Mercury's resonant rotation from secular orbital elements

TL;DR

This paper derives precise secular orbital elements for Mercury using recent ephemerides and observations, providing key data for understanding Mercury's rotation and interior structure, and introduces a method to calculate its orbital planes.

Contribution

It offers updated orbital parameters for Mercury based on recent data and presents a new approach to determine the planet's orbital plane orientations.

Findings

Mean orbital period of Mercury: 87.96934962 days

Spin rate assuming perfect resonance: 6.138506839 deg/day

Longitudinal displacement at the equator: approx. 67 m/year

Abstract

We used recently produced Solar System ephemerides, which incorporate two years of ranging observations to the MESSENGER spacecraft, to extract the secular orbital elements for Mercury and associated uncertainties. As Mercury is in a stable 3:2 spin-orbit resonance these values constitute an important reference for the planet's measured rotational parameters, which in turn strongly bear on physical interpretation of Mercury's interior structure. In particular, we derive a mean orbital period of 87.96934962 $\pm$ 0.00000037 days and (assuming a perfect resonance) a spin rate of 6.138506839 $\pm$ 0.000000028 degree/day. The difference between this rotation rate and the currently adopted rotation rate (Archinal et al, 2011) corresponds to a longitudinal displacement of approx. 67 m per year at the equator. Moreover, we present a basic approach for the calculation of the orientation of the instantaneous Laplace and Cassini planes of Mercury. The analysis allows us to assess the uncertainties in physical parameters of the planet when derived from observations of Mercury's rotation.