The interior structure of Mercury constrained by geodetic data

Given its unusually high bulk density, Mercury represents a world unique among other terrestrial planets in the solar system. While the general aspects of its interior, such as the core size, are relatively well constrained by the measurements of the planet mass, radius, and moments of inertia, details of its mantle viscosity and thermal profile are still relatively unknown. Recent estimates of Mercury‘s tidal Love number k₂, along with a surprisingly low moment of inertia factor (MoIF) obtained from the MESSENGER gravity data [1] indicate a weak mantle with a CMB viscosity potentially as low as 10¹³ Pa s [2, 3]. Alternative estimates of Mercury‘s MoIF based on laser altimetry (e.g., [4]) would allow a higher CMB viscosity [2] but were reported by [3] as only marginally consistent with the newest value of k₂ from [1].

In this work, I construct interior models of Mercury constrained by a set of geodetic observables including the mean density, polar MoIF, relative moment of inertia of the mantle, and the tidal Love numbers k₂ and h₂. The acceptable interiors are seeked by means of Bayesian inversion. The core is modelled as an Fe-Si-C-S alloy [5] and the mantle is either considered homogeneous (Case A) or endowed with two possible bulk chemical compositions (Case B), derived from the composition of surface lavas [6, 7]. The density and elastic properties of the mantle in Case B are calculated with the thermodynamic software Perple_X [8]. At the CMB, I prescribe a distinct layer, which might either correspond to crystallised Fe-S at the top of the core or to a denser material at the base of the mantle (see also [9]). For the constraining MoIF, I choose the laser altimetry-derived value [4] in one set of samples and the gravity-derived value [1] in the second set of samples.

The presence of a distinct CMB layer with homogeneous fitted properties (density, viscosity) alleviates the need for a weak mantle. While the CMB layer’s viscosity tends to values below 10¹⁸ Pa s, the posterior probability distribution of the lower-mantle viscosity peaks above 10²⁰ Pa s. Moreover, the models from Case B tend to prefer higher values of MoIF~0.34 and cannot be easily reconciled with the lower gravity-derived estimate. In the inversion with laser altimetry-derived MoIF, the CMB layer is predicted to have thickness between 40-160 km and a wide range of possible densities. Outer radius of the liquid part of the core peaks around 1990 km and the temperature above the CMB layer is typically below 1600 K. Silicon content in the outer core peaks around 7 wt%, while sulfur and carbon represent a minor component (2-3 wt%).

[1] Genova et al. (2019), doi:10.1029/2018GL081135.

[2] Steinbrügge et al (2021), doi:10.1029/2020GL089895.

[3] Goossens et al. (2022), doi:10.3847/PSJ/ac4bb8.

[4] Bertone et al. (2021), doi:10.1029/2020JE006683.

[5] Knibbe et al. (2021), doi:10.1029/2020JE006651.

[6] Namur et al. (2016), doi:10.1016/j.epsl.2016.01.030.

[7] Nittler et al. (2018), doi:10.1017/9781316650684.003.

[8] Connolly (2009), doi:10.1029/2009GC002540.

[9] Hauck et al. (2013), doi:10.1002/jgre.20091.