Other
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.t11c0407v&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #T11C-0407
Other
3210 Modeling, 5134 Thermal Properties, 5418 Heat Flow, 8121 Dynamics, Convection Currents And Mantle Plumes, 8125 Evolution Of The Earth
Scientific paper
Secular cooling of terrestrial planets is largely controled by the heat transport through thermal convection of the silicate mantle enclosing the metallic core. Similar to the well known temperature dependence of mantle viscosity η (T) (Tozer, 1972), thermal conductivity k, dominated by a lattice dynamical component klat as in Hofmeisters (1999) model, also decreases with temperature. In previous work we have shown, for isoviscous models, without mantle core coupling, that secular cooling is substantially delayed, by one to two billion year, for models based on variable k(T,P), compared to constant k models. Here we present results of numerical mantle convection models for both variable k(T,P) and η (T,P) and we also consider thermal coupling between the mantle and the core, represented by an isothermal heat reservoir. The results show that the delay in secular cooling of the models with variable conductivity of the Hofmeister type, compared to the constant k model, is a robust feature also in mantle convections models with variable viscosity. These results show that thermal conductivity, like viscosity, playes a major role in controling planetary thermal evolution. The thermal evolution of the core is driven by conductive heat transport through the core mantle boundary (CMB) and is therefore also sensitive to the particular model of thermal conductivity. In this respect the radiative conductivity component krad, related to phonon transport is particularly important as it increases with temperature and plays a stabilizing role in the bottom thermal boundary layer. Our results show that the temperature contrast δ T across the thermal boundary layer at the bottom of the mantle continues to increase from a zero initial value to values between 400 K and 800 K, for variable k and constant k respectively and within the variable k models δ increases with the relative contribution of the radiative conductivity krad. The smaller temperature contrast across the bottom thermal boundary layer for the variable k models is reflected in a reduced tendency for plume formation at the CMB in the variable k models. The heatflux from the core shows an increasing tendency with time and strong fluctuations originating from thermal interaction of the core with cold downwellings and hot upwellings in the mantle convective flow at the CMB. We varied the initial CMB temperature between 3273 and 4273 K and found that high TCMB results in higher cooling rates, stronger time dependence and an increased contribution of the radiatiative conductivity krad. In general we found the cooling rate of the mantle to be strongly time dependent, fluctuating around values of 100 - 150 K/Gyr. These mean values are fairly constant or even slightly increasing with time, illustrating the buffering effect of the core heat flux, increasing with time.
Rainey E. S.
van den Berg Arie P.
Yuen Dave A.
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