle
and thus the number
of
individual hot spotsneededto remove that heat. Adopting, as
above,the 15% value [Sclater et al., 1980] as an upper bound
to the fraction of Venusheat lossgeneratedby crustalradioactivity, the contribution of volcanismat a Hawaiian-type hot
spotto thetotalheatlosswouldstillbeabout1partin 10'•.
A planetwith 10'• hot spotsasactiveasHawaiiwouldhavea
total volcanic flux sufficientto affect the planetary surface
profoundlyon geologicallyshorttime scales.If the global rate
Qv= p(C•,
AT+ Amf)dt
(2) of heatlossQ is contributedentirelyby hot spotvolcanism,the
wherep andC•,arethedensity
andspecific
heat,respectively,
of planetaryvolcanicflux is
thevolcanic
material,AHœis theheatof fusionof themagma,
dV*
Q
dV
ATisthediffere.!•,
between
theeruption
temperature
ofthe
magma
and
the•,,ii•i•i•ent
surface
temperature,
and
dV/dt
isthe
volumetricflux of magmawith time.Shaw[-1973]hasestimated
thattheHawaiianhotspothasproduced
8.5 x 105km3 of lava
between the time of formation
of the bend in the Hawaiian-
Emperorchainand the present.Adoptingthe figure42 m.y. for
the age of the bend I'Dalryrnpleand Cla•tue, 1976] gives an
averagefor dV/dt of 2 x 10-2 km3/yr,thoughvalueslargerby
up to a factor of 5 havebeenappropriatefor time.intervalsof a
few million years [Shaw, 1973]. To estimatethe averagevolcanic heat loss contributedby the Hawaiian hot spot for the
last42m.y.,weassume
p = 2.8g/cm
3,C•,= 1.2J/gK, AHœ=
400 J/g, and AT = 1300 K [Solomonet al., 1981]. Then from
(2),Qo- 3.5 x 109W, or 0.008%oftheearth'sglobalheatloss.
The total contributionof hot spot volcanismto the overall
heat lossof the earth is minor [Sclater et al., 1980]. Of all the
proposedhot spots [e.g., Morgan, 1971; Burke and Wilson,
1976], Hawaii hashad probablythe greatestvolumetricflux of
magmain recentgeologichistory.Thus whetherthe important
terrestrialhot spotsnumber20 [Morgan, 1971] or closerto 100
[Burke and Wilson, 1976], the summed contribution of volcanicallydeliveredheatfrom suchhot spotsto globalheat flow
is less than 1%.
If a hot spotwith the samemagmaticflux as Hawaii were on
Venus, the rate of heat loss due to volcanism would be some-
what lessthan on earth becauseof the higher surfacetemperatureand thereforethe lesserextentof cooling.For an eruption
temperatureand other parametersequal to thoseassumedfor
theearth,equation
(2)givesQ,= 2.5 x 109 W, or 0.007%of
the global Venus heat lossif the heat lossper massof Venus
and the earth are equal.Thus to removethe total heat flux from
Venusby volcanismat hot spotswith ratesof volcanicactivity
similarto thatof Hawaii,a totalof 10'• suchhotspotswouldbe
needed.This figureamountsto an averageof one 'Hawaii' on
dt
-
p(C•, AT + AHœ)
(3)
For Q = 34 x 10•2 W andotherparameters
asadoptedabove,
dV*/dt = 280km3/yr.Averagedoverthesurfaceof Venus,this
fluxamountsto a rateof surfaceburialof 6 x 10-7 km/yr,or
an additional
1-km thickness of new volcanic material over the
entireplanetevery2 m.y. This globalburial rate is comparable
to the estimatedlower bound on the burial rate for Io [Johnson
et al., 1979].
