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Regional tectonic analysis of Venus as part of the Pioneer Venus guest investigator projectOver the past year, much of the tectonic analysis of Venus we have done has centered on global properties of the planet, in order to understand fundamental aspects of the dynamics of the mantle and lithosphere of Venus. We have developed convection models of the Earth and Venus. These models assume whole mantle internally-heated convection. The viscosity is temperature, volatile-content, and stress dependent. An initial temperature and volatile content is assumed, and the thermal evolution is tracked for 4.6 billion years. During this time, heating occurs by decay of radiogenic elements in the mantle, and degassing and regassing of volatiles takes place at the surface. For a model assuming plate tectonics as the primary heat loss mechanism, representing the Earth through most of it's history and perhaps Venus' earlier history, degassing of the mantle was found to occur rapidly (approximately 200 My) over a large range of parameters. Even for parameters chosen to represent extreme cases of an initially cool planet, low radiogenic heating, and large initial volatile complement, the mantle water content was degassed to an equilibrium value in about 2 By. These values may be applicable to the early Venus, if a large, Moon-forming impact on Earth resulted in efficient heating and loss of water, leaving Venus with a comparably greater volatile budget and less vigorous early convection. It may therefore be impossible to retain large amounts of water in the interior of Venus until the planet cools down enough for the 'cold-trap' effect to take place. This effect traps crust forming melts within the mantle due to a cusp in the solidus, causing these melts to refreeze at depth into a dense eclogite phase, which will inhibit ascent of this material to the surface. This effect, however, requires a hydrous mantle, so early loss of water might prevent it from taking place. Since without plate tectonics there is no mechanism for regassing volatiles into the mantle, as occurs on Earth at subduction zones, this means the interior of Venus would at present be almost completely dry. We have also calculated argon degassing, and mantle flow velocities. viscosities, and cooling rates in these models, and these values can provide constraints on present day mantle dynamics.
Document ID
19940009088
Acquisition Source
Legacy CDMS
Document Type
Contractor Report (CR)
Authors
Williams, David R. (Arizona State Univ. Tempe, AZ, United States)
Date Acquired
September 6, 2013
Publication Date
September 9, 1991
Subject Category
Lunar And Planetary Exploration
Report/Patent Number
NAS 1.26:194079
NASA-CR-194079
Accession Number
94N13561
Funding Number(s)
CONTRACT_GRANT: NAG2-690
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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