Variations of Heat Flux and Elastic Thickness of Mercury derived from Thermal Evolution Modeling

The very low obliquity of Mercury causes important surface temperature variations between its polar and equatorial regions [1]. At the same time, its 3:2 spin orbit resonance leads to longitudinal temperature variations [2]. The combination of these two effects creates a peculiar surface temperature distribution with equatorial hot and warm poles, and cold poles at the geographic poles of the planet. Models that considered the insolation pattern were found compatible with the low-degree shape and geoid from MESSENGER [3]. The models of [3] showed that the insolation pattern imposes a long wavelength thermal perturbation throughout the mantle, whose temperature distribution is strongly correlated with the surface temperature variations. In addition to surface temperature variations, lateral variations of crustal thickness can also affect the temperature distribution of the lithosphere and mantle as it was suggested for Mars [4]. With the topography and gravity data from MESSENGER, a series of models of Mercury’s crustal thickness have been derived assuming constant or variable crustal density, based on the composition of the surface [5].

In this study we include crustal thickness and surface temperature variations of Mercury in the geodynamical code GAIA [6], similar to [4]. We tested several crustal thickness models from [5]. All the simulations are carried in a full 3D spherical geometry, use the extended Boussinesq Approximation, and consider core cooling and radioactive decay. We also use a pressure- and temperature-dependent viscosity in the mantle. The crust is  enriched in heat producing elements (HPEs) compared to the depleted mantle according to a fixed enrichment factor. We model the entire thermal evolution of Mercury to determine the variations of surface and core-mantle boundary heat fluxes in addition to the temporal evolution and distribution of the elastic lithosphere thickness.

Our models indicate that the surface temperature variations of Mercury induce a long-wavelength pattern on both the elastic lithosphere thickness and the heat fluxes, while the crustal thickness variations lead to smaller scale variations of the two quantities. Our models show that different geochemical terranes such as the North Volcanic Plains (NVP) or the High Mg-Region [7] could have experienced drastically different thermal histories throughout the evolution of Mercury.

Future data from the BepiColombo mission [8] will provide a better resolution for the gravity and topography of Mercury, as well as measurements of its surface composition. These data could be used to provide additional estimates of the elastic lithosphere thickness and to constrain the time of formation of the associated geological features. This will help to improve our geodynamical models and in turn constrain Mercury’s thermal evolution.

References:

[1] Margot et al., 2012. [2] Siegler et al., 2013. [3] Tosi et al., 2015. [4] Plesa et al., 2018. [5] Beuthe et al., 2020. [6] Hüttig et al., 2013. [7] Weider et al., 2015. [8] Benkhoff et al., 2021.