A new computer model finds that heat in Venus' hot atmosphere could have led to a cooling effect within the planet's interior.

Venus is often nicknamed Earth's evil twin because although it is a similar size to Earth, it is shrouded in a thick, inhospitable atmosphere that is considered an extreme form of the greenhouse effect, created by carbon dioxide, sulphur dioxide and water vapour spewed out from thousands of volcanoes in Venus' youth. Today's surface temperature of 470 degrees Celsius, forced by this extreme greenhouse effect, could have been a lot higher in the past.

Much of a planet's heat originates from within its interior, through the radioactive decay of elements and from the left-over heat of planetary formation. In Venus' case there is enough heat to partially melt portions of the upper mantle, which rise up to penetrate the crust at volcanic vents, extruding lava onto the surface and gases into the atmosphere.

The more greenhouse gases the hotter the atmosphere, and one might expect that to lead to more volcanism. "But at a certain point this process turned on its head – the high temperatures caused a partial mobilization of the Venusian crust, leading to an efficient cooling of the mantle, and the volcanism strongly decreased," says lead author Lena Noack from the German Aerospace Center (DLR) in Berlin. "This resulted in lower surface temperatures, rather comparable to today’s temperature on Venus, and the mobilization of the surface stopped.”

Noack and colleague Doris Breuer arrived at this conclusion by constructing a model that described Venus' hot atmosphere as ‘coupled’ to a 3D model of the planet’s interior.

“Interestingly, due to the rising surface temperatures, the surface is mobilized and the insulating effect of the crust diminishes,” says Noack. “The mantle of Venus loses much of its thermal energy to the outside. It’s a little bit like lifting the lid on the mantle: the interior of Venus suddenly cools very efficiently and the rate of volcanism ceases. Our model shows that after that ‘hot’ era of volcanism, the slow-down of volcanism leads to a strong decrease of the temperatures in the atmosphere.”

The model also threw up another surprise in that the volcanic resurfacing takes place at different locations at different times, with a handful of active volcanoes remaining after the atmosphere has cooled enough to immobilize the crust.

Unusual surface hot spots have recently been detected by ESA's Venus Express mission, but as yet, no active volcanoes have been identified. The hunt continues.