What’s Up With Mercury's 2nd-Degree Shape?

Abstract:

The long-wavelength topography and geoid of a planet are basic observations fundamental to understanding the planet’s thermal and dynamical history. Observations by the MESSENGER spacecraft have significantly reduced the uncertainty in the spherical harmonic 2nd-degree (l₂) topography and gravity coefficients. Similar to those of the Moon, the long wavelength shape and geoid of Mercury are significantly out of hydrostatic equilibrium [Perry et al., 2015]. The diversion from equilibrium of the Moon has been attributed to orbital evolution and the “freezing-in” of a fossil bulge. With respect to Mercury, the disequilibrium of the l₂ shape and geoid is unlikely to be due to its orbital history [Matsuyama and Nimmo, 2009].

Non-hydrostatic models can explain the gravity and shape of Mercury. Buoyancy from thermal anomalies isostatically supporting the surface falls short of reproducing the observed l₂ admittance and topography. We explore three scenarios that can generate high admittances at degree-2: flexural/membrane loading on the surface, buoyant structures within the mantle, or topography on the core-mantle boundary. We discuss both isostatic and dynamic models of compensation, and include variations of viscosity structure and elastic properties. However, typical sources of these mechanisms (e.g. large volcanic provinces that collectively have symmetry about the equator or mantle convection with a strong l₂ component) are not obviously present on Mercury.