Spin states and moments of inertia of Venus, Europa, and Ganymede

by Dr Jean-Luc Margot (UCLA)

Danjon (Observatoire de Paris)


Observatoire de Paris


Earth-based radar speckle tracking observations in 2006–2020 improved the
knowledge of the spin axis orientation of Venus by a factor of 5-15 compared to Magellan estimates. They also enabled the first measurement of the spin precession rate and moment of inertia of Venus. I will describe prospects for improving the estimates and other geodesy aspects at Venus. Radar speckle tracking observations of Europa and Ganymede in 2011-2023 yielded estimates of their spin axis orientations to 0.01 degrees. These measurements conform to the expected 30-year precessional cycle and provide insights into the moons’ Cassini States. I will discuss new scientific prospects associated with these observations. First, the spin state can reveal the presence of a subsurface ocean: a decoupling between the icy shell and the interior results in a different obliquity than that of a solid body. Second, an angular deviation from the strict Cassini state enables estimates of energy dissipation. Third, a measurement of librations, if detectable, would enable the first measurement of the shell’s moment of inertia and bounds on the rheology and thickness of the shell. Fourth, the obliquity may explain remarkable surface features, such as the distribution and orientation of cycloids, strike-slip faults, and lineaments on Europa. Fifth, knowledge of the obliquity is required to enable tidal heating calculations. Finally, these measurements are expected to facilitate Clipper and JUICE operations and prevent initial, large mapping errors in spacecraft data products.