From the March 2012 issue

Is the slow rotation of Venus a result of a geologic cataclysm?

Igor Mikheev, Dallas, Texas
By | Published: March 26, 2012
By analyzing Venus’ cloud layers, scientists know the planet spins slowly in the direction opposite most of the other planets in our solar system. Its slow rotation is a result of its spin direction flipping sometime in the past. However, astronomers aren’t sure what caused this unusual spin. NASA
The slow rotation period of Venus (it takes 243 Earth days to complete one full rotation) and the orientation of its spin axis to its orbital plane (177°, so it spins in the direction opposite its rotation around the Sun) are two puzzles that planetary scientists have not yet solved. Venus’ crater distribution is roughly uniform over the planet, and few small craters are present. When comparing Venus to other cratered bodies, scientists hypothesized that the surface is relatively young. Its age then led to the suggestion of a global cataclysm — an extensive buildup of heat over time that lead to an episode of global volcanism and lava flows. However, there is no evidence of such an episode: Astronomers haven’t dated rocks on Venus yet.
It appears unlikely that resurfacing, if it occurred, led to the slow rotation rate of Venus, but it certainly may have caused some changes to the speed. Plate tectonics can affect the rotation rate through redistribution of mass. As the surface expanded or contracted (due to the buildup of heat and subsequent cooling through the release of heat from volcanism), it could have led to changes in the angular momentum and, hence, rotation rate.

Multiple impacts may have contributed to Venus’ unusual angular momentum compared to other planets in the solar system. Such impacts eventually could have forced Venus to spin in the direction opposite its rotation and also given it a different spin rate. Tides resulting from the Sun — both thermal (in Venus’ deep atmosphere) and gravitational (in the planet’s solid body) — now control Venus’ rotation rate.
There’s a possibility that atmospheric drag slowed the planet and switched its rotation direction, but the atmosphere would have had to rotate in the same direction in the planet’s past. Scientists know little about the evolution of Venus’ atmospheric circulation, so we cannot prove that the drag led to the planet’s slow rotation rate. — Sanjay S. Limaye, University of Wisconsin-Madison