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Unraveling the interior dynamics of planetary bodies Partial melting and the thermo-chemical evolution of terrestrial planets N. Tosi, A.-C. Plesa, C. Huettig, M. Maurice, S. performance scales nearly linearly with up to several Padovan, A. Nikolaou, Department of Astronomy thousand cores (Figure 1). This makes Gaia one of and Astrophysics, Technische Universität Berlin the few mantle convection capable to make effective use of large-scale computational infrastructures. In Short • Motivation: Surface features like volcanoes and tectonic plates observed on terrestrial bodies of the Solar System are a direct consequence of the dynamics of the deep interior, which is strongly affected by the production of partial melt. • Goals: Understand the thermo-chemical evolution of the interior of terrestrial bodies by linking present-day observations with state-of-the-art numerical simulations of solid-state convection in planetary mantles. • Constraints: Space missions deliver data on the morphology and composition of the surface and crust of planetary bodies, on their gravity and magFigure 1: Code performance using a grid with 11.5 × 106 (red netic field, and surface heat flow, all of which can line) and 54 × 106 (green line) computational points. With the be used to constrain models of the thermochemi- largest mesh, Gaia scales nearly linearly with up to 54000 cores. cal evolution of the interior. Mercury’s Thermal Evolution and Impacts • HPC features: To investigate the interior dynamThe surface of Mercury is covered by craters, ics of planetary bodies we use the mantle convecmostly formed in the early phases of the planet’s evotion code Gaia: a finite volume, massively parallel lution. Each impact generates a shock front which code capable to run efficiently on several tens of propagates in the interior. The associated shockthousand cores. energy is ultimately converted into a temperature anomaly which, for very large events, can interact Solid-state mantle convection is the primary mecha- with and modify the underlying convection pattern nism that controls the dynamics and thermal evolu- (Figure 2). tion of the Earth and terrestrial planets. Thermal and compositional buoyancy drive the slow creeping flow of silicate mantles, which is ultimately responsible for the heat transport from planetary interiors to the surface and for the formation of geological structures impact such as volcanoes, rifts, and tectonic plates. location As the style of convection is known rather poorly either from seismological observations – available solely for the Earth and to a lesser extent, the Moon, and representative only of the present-day state of Temperature 0 1 the mantle – or, indirectly, from the study of geological structures, our main understanding stems from Figure 2: Temperature distribution in the interior of Mercury numerical simulations of the dynamics of the mantle. shortly after a basin-forming impact. Such phenomena strongly In the present project we perform numerical simu- modify the planform of convection and the production of partial lations of thermochemical mantle convection using melt. the 2D-3D code Gaia [1], with the goal of constraining the thermochemical evolution of the interior of We focused on the effects of large impacts on Mercury, the Moon, and Mars. Gaia’s performance the melt production in the basin area and on the has been tested using meshes with up to 54 million depth of the source region for this melt. Our models grid-points. For a sufficiently large mesh, the code show that the properties (volume and depth of the bep00041 source region) of the melt sheets observed within large basins are mostly the result of the state of the mantle at the time of impact. By tuning the models to the properties of the well-studied melt sheet of the Caloris basin, our model can explain the observed properties of the major impact basins on Mercury. This result indicates that the combination of numerical models of the effects of large impacts on the interior dynamics with remote sensing observations of the melt sheets can place important constraints on the mantle evolution as a function of time. Present-day mantle plumes on Mars The presence of mantle plumes at present-day in the interior of Mars has been inferred from geologic mapping of young lava flows of only several million years in the Elysium and Tharsis volcanic provinces. Another indication comes from the analysis of the Martian meteorites, which suggests that the temperatures derived from shergottitic samples, which are the youngest samples from Mars, possibly indicate the presence of thermal anomalies in the interior of Mars. However, whether such thermal anomalies in the interior of Mars are active at present and how long have they been stable underneath large volcanic centers on Mars is unknown (Figure 3). We have started investigating what are the condi- InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport), due to launch in 2018 (http://insight.jpl.nasa.gov), will aim at improving our understanding of the thermochemical evolution and present-day state of Mars [2]. Early Stage of Planetary Evolution and Mantle Mixing On Earth and other terrestrial bodies, large scale compositional heterogeneities may have formed during the early stage of planetary evolution by fractional crystallization of a liquid magma ocean or later by partial melting of the mantle. The mixing efficiency of such heterogeneities is directly linked to the convection vigor and style (i.e., mobile or stagnant lid). In order to make reliable predictions of the mixing efficiency in a 3D geometry using a strongly temperature-dependent viscosity as appropriate for the interior of terrestrial bodies, numerical simulations with high spatial resolution need to be employed (Figure 4). Such simulations need to be carried out until they reach statistical steady state in order to accurately quantify mantle mixing and to derive suitable scaling laws. Because of these requirements, up to now, most of the studies of mixing in a 3D geometry have been limited to simple isoviscous models. Nevertheless, thanks to the good parallel scalability of our code, Gaia we will be able to investigate scenarios with strongly variable viscosity for 3D configurations. (a) (b) 0.9 Temperature 1 Figure 4: 3D numerical simulations of thermal convection with a strongly temperature-dependent viscosity that will be used to accurately quantify mantle mixing in the stagnant lid regime. WWW Figure 3: Present-day mantle plumes in the interior of Mars shown together with the MOLA topography for the southern highlands. https://sites.google.com/site/nictosi/ More Information [1] C. Hüttig, N. Tosi, W.B. Moore (2013), tions leading to the formation and survival of mantle Phys. Earth Planet. Inter., doi: plumes in the interior of Mars up to the present-day. 10.1016/j.pepi.2013.04.002 Preliminary results show that while the depth dependence of the viscosity plays an important role for the [2] Plesa et al., (2016). J. Geophys. Res. – Planets, manuscript in press. presence of strong thermal anomalies, the crustal structure may influence their location. Furthermore, Funding our models together with the upcoming seismic and heat flow measurements from the NASA mission Helmholtz Gemeinschaft (VH-NG-1017) bep00041