Convective thermal evolution of the upper mantles of Earth and Venus

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Abstract
On Earth the present‐day rate of heat loss is about twice the heat generation rate; on Venus it is about half. Though this rough balance may be due to a feedback mechanism between mantle temperature and heat loss, it is difficult to see how such a mechanism can occur on timescales of 1 Ga or less when the upper mantle of the Earth is thought to be cooling at about 40°C Ga−1. On Venus a decrease in surface heat flux presumably occurred at the end of the catastrophic resurfacing event at ∼500 Ma. Parameterized convection models relate heat flux to Rayleigh number by the exponent β. Such models using a range of viscosities and values of β from 0.2 to 0.3 show that the effect of a sudden decrease in surface heat flux is to cause an independently convecting upper mantle to increase in temperature by 100–500°C over 1 Ga, whereas, if whole mantle convection occurs, the temperature change is less than 60°C. An increase in mantle temperature of 200°C or more will affect mantle viscosity, lithospheric thickness and melt generation rate, all of which may affect the feedback mechanism.