Enceladus' South Polar Terrain Geology: New Details from Cassini ISS High Resolution Imaging

Kurzfassung

The Cassini spacecraft executed a close flyby of Enceladus on August 11 (altitude: 50km); two more are planned for October 9 (altitude: 25 km), and October 31 (altitude: 196 km). High resolution (as fine as 7m/pixel) images of known geologically active features in the South Polar Terrain (SPT) have been returned to investigate how plume eruptions, tectonism, and seismicity alter the surface and to reveal how the SPT has evolved over time. We examined six known eruption sites (Spitale and Porco 2007, Nature 449, 695697) along Cairo, Baghdad, and Damascus Sulci, as well as inactive portions of the "tiger stripes" and bright fractured terrain in adjacent areas. We also obtained contiguous ISS broadband multispectral mosaics of the entire SPT region to refine our geological and digital terrain maps and to search for volcanically and tectonically driven temporal changes. The highestresolution images show ice blocks up to tens of meters in size that are widely but nonuniformly distributed over a variety of terrain units. The upraised flanks and valley walls of active tiger stripes are mantled in places by smooth fluffylooking deposits, most likely accumulations of coarsegrained plume fallout. With increasing lateral distance from the stripes, the smooth upraised flank deposits grade into rounded, platytextured, elongate hills and a conspicuous system of quasiparallel knobby ridges and grooves that have spacings and dimensions comparable to the tiger stripe flanks themselves. Peculiar narrow lenticular ridges, perhaps emplaced by extrusion or as icy pyroclastic deposits, rise from tens to hundreds of meters along the medial fissures of some tiger stripes. On regional scales, the ends of the tiger stripes are bounded by a complex network of fractured terrain, within which can be found numerous transform faults that lie at high angles relative to the trends of the tiger stripes. Observed offsets along these transforms and an absence of lateral symmetry of the displaced terrains suggest that tiger stripes are not exact analogs to classic terrestrial oceanic rifts. Instead, any possible tectonic divergence is more likely a result of the superposition of many regionally and temporally distributed spreading centers.