Mars moon ephemerides after 14 years of Mars Express data

TL;DR

This paper presents updated ephemerides for Mars' moons Phobos and Deimos, integrating 141 years of observational data and spacecraft measurements, with improved models and analysis of libration and tidal parameters.

Contribution

It provides the most comprehensive ephemerides for Mars' moons to date, incorporating extensive historical and recent data, and explores the moons' libration and tidal properties.

Findings

Libration amplitude of Phobos is 1.09 +/- 0.01 degrees.

Discrepancies identified between MRO and MEX astrometric data.

Predicted positional accuracy of a few hundred meters for upcoming years.

Abstract

The Mars Express (MEX) mission has been successfully operated around Mars since 2004. Among many results, MEX has provided some of the most accurate astrometric data of the two Mars moons, Phobos and Deimos. In this work we present new ephemerides of Mars' moons benefitting from all previously published astrometric data to the most recent MEX SRC data. All in all, observations from 1877 until 2018 and including spacecraft measurements from Mariner 9 to MEX were included. Assuming a homogeneous interior, we fitted Phobos' forced libration amplitude simultaneously with the Martian tidal k2/Q ratio and the initial state of the moons. Our solution of the physical libration 1.09 +/- 0.01 degrees deviates notably from the homogeneous solution. But considering the very low error bar, this may essentially suggest the necessity to consider higher order harmonics, with an improved rotation model, in the future. While most data could be successfully fitted, we found a disagreement between the Mars Reconnaissance Orbiter and the Mars Express astrometric data at the kilometer level probably associated with a biased phase correction. The present solution precision is expected at the level of a few hundreds of meters for Phobos and several hundreds of meters for Deimos for the coming years. The real accuracy of our new ephemerides will have to be confirmed by confrontation with independent observational means.