Terrestrial hot spotscontribute heat to the lithosphereby
mechanisms
other than extrusivevolcanism.Lithosphericthinning beneathhot spotsleadsboth to an enhancedconductive
heatflux and to modestelevationsin surfacetopography,such
as the Hawaiian swell associatedwith the hot spot beneath
Hawaii [Detrick and Crough,1978]. This lithosphericthinning
occurson time scalesshort compared with the characteristic
time for lithosphericheatingor coolingby conduction[Detrick
andCrough,
1978];asthenoSPheric
stoping,
or in situdelamination and sinkingof the lower lithosphere,may be involvedin
the rapid thinningprocess[Turcotte, 1982].The lithosphereis
apparently thinned by about a factor of 2 to 3 beneath the
Hawaiian hot spot [Detrick and Crough, 1978; Crough, 1978;
Detrick et al., 1981;Sandwell,1982;yonHerzen et al., 1982]. Ifa
large number of hot spots are operative on Venus, then the
conductivegradient would be lessenedand the effectivelithospheric thicknessincreasedin those areas distant from hot
spotscomparedwith the global averagescalculatedin the last
section.
Theseideasmay be quantifiedas follows.Let • be the globally averagedheat flux, let q0 be the averageheat flux in areas
distantfrom hot spots,letf be the fractionalarea of planetary
surfaceoccupiedby hot spots,and let r/q0 be the conducted
heat flux in the vicinity of a typical hot spot.The quantity r/
may be regarded as the factor by which the lithosphere is
9244
SOLOMON
ANDH•D: LITHOSPHERIC
HEATTRANSPORT
ONVENUS
characteristicallythinned by someunspecifiedprocessat a hot
spot.Clearly,
Large Volcanic Construct
Pervasive
Volcanic
Centers
H•ghlands
Roll•ng
O =fnqo + (1 -f)qo + qv
(4)
Lowlands
Plains
•,•'
•
///
whereq• is the contributionto the globalheat flux deliveredby
volcanism.
If volcanicallydeliveredheat is smallcomparedwith conducted heatat hot spotsand elsewhere
(i.e.,q, ,• •), thenq0 is given
by
o
qo= 1--f +fil
(5)
Under the assumptionthat the Hawaiian hot spotis a guideto
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,
q0 •> •/2. That is, the lithosphericthicknessfar from hot spots
can be increasedby a factor only as large as 2 comparedwith
the global averagederivedin 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
near equality only if f •_ 1. Lithosphericthicknesses
approaching typical terrestrial valueswould be possible,but only in a
few areasof very limited spatialextent.
If, in contrast, lithosphericheat transfer on Venus occurs
dominantly by magma transport at individual hot spots(i.e.,
q• -• 0 in equation(3)),the averagelithosphericthicknessis not
constrainedby global heat flow [cf. O'Reilly and Davies,1981]
and can be considerablygreaterthan estimatesobtainedin the
last section.In particular, the averagelithosphericthickness
can be greaterthan the 100-kmdepth of isostaticcompensation
inferred from the gravity anomaliesover a number of topographicallydistinctfeatureson the planet [Phillipset al., 1979,
1981; Reasenberget al., 1981]. Thus large topographicrelief
couldbe supportedpassivelyand on nearly a globalbasiseither
by local isostaticcompensationor by regional compensation
and lithosphericstrength.
The hypothesisthat hot spot volcanism dominates lithosphericheat transferon Venus(Figure 5) cannotbe excludedon
the basisof our presentknowledgeof the surface.The hypothesiscarriesimportant implicationsfor the interpretatonof surface physiographicfeatures (Table 1). The Venus surface,if
most of the heat loss occursby volcanism,should be densely
Hot
Spot
Volcanism
Hypothesis
Fig. 5. A schematicillustrationof the hot spot volcanismhypothesisfor lithosphericheat transport on Venus. Volcanismcarriesthe
majority of the heat throughthe lithosphere,and activevolcaniccenters on the surface of Venus should be widespread.The highland
terrain consistsof large volumes of volcanic depositsand probably
thickenedcrust.The surfacematerial is generallyvolcanicand geologically young.
sideredin this paper, and the implicationsof their model involve a combination of aspectsof the scenariosdepicted in
Figures4 and 5.
