Gabriel Tobie

Publications HAL

1 – Tidal control of water-rock interactions and ocean formation  in icy worlds :

In the framework of the ANR OASIS that I coordinated from 2017 to 2021, we studied the coupling between tidal deformation and water-rock-organics interaction combining experimental and modeling approach. A major outcome of this project was to show that aqueous alteration of silicate minerals is effecient even at subfreezing temperatures, in the presence of ammonia (Zandanel et al. 2022). This implies that, as soon as first water melt appears in icy worlds, water-rock interactions rapidly altered the silicate mineral phase. We further showed that the hydrothermal activity and production of hydrogen we observed at present on Enceladus should be relatively recent (< 100 Myr, Zandandel et al. 2021) if the main source is olivine alteration. Another possibility is that H₂ results from aqueous alteration of organics. First experiments on organic alteration were performed during the OASIS project and are now studied in more details in the ERC Promises project (PI C. Sotin, 2022-2027) and ANR OSSO BUCO (PI B. Reynard, 2023-2027), for which I’m participating as Workpackage lead. The fate of salts and volatiles resulting from aqueous alteration will be studied in two other ANR projects (EXOTIC-ICES, PI L. Bove, 2023-2027; CAGES, PI A. Pakhamova, 2023-2027) for which I’m the lead for the LPG partner.

The tidally-controled evolution model developed during the ANR OASIS has  been also recently applied to Mimas in order to understand in which conditions an ocean may exist at present in this small moon (Lainey et al. 2024). My contribution to this work led by V. Lainey has been to quantify the amount of tidal dissipation required to explain the existence of such an ocean and the timescale of its formation. These computations demonstrated that the ocean formed less than 20 Myr ago, and possibly not more than 5 Myr ago if its rocky core has properties similar to Enceladus. In this condition, Mimas may have active hydrothermal processes in its core despite its apparent inactive and old surface.

2 – Coupling between ocean and ice shell dynamics in icy worlds :

In parallell to this work on heat generation and aqueous alteration of the rocky core of icy moons, I also participated to the modeling of ocean/ice dynamical coupling in the framework of the ANR COLOSSe (2020-2024) led by G. Choblet, for which I’m in charge of one workpackage. The main motivation of this project is to better understand how the ocean dynamics may transport heat and chemical compounds from the seafloor to the outer ice shell, and what signatures at the icy surfaces may result from oceanic interaction. One of the main outcomes was to show that the ocean dynamics may produce significant variations in ice thickness if the outer shell is in a conductive thermal state (Kihoulou et al. 2023).  We showed that the observed long-wavelength topography observed by Cassini may be a direct consequence of the ocean dynamics on Titan (Amit et al. 2020, Terra-Nova et al. 2023) and on Enceladus (Bouffard et al. 2024). The simulations performed for Enceladus further demonstrated that material released from hydrothermal vents at the seafloor can be very efficiently transported upward through the ocean to the plume source in timescales of a few days to a few weeks (Bouffard et al. 2024).

Finally, to better characterize the connections between the predicted internal dynamics and the observed surface activity on Enceladu by Cassini,  I participated to the analysis and interpretation of  the Cassini data to provide new constraints on the spectral diversity  (Robidel et al. 2020), the tectonic activity at Enceladus’s soutgh pole  (Rossi et al. 2020) and the tidal control of jet activity (Soucek et al. 2024).  We thus identified recently active regions outside the south polar region, which may also be related to the tidally-induced seafloor hotspots (Robidel et al. 2020). And we explained how the observed variations in plume acticity can be used to infer the tidally-modulated  dynamics of the water-filled faults at Enceladus’ south pole  (Soucek et al. 2024)

 

3 – Tidal dissipation in rock mantles: from Io, Europa to Trappist-1 planets

 In the case of Jupiter’s moons, Io, and potentially Europa, the tidal dissipation in the silicate mantle is large enough to induce silicate melting. In order to determine the feedback between tidal dissipation and internal melting, as part of M. Kervazo’s PhD thesis I supervised, we develop new rheological laws for rocky mantles taking into account silicate melt and associated shear and bulk dissipation. We showed in the case of Io that bulk dissipation, which is usually ignored, can strongly change the dissipation process and the spatial distribution of generated heat (Kervazo et al. 2021). By exploring a variety of interior parameters, we then showed how the melt content and distribution may be inferred by future space missions to Io (Kervazo et al. 2022). This study received the first award of “best student-led article” by the Icarus Journal in early 2023 and Mathilde Kervazo was awarded “Prix de these 2023” from Comité National Français de Géodésie et Géopysique. A third study dedicated to the role of accumulated melt in Europa’s silicate mantle, following a first study highlighting the conditions of melting in Europa’s mantle (Behounkova et al. 2021) is currently under finalization and should be submitted to Planetary Sciences Journal soon.

The Trappist-1 system shares many similarities with the Galilean moon system, with comparable tidal forcing. Trappist-1 planets have, however, larger size comparable to the Earth, which implies different interior properties. In collaboration with Emeline Bolmont and Alexandre Revol from the Observatory of Geneva, we have adapted the LPG-tide model to the Trappist-1 case and explored the consequences of tidal dissipation to their heat budget (Bolmont et al. 2020) and to the rotation evolution (Revol et al. 2023). The coupling between atmospheric and internal tidesfor the case of Venus is currently investigated in detail in the frame of Yann Musseau’s thesis which I co-supervise with Caroline Dumoulin. A first paper highlighting the role of mantle viscosity on the rotational state of Venus is currently under finalization and should be submitted soon to Journal of Geophysical Research.

 

4 – Preparation of the Juice and Europa Clipper mission and future explorations of icy worlds

I’m member of the JUICE  and Europa Clipper teams as co-Investigator of the 3GM/Juice instrument, team member of the MAJIS/Juice instrument, and co-investigator of the SUDA/Clipper instrument. Since 2017, I have co-led, with Tim Van Hoolst, the Working Group 1 Interior/Geophysics of the Juice mission. To support observation planning, I have initiated the development of the “planetary-coverage” tool (https://docs.planetary-coverage.org/) developed at LPG/OSUNA by B. Seignovert in partnership with ESAC (Madrid). This tool is now used as a reference tool by the SOC Juice team. Since September 2022, I’m member of the Juice Clipper Steering Committee, including 9 experts of each team, whose goal is to identify scientific synergies between the two missions. The “planetary-coverage” tool is particularly useful to assess multi-instrumental synergies between the two missions. I  contributed to the writing of two synthesis papers on the interior sciences that will be done by the two missions (Roberts et al. 2023, Van Hooslt et al. 2024). Moreover, the “LPG-tide” code that I developed was used to predict the geophysical measurements  to be performed by Juice at Ganymede (Steinbrugge et al. 2023), as well as the possible detection of gravitational atmospheric tides on Titan by Dragonfly (Charnay et al. 2022).