QUOTE (ljk4-1 @ Nov 7 2005, 09:30 AM)
Actually, should a large impact create a moon? My guess is that if Venus did get hit by something massive and powerful enough to practically flip it over, slow down its spin rate, and have it rotate in the opposite direction of most other major worlds, Venus was lucky enough to have survived intact, forget having a moon in the outcome.
Or maybe Venus was hit BY its own moon.
There are good sources on the Net for things being wildly speculated about here.
To pick just one: the angular momentum of a planet-moon system would remain the same after an impact as before. Such a collision couldn't make the system go backwards unless it was going backwards in the first place.
For the Moon to be created from a collision, it had to be a glancing blow. A straight-on punch would not liberate a lot of material.
A collision would be necessary if Venus's rotation was set in place as its formation concluded. But there are suggestions that tidal and thermal dynamics of its massive atmosphere may have "set" the rotation at the current rate. If so, the initial rotation is irrelevant/unknowable. In fact, the low inclination suggests that there is some sort of sun-driven factor: a collision that reordered Venus's rotation radically would be unlikely to leave the axis so nearly perpendicular to its orbit -- although that kind of coincidence is not impossible.
The rotation is only one oddity, not two or more. Given that the rotation is so slow, the reversal is not so odd -- the difference between rotating slowly E-W or slowly W-E is not nearly so big a difference as the slow rotation in the first place. Consider that a point on Venus's equator is rotating at about the speed of a fast *walk*!!! If it were moving the other direction, but just as slowly, that would only be a difference of a few km/h.
Also, the relative relationship between the day and year of Venus is not an "extra" oddity. Given the slow rotation, that follows as a logical consequence.
There is a second mystery regarding Venus's rotation, however, and that is why it is [almost] synchronized so as to show [almost] the same face towards the Earth at every conjunction. Mathematically, it would seem impossible for the tidal attraction to make this happen... but if this is a coincidence, it is a remarkable one.