Open Access Publications from the University of California

## Investigation of the interior of Mercury through the study of its gravity, topography, and tidal response

• et al.
Abstract

With the goal of furthering our understanding of the interior structure of Mercury, this work tries to answer the following two questions. What can the response of the planet to solar tides reveal about the interior structure? What is the thickness of the crust and what are the implications for the interior?

By comparing the models developed here for the tidal response of Mercury with the response measured by the MErcury Surface Space ENvironment GEochemistry and Ranging spacecraft (MESSENGER), the rheology of the mantle of the innermost planet is investigated.

The measured tidal deformation indicates that, unless the rigidity of mantle materials is unexpectedly high, the mantle is relatively cold.

Geochemical arguments based on the composition of the surface of Mercury as measured by MESSENGER have been used to put forward the hypothesis of the existence of a solid FeS layer at the bottom of the mantle.

The tidal modeling indicates that the presence of the FeS layer is unlikely.

To further constrain the interior structure of Mercury the thickness of the crust is calculated by computing geoid-to-topography ratios over the surface of the planet.

The inferred crustal thickness, $35\pm18$ km, has three interesting implications.

First, this relatively thin crust allows for the possibility that basin-forming events excavated mantle material from Mercury's mantle.

If this material is still exposed on the surface it can potentially be observed by instruments onboard MESSENGER and future missions at Mercury.

Second, the volume of silicate materials present in the crust of Mercury represents about 10\% of the total silicate materials in the planet, the largest value among the terrestrial planets. This implies that Mercury had the highest efficiency of crustal production.

Finally, by combining the estimate of the crustal thickness with the measured abundances of heat-producing elements on the surface of Mercury a lower bound can be placed on the amount of heat production in the mantle at a time following the accretion and differentiation of the planet, approximately 4.45 Ga ago.

This information is useful for future models of the thermochemical evolution of the innermost planet.