Mercury's gravity field as constrained by the tectonic record

Kurzfassung

Due to MESSENGER’s elliptical orbit, Mercury's gravity field is unevenly known with resolution ranging from up to only about spherical harmonic degree 10 (∼1,500 km wavelength) in the southern hemisphere to degree 90 and up to 160 in the north (∼100–170 km; [1]). Such low-resolution gravity field represents a major obstacle to our understanding of the planet's interior structure and geodynamic history [2]. One objective of the BepiColombo mission is to improve Mercury's gravity field models using the Mercury Orbiter Radio Science Experiment (MORE) [3]. However, even after the extended mission, the gravity field is predicted to only reach about degree 40-50 globally. Some previous works have attempted to increase the resolution of the gravity field based on the expectation that, at sufficiently high harmonic degree, gravity should be correlated to topography [4]. Yet, at the measured wavelengths on Mercury, lithosphere deformations from geologic loads are expected to prominently affect the planet's gravity field, making gravity-from-topography models highly non-unique [2,4].

Before high-resolution gravity measurements were made available on the terrestrial planets, studies have commonly used elevation data and tectonic structures to investigate lithosphere deformations and the presence of subsurface loads [5]. Detailed characterization of subsurface loads and lithosphere deformations can in turn be used to break part of the non-uniqueness in the predicted, high-degree gravity, field. In this work, we combine topography data and tectonic deformations [6,7] together with a lithosphere loading model [8] to provide high-resolution gravity field and interior structure models of Mercury.

Using a global tectonic catalog together with analysis of elevation profiles across tectonic landforms, our previous works have estimated lateral variations in tectonic strain [6,7]. The estimated average shortening reflects Mercury’s global contraction (with values of ranging from 6 to 8 km), while lateral variations have been attributed to local lithosphere deformations from geologic loads [7]. While lateral cooling efficiency due to crustal thickness or surface temperature variations can induce lateral variations in planetary contraction, these effects are expected to only moderately affect contraction [9] and would not fully explain the observed large variations. Here, we convert the estimated tectonic strain to membrane–flexural uplift and subsidence using a spectral transfer function similar to an admittance [7]. Lithosphere displacements are then used together with topography data to predict high-resolution gravity field models of Mercury. The models are patched to MESSENGER's observed gravity field based on the local degree-strength. A by-product of this inversion is the planet’s interior structure, consisting here of a crust with laterally variable thickness and mantle with laterally variable density.

The estimated gravity field and interior structure maps will be presented at the conference.