Most of the 5000 known exoplanets today have a radius between 1.0 and 4.0 Earth radii, i.e. they appear to be large versions of Earth, so-called super-Earths, and small versions of Uranus or Neptune, so-called mini-Neptunes. Both prototype planets are as different as one can imagine: Earth, the rocky planet supporting life with an atmosphere and a magnetic field shielding it from solar radiation, and Neptune, the ice giant predominantly made of the planetary ices water, ammonia, and methane with a peculiar magnetic field structure. While we have a fairly good understanding of Earth’s interior, Neptune and its similar neighbor Uranus remain puzzling and have been therefore the main subjects studied within this fellowship. The magnetic fields of the ice giants are highly non-dipolar and their axes have large tilts with respect to the planets’ spin axes. To date, there are no conclusive models for their magnetic dynamos based on valid interior structure models, that reproduce the correct age of the planets (4.56 billion years). The key to solve this problem is to identify the regions, where the magnetic dynamos are driven, which could be either in a thin outer shell or in the deep interior inside the ice-rich region atop the rocky core.
The main objective of the project xICE was the characterization of materials that are predicted to be relevant for the deep interiors of our Solar System’s ice giant planets Uranus and Neptune, as well as their exoplanetary cousins. Uranus and Neptune are typically modeled as adiabatic planets that consist of three differentiated layers, i.e. a hydrogen-helium-rich outer mantle, a ice-rich inner mantle, and a rock-rich core. Typical thermodynamic conditions span thousands of Kelvin in temperature and several Mbar in pressure. Such models, however, have been unable in the past to explain the thermal evolution, interior structure, and magnetic field geometry in a consistent way. Therefore, recent models have proposed to consider thermal boundary layers, compositional gradients, and interface effects to match observational constraints. Of special importance for this endeavor of improvement is the understanding of the core-mantle boundary and the rock/ice mixing behavior. Specifically, it was the project’s goal to investigate physical processes at the core-mantle boundary that could provide hints on how thermal boundary layers and interfaces could be potentially formed and sustained in ice giants.
