Rotation of Venus

Q: Why does Venus rotate clockwise when viewed from the North, while the other planets rotate counterclockwise?

There are several theories about Venus' rotation, but no scientific consensus has emerged. A strong possibility is that Venus' atmosphere provided enough torque to slow down and even reverse the planet's spin. Venus has a super-rotating atmosphere: the gas circles the planet every four Earth days, while a sidereal day (the time it takes a star to return to its apparent position in the night sky) on Venus lasts 243 Earth days. Given such a mismatch between the atmospheric rotation speed and the planetary rotation speed, it makes sense that Venus' atmosphere might have altered its rotation. A similar process may have taken place on Saturn's moon Titan. Interestingly, new observations suggest that Venus' rotation rate is not constant -- instead it changes due to fluctuations in the rate of angular momentum transport between the atmosphere and the planet.

The other leading theory about Venus' unusual rotation is that a dwarf planet-sized object crashed into Venus early in the evolution of the Solar System, reversing its direction of rotation. Such giant impacts are expected to be common during planet-building. For example, Earth's moon was likely formed from the debris ejected during a collision between young Earth ("Gaia") and a Mars-sized object ("Theia"). Mercury's structure, consisting of a heavy iron core with an undersized rocky mantle, also suggests that it was the victim of a giant impact that ejected most of the mantle material.

-Sally Dodson-Robinson