Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p32d0571s&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P32D-0571
Physics
5418 Heat Flow, 5480 Volcanism (8450)
Scientific paper
After a lava is extruded onto the surface of a planetary body with an atmosphere, it largely cools by a combination of radiation and atmospheric convection. Previous work has considered these two modes to be independent, which in practice means they can be calculated separately and summed. This assumption holds on planetary bodies with transparent atmospheres such as on Earth. On Venus, however, the dominantly CO2-rich atmosphere absorbs strongly in the infrared. The absorption is sufficiently strong in a number of bands that a significant amount of thermal energy is absorbed in the atmosphere over the length scales governing convection (i.e., the thermal boundary layer thickness). This coincidence of length scales causes radiation and atmospheric convection to be strongly coupled on Venus. Convective and radiative heat fluxes cannot be calculated separately and summed. I solve the problem of coupled radiation and convection by using a non-gray approximation for combined radiation and conduction in the convective boundary layer over all relevant wavelengths, and then calculate the total heat flux using a boundary-layer analysis of free-convection heat flux. Results indicate that the heat flux from lavas on Venus is less than would be estimated if the atmosphere were transparent. This reduction in heat flux from radiative coupling is due to the dampening of thermal gradients within the boundary layer, which increases the boundary layer thickness, and hence reduces heat flux. Calculations for lavas on Venus indicate that lavas there should cool 30 to 40 percent more slowly than their equivalents on Earth. (This result is an improvement over previous calculations that assumed that the atmosphere was transparent over the length scales relevant to convection. Those calculations suggested that lavas cooled more rapidly than on Earth.) That lavas cool more slowly on Venus than previously recognized extends our understanding of how some lavas on that planet reach extraordinary lengths, sometimes thousands of kilometers, before solidifying.
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