Lander radio science experiment with a direct link between Mars and the Earth

Mars' Orientation and rotation Parameters (called here MOP): precession, nutation, polar motion and length of day (LOD) variations, are related to the interior of the planet as well as to the dynamics of its atmosphere. The MOP can be determined using the Doppler shift on radio signal data due to the motion of a probe landed on Mars relative to tracking stations on Earth. In this paper we perform numerical simulations for assessing the precision on the determination of the MOP using Direct-To-Earth (DTE) X-band Doppler measurements for a nearly equatorial lander. We then discuss how a better knowledge of the MOP could improve our understanding on the interior structure. The sensitivity of such a DTE radio link to these rotation parameters is first investigated. This shows that the latitude of the landing site must be higher than 20° to detect the Chandler Wobble component of the polar motion in DTE Doppler data and must be at least 40° to get tight constraints on it. It is found that the precision in the determination of the seasonal LOD variations will be significantly improved after about 350 days of operation, reaching the 5% level after 550 days, thereby better constraining the CO2 mass budget in the Martian atmosphere and ice caps. The current precision in the precession rate (25 milliarcsecond (mas) per year) will be matched after 150 days of mission. An uncertainty of less than 5 mas/year will be reached after 700 days, improving the precision on the polar moment of inertia by a factor of five. The precision on the determination of the amplitudes of nutation is estimated at a few mas after one Martian year of mission (about 12 mas on the prograde/retrograde semi and terannual amplitudes) allowing for the detection of the contribution expected from the liquid core. Considering Mars with a liquid core in accordance with recent geodesic measurements, the Free-Core-Nutation (FCN) period is estimated with a precision of less than 10 days after 550 days of mission in case of an actual FCN period equal to −240 days and with a precision of 1 day after 300 days of observation for an FCN period at −230 days, which is close to the forcing terannual period of 228.9 days (resonance). It is shown that only 100 days of mission duration are needed to detect the liquid core contribution to the nutation near the resonance from the estimates of the amplitudes of nutation. Finally, the precisions on core physical parameters are inferred from the FCN period estimate. We show that a precision of 30% for the core moment of inertia and 5% for the dynamical flattening can be obtained after one Martian year of mission duration.

► Direct to Earth Doppler sensitivity to Mars orientation parameters (MOP) is deeply examined.
► Realistic precisions on MOP are obtained from numerical simulations.
► Liquid core contribution on nutation amplitudes can be observed.
► Core moment of inertia and dynamical flattening can be strongly constrained.