Venus has an extremely slow rotation rate, to the point that it actually rotates slower than it revolves around the sun. In other words, its day is longer than its year. With the exception of Mercury, which has some resonance with the sun, the Venus day is by far the slowest of any planet, and for that matter, non-tidally locked object in the Solar System. Why is that?
Not only is the rotation slow, it is retrograde, thus rotates in the opposite direction to the other planets - which may be linked to the slow rotation. The atmosphere also rotates much faster.
There are several theories as to why Venus has a slow retrograde rotation, according to Why Venus Spins the Wrong Way (Franzen, 2001), states that a theory of this phenomenon states that
Venus initially spun in the same direction as most other planets and, in a way, still does: it simply flipped its axis 180 degrees at some point. In other words, it spins in the same direction it always has, just upside down, so that looking at it from other planets makes the spin seem backward.
The mechanism behind this, according to the theory, is that
sun's gravitational pull on the planet's very dense atmosphere could have caused strong atmospheric tides. Such tides, combined with friction between Venus's mantle and core, could have caused the flip in the first place.
However, in the same article, some researchers argue that the planet simply slowed to a standstill and reversed due to the same factors as the previous theory as with tidal effects of other planets - that it simply settled into its own stable rotation.
To add to the mystery, the article Venus Spinning Slower Than Thought—Scientists Stumped (Major, 2012) suggest that the rotation is slowing down even further, with
According to the new data, Venus is rotating 6.5 minutes slower than it was 16 years ago, a result that's been found to correlate with long-term radar observations taken from Earth.
2 possible mechanisms for this slow down are:
friction from the thick atmosphere
an exchange of angular momentum with Earth
With the new data, there is still no definitive answers, but very interesting theories.
Nobody really knows for sure. And there's two, nay three more odd things about it's rotation on its own axis, namely:
- It is the only planet in the Solar system that rotates retrograde, i.e. clockwise, when all other planets rotate prograde, or anticlockwise, on their axes,
- Latest findings (merely a good month ago as of writing this answer) revealed that its rotation period is actually increasing, i.e. its axial rotation velocity is slowing down, and
- Its sidereal rotation period, or duration of one Venusian day, is longer than it takes for it to complete one orbit around the Sun, or one Venusian year.
Venus is indeed an oddball. Some theories explaining this strange Venusian day cycle go like this:
Astronomers think that Venus was impacted by another large planet early in its history, billions of years ago. The combined momentum between the two objects averaged out to the current rotational speed and direction. source
One possibility is that Venus rotated normally when it first formed from the solar nebula, and then the tidal effects from its dense atmosphere might have slowed its rotation down. source
"It is difficult to find a mechanism that will cause the average rotation rate to change this much in only 16 years," said Venus Express project scientist Håkan Svedhem. "The origin of this could lay in the solar cycle or in long-term weather patterns that modify the atmospheric dynamics. But this puzzle is not yet solved." source
The rotation period of Venus may represent an equilibrium state between tidal locking to the Sun's gravitation, which tends to slow rotation, and an atmospheric tide created by solar heating of the thick Venusian atmosphere. The 584-day average interval between successive close approaches to the Earth is almost exactly equal to 5 Venusian solar days, but the hypothesis of a spin–orbit resonance with Earth has been discounted. source
And so on, ad infinitum ...