HR: 08:00h
AN: P11F-01 INVITED    [Abstracts]
TI: Cold Fire: The Geology and Geophysics of Enceladus
AU: * McKinnon, W B
EM: mckinnon@wustl.edu
AF: Dept. Earth Planet. Sci. and McDonnell Center for Space Sci., Washington Univ., Saint Louis, MO 63130, United States
AB: The mythological Enceladus is buried beneath Mt. Etna, and is responsible for its tremors and volcanism. Enceladus, the 500-km-wide moon of Saturn, is no less active, but the source of its geological vigor is one of the great challenges of contemporary planetary science. Data from Cassini has revolutionized our view of this world. It is rock-rich, 56-59% rock by mass, yet its surface is nearly pure water ice. Most of Enceladus has been resurfaced, and active plumes of vapor and micron-sized ice particles are erupting from a set of semi- parallel fissures ("tiger stripes") at its south pole. Thermal emission from the south polar terrain (SPT) of 5.8 ± 1.9 GW is measured directly. The SPT is also ringed with distinctive compressional fold and/or thrust belts. Enceladus is thus the icy satellite equivalent to Io, in terms of phenomena and data type, i.e., traditional visual imaging and infrared spectroscopy are augmented by heat flow determination; in addition, Cassini can and has flown through the plume and measured its composition. The source of Enceladus' thermal output must be tidal heating associated with its 2:1 e-type mean-motion resonance with larger Dione. The magnitude of its heat output can be explained by a suitably dissipative interior, but one that in all likelihood is differentiated and contains an internal ocean ("aquasphere"), because solid-body dissipation is inadequate. Its present heat flow cannot be maintained over geologic time, however, which implies that Enceladus' eccentricity is time variable (presently decreasing) and/or that the source of Saturn's dissipation (its ` Q') has also varied (decreased) over geologic time. Where in Enceladus is tidal strain energy dissipated, and why is it concentrated in one geographic locale? Is it in the rock core, the overlying warm (in a homologous sense) ice mantle, or in the cold lithosphere, along vertical, "tiger-rific" shear zones, or some combination? Core dissipation is unlikely to be quantitatively sufficient, whereas models of mantle and/or lithospheric dissipation are under active development. And what is the relation between the present activity at the SPT and the rest of the surface, most of which appears geologically youthful and also betrays evidence of high heat flows (viscously relaxed craters, groove-like extensional terrain)? Ammonia and salts have been suggested as antifreezes in Enceladus' ocean, facilitating plume eruptions. To date neither have been detected in the plumes or in the related E-ring torus, and the limits on Na are particularly stringent. The latter sets a severe constraint on the extent of water-rock interaction within Enceladus, if the proximate plume source is ocean water. An alternate view posits that the plumes are driven by degassing clathrate; this hypothesis is attractive in terms of physical chemistry but a clathrate- dominated mantle is too stiff (as presently measured) to be compatible with Enceladus' active tectonics. Whatever the cause and path for Enceladus' activity, it promises to greatly clarify our theoretical understanding of how icy worlds work.
DE: 5418 Heat flow
DE: 5422 Ices
DE: 5430 Interiors (8147)
DE: 5480 Volcanism (6063, 8148, 8450)
DE: 6280 Saturnian satellites
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting