Mercury’s lithospheric thickness and crustal density, as inferred from MESSENGER observations

Abstract:

The gravity field and topography of Mercury measured by the MESSENGER spacecraft have provided insights into the thickness of the planet’s elastic lithosphere, T_e. We localized the HgM006 free-air gravity anomaly and gtmes_125v03 shape datasets to search for theoretical elastic thickness solutions that best fit a variety of localized coherence spectra between Bouguer gravity anomaly and topography. We adopted a crustal density of ρ_crust =2700 kg m^-3 for the Bouguer gravity correction, but density uncertainty did not markedly affect the elastic thickness estimates. A best-fit solution in the northern smooth plains (NSP) gives an elastic thickness of T_e =30–60 km at the time of formation of topography for a range of ratios of top to bottom loading from 1 to 5. For a mechanical lithosphere with a thickness of ~2T_e and a temperature of 1600 °C at the base, this solution is consistent with a geothermal gradient of 9–18 K km^-1. A similar coherence analysis exterior to the NSP produces an elastic thickness estimate of T_e =20–50 km, albeit with a poorer fit. Coherence in the northern hemisphere as a whole does not approach zero at any wavelength, because of the presence of variations in crustal thickness that are unassociated with elastic loading.

The ratios and correlations of gravity and topography at intermediate wavelengths (harmonic degree l between 30 and 50) also constrain regional crustal densities. We localized gravity and topography with a moving Slepian taper and calculated regionally averaged crustal densities with the approximation ρ_crust=Z_l/(2πG), where Z_l is the localized admittance and G is the gravitational constant. The only regional density estimates greater than 2000 kg m^-3 for l=30 correspond to the NSP. Density estimates outside of the NSP were unreasonably low, even for highly porous crust. We attribute these low densities to the confounding effects of crustal thickness variations and Kaula filtering of the gravity dataset at the highest harmonic degrees, both of which tend to introduce a downward bias to crustal density estimation. An alternative analysis—which corrected for crustal thickness variability and was restricted to regions with gravity/topography coherence greater than 0.6—yielded an aggregate crustal density of ρ_crust=2602 ± 470 kg m^-3 for Mercury’s high northern latitudes.