Advances in MLA and BELA altimetry crossover analyses for Mercury geodesy

The analysis of the wide-range of data retrieved by the NASA MESSENGER mission to Mercury allowed for a transformative improvement of our knowledge of the planet [1], which is now viewed as a cornerstone for our understanding of the evolution of our Solar System, as well as for the interpretation of exoplanetary systems. Our work focuses on Mercury’s geodetic parameters, and on the role of on-board laser altimeters in their determination. MESSENGER Laser Altimeter (MLA) enabled not only the first topographic and thermal models of the planet, but also supported MESSENGER’s orbit determination, and more recently allowed for a first estimation of the vertical tidal displacement from orbit [2], characterized by the h₂ Love number, through crossovers analysis. The analysis of altimetry data also contributed through orbit determination to the retrieval of the Hermian gravity field, as well as to independent solutions for Mercury’s orientation. However, due to MESSENGER’s highly eccentric orbit as well as its high latitude periapsis, altimetry observations were collected in the northern hemisphere (and mainly above latitude 60°-70°), thus limiting Mercury’s ground surface coverage. The BepiColombo Laser Altimeter (BELA) onboard the ESA Mercury Planetary Orbiter (MPO) will be the second altimeter to orbit Mercury. The lower altitude apoapsis of MPO and the lower latitude periapsis would provide a more uniform altimetry mapping of Mercury’s surface than MLA, yet complementary. As a result, combining observations from MLA and BELA could significantly improve our knowledge of the geodetic parameters of Mercury, and help solve current open questions on its internal structure and composition.

We first focus on MLA and perform an updated crossover analysis following [2], using the pyXover software package with a recent orbit solution including an advanced modeling of non-gravitational forces which proved to be more consistent with altimetry crossovers [3]. We present our updated solutions for orbit and geodetic parameters, and we discuss the impact of the updated input models. We also perform closed loop simulations, see e.g. [4], using realistically simulated BELA altimetric ranges and planned orbits, adopting the latest expected accuracies from the literature. We introduce realistic perturbations on our knowledge of each ground track, as well as on global parameters such as orientation parameters (north pole direction, rotation rate, libration amplitudes) and on a reference Love number h₂, and evaluate their recovery. We then present how the geodetic parameters can be improved from a common fit of MLA and BELA altimetry data. In addition to considering crossovers from each separate dataset, we search for crossovers between the ground tracks of two altimeters. We discuss how the geodetic parameters would be improved from the larger number of crossovers at lower latitude.

[1]: Solomon et al., 2018. Cambridge University Press.

[2]: Bertone et al., 2021. Journal of Geophysical Research: Planets.

[3]: Andolfo et al., 2024. Journal of Guidance, Control, and Dynamics.

[4]: Desprats et al, 2024. EGU General Assembly 2024.