| Institution(s) supplémentaire(s) | Department of Geology, University of Liège, 4000 Sart Tilman, Belgium; Department of Earth and Environmental Sciences, KU Leuven, 3000 Leuven, Belgium; Royal Observatory of Belgium, Avenue Circulaire 3, Brussels 1180, Belgium; Department of Mechanical and Aerospace Engineering, Sapienza University of Rome, 00184 Rome, Italy |
Abstract | Unique physical and chemical characteristics of Mercury have been revealed by measurements from NASA’s MESSENGER spacecraft. The crust of Mercury was built over the first billion years of the planet by intense volcanic activity. Mantle melting and emplacement of lava to the surface produced a secondary magmatic crust varying spatially and over time in composition. These surface compositions for major elements can be converted to expected mineralogy based on phase equilibria under highly reducing conditions. The crustal mineralogy is supposed to be dominated by silicates (olivine, clinopyroxene, orthopyroxene, plagioclase, trydimite) and CaMg sulfides (Namur and Charlier 2017). Crustal mineralogy and porosity (produced by impacts and regolith formation) have implications for the density of the crust. The silicate mineralogy at the surface translates to pore-free crustal densities of 2,800-3,150 kg.m-3. Maximum crustal density (3,100-3,150 kg.m-3) is found in High-Mg regions that are modelled to be forsterite-dominated and plagioclase-poor. The lightest crust (2,750-2,800 kg.m-3) is found in Al-rich regions such as the North Volcanic Plain that are plagioclase-dominated. Calculations of the thickness of the crust can be made using the MESSENGER gravity and topography data and lateral variations of crustal density should be considered. Using the assumptions that the surface density is representative for the density at depth, we find that the calculated local thickness of the crust is correlated with the degree of mantle melting (Beuthe et al. 2020). Low-degree melting of the mantle below the Northern Volcanic Plains produced a thin crust while the highest melting degree in the ancient High-Mg region produced the thickest crust, excluding mantle excavation by an impact in that region. However, uncertainties remain about the abundance of oxygen in surface rocks, with implications for the mineralogy that could be dominated by silicates and abundant Si metals, potentially produced by smelting reaction of lavas with graphite (McCubbin et al. 2017). The unconstrained abundance of metal phases and graphite in crustal rocks would require crustal densities to be reevaluated. Independent approaches to evaluate crustal density and porosity from gravity data are also necessary. These interpretations and their caveats will be discussed in light of the MESSENGER data and the BepiColombo measurements that are expected from 2025.
References: Beuthe et al. 2020 GRL 47: e2020GL087261; McCubbin et al. 2017 JGR Planets 122: 2053-2076; Namur and Charlier 2017 Nature Geosci 10: 9-13.
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