Mercury’s gravity field and geology after MESSENGER: the Mercury Orbiter Radio-Science Experiment (MORE) perspective

The NASA MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission returned a wealth of data for the geophysical characterization of Mercury and its internal structure. The Mercury Laser Altimeter (MLA) and the Radio Science (RS) system were fundamental for the determination of the planet topography, gravitational field, rotation, and tides (e.g. Genova et al., 2019). Particularly, the radio science data combined with the topography allowed to derive global gravity field maps. Because of the high eccentricity of MESSENGER’s orbit, the gravity field was estimated up to the degree and order 80 of spherical harmonic expansion for the regions with latitude > 67 N. This has been possible thanks to the lower altitudes reached during the last phase of the mission (up to 25 km). For the same reasons the resolution in the southern regions is much lower, rarely exceeding degree 12. A comparison between maps of gravity anomalies, topography, and surface features shows that the gravity field is overall well correlated with topography. Large positive gravity anomalies are associated with major impact basins (e.g. Caloris and Sobkou) while negative anomalies correlate with smaller impact craters (e.g. Rachmaninoff crater), especially in the northern polar regions. Another large positive anomaly is associated with the Northern Rise (a regional uplift located toward the northern pole). The origin of the latter geologic feature (and the associated gravity anomaly) is still debated and it remains one of the main open question in Mercury’s geophysics. The northern polar regions, characterized by the emplacement of young smooth volcanic units, are well correlated with negative gravity values. Finally, while the regions at latitudes > 15 S have a good coverage of gravity data, it is difficult to make specific assumptions on the gravity field associated to morphotectonic structures located in the southern hemisphere. The ESA BepiColombo mission has been successfully launched in October 2018 and is currently cruising toward Mercury with another radio science experiment onboard: the Mercury Orbiter Radio-Science Experiment (MORE). This experiment will provide a uniform coverage of the hermean gravity field, filling the gap inherent in the MESSENGER determinations. Indeed, the nearly circular orbit of BepiColombo will allow to reach the degree 50 of spherical harmonic expansion in the southern hemisphere of Mercury, shedding new lights on the geophysics of this region of the planet. Among the other interesting features located toward the southern pole, the Enterprise Rupes, dislocating the northwestern rim of the Rembrandt impact basin, is a regional, about 1000-km-long, tectonic feature showing up to 2-3 km elevation drops across. The emplacement of the latter contractional feature might have resulted into a local significant thickening of the lithosphere, which is one of the most interesting targets for the geophysics of the southern hemisphere. Here, using MESSENGER topography, the Mission Analysis, Operations, and Navigation Toolkit Environment (MONTE) for BepiColombo mission simulation, and a series of geological assumptions, we derive the MORE expected results in terms of gravitational field maps. Our results confirm that MORE will improve the resolution and accuracy of the Mercury’s gravity field, especially for the southern hemisphere, allowing the correlation between gravitational anomalies and regional morphotectonic structures, including the Enterprise Rupes, whose putative gravity anomaly should be detectable by BepiColombo.