Ganymede: A Frozen Enigma

Abstract

In 2022, ESA plans to launch the JUICE (JUpiter ICy moons Explorer) mission which will spend at least three years making detailed observations of Jupiter and three of its largest moons, Ganymede, Callisto and Europa. These moons are currently a hot topic within the science community as their interiors might include oceans consisting of liquid water. These oceans could provide life, but at the moment little is known about the exact composition and structure of these interiors. Only Earth based observations and a few fly-by’s have been performed to measure the characteristics of these moons. The JUICE mission will provide more detailed information on the moons through fly-by’s. This thesis research will focus on Ganymede as JUICE will be the first human-developed satellite to orbit thismoon. Ganymede stands out as a potential scientific target due to several specific reasons; the most remarkable being it’s intrinsic magnetic field. Only two other solid bodies within the Solar System generate such a magnetic dipole field (Earth andMercury). The complex interactions of this magnetic fieldwith Jupiter’smagnetic field are unique and could provide a lot of new knowledge when studied. Measurements from Galileo and the Hubble space telescope suggest that a subsurface layer of (saline) water is present within the moons interior. Saline water could be a good conductor of electricity, generating the magnetic field. The magnetic field of Ganymede could also point towards a complex core, which is another possibility for the generation of this field. It could be that the core of Ganymede consists of liquid, iron rich elements which generate and maintain this magnetic field. Unfortunately, current models of the gravitational potential field and the interior of Ganymede are still uncertain. A precise gravitationalmodel of Ganymede could provide a lot of information about this interior. An orbiter or in-situ probes are required to achieve high precision gravitational potential field models. JUICE is expected to obtain a model of Ganymede’s gravitational potential field of at least degree and order 15. This thesis will provide insight in how different possible internal density distributions of Ganymede influence the gravitational potential field of the moon. Thisway,when JUICE obtains more information on the gravitational potential field of Ganymede, variations within this field can directly be utilized to determine what interior aspects could cause these variations. From 44 billion 1D homogeneous models considered during this research, only 260 adhered to current known characteristics of Ganymede. Certain elements and water phases are present in all models: a pure iron or iron-sulfide core, a silicon mantle, an ice VI layer together with an liquid ocean and a outer crust consisting of Ice Ih. Dependent on the exact layer thicknesses within a model, intermediate ice phases, ice III and V, can also be present. Layer correlations between the 260 models were analyzed and fourteen models where selected for further research. These models were combined with different boundary and density variations to obtain different 3D heterogeneous models. Gravitational potential simulations for spherical harmonics coefficients up to order/degree 48 were performed. It was found that several relations exist between gravitational potential field data and internal density distributions within Ganymede. If one can effectively correct gravitational potential field signals for measurable components within Ganymede’s interior, several sets of internal structures emerge. Furthermore, taking into account the established limitations and correlations between layers, the gradient of the gravitational signal power over spherical harmonics degree can be directly related to the thickness of an interiors ocean. Several distinguishable models show that the presence of ice III, and to a lesser extent ice V, increase the gravitational signal power of a model. When combined with the correlations found between internal layers during this research, one could even establish an accurate first order approximate of Ganymede’s internal composition. These results, together with measurements performed by JUICE, will provide numerous new insights on Ganymede’s frozen enigma.