Year: 2025
Type: dissertation
Abstract: Improving our knowledge of Venus's internal structure is essential for understanding its history and providing insight into why the evolutionary path of this "twin sister" of Earth has diverged so radically from ours, leading to the inhospitable planet we see today. Five Venus missions are in development for exploration over the next decade, with EnVision (ESA) and VERITAS (NASA) both including a geophysical investigation among their objectives. Examining the rotational dynamics of a planet often allows us to constrain properties of its interior. The motion of its spin axis can be described in two ways: its orientation relative to the celestial sphere (precession, nutation), or its orientation on the planetary surface itself (polar motion). These two types of motion bring complementary clues about the planet's interior. The precession of Venus was recently measured for the first time, but the uncertainty remains too large to constrain interior models. As for Venus's polar motion, it has never been measured. In this PhD research, we characterize Venus's polar motion, focusing on the signatures of its interior and atmosphere, to support the geophysical interpretation of a future detection of polar motion. We develop an analytical model of planetary polar motion that includes the solar gravitational torque, typically negligible for fast-rotating planets, but significant for the slowly rotating Venus. Our model also accounts for centrifugal and tidal deformations of a viscoelastic mantle, atmospheric super-rotation and short-term dynamics, and core-mantle coupling effects. This comprehensive model allows us to analyze how the period, damping, and amplitude of the free polar motion depend on the interior properties and processes. The short-period forced polar motion is also computed to aid in a future detection of the polar motion signal. In a second aspect of the PhD, this theoretical work could be applied to the characterization of the observational signature of Venus's polar motion. Relevant observations include the tracking of surface features via radar imaging and the recovery of the gravity field from spacecraft tracking of an orbiter. Compared to Earth and Mars, Venus's free polar motion is expected to be much slower, with a Chandler period of 16000~years, and of significantly larger amplitude, with an offset of 50~km between the spin pole and the figure pole. Given these differences, modeling Venus's polar motion in observational data will require approaches different from those adopted previously for Earth and Mars. The inversion of a comprehensive kinematic rotation model from synthetic observations, together with our dynamical model of Venus's polar motion, will enable predictions of how future missions could constrain the physical and thermo-chemical properties of Venus's interior.
Author: Phan, Pierre-Louis
Institution: Laboratoire Temps Espace
Language: English
Topic: Astro and Planetary Science
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7140154318