Finding similarities between the deep mantle structures of Earth & Mercury

Abstract

Large Low Shear-Velocity Provinces (LLSVPs) in Earth have been recognized in seismic data for several decades. The anomalies are however still poorly understood for reasons including their inconvenient location in the deep mantle, inhibiting progress in our understanding. Comprehension of these structures is important, because of their speculated major role in the formation and evolution of the Earth, and her residents. The gravity anomaly associated with the LLSVPs, the C22-lobs, are roughly observed in the gravity field of Mercury too. Therefore, the gravity and topography data about Mercury is assessed for indications of similar deep mantle features as observed in Earth. The advantage of Mercury is its thin mantle, that allows for a relatively shallow location of the core-mantle boundary (CMB), hence shallow deep mantle anomalies. The cause of the thin mantle is yet unknown, currently suggested explanations include vaporization of the mantle due to the immense heat of the solar nebula or a hit-and-run collision scenario that ripped off the mantle of Mercury. In this research it is investigated how deep mantle structures like LLSVPs and CMB topography could account for the gravity field of Mercury. It is concluded that deep mantle structures with comparable size and density characteristics as the LLSVPs in Earth could exist. Compensation by CMB topography would require a rough CMB. The results are highly affected by the limited knowledge about crustal compensation in Mercury and the truncation of the gravity field. Better crustal models of Mercury could be obtained by investigating local gravity anomalies in relation to topographic features, considering flexural isostasy and using normal modes. In addition to that, BepiColombo is currently en route to Mercury to gather more global data of Mercury, that would highly aid in our understanding of the crust and mantle of Mercury. A relation comparable to the hypothesised relation between hotspot volcanism and LLSVPs on Earth is also investigated for Mercury. A relation was not necessarily observed, however it was also not ruled out and requires more attention as soon as the crustal models have improved and more global coverage of volcanic features on Mercury is available.