HR: 09:30h
AN: P11F-07    [Abstracts]
TI: The Role of Ammonia in the Evolution of Enceladus
AU: Freeman, J
EM: freeman.justin@gmail.com
AF: RSES, Australian National University, Acton, ACT 0200, Australia
AU: * Stegman, D
EM: dave.stegman@sci.monash.edu
AF: School of Mathematical Sciences, Monash University, Clayton, VIC 3800, Australia
AU: May, D
EM: david.may@sci.monash.edu
AF: School of Mathematical Sciences, Monash University, Clayton, VIC 3800, Australia
AB: A large internal density anomaly, most likely an ice diapir, is inferred to play a central role in a sequence of globally significant events 1) true polar wander induced by the ice diapir so that the region of anomalous activity has become situated at the south pole (Nimmo et al., 2006), 2) formation of large fractures due to the tectonic stresses generated as a consequence of the elastic lithosphere's reorientation (Melosh, 1980), 3) subsequent motion along the fractures producing frictional heating and water vapor which is deposited on the surface of the surrounding region where it recondenses (Nimmo et al., 2007). Recent models of shear heating along the tiger stripes (Nimmo et al., 2007) not only appear to account for nearly all of observed surface heat flow 5.8±1.9 GW (Spencer et al., 2006) but provide a good match to the surface distribution of temperature. Models of purely thermal convection that require between 3-5 GW of internal heating in order to successfully develop degree-one features (Grott et al., 2007) are problematic for two reasons: 1) nearly all of the observed 6 GW coming out of the south pole is produced near the surface (Nimmo et al., 2007) and 2) several independent analyses estimate the maximum tidal dissipation available for internal heating in the range between 0.1-0.5 GW. There are at least three notable features of Enceladus which remain unexplained: 1) origin of the ice diapir within the interior, 2) origin of a subsurface ocean beneath the south polar region which, by inference, must exist to allow sufficient shear velocities along the fractures to produce the observed amount of surface heat flow, and 3) the origin of the asymmetry in Enceladus' surface deformation (that is, IF the tectonic fractures were indeed generated by the reorientation of the satellite, which changes the stress pattern globally, why did these fractures only form in one place?) We propose that a compositional diapir of pure water ice generated by the differentiation of an ammonia-water ice mantle can reconcile these aspects into a self-consistent geodynamic evolution of Enceladus. Ammonia has been observed on numerous other icy bodies in the outer solar system, including Jovian satellites (Spohn and Schubert, 2003; Nagel et al., 2004), Charon (Cook et al., 2007), and even Kuiper Belt objects such as Quaoar (Jewitt and Luu, 2005), so it's quite conceiveable ammonia is present in Enceladus as well. We demonstrate that the relatively small amount of available tidal dissipation is adequete for a differentiated layer to form at the core-mantle boundary and present 2-D and 3-D numerical models and scaling analysis for different scenarios of how the diapir might rise to the surface. We show that the creation of a regional subsurface ocean is a natural consequence of a pure water ice diapir reaching the surface. The subsequent assymetry of large tectonic fractures on the surface will be more likely as stresses concentrate over the subsurface ocean where the elastic layer is decoupled from the underlying viscous ice shell. Several lines of evidence are addressed such as how processing of the vapor plume by magnetospheric ion irradiation may reconcile the apparent lack of observed ammonia components in the plumes vapour content or on the surface (Loeffler et al., 2006). These chemical species (NH₃, N₂ and N⁺) have been observed (Smith et al., 2005) in small quantites, but there is some indication ammonia is an impurity present in larger amounts increasing with depth as inferred from radar albedo measurements (Ostro et al., 2007).
DE: 6280 Saturnian satellites
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting