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Mechanisms for Lithospheric Heat Transport on Venus Implications For 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 hypoth- esisthat 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 the levelsof 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 convec- tion 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 transport occursprimarily 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
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