HR: 11:35h
AN: P22A-06    [Abstracts]
TI: Large Icy Diapirs and Small Icy Satellites: Reorientation of Mini-Moons
AU: * Pappalardo, R T
EM: robert.pappalardo@colorado.edu
AF: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309-0392 United States
AU: Nimmo, F
EM: fnimmo@emerald.ucsc.edu
AF: Dept. Earth Sciences, University of California, Santa Cruz, CA 95064 United States
AU: Moore, J M
EM: jeff.moore@nasa.gov
AF: Ames Research Center, MS 245-3, Moffett Field, CA 94035-1000 United States
AB: A class of icy satellites ~400 - 500 km in diameter are large enough that they can experience significant tidal heating (Miranda and Enceladus), while small enough that rising diapirs could significantly perturb the bodies' moments of inertia. If these "mini-moons" are heated sufficiently for differentiation and/or internal convection, the resultant low-density plumes may affect inertia moments enough to trigger satellite reorientation relative to the primary planet. Here we emphasize the case of Miranda and its three large coronae, which are inferred to have formed above large-scale diapiric upwellings [1]. Their locations on the greatest inertia axis (near the south pole) and the intermediate inertia axis (along the leading-trailing axis) suggest that coronae are negative density anomalies that have led to satellite reorientation [2]. The Miranda reorientation hypothesis is further supported by fresh crater distributions [3] and structural evidence [4]. True polar wander of the Earth, promoted by convection and associated continental drift, depends on the perturbation timescale relative to the timescale of viscous relaxation [e.g. 5]. Applying similar arguments to a warm and hydrostatic Miranda, we find that a corona-scale (60° width) icy diapir 100 kg/m³ less dense than its surroundings could induce significant (tens of degrees) reorientation, if the ratio of the diapir lifetime τ_conv to the ice shell relaxation time τ_R exceeds 10³. For Enceladus, large reorientations can occur even more readily, if τ_conv / τ_R exceeds 10². In contrast, a cold mini-moon with a frozen-in hydrostatic figure (Mimas and Proteus) would not easily reorient, as a density anomaly would need to overcome the large difference in principal moments [6]. We find that this latter scenario is consistent with the 130 km crater Herschel not having reoriented Mimas. Mini-moon reorientation allows for some interesting possibilities: a large density anomaly could reorient a warm moon while the same anomaly would not induce reorientation if that moon were cold; cooling of a thermally buoyant diapir might allow reorientation of a mini-moon back to its original orientation; and a mini-moon could experience multiple reorientation events if individual diapiric plumes form sequentially.
[1] Pappalardo, R.T. et al., JGR, 102, 13369-13379, 1997. [2] Janes, D.M. and H.J. Melosh, JGR, 93, 3127-3143, 1988. [3] Plescia, J.B., Icarus, 73, 442-461, 1988. [4] Pappalardo, R.T., Ph.D. thesis, Arizona State Univ., 1994. [5] Tsai, V.C. and D.J. Stevenson, JGR, submitted. [6] Melosh, H.J., EPSL, 26, 353-360, 1975.
DE: 6280 Saturnian satellites
DE: 6290 Uranian satellites
SC: Planetary Sciences [P]
MN: Fall Meeting 2005