HR: 1340h
AN: P23A-0235    [Abstracts]
TI: Coronae Formation on Venus by Rayleigh-Taylor Instability
AU: * Hoogenboom, T
EM: trudi@jpl.nasa.gov
AF: Jet Propulsion Laboratory, 4800 Oak Grove Drive MailStop 183-501, Pasadena, CA 91109 United States
AU: Houseman, G A
EM: greg@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS29JT United Kingdom
AB: In this study we explore the idea that coronae have formed on Venus as a result of gravitational (Rayleigh-Taylor) instability of the mantle lithosphere. The lithosphere is represented by a system of stratified homogeneous viscous layers (low-density crust over high-density mantle, over a lower density layer beneath the lithosphere). A small harmonic perturbation imposed on the base of the lithosphere is observed to result in gravitational instability under the constraint of assumed axisymmetry. Numerical solutions are obtained for viscous strain on the order of 200 percent or more. Topography develops with time under the influence of dynamic stress associated with downwelling or upwelling, and spatially variable crustal thickening or thinning. Axisymmetric Rayleigh-Taylor instability can produce the scale of uplift and most of the topographic forms displayed by coronae. A central upwelling mechanism is inferred for some coronae (e.g. group 3a, rim surrounding elevated central region coronae), whereas central downwelling is required for others (e.g. Group 8, depressions). Observed average coronae radii are consistent with a lithospheric thickness of only 60 km. The sign of topography is not diagnostic of central upwelling or downwelling because crustal thickness variations induced by the deformation may cause a topographic high above a downwelling or a low above an upwelling when the opposite would be observed in the absence of a crustal layer. The model calculations show, however, that the gravity anomaly is always negative above a downwelling, although its amplitude may be diminished by deformation of the crust. Using the ratio of peak gravity to peak topography anomaly removes uncertainty associated with lithospheric thickness and the density contrast between the crust and mantle. This measure depends primarily on the ratios of crust to lithospheric thickness and strength. The variation of observed gravity to topography ratios suggests spatial variability in the relative strength and thickness of the crust. Average values are consistent however with a low-density crustal layer only 10 to 20 km thick.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8149 Planetary tectonics (5475)
DE: 6295 Venus
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting