Probing planetary interior structure and processes with high-precision rotation measurements

Profound developments in our understanding of the Earth, Moon, and other planetary bodies have been enabled by rotation studies.  I will describe the application of a new Earth-based radar technique that enables high-precision measurements of planetary spin states and provides powerful probes of planetary interior structure and processes.

Observations of radar speckle patterns tied to the rotation of Mercury establish that the planet exhibits periodic variations in rotation period or length of day.  The amplitude of the oscillations show that the mantle of Mercury is decoupled from a molten outer core.  These data enable precise estimates of Mercury's moment of inertia and the size of its core.

Observations of Venus show that its spin period is not constant. The length of day exhibits ~70 ppm variations that are consistent with percent-level changes in atmospheric angular momentum transferred to the solid planet.  Monitoring these fluctuations provides important constraints on the atmospheric dynamics of Venus, which is key to elucidating its superrotation and the generation of recently discovered planetary-scale structures in the atmosphere.  The radar observations provide the most realistic near-term prospect of measuring Venus's moment of inertia and the size of its core.

Measurements of Europa's obliquity suggest that the ice shell is decoupled from the interior, strengthening the case for a global subsurface ocean.  The amplitude of the oscillations in length of day depends on the rheology and thickness of the ice shell, perhaps the most important determinants of Europa's astrobiological potential.