Breuer, D.

Grott, M.

Spohn, Tilman

English

Institute of Transport Research:Publications

The Thermo-Chemical Evolution of Mercury revisited. M. Grott1, D. Breuer1, and T.Spohn1, 1German Aerospace Center, Berlin, Germany (matthias.grott@dlr.de)Introduction: The thermal evolution of Mercury andthe history of volcanism on the planet critically dependon the abundance of heat producing elements and the rateof heat transport in the planetary interior. With MES-SENGER in orbit around Mercury since March 2011,new data on the surface abundance of radioactive ele-ments as well as the planet’s moment of inertia have beenobtained. Data from the gamma ray spectrometer indi-cate an average surface abundance of 1150±220 ppm K,220±60 ppb Th, and 90±20 ppb U [1]. This implies aK/Th ratio of 5200±1800, comparable to other terrestrialplanets.The moment of inertia factor C/MR2 was estimatedto be 0.353, and the normalized mantle moment of iner-tia Cm/C was estimated to be 0.452. These values indi-cate the presence of a large iron core, and while previousestimates for the core size were close to 1800 km, newinterior structure models require core sizes of 2000-2100km [2]. Bulk sulfur contents required to satisfy theseconstraints are close to 10 % [2], and SO2 could be thevolatile driving pyroclastic volcanism [3]. Photogeolog-ical evidence suggests that volcanism was a globally ex-tensive process even after 3.8 Gyr [4], and the northernplains were likely emplaced in a flood lava mode by high-temperature, low-viscosity lava. Elemental abundancesof Mg, Al, and Ca indicate that surface rocks were de-rived from partial melts at high melt fractions between20 and 30 % [5], indicating a composition intermediatebetween basalt and terrestrial komatiite.Major constraints for thermal evolution models arethe inferred small radial contraction of Mercury [6] andits magmatic history [4]. While previous models re-quired a large bulk content of long-lived heat producingelements in the planetary interior to minimize planetaryheat loss [7], recent models have shown that the presenceof a thermally insulating regolith layer is sufficient toslow planetary cooling even for more volatile rich com-positions [8].Modeling: Using the above constraints, we havereinvestigated the coupled thermal and crustal evolutionof Mercury. Thermo-chemical evolution models are cal-culated using the model by [8], which takes the presenceof a thermally insulating regolith layer into account. Weconsider models with a core size of 2050 km, which sig-nificantly reduces the silicate fraction of the planet alongwith its inventory of heat producing elements. Further-more, the mantle Rayleigh number is reduced by a factorof three with respect to models with a core radius of 1850km. We tie the bulk concentration of heat producing ele-Figure 1: (a) Histogram of successful models as a func-tion of crustal enrichment factor λ. (b) The cessationtime for crustal production as a function of Λ for suc-cessful models. (c) Upper mantle temperature Tm as afunction of time for successful models. Lines ending be-fore 4500 Myr indicate the cessation of mantle convec-tion.1376.pdf43rd Lunar and Planetary Science Conference (2012)2ments through the enrichment factor Λ to their measuredsurface concentrations [1]. For the concentrations givenabove, an enrichment factor of one corresponds to a bulkuranium content of 90 ppb, while an enrichment factor offour would correspond to 22.5 ppb bulk uranium content.Parameters most strongly influencing model resultsare the initial upper mantle temperature Tm, the mantleviscosity η, the initial temperature jump across the core-mantle boundary ∆T , the crustal thermal conductivitykc, the regolith thickness Dreg , the volume change uponmantle differentiation δV/V , and the enrichment factorof radiogenic elements in the crust as compared to theprimordial mantle Λ. We have run more than thirty thou-sand Monte-Carlo simulations varying these parametersbetween 1600-2000 K, 1019-1021 Pa s, 0-300 K, 1.5-4W m−1 K−1, 0-5 km, 1-5%, and 1-4, respectively, andrejected all models which show radial contraction in ex-cess of 3 km. In addition, models resulting in crustalthicknesses of less than 20 km or models continuing toproduce crust to the present day have been disregarded.These constraints are satisfied by only a few percent ofthe calculated models.Results: Fig. 1 shows the results of the calculationsfor 393 successful models, where models which show aradial contraction of less than 3 km are shown in green,while models satisfying the tighter constraint of a totalcontraction less than 2 km are shown in red. A histogramgiving the fraction of models for a given enrichment fac-tor is shown in Fig. 1a, and only models with Λ < 3satisfy the constraints posed by the global contractionand crustal evolution. These low values of the enrich-ment factor are consistent with melts derived at high meltfractions and a komatiitic composition [5].The time of cessation of global crustal productiontDc is given in Fig. 1b as a function of the crustal en-richment factor, where we have ruled out models withpresent day volcanic activity. Λ and tDc are negativelycorrelated as expected, because larger crustal enrichmentfactors result in more efficient planetary cooling and anearlier cessation of global crustal production. Crustalgrowth continues up to at least 2.5 Gyr, consistent withthe reported young ages for some volcanic planes onMercury [4]. Successful models have average crustalthicknesses between 20 and 80 km.Typical temperature profiles for successful modelsare shown in Fig. 1c, where the upper mantle temper-ature Tm is shown as a function of time. Although aver-age mantle temperatures differ by up to 300 K, all modelshave similar cooling histories with secular cooling ratesaround 50 K Gyr−1. Therefore, mantle temperatures arenot strongly constrained by thermal evolution models,and warm as well as cold models are equally plausible[2]. Lines ending before 4500 Myr indicate the cessationof mantle convection, at which point the upper mantletemperature Tm is no longer defined. Therefore, slug-gishly convecting models as well as purely conductivemodels are consistent with the constraints posed by Mer-cury’s global contraction and magmatic history. Man-tle viscosity, crustal thermal conductivity, regolith thick-ness, and the volume change upon mantle differentiationare not constrained by our models.If sulfur is the light element in Mercury’s core, sulfurcontents above 6% are needed to prevent large scale corefreezing, which would result in significant radial contrac-tion in excess of the observed values. However, if siliconinstead of sulfur would be the light core constituent, corefreezing would not result in significant amounts of radialcontraction, because silicon partitions almost equally be-tween solid and liquid phases on the iron rich side of theeutectic [9]. In this case, core temperatures in excess of1860 K would be required to sustain a liquid outer core[10], and such high temperatures are reached by only afew models investigated here.Conclusions: Thermo-chemical evolution modelsfor Mercury using observed abundances of heat produc-ing elements and large core sizes are consistent with thesmall radial contraction and magmatic evolution of theplanet if small crustal enrichment factors between 1 and 3are assumed. This finding is consistent with a komatiiticcomposition for the crust, which is likely derived fromsource regions with high melt fractions [5].References [1] Peplowski et al., Science, 333, 1850-1852, 2011. [2] Rivoldini et al., Icarus, 213, 451-472,2011. [3] Kerber et al., EPSL, 285, 263-371, 2009. [4]Head et al., Science, 333, 1853-1855, 2011. [5] Nittleret al., Science, 333, 1847-1849, 2011. [6] Watters et al.,EPSL, 285, 283-296, 2009. [7] Hauck et al., EPSL, 222,713-728, 2004. [8] Grott, et al., EPSL, 307, 135-146,2011. [9] Kuwayama and Hirose, Am Mineral. 89, 273-276, 2004. [10] Fei et al., LPSC 42nd, 1949, 2011.1376.pdf43rd Lunar and Planetary Science Conference (2012)

The Thermo-Chemical Evolution of Mercury Revisited

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