Using MESSENGER Data to Model the Thermochemical Evolution of Mercury's Interior

Introduction: Mercury, the smallest terrestrial planet, has been classified as a geochemical endmember of the inner solar system, but is the solar system’s least studied planet [1]. Understanding how Mercury’s interior evolved will provide insights about the formation conditions of the inner solar system, how magma evolves under highly reduced conditions, and the best constraints for modeling how mass and heat are transferred in a thin mantle. MESSENGER provided information about the surface composition of Mercury from three spectrometers (the XRS, GRS and NS); and used geophysical instruments (the MDIS, MLA, and Radio Science systems) to determine both the surface geological features and investigate the core and mantle’s composition and structure [2]. These data indicated Mercury is made up of four layers: a crust (~35 km thick), a mantle (365 km), and a core (2040 km) [3,4]. This data placed constraints on Mercury’s tectonic and thermal history, as lobate scarps were found indicating that tectonic shortening has occurred [5], and that Mercury’s crust is generally richer in S and Mg and poorer in Fe than the other terrestrial planets [6]. The specific compositional differences of the surface have been split into nine regions [7]; here, we focus on the Northern Volcanic Plains to determine how the interior of Mercury thermochemically evolved. The Northern Volcanic Plains, which formed about 3.5 Ga [8], is the largest smooth igneous deposit on the surface of Mercury. This was the last major volcanic depositional event on the Mercurian surface, and as such will provide insights into Mercury’s interior. Under additional assumptions, such as a homogeneous mantle composition and no significant magma fractionation occurred during ascent, the major element composition of the NVP can constrain both the source rock of the mantle, and the melting processes in the mantle. Motivation: Prior thermochemical evolution models of Mercury used an Earth-based solidus which accounts for a significantly higher Fe concentration and oxidation state than the MESSENGER data indicates [911]. The high S and low Fe concentrations on the surface, and a high metal/silicate ratio indicate low oxygen fugacity (fO2) conditions during the planet’s formation [12]. Given the large effect of Fe on the solidus temperature, any melting model of Mercury’s interior would greatly overestimate both the melt amount and composition generated at any point in time (Figure 1). These previous models would not provide enough melt to form the large expanse of the NVP. There are also no self-consistent thermochemical evolution models for Mercury’s interior that account for the evolution of the mantle as the interior of Mercury thermochemically evolved, as they all assume the mantle is one uniform temperature.