Thesis 2023

1- Ganymede, numerical simulations, fluid layers

The ESA Juice (Jupiter Icy Moons Explorer) launched in April 2023 will explore Ganymede, Jupiter’s largest moon in great details from 2034. Besides being the largest moon in the solar system, Ganymede is unique with its internal magnetic dynamo. It also belongs to the family of ocean worlds with the presence of a salty water ocean under an icy crust several tens of kms thick. The present work will simulate the formation of both the water ocean and the iron-rich liquid core after developing state of the art numerical tools describing the thermal evolution of two-phase material. These simulations will help interpret the future observations of the Juice mission.

Ganymede is the most differentiated moon in the solar system with an hydrosphere and a refractory core. The hydrosphere includes a salty water ocean squeezed between an icy crust and a High- Pressure (HP) ice layer. The presence of a liquid layer raises the question of its potential habitability. The refractory core possesses a liquid metallic core that sustains a magnetic dynamo. The Juice mission, launched on April 13, 2023, will reach the Jovian system in 2031 and will start orbiting Ganymede in 2034. At the same time, the NASA missions Europa Clipper and Dragonfly will explore Europa and Titan, respectively. These three moons belong to the Ocean Worlds family. Before these missions reached their targets, it is critical to develop thermal evolution models which can handle the complexities of the differentiation processes of these Ocean Worlds.

Although the formation of the Jovian system is still debated, it is well established that Jupiter’s moons formed in a colder environment than that of the terrestrial planets. Thus, their differentiation processes have been slower. Two-phase flow has been investigated to explain early processes on Earth (e.g. crystallization of the magma ocean and formation of the liquid iron core) or local processes (e.g. migration of magma from the mantle depths to the mid-ocean ridges). The work will focus on the two-phase processes at work inside Ganymede (or other ocean worlds) as the interior heats up due to decay of the long-lived radioactive elements. These processes include a first phase of dehydration leading to a large hydrosphere and a second phase leading to the formation of a liquid iron core. Because these processes are slow and global, they will affect the surface tectonics and the characteristics of the magnetic field. The science objective of the thesis is to determine the timing of the hydrosphere/rock differentiation and the iron core/silicate differentiation.

The development of a 3D spherical code will be achieved in a series of steps leading to the following tasks. 1) a 1D code developed by Stéphane Labrosse (co-advisor located at ENS Lyon) will be tailored to Ganymede’s characteristics. This will help determine the critical parameters and the limits of such an approach. 2) Partial melt and dehydration processes will be added to the 3D spherical code OEDIPUS developed by Gael Choblet (co-advisor at LPG Nantes) to model thermal evolution of one- phase material. Results will be compared with the 1D approach to validate the 3D approach. 3) Once validated, the code will be applied to the formation of Ganymede’s ocean. Implications for its composition, and the volume change and related surface tectonics will be inferred. 4) writing of a paper on the formation of the hydrosphere to be published in an international peer-reviewed journal. The paper will be made available in the HAL depository. 5) The 3D code will then be tailored to the iron-alloys differentiation. A first step will be a bibliography on the properties of iron-alloys at the (P,T) conditions relevant to Ganymede’s interior. 6) determination of the critical parameters controlling the timing of the iron core differentiation. 7) Investigation of the role of Carbon rich molecules during this process. 8) Application to Ganymede with predictions of the iron-rich core size, a parameter that will be measured by the Juice mission. 9) Writing of a paper on the formation of the iron-rich core. This paper will be submitted to an international peer-reviewed journal. Once accepted, the paper will be made available in the HAL depository. 10) codes and restart files will be archived according to the ERC DMP and following the rules at Nantes University. 11) Outreach events during ‘Fete de la Science’ will be organized.

> Submit an application file on the 3MG doctoral school website.