Heat Loss and Tectonic Style of Venus

The tectonic style of a terrestrial planet depends strongly on the mechanisms of heat release from the mantle through the lithosphere to the surface. Three types of lithospheric heat transfer have been proposed. (1) Lithospheric conduction, (2) (hot spot) volcanism, (3) plate recycling (mainly at spreading plate margins). In the case of the Earth the total heat flow is determined by plate recycling 65%, heat conduction through the lithosphere 20%, decay of radioactive elements in the crust 15%, hot spot volcanism <1%. Scaling the mean surface heat flow density of the Earth to venusian conditions leads to 66 mW/m². In the case of Venus plate tectonics play only a minor role. Thus, two processes remain for heat release: (hot spot) volcanism and conduction. The term "hot spot" is written in brackets because volcanism on Venus occurs globally, not necessarily associated with hot spots. The volcanic lava production has been estimated from Venera 15/16 scenes. Arecibo and Magellan images revealed that the surface character south of 30° N is very similar to the area covered by Venera. The main results of the estimation are: (i) The maximum thickness of the plain lavas is 3 km. (ii) With plain lava thicknesses larger than 200 m the lava production from central volcanoes is negligible, (iii) Two age models have been used for the mean age of the area obseved: Δt₁ = 10⁹ a, Δt₂ = 400 x 10⁶ a. Δt₁ leads to the maximum lava production rate of 3 km³/a compared to 20 to 25 km³/a of the Earth; this gives a maximum contribution of 0.75mW/m² to the heat flow density of Venus, i.e. about 1%. This implies that either heat conduction is the only dominating process for heat release or there is a hidden reservoir of the "missing basalt" somewhere or there is another unknown tectonic process. Assuming pure conduction and correcting the surface heat flow density for radioactive elements in the crust leads to a thickness of the thermal lithosphere of 45km. A reservoir for the "missing basalt" could be basaltic underplating to a depth of 100 km. This gives a contribution of about 20 mW/m² with the age model δt₂ to the heat flow density from first order calculations. While the tectonic style of the Earth can be described to be linear formed at the plate margins, the surface of Venus is characterized by global spotty volcanism. The surface is more dominated by volcanic landforms than in the case of the Earth despite the relatively low lava production rate with a maximum of 3 km³/a. As plate tectonics is a minor process on Venus, conduction through a rather thin lithosphere should play an important role for heat release.