The overturn of mantle cumulates following the crystallization of the lunar magma ocean – particularly the sinking of ilmenite-bearing cumulates (IBC) – provides an explanation for several aspects of the Moon’s evolution. However, the growth of a stagnant lid due to the temperature dependence of the viscosity tends to prevent IBC from sinking. Here, we investigate the dynamics of the overturn based on a composition-dependent rheology coupling diffusion and dislocation creep of major lunar mantle minerals via three alternative mixing models: isostrain, isostress and Minimized Power Geometric (MPG). The pre-overturn structure is obtained from a fractional crystallization model of the lunar magma ocean, which predicts the formation of a 36-km-thick IBC layer. The possibility of overturning this layer strongly depends on the choice of the rheological mixing model. The isostress model allows for a rapid and complete overturn, while the isostrain and the experiment-based MPG models do not allow IBC to sink. If IBC started sinking and mixing with the underlying mantle during magma ocean solidification, IBC could be initially distributed across a layer with a thickness of up to 150 km, whose partial overturn is always possible, independent of the rheological model. These results highlight the importance of improving rheological models for relevant lunar materials. In all cases, the overturn occurs via small-scale instabilities.

The InSight mission [1] landed in November 2018 in the Elysium Planitia region [2] bringing the first geophysical observatory to Mars. Since February 2019 the seismometer SEIS [3] has continuously recorded Mars’ seismic activity, and a list of the seismic events is available in the InSight Marsquake Service catalog [4]. In this study, we predict present-day seismic velocities in the Martian interior using the 3D thermal evolution models of [5]. We then use the 3D velocity distributions to interpret seismic observations recorded by InSight. Our analysis is focused on the two high quality events S0173a and S0235b. Both have distinguishable P- and S-wave arrivals and are thought to originate in Cerberus Fossae [6], a potentially active fault system [7]. Our results show that models with a crust containing more than half of the total amount of heat producing elements (HPE) of the bulk of Mars lead to large variations of the seismic velocities in the lithosphere. A seismic velocity pattern similar to the crustal thickness structure is observed at depths larger than 400 km for cases with cold and thick lithospheres. Models, with less than 20% of the total HPE in the crust have thinner lithospheres with shallower but more prominent low velocity zones. The latter, lead to shadow zones that are incompatible with the observed P- and S-wave arrivals of seismic events occurring in Cerberus Fossae, in 20° - 40° epicentral distance. We therefore expect that future high-quality seismic events have the potential to further constrain the amount of HPE in the Martian crust. Future work will combine the seismic velocities distribution calculated in this study with modeling of seismic wave propagation [8, 9]. This will help to assess the effects of a 3D thermal structure on the waveforms and provide a powerful framework for the interpretation of InSight’s seismic data. [1] Banerdt et al., Nat. Geo. 2020; [2] Golombek et al., Nat. Comm. 2020, [3] Lognnoné et al., Nat. Geo. 2020, [4] InSight MQS, Mars Seismic Catalogue, InSight Mission V3, 2020, https://doi.org/10.12686/A8, [5] Plesa et al., GRL 2018, [6] Giardini et al., Nat. Geo. 2020, [7] Taylor et al., JGR 2013, [8] Bozdag et al., SSR 2017, [9] Komatitsch & Tromp, GJI 2002.