GBT24B-428

Interior structure of Europa and Ganymede

Abstract

We request 10 sessions of 45 minutes each to expand upon our measurements of the spin axis orientations of Europa and Ganymede with a unique observational technique available only with Goldstone-GBT at X-band. Our 2011-2023 radar speckle tracking observations provide initial estimates of spin axis orientations. The 2024B geometry will enable observations at previously unsampled baseline orientations, which is necessary to improve our preliminary estimates and sample temporal variations in obliquity over the 30-year precessional cycle. Measuring the spin states of the Galilean satellites holds the key to characterizing fundamental interior and surface properties. 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 would enable the first measurement of the shell's moment of inertia, placing definite 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. Finally, the obliquity is required to enable tidal heating calculations.

Investigators

Name	Institution
Jean-Luc Margot *	California at Los Angeles, University of
William Armentrout	Green Bank Observatory

* indicates the PI