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G. Schubert (Department of Earth & Space Sciences, Institute of Geophysics & Planetary Physics, University of California, Los Angeles), B.J. Travis (EES-2, Los Alamos National Laboratory)
Chondritic meteorites are so named because they nearly all contain chondrules - small spherules of olivine and pyroxene that condensed and crystallized in the solar nebula and then combined with other material to form a matrix. Their parent bodies did not differentiate, i.e., form a crust and a core. Carbonaceous chondrites (CCs) derived from undifferentiated icy planetesimals. CCs exhibit liquid water-rock interactions.
CCs contain small but significant amounts of radiogenic elements (e.g., 26Al), sufficient to warm up an initially cold planetesimal. A warmed-up phase could last millions of years. During the warmed-up phase, liquid water will form, and could evolve into a hydrothermal convective flow. Flowing water will affect the evolution of minerals.
We report on results of a numerical study of the thermal evolution of CCs, considering the major factors that control heating history and possible flow, namely: permeability, radiogenic element content, and planetesimal radius. We determine the time sequence of thermal processes, length of time for a convective phase and patterns of flow, amount of fluid flow throughout the planetesimals, and sensitivity of evolution to primary parameters.
We use the MAGHNUM code to simulate 3-D dynamic freezing and thawing and flow of water in a self-gravitating, permeable spherical body. Governing equations are Darcy's law, mass conservation, energy conservation, and the equation of state for water, ice and vapor mixtures.
We have simulated the evolution of heating, melting of ice, subsequent flow and eventual re-freezing for several examples of carbonaceous chondrite planetesimals. We have demonstrated that hydrothermal convection should occur for a range of parameter values and would last for several millions of years. Roughly half the interior of simulated planetesimals experience water fluxes of 100--200 pore volumes. High pore volume flux facilitates extensive chemical reactions.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.