The Effect of Mechanical Boundary Conditions on the Secular Cooling of Planetary Mantles.

Details The Effect of Mechanical Boundary Conditions on the Secular Cooling of Planetary Mantles. The Effect of Mechanical Boundary Conditions on the Secular Cooling of Planetary Mantles.

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.t13c0535s&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #T13C-0535

Physics

5400 Planetary Sciences: Solid Surface Planets, 5430 Interiors (8147), 8125 Evolution Of The Earth (0325), 8130 Heat Generation And Transport, 8147 Planetary Interiors (5430, 5724, 6024)

Scientific paper

It has been suggested that the lack of an appreciable Venusian dipolar magnetic field has resulted from the absence of lithospheric participation in Venusian mantle convection for the past 0.5-0.75 billion years (during this period mantle convection has been limited to the thick stagnant-lid regime). In contrast to convection in the stagnant-lid regime, terrestrial mantle convection (featuring the subduction of oceanic lithosphere) is particularly efficient at cooling the Earth. Moreover, participation of the Earth's outer thermal boundary layer in terrestrial mantle convection carries cool material deep into the planetary interior and influences heat flow at the core-mantle boundary. Thus, lithospheric subduction influences the geodynamo that originates with compositional convection in the conducting outer core. In this study, we examine the thermal evolution of terrestrial planets by modelling mantle convection in spherical shells with time decaying heat sources and core cooling. Our calculations are performed using the spherical shell mantle convection code TERRA. Similar bulk concentrations of U and Th and K/U ratios to those estimated for the Earth are assumed. Free-slip and rigid mechanical boundary conditions are imposed at the surface of the models to simulate planets with active and inactive lithosphere subduction, respectively. Our calculations extend over periods equating with hundreds of millions of years. We add different amounts of excess heat to statistically steady (i.e., showing neither long term cooling nor warming) initial thermal fields and then examine the effect of the mechanical boundary conditions on secular cooling. Upon obtaining solutions that give present day secular cooling rates in the range estimated for the Earth, we repeat our calculations for a planet with a rigid mechanical surface. All calculations start from an initial condition obtained with a free-slip surface. Thus, by introducing a rigid surface, we implicitly examine the effect of the cessation of the upper lithosphere's participation in mantle convection. In the absence of plate tectonics the extraction of heat from the core-mantle boundary to the overlaying mantle is reduced. We consider the necessary conditions in the mantle to halt thermal convection in the core and hence the generation of the global magnetic field. We discuss our results in the context of the sequence of events that may have unfolded in Venus's recent past and consider whether plate tectonics may be a critical ingredient in sustaining the Earth's magnetic field.

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