My Thesis in 400 Words: Rafael Fuentes

Rafael Fuentes, an iREx student at McGill University, finished his Ph.D. in 2022. Here, he summarizes the research project he carried out during his studies:

Developing a model for the interiors of giant planets is crucial to understanding their internal structure and evolution. When combined with astronomical observations, these models can help provide insight into how these types of planets formed. Observations of giant planets have revealed a number of intriguing puzzles – for example, measurements taken by NASA’s Juno mission show that Jupiter’s core is larger than we predicted! A key element to solving these observational puzzles is determining how well-mixed the interiors of these planets are.

Four giant planets juxtaposed against a black background.

A montage of giant planets in the Solar System. From right to left: Jupiter (Juno perijove 6, Credit:NASA/SwRI/MSSS/Gerald Eichstädt/Seán Doran); Saturn (Cassini, Credit: NASA/JPL-Caltech/Space Science Institute); Uranus and Neptune (Hubble, Credits: NASA/ESA/A. Simon (NASA Goddard Space Flight Center), and M.H. Wong and A. Hsu (University of California, Berkeley)).

During my Ph.D., I created a model for how material flows and mixes inside giant planets. In this model, convective flows are cooled from the surface (which mimics the outer envelope of the planet). These flows advance inward into a neighboring stable region with a gradient of heavy elements (which mimics the planet’s core).

Using this model, I investigated several interesting questions: How quickly does material in the convection zone move inward? How are heavy elements mixed through the convective boundary? Further, under certain circumstances, a gradient of heavy elements can trigger the formation of multiple convective layers (just like the many layers in a caffe latte which form due to the very same physics). So, another question is: Can the interior of gas giants undergo layered convection?

To explore answers to these questions, I created two-dimensional simulations of giant planet interiors. I found that the rate at which the convective zone moves inward depends on how quickly it cools down and how large the planet’s initial composition gradient is. A larger internal gradient in the composition of the planet stabilizes the fluid against convection and slows down its evolution. Further, I found that turbulent motions near the boundary of the convective zone carry enough energy to lift and mix heavier fluid up from below. This process is called “entrainment” and is the mixing process acting at the convective boundary. I also discovered that the vigorous mixing and turbulence of the outer convection zone prevents the formation of multiple convective layers. These results challenge current one-dimensional models, which predict that multiple layers can form and survive within these planets. However, there is still work to be done by researchers to connect one-dimensional and multi-dimensional models. I look forward to many years of building upon my work and to applying the lessons learned here to more complex simulations in the future.