Colloquium Amirhossein Bagheri — ETH Zürich, Switzerland

Using tides to reveal the interior and origin of planetary bodies – Application to Mars-Phobos-Deimos and Pluto-Charon systems.

Abstract: Studying the present-day interior properties and the mechanisms that govern their evolution can provide insight into their origin and formation processes. With the lack of seismic measurements for the majority of planetary bodies, geodetic observations such as tidal response can play an important role in understanding their interior properties.Here, we focus on the Mars-Phobos-Deimos and Pluto-Charon planetary systems, using tidal and thermal evolution models to study their interior structure and formation history. Robust geodetic measurements for Mars, including its response to the gravitational attraction of the Sun and Phobos, as well as its mass and moment of inertia, have been used to constrain interior models created with thermodynamic calculations and viscoelastic models, allowing us to obtain seismic wave velocity profiles, viscoelastic behavior, and core size and composition. Using the constraints obtained from inversion and a new tidal evolution model, we studied the orbital evolution of Phobos and Deimos. We found that the commonly used tidal evolution model is inaccurate for highly eccentric orbits and does not take into account dissipation due to libration, which necessitates considerable modifications to the model. Based on this study, we propose a new model for the origin of Phobos and Deimos, suggesting that they were formed by the disruption of a common progenitor due to an impact with an oncoming object. We also used the results obtained from orbital evolution modeling to constrain the interior structure of Phobos and Deimos, showing how measurements of tidal deformation can help better understand their interior structure.The study also examines the Pluto-Charon binary using a coupled tidal-thermal evolution model, which reveals that Pluto has maintained a subsurface water ocean due to internal heating, while Charon has not. The results suggest that the two bodies experienced a period of severe tidal heating after Charon’s formation until reaching a stable 1:1 resonance.