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ExoMars Radioscience (LaRa), a Space Geodesy Experiment to

Véronique Dehant, S. Le Maistre, M. Yseboodt, M.J. Péters, Ö. Karatekin, B. Van Hove, A. Rivoldini, R.M. Baland, T. Van Hoolst Royal Observatory of Belgium (ROB)

Zecliptic - Orientation in space Objectives Rotation → precession & nutation Axis - Orientation in → polar motion Precession Nutation - Rotation rate e = e + e → length-of- variations 0

Modeling of Yecliptic - interior of - core

- dynamics (CO2 - core dimension  = 0 + 

sublimation/ condensation process) - core state Xecliptic - interior evolution - habitability

Principle of the experiment Instrument

Coherent transponder -band radiolink (no instrumental phase) in X-band RX TX Uplink at 7173.87 MHz (-to-space) One RX antenna receiving from Earth Large antennas at Coherent Downlink at 8428.58 MHz (space-to-Earth) Two TX antennas transmitting to Earth ground station transponder for observation [30°,45°] budget so that at long distances, ground Doppler effects related to the stations can lock on the signal 64m / 70m antennas for long distances between Two-way Doppler measurements between a lander revolutions and rotations of Mars - closed-loop Doppler receiver Mars and Earth and 34m / 35m antennas at on Mars and a ground station on Earth and the Earth - open-loop receiver when SNR smaller distances (NASA DSN, Frequency reference (maser) on Earth (Signal-to-Noise Ratio) too small Ground Stations, ESA ESTRACK tracking stations)

Simulations Max. Mars Orientation Max. MOP change in Geophysical Model Parameters (MOP) amplitude the lander Properties position Nutation in Trigonometric series 10.3 m 10.3 m obliquity (e) (annual, ½, 1/3, …) Cf (Mars core moment of Nutation in Trigonometric series 29.4 m 12.3 m inertia) () (annual, ½, 1/3, …) We investigated the signature in the Doppler of Mars Orientation Parameters from Core state, Liquid Core effect Liquid core 0.7 m on 0.4 m core , core which we determined strategies of observation and of data analysis. amplification in the d, density nutations 0.4 m on de These signatures and strategies are related to: Precession Rate Rate 0.8 m 0.8 m C (Mars - Amplitude of nutations, Amplitude of liquid core contribution to nutations (uncertainty of moment of - Amplitude of variations 2 mas/y) inertia) Length-of-day (LOD) Trigonometric series 11.8 m 11.8 m Atmosphere - Amplitude of Polar motion variations (annual, 1/2, 1/3, …) and caps - Dependence on lander and longitude, Polar motion (PM) Trigonometric series 1.2 m 1.2 m dynamics - Dependence on mission duration Estimated uncertainties on the amplitudes of - Aptitude to communicate or visibility the semi- and ter-annual pro- and retro- typical noise on Doppler measurements ( 0.05 mm/s) grade nutation as a function of the mission duration. The uncertainties are We further studied the synergies with other missions: superimposed with the expected effect that - InSIGHT (Interior exploration using Seismic a planet with a liquid core would have on those nutations (grey boxes). Investigations, Geodesy, and Heat Transport) / RISE (Rotation and Interior Structure At least 250 days of mission are necessary to InSIGHT ExoMars detect the liquid core signature in the semi- Experiment) & SEIS → interior of Mars annual prograde amplitude and at least 500 - TGO ( ) → Maximal signature in the Doppler observable days to get it in the ter-annual retrograde as a function of the lander latitude amplitude.

Conclusions and expectations LaRa is a Belgian instrument selected on the ExoMars 2020 2016 surface platform of Roscosmos/ESA mission. LaRa will provide information on interior of Mars, its evolution,

as well as CO2 sublimation/condensation process between the atmosphere and the ice caps.

Additional simulations (not presented here) in showed that: - synergies between InSIGHT / RISE and LaRa are valuable; 2020 © ESA - synergies with all atmospheric data are very valuable too. © ESA

Acknowledgments: This work was financially supported by the Belgian PRODEX program managed by the in collaboration with the Belgian Federal Policy Office.