Mars moon ephemerides after 14 years of Mars Express data

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. We present new ephemerides of the Mars moons benefitting from all previously published astrometric data to the most recent MEX SRC data. Observations from 1877 until 2018 and including spacecraft measurements from Mariner 9 to MEX were included. Assuming a homogeneous interior, we fitted the forced libration amplitude of Phobos simultaneously with the Martian tidal k(2)/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. Considering the very low error bar, however, 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 current solution precision is expected at the level of a few hundred meters for Phobos and several hundred meters for Deimos for the coming years. The real accuracy of our new ephemerides will have to be confirmed by comparison with independent observational means.

Automated summary

The Mars Express mission has provided accurate astrometric data of the Mars moons Phobos and Deimos, but further investigation into higher order harmonics and an improved rotation model is needed to improve accuracy. Discrepancies between astrometric data from different spacecraft suggest a biased phase correction may be affecting the results. The current precision of the ephemerides is expected to be in the range of a few hundred meters for Phobos and several hundred meters for Deimos, with the need for confirmation through independent observational means.