Geodesy at Mars: from gravity, rotation and tides to the deep interior of the red planet.

Abstract:

Space geodesy uses satellite observations in order to obtain information on Earth’s shape, rotation and gravity. Applied to planets for half of a century now, space geodesy allowed planetary scientists to drastically improve our understanding of many worlds of our solar system. In particular, 40 years of observations from orbit and from the surface of Mars provided strong constraints on Mars’ interior. The gravity and rotation experiments are mostly based on two-way (i.e. round trip) radio links between in situ spacecraft and ground stations on Earth. Benefiting from the high RF output power of the Earth tracking stations, as well as from the high stability of their clocks (maser), the two-way Doppler shifts are accurately measured by comparing the frequency of the ground-based reference signal with the frequency of the radio signal coherently transponded back to Earth by the spacecraft. Accumulated over decades from several Martian orbiters, radio-tracking data allowed to determine the static gravity field of Mars to degree and order 120 when expanded in spherical harmonics. The accuracy reached on the low degree gravity coefficients allowed to precisely measure the time variations of the degree-2 and degree-3 zonal coefficients, improving thereby our understanding of the seasonal mass exchange between the atmosphere and the icecaps due to the sublimation/condensation of CO2, as well as the tidal contributions. Radio-tracking data from landers and rovers, such as Viking, Pathfinder and Opportunity, have been used to constrain the interior of Mars as they are well suited to determine Mars’ rotation and orientation changes in space. In particular, these data provided a very accurate estimate of the precession rate. Combined with degree-2 gravity coefficients estimate from orbiters, the precession rate enabled to infer the moment of inertia of the whole planet with 0.02% precision. The radio science experiments of the InSight lander, RISE, and of the ExoMars 2022 surface platform, LaRa, are now aiming at detecting the signature of the liquid core in the nutation of Mars’ spin axis, in order to confirm from independent measurements the state of the core and further constrain its size, composition, and shape.