CONCLUSIONS
Without more detailed information on the Venus surface,all
surface material should, in either case, consist of volcanic de-
of the mechanismsfor lithospheric heat transfer considered
here (lithospheric recycling, conduction, and hot-spot volcanism)shouldbe regardedas potentiallyimportant for Venus.
Though eachof thesemechanismshasbeentreatedindividually
in this paper, weightedcombinationsof mechanismsmust, of
course,also be considered[e.g., Phillipsand Malin, 1982]. The
possibilitythat the dominant mechanismearly in the history of
the planetwasdifferentfrom that at present[e.g.,Phillipset al.,
1981] shouldbe recognizedas well.
Each of the possibleend-membermodels for lithospheric
heat transfer on Venus carries different implicationsfor the
detailed characteristicsof Venus landforms. The predicted
characteristics
(Table 1) shouldserveasa guideto the interpretation of existingand future imaging and topographicdata on
the Venussurface.For each of the hypotheticalmodelsfor heat
transfer,however,there is a clear implication that either many
of the surfacegeologicalunits or much of the surfacetopography is geologicallyyouthful.
A common presumptionin discussionsof comparativeplanetary evolutionhas beenthat planetary sizeplaysa crucial role
in controlling the internal heat budget and the volcanic and
tectonicresponses
to global heat loss.The comparisonof Venus
and earth holds a key spot in the test of this presumption
[Phillipset al., 1981;Head andSolomon,1981]. Establishingthe
natureof the tectonicand volcanichistoryof the Venussurface
and inferring the dominant mechanismsfor lithosphericheat
transferon that body remain elusivegoalsof the highestpriority for our understandingof the evolution of the terrestrial
planets,includingour earth.
posits.In particular, the Venus highlandswould most likely
have beenformedby volcanicconstruction.Tectonicactivity in
the absenceof large-scalehorizontal motions should be principally restrictedto that associatedwith verticalmotionsof the
lithosphere.
The Phillips and Malin [1982] model for hot spot tectonics
on Venus includesboth volcanictransport of heat and lithosphericthinning beneathmajor volcaniccenters,with the latter
processmaking a greatercontributionto the global heat loss.
This modelmay be viewedas a combinationof the mechanisms
of lithosphericconduction and hot spot volcanismas con-
Acknowledgments.Discussionsat the VenusTectonicsWorkshop
(April 20-21, 1982),sponsoredby the Lunar and Planetary Institute
and organizedby Roger Phillips,were helpfulin sharpeningthe argumentspresentedhere.We appreciateconstructivereviewsof an earlier
draft of this manuscript by David Armbruster, Kevin Burke, Rob
Comer, Ken Goettel, Bill Kaula, Mike Malin, and GeorgeMcGill. We
thank Ray Arvidson,Bob Detrick, Bill Kaula, George McGill, Roger
Phillips,JerrySchaber,and Don Turcottefor preprintsof papersprior
to publication,and Dorothy Frank and Jan Nattier-Barbara for their
assistancein the preparation of this manuscript.This researchwas
supportedby NASA grants NSG-7081 and NSG-7297 at MIT and
grantsNGR-40-002-088 and NGR-40-002-116 at BrownUniversity.
covered with thousands of distinct centers of current or recent
volcanic activity. Most of the physiographicfeatureswould
have surfaceslessthan 10 m.y. old, thougha few areasof older
terrain may have escapedrecentresurfacing.If the majority of
the heat deliveredat hot spotsis due to lithosphericthinning,
then hot spotswould nearly have to coverthe surfaceof Venus
for the lithosphericthicknessfar from hot spotsto be substantially greater than the globally averagedvalue. Much of the
SOLOMON AND HEAD: LITHOSPHERIC HEAT TRANSPORT ON VENUS
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Anderson, D. L., Plate tectonicson Venus, Geophys.Res. Lett., 8.
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