Nowadays, Venus is a sweltering hellscape with no liquid water on its surface, where temperatures exceed 450掳C 鈥� hot enough to melt lead. But it may have had an ocean billions of years ago, and its tides could have slowed the planet鈥檚 rotation to make its climate relatively temperate.
Venus is an extremely slow rotator. It makes a full turn just once every 243 Earth days, which is slower than any other planet in the solar system. In the deep past, it may have spun faster.
Observations of Venus鈥檚 geology and clouds have indicated that early in its history, the planet may have been more similar to Earth, with a liquid water ocean on its surface.
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Michael Way at NASA鈥檚 Goddard Space Flight Center and his colleagues calculated how an ocean on young Venus could have slowed down its spin from a much faster rotation shortly after it formed聽and made it more habitable. They presented their research at the Lunar and Planetary Science Conference in Texas on 21 March.
Like Earth鈥檚 ocean, Venus鈥檚 would have had tides, which can slow down a planet鈥檚 rotation as the sun or a moon pulls on the water. Because Venus is closer to the sun, those tides would have been stronger than on Earth.
Way and his colleagues found that if Venus started out spinning at about the same rate as Earth does now, these tides could have reduced the spin rate by five Earth days every million years. At that pace, it would take less than 50 million years to slow Venus down to its current rotation rate 鈥� an extremely fast braking by planetary standards.
A slower rotation rate plus a water ocean would probably have created huge cloud banks that would have reflected away much of the sunlight that makes Venus so hot now, says Way.
鈥淰enus stays very cool because the clouds are blocking that solar radiation.鈥� That would have made it much more hospitable to any ancient life. Then, a runaway greenhouse gas effect heated it up to the blazing hot but slow-rotating world we see today, where life is all but impossible.
