>> Saturday, July 25, 2009
The Mother asked: How is it that Venus spins backward? Shouldn't the momentum that created the solar system keep everything spinning the same direction?
In theory, I believe you're right. Of the planets in the solar system, all of them orbit in the same direction so it makes sense that they'd rotate in that direction, too. The fact that so many planets rotate in the same direction (i.e. the same direction they're orbiting in) argues it, too.
But Venus doesn't and Uranus is tilted so far on it's axis (~98 degrees) that, depending on own's perspective, it can also be said to rotates backward. But really it's more lopsided.
So, why Venus and Venus only. Well, Venus is not the only item with retrograde motion. Several moons around Jupiter, Saturn and Neptune orbit in a retrograde way. Several of these items are believed to be captured Kuiper belt items. Kuiper belt items are items like comets and some very far flung planetary-type bodies that are far beyond the edge of the planets. Some objects, like comets, we see every so often because they have highly elliptical orbits and they can crash into planets (as happened to Jupiter a few years ago) or be captured, in theory, by the gravitational pull of a planet. As these items have their own spin and orbit (and will be going very fast as they get closer to the Sun), they can readily get caught by a planet, but not necessarily be in plane or going in the same direction when caught (it depends on whether it's incoming or out going and from what direction they approach the planet that captures them).
That, of course, doesn't apply to Venus. Venus is very similar in composition and characteristics to Earth and has a very circular orbit (which would be unlikely in a captured high speed body), in fact the most circular orbit in the solar system. So, why would it spin backwards even at a very slow rate?
The easy answer is, we don't know. There are speculations, of course. One is that the current year/day represents an equilibrium state between gravitational tidal locking by the Sun that tends to slow the rotation rate, and an atmospheric tide created by the solar heating of Venus' thick atmosphere. Ironically, in addition, the periods of Venus' rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known.
But there are other speculations. Alex Alemi and David Stevenson of the California Institute of Technology, using their models, believe that Venus once had a moon (it doesn't have one now) that had been formed from a cataclysmic impact event. Ironically, they believe the retrograde motion now is the result of another cataclysmic impact that changed the rotation and, ironically, pulled the moon back into the planet. This, they believe, happened billions of years ago. Since the surface of the moon is highly volcanic with a new surface, much like the Earth has, there's no indication of any such impacts on the planet's surface.
Just goes to show that there are many strange things going on in the big wide universe. And we've got a long ways to go before we get more than scratching the surface on figuring out how what we can see today happened. By then, of course, we'll likely have a whole new set of mysteries to figure out.