Surface Processes and Tectonics in the Outer Solar System: Insights from the Saturnian Moons Titan and Enceladus
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Surface Processes and Tectonics in the Outer Solar System: Insights from the Saturnian Moons Titan and Enceladus

Abstract

Icy satellites of the outer solar system have become the primary target for planetary exploration because of their relevance to understanding of solar-system evolution and to the origin of life. Despite this importance, it remains unclear how different combinations of tectonic deformation, climate conditions, and surficial and interior processes have shaped geologically diverse paths of satellite evolution, as evident from their widely different surface morphologies. Here I address this fundamental question by conducting geological mapping of Enceladus and Titan, the two end-member icy satellites of Saturn; Enceladus has tectonic activity expressed by erupting plumes along active faults while Titan has a thick atmosphere that exerts strong control on its surface processes and hence surface morphologies. My studies on Enceladus focus on two subjects: (1) the transport time scale for nanoparticles of silica from the ocean floor to the erupting plumes and (2) the role of the non-tidal stress in controlling the phase lag of time-varying plume fluxes that share the same periodicity with the diurnal tide. I assess the transport time scale of silica particles based on experimentally determined scaling relationships for convection systems under rotation and entrainment of particles in thermally-driven convecting fluids. The physics-based analytical relationships obtained from this approach allow the establishment of the size of the silica particles to the thermal regime of the core, which in turn provides the basis for estimating the transport time scale of the particle through the ocean, which I find to be on the order of months. To assess the role of the non-tidal stress in controlling the phase lag of plume eruption on Enceladus, I conducted detailed structural mapping along geyser-hosting faults zones (i.e., the informally named tiger stripes in the literature). My mapping shows that the geysers are preferentially located at local extensional structures along overall strike-slip faults. In order to have simultaneous strike-slip fault motion and local development of extensional structures along the strike-slip faults, coeval shear and tensile failure is required. Imposing this condition and assuming that the peak-eruption time is the result of the superposed tidal and non-tidal stresses reaching the maximum tensile-stress value, I am able to use a stress-decomposition model to determine the static non-tidal stress field along geyser-hosting faults. The required non-title stress field is best explained by lateral viscous flow induced by the gradient of gravitational potential stored in an unevenly thick ice shell. My research on Titan focuses on the geomorphological response in space and time to climate change and tectonic deformation. In this end, I established the spatial distribution and temporal relationships among morphologically distinctive terrains through mapping in the South Belet and Soi Crater regions. The major finding of the work is that dunes and lakes are the youngest geomorphologic units resulting from the youngest climate condition that are superposed on top of hummocky, labyrinth, pitted, and mountainous terrains. The presence of dune fields requires aeolian transport, the lake and labyrinth terrains surface and subsurface fluid-flow activities, and the pitted terrain removal of volatile materials. The oldest mountainous terrain is best explained by early tectonic deformation. The spatial distribution of dunes and lakes is consistent with the global mapping results that climate-sensitive terrains are distributed symmetrically with respect to the equator, reflecting the symmetry of the atmosphere circulation.

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