Figuring out Venus's rotation has always been tricky. Even though this planet is our next-door neighbor, early astronomers could never determine something as simple as its spin rate.
Venus hid that fact, and many others, behind a thick layer of clouds. Despite centuries of scrutiny by observers, visual telescopes could only see as deep as the cloud tops.
It wasn't until 1961 -- after the advent of radio astronomy -- that astronomers could peer through the clouds to the surface. What they found was baffling: Venus spins the wrong way. Unlike the other terrestrial planets, Venus rotates retrograde, in the direction opposite its revolution around the Sun. Astronomers then had to figure out why. To date the leading theory has been that Venus suffered an impact early in its history. Perhaps something hit our sister planet hard enough, and at such an angle, to set the planet spinning backward.
Recently, new models have suggested that an impact might not be the cause after all. Alexandre Correia and Jacques Laskar (Astronomie et Systemes Dynamiques, France) report that models of Venus suggest that the planet originally rotated "normally" and over time the spin axis flipped by itself.
Correia and Laskar explain that the planet's dense atmosphere may be the culprit. Over time, gravitational torque and frictional forces between the atmosphere of Venus and the planet itself slow the rotation to the point where it actually began to spin backward. The models' predictions are further strengthened because the researchers obtained the same result regardless of the planet's initial obliquity or period. Correia and Laskar published their results in the June 14th issue of the journal Nature.
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