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Magnetism and thermal evolution of the terrestrial planets

Stevenson, David J. and Spohn, Tilman and Schubert, Gerald (1983) Magnetism and thermal evolution of the terrestrial planets. Icarus, 54 (3). pp. 466-489. ISSN 0019-1035.

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Of the terrestrial planets, Earth and probably Mercury possess substantial intrinsic magnetic fields generated by core dynamos, while Venus and Mars apparently lack such fields. Thermal histories are calculated for these planets and are found to admit several possible present states, including those which suggest simple explanations for the observations; while the cores of Earth and Mercury are continuing to freeze, the cores of Venus and Mars may still be completely liquid. The models assume whole mantle convection, which is parameterized by a simple Nusselt-Rayleigh number relation and dictates the rate at which heat escapes from the core. It is found that completely fluid cores, devoid of intrinsic heat sources, are not likely to sustain thermal convection for the age of the solar system but cool to a subadiabatic, conductive state that can not maintain a dynamo. Planets which nucleate an inner core continue to sustain a dynamo because of the gravitational energy release and chemically driven convection that accompany inner core growth. The absence of a significant inner core can arise in Venus because of its slightly higher temperature and lower central pressure relative to Earth, while a Martian core avoids the onset of freezing if the abundance of sulfur in the core is ⪆15% by mass. All of the models presented assume that (I) core dynamos are driven by thermal and/or chemical convection; (ii) radiogenic heat production is confined to the mantle; (iii) mantle and core cool from initially hot states which are at the solidus and superliquidus, respectively; and (iv) any inner core excludes the light alloying material (sulfur or oxygen) which then mixes uniformly upward through the outer core. The models include realistic pressure and composition-dependent freezing curves for the core, and material parameters are chosen so that the correct present-day values of heat outflow, upper mantle temperature and viscosity, and inner core radius are obtained for the earth. It is found that Venus and Mars may have once had dynamos maintained by thermal convection alone. Earth may have had a completely fluid core and a dynamo maintained by thermal convection for the first 2 to 3 by, but an inner core nucleates and the dynamo energetics are subsequently dominated by gravitational energy release. Complete freezing of the Mercurian core is prohibited if it contains even a small amount of sulfur, and a dynamo can be maintained by chemical convection in a thin, fluid shell.

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Stevenson, David J.0000-0001-9432-7159
Additional Information:© 1983 by Academic Press, Inc. Received August 20, 1982; revised January 31, 1983. We acknowledge the support of the Deutsche Forschungsgemeinschaft, the National Aeronautics and Space Administration through Grant NSG 7315 to UCLA, the National Science Foundation through Grant EAR-8206383 to the California Institute of Technology, and the use of UCLA computing funds. We also acknowledge useful conversations with Pat Cassen and Ray Reynolds. Most of the calculations reported here were performed in 1979, and the first author wishes to acknowledge the persistence of various colleagues in urging completion and publication.
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Deutsche Forschungsgemeinschaft (DFG)UNSPECIFIED
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Caltech Division of Geological and Planetary Sciences3820
Issue or Number:3
Record Number:CaltechAUTHORS:20131015-131349951
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Official Citation:David J. Stevenson, Tilman Spohn, Gerald Schubert, Magnetism and thermal evolution of the terrestrial planets, Icarus, Volume 54, Issue 3, June 1983, Pages 466-489, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41923
Deposited By: Tony Diaz
Deposited On:16 Oct 2013 19:44
Last Modified:03 Oct 2019 05:53

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