Bernt, Irene

Breuer, Doris

Collinet, Max

Plesa, Ana-Catalina

Schwinger, Sabrina

Institute of Transport Research:Publications

Thermochemical modeling of the lunar mantle - Effects of an initially lay-ered composition• Irene BerntGerman Aerospace Center (DLR), Institute for Planetary ResearchIrene.Bernt@dlr.de• Ana-Catalina PlesaGerman Aerospace Center (DLR), Institute for Planetary Research• Sabrina SchwingerGerman Aerospace Center (DLR), Institute for Planetary Research• Max CollinetGerman Aerospace Center (DLR), Institute for Planetary Research• Doris BreuerGerman Aerospace Center (DLR), Institute for Planetary ResearchThe Moon was initially covered by a global magma ocean as a consequence of the Moon-forming impact. Upon cooling,the magma ocean underwent fractional solidification, which led to an initially layered composition in the lunar mantle.The aim of our work is to investigate the effects of the layered initial mantle composition on the convection and thesubsequent melting of the lunar mantle.In our work we use the mantle convection code GAIA [1] to model the thermochemical evolution of the Moon. Weconsider a compositionally heterogeneous mantle and compare our results with a homogeneous lunar mantle scenario.For both scenarios (i.e., homogeneous and heterogeneous mantle), we use a petrological model [2] to calculate the initialdensity, solidus and liquidus curves, density change due to depletion, and, in case of a heterogeneous lunar mantle, theinitial temperature profile and the initial layered structure. Our models use a 2D quarter cylinder geometry. We employan Arrhenius law to calculate the temperature and depth dependent viscosity, and we account for core cooling, radioactivedecay, and mechanical mixing. The mantle composition is tracked via a particle-in-cell method [3], where tracer particlescarry information about material properties such as density, melting temperature, degree of depletion, and amount of heatproducing elements. We account for latent heat consumption during melting and consider both the increase of solidusand the changes in density of the residual material due to mantle depletion. Our models track the timing of the meltingevents and how much of the components melted.We calculate the amount of secondary crust produced during the evolution and require that successful models fit thepresent-day secondary crust thickness with values between 2 and 10 km. This range accounts for basaltic lava flows thatcomprise less than 1% of today’s crust [4] and the Mg-suite rocks, that account for about 6% to 30 %, though recentfindings show that at least some rocks of the Mg-suite originated from impact melts [5].Our results show that a model with a homogeneous initial mantle composition either produces too much crust to matchtoday’s estimates, or needs a very cold initial temperature profile, which is unlikely to be established from a solidifyingmagma ocean, and then produces melt too late to match the ages of the Mg-suite rocks. The heterogeneous models canmatch the estimates when at least part of the IBC layer takes part in the mantle convection.References [1] Hüttig C. et al. (2013) PEPI; [2] Schwinger S. and Breuer D. (2021) PEPI; [3] Plesa A.C. et al. (2013)IGI Global; [4] Head J.W. (1976) Rev. Geophys.; [5] White L. F. et al. (2020) Nature Astron.1Figure 1: Convection in an initially layered lunar mantle. In panel a) the initial layered composition is shown (usingthe density field) directly after the solidification of the lunar magma ocean, whereas in panel b) the same case is shown500 million years later. The models use an anorthositic crust with a low thermal conductivity of 2 Wm−1K−1 and fourmantle compositional layers (top to bottom): IBC = ilmenite bearing cumulates; Cpx = clinopyroxene rich cumulates;Opx = orthopyroxene rich cumulates, Olv = Olivine rich cumulates. The figure is shown to illustrate the convection in amultilayered lunar mantle in a case where IBC takes part in the convection.2

Thermochemical modeling of the lunar mantle - Effects of an initially layered composition

https://elib.dlr.de/191376/1/Abstract_Ada_Lovelace2022_final.pdf

Thermochemical modeling of the lunar mantle - Effects of an initially layered composition

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