Proposal of a thesis subject at LPG

"Inferring tidal and loading effects in the gravity field of Mars and Venus"
In September 2022, in Nantes, France

In absence of global seismic data, gravimetric measurements from the orbit are powerful tools to probe the internal structure and thermal state of planetary bodies. They are widely used to estimate crustal and lithospheric thicknesses as well as the dynamic regime of the mantle (dynamic support of topography, number of plumes…), on all terrestrial planets and even some moons. However, variations of low degree coefficients of the gravity field on different timescales (diurnal, seasonal, decadal) can also be used to constrain the viscoelastic response of the solid body to various forcing. This includes deformations due to diurnal tides, surface load variations due to seasonal cycles, or even long-term relaxation due to rotational state variations. The amplitude of the response is controlled by the mechanical properties of the internal layers. Monitoring the time variations of the main gravity field coefficients on these different timescales is thus of a great help to better constrain the radial structure of the planet (size and state of the core) but also the viscosity of the mantle. On Mars, the seasonal cycle of condensation and sublimation of the atmospheric CO2at the polar caps is the main contribution, after the solar tides, to the short-term temporal variations of the gravity field. Smaller variations are expected due to the variations of the tidal effects arising from its orbit eccentricity around the Sun. A weaker contribution should also occur through the viscoelastic relaxation of the interior in response to variations of surface loading at much longer timescales responding to the changes of the obliquity of the rotation axis of the planet. These small amplitude variations, not yet quantified, may be retrieved by reprocessing the radio tracking data acquired by the various missions in operation around Mars during the last 25 years. On Venus, besides tidal fluctuations, gravity changes are also expected from the modulation of the atmosphere-interior coupling, resulting in loading variations at the surface. A small secular variation should also occur due to the viscoelastic relaxation of the planet in response to the spin change. These effects have not been detected with the current gravity field solution but its accuracy should be significantly improved with the future orbiter missions to this planet (EnVision, VERITAS). Combining the measurements acquired by the ESA’s EnVision mission with those acquired by future NASA’s VERITAS and past Magellan data will provide an unique opportunity to quantify the annual to decadal variations of the gravity field. The work proposed in this thesis will be essential to prepare the future observation campaigns and scientific outcomes of the EnVision Radio Science and Gravity Experimentled by LPG with the support of CNES.

The objectives of the thesis are :
- To (re-)process the tracking data of the Martian spacecraft (Mars Global Surveyor, Mars Odyssey, Mars Reconnaissance Orbiter, MAVEN of NASA, Trace Gas Orbiter of ESA) in order to refine the time-variable gravity solution of Mars. This tracking data processing will be performed with the GINS orbitography software developed by CNES and used at LPG.
- To model the viscoelastic response of Mars using an updated model of internal structure, in order to interpret the results of 1). This model has been developed at LPG and already applied to icy satellites and Venus. It will take into account the tidal and loading effects at different spatial wavelengths and time scales.
- To compute the predicted short-term temporal variations of the Venusian gravity field due to the atmosphere/solid body coupling, using the viscoelastic model already used in 2), assuming various internal structure and states that are consistent with Venus rotational evolution.
- To quantify the expected amplitude of the secular variations of the Venusian gravity field in the tracking data of the future VERITAS and EnVision missions that should be launched to the planet in 2028 and 2031, respectively. The simulations will use the mission scenarios for the tracking data noise budget and coverage and will be performed with the GINS software.


Good knowledge in orbital mechanics of artificial satellites, in harmonics analysis, in shell script and fortran. Good knowledge in internal geophysics of the Earth or terrestrial planets.
Speciality of the Master: Space engineering, Earth and planetary interior modeling.


Further information
The 3-year doctoral research grants are awarded to young graduates holding a Master delivered by a university or an equivalent diploma. This theoretical and practical research training has to lead to a thesis defence. The contract must be equally co-financed by CNES and a financial partner (public organisation, industrial, region, etc.). This financial partner must be found by the topic suggesting laboratory. The gross amount charged to the co-financing partner is € 53,685 for the full three years.
To apply, contact Pascal Rosenblatt at