Enceladus' South Polar Terrain Geology: New Details from Cassini ISS High Resolution Imaging
2008
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.
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