Tidal Dynamics of Laterally Heterogeneous Planetary Bodies

Planetary bodies experience deformation in response to tidal forces from external gravitating objects. Changes to planetary gravity fields produced by this forcing depend on internal rheological structure. For example, a spherically symmetric body's response to tides depends on the presence, thicknesses, and effective shear moduli of internal layers (e.g., the core, an ocean) and follows the spatial pattern of driving gravitational potentials. However, bodies that exhibit substantial deviation from spherical symmetry can exhibit a more complicated response to forcing that includes power both at short (i.e., regional-scale) and long (i.e., global-scale) wavelengths. In this thesis, we explore the impact of lateral heterogeneities in internal rheological structure on the tidal responses of Enceladus, the second major moon of Saturn, and Earth's Moon. We begin by exploring the impact of faults and lateral variations in ice shell thickness on the long-wavelength gravitational response of Enceladus (Chapter 2). Using finite element models of Enceladus' crust, we demonstrate substantial deviation between the longest-wavelength components of response to tidal forcing (i.e., the diurnal degree-2 Love numbers) and those expected for a spherically symmetric model of Enceladus. We further show that structural heterogeneities predominantly produce deformation over spatial scales similar to the size of these features. Based on this finding, we develop a methodology to precisely characterize regional variations in crustal thickness using local observations of surface strain at Enceladus (Chapter 3). We demonstrate that this strain can be readily inferred using measurements of the motion of the surface over the tidal cycle (e.g., via interferometric synthetic aperture radar or InSAR). We then adapt our finite element models to explore relationships between slip on fault structures over Enceladus' south polar region (i.e., the Tiger Stripes) and geological activity observed for the body (Chapter 4). We find a close correspondence between the double-peaked and asymmetric timing of strike-slip motion and observed diurnal variations in the brightness of a large plume emanating from Encealdus' South Pole. Based on this finding, we suggest that rifts along the Tiger Stripes experience periodic localized extension to modulate cryovolcanic activity. Finally, we use observations of the spherical harmonic degree-3 variability in the lunar gravity field to infer the presence of a 2-3% internal hemispheric, near-to-far- side variation in shear modulus of the Moon's mantle (Chapter 5). We demonstrate that a 100-200K hemispheric difference in mantle temperature can explain this inferred shear modulus structure, and suggest that internal asymmetries may be a remnant of processes which formed surface mare basalts ~4 Gya. Our work demonstrates future potential for the use of tidal tomography to characterize the deep interior structure and geodynamics of planetary bodies throughout the solar system.