HR: 1340h
AN: P33A-0228    [Abstracts]
TI: Laboratory Experiments on Transient Convection in a Fluid With a Temperature Dependent Viscosity: Investigating the Unusual Coexistence of Coronae and BAT Highlands on Venus
AU: * Robin, C M
EM: crobin@physics.utoronto.ca
AF: Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7 Canada
AU: Thalayan, V
EM: vidt@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Jellinek, A M
EM: mjellinek@eos.ubc.ca
AF: Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AB: The coexistence of Venusian highlands, attributed to long-lived mantle upwellings, and dynamically-supported coronae, attributed to short-lived mantle 'thermals', is difficult to reconcile with models of Rayleigh-Bénard convection under thermally steady-state conditions. In particular, whereas thermals are a characteristic feature of ``stagnant lid'' convection, long-lived plumes likely require stronger mantle cooling due to ``active lid'' convection such as the subduction and large-scale mantle stirring characteristic of the Earth. However, the inferred young surface age of the planet has been explained previously in terms of a catastrophic resurfacing event, a scenario that permits the possibility that the planet is currently in a thermally transient regime. Accordingly, we use an extensive series of laboratory experiments on convection at high Rayleigh number (Ra ~ 10⁷) with additional stirring imposed from above to investigate the thermal transient following a global resurfacing event. We analyze the transitions in the structure and heat transfer properties of the flow as the system evolves from stagnant lid to active lid convection (transient regime 1), and from active lid to stagnant lid convection (transient regime 2). In regime 1 the hot thermal boundary layer is overrun as cold material spreads, forming a low viscosity ``squeeze'' layer. Ahead of the cold front convection is in the form of intermittent thermals, which have a comparable viscosity to the interior fluid and a period of formation that depends partly on the rate of imposed stirring. Behind the cold front conditions exist in which convective instabilities of the squeeze layer can lead to coexisting long-lived, low viscosity plumes. Such a regime persists until the cold material covers the bottom boundary. The local basal heat flux during this transient depends on the temperature of the cold material, which is governed by stirring rate, and can exceed the steady-state heat flux for active lid convection. Thermally steady-state (active-lid) conditions, indicated by an interior temperature θ ≈ 0.5, are recovered after a few overturns. In regime 2 a gradual transition from plume- to thermal-dominated convection occurs as θ evolves from 0.5 → 0.76. The corresponding local basal heat flux evolves to the (lower) stagnant lid value. Applied to Venus, our results suggest that the coexistence of highlands and coronae is consistent with a transient mode of convection following a catastrophic lithopsheric overturn. The corresponding large local heat flux may have implications for a short-lived Venusian dynamo.
DE: 5418 Heat flow
DE: 5455 Origin and evolution
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8130 Heat generation and transport
DE: 8147 Planetary interiors (5430, 5724, 6024)
SC: Planetary Sciences [P]
MN: Fall Meeting 2005