CONVECTIVE THERMAL EVOLUTION OF THE UPPER MANTLES OF EARTH AND VENUS

On Earth the present-day rate of heat loss is about twice the heat gen eration rate; on Venus it is about half. Though this rough balance may be due to a feedback mechanism between mantle temperature and heat lo ss, it is difficult to see how such a mechanism can occur on timescale s of 1 Ga or less when the upper mantle of the Earth is thought to be cooling at about 40 degrees C Ga-1. On Venus a decrease in surface hea t flux presumably occurred at the end of the catastrophic resurfacing event at similar to 500 Ma. Parameterized convection models relate hea t flux to Rayleigh number by the exponent beta. Such models using a ra nge of viscosities and values of beta from 0.2 to 0.3 show that the ef fect of a sudden decrease in surface heat flux is to cause an independ ently convecting upper mantle to increase in temperature by 100 - 500 degrees C over 1 Ga, whereas, if whole mantle convection occurs, the t emperature change is less than 60 degrees C. An increase in mantle tem perature of 200 degrees C or more will affect mantle viscosity, lithos pheric thickness and melt generation rate, all of which may affect the feedback mechanism.