Mercury's gravity, tides, and spin from MESSENGER radio science data

TL;DR

This study uses MESSENGER radio science data to refine Mercury's gravity field, spin state, and tidal response, providing insights into its interior structure and thermal state.

Contribution

It presents a detailed gravity and spin model of Mercury using extensive radio tracking data, improving previous estimates and constraining interior properties.

Findings

Mercury's mass parameter GM measured with high precision.

Second-degree gravity coefficients confirmed with better than 0.4% accuracy.

Tidal Love number k2 suggests a hotter, weaker mantle.

Abstract

We analyze radio tracking data obtained during 1311 orbits of the MESSENGER spacecraft in the period March 2011 to April 2014. A least-squares minimization of the residuals between observed and computed values of two-way range and Doppler allows us to solve for a model describing Mercury's gravity, tidal response, and spin state. We use a spherical harmonic representation of the gravity field to degree and order 40 and report error bars corresponding to 10 times the formal uncertainties of the fit. Our estimate of the product of Mercury's mass and the gravitational constant, $GM = (22031.87404 \pm 9 \times 10^{-4})$ km$^{3}$s$^{-2}$, is in excellent agreement with published results. Our solution for the geophysically important second-degree coefficients ($\bar{C}_{2,0} = -2.25100 \times 10^{-5} \pm 1.3 \times 10^{-9}$, $\bar{C}_{2,2} = 1.24973 \times 10^{-5} \pm 1.2 \times 10^{-9}$) confirms previous estimates to better than 0.4\% and, therefore, inferences about Mercury's moment of inertia and interior structure. Our estimate of the tidal Love number $k_2 = 0.464 \pm 0.023$ indicates that Mercury's mantle may be hotter and weaker than previously thought. Our spin state solution suggests that gravity-based estimates of Mercury's spin axis orientation are marginally consistent with previous measurements of the orientation of the crust.