Could Venus and Earth once have been a single planet?

[venus (Lydia)]

I was just wondering about this today while reading about how the Moon was formed. I thought that if an impact had occured on a larger planet and split that planet in two could that account why Venus has a retrograde rotation?

[renormalised (Carl)]

No, the dynamics of the system are all wrong for Earth and Venus to have been a single planet split in two. They would have markedly different orbits to what they have now if they had started out in the way you mentioned, plus there's no known way to produce two equally sized planets from one larger one via collision. In any impact that might be big enough to fracture such a large planet, the resulting planet would be larger than the original as it would absorb much of the impactor's mass. In any case, nature has a habit of producing lots of smaller bodies rather than larger ones. For every Mars sized body that was around, there were 10 or more Moon sized bodies...however there were very few Earth sized planets in the system. 2, as it turned out. The reason why Venus has retrograde rotation is because of its massive atmosphere and how it interacts with the Sun. Short of going into the physics of the interaction, what basically happened is that the young Sun, which was far more active than what it is today, slowed down Venus' rotation by ablating its upper atmosphere away and taking away the extra energy the atmosphere gained by friction on the planet's surface...in essence the massive atmosphere slowed down the planet's rotation, gaining energy via transfer of angular momentum and the solar wind removed that energy by removing the upper atmosphere of the planet. It eventually slowed till it almost stopped, then started back up again in the opposite direction. At least, that's the theory. It may have also suffered a massive collision just like Earth did early on in its formation. Also, tidal interactions would've contributed to the slowing down of Venus's rotation as well. Venus has a curious spin-orbital resonance with the Earth, which may or may not be due to the Earth having locked Venus into a spin-orbital resonance at some time in the past.

[venus (Lydia)]

Hmm one reply, does not bode well for my hypothical...
Surely there must be thousands of scenarios for a planet to split?

[renormalised (Carl)]

Any collision has to follow the laws of physics, so any result from a collision of two bodies must comply with those laws. A head on collision will complete disintegrate both objects and the resulting debris would coalesce into smaller objects, or one large object and a few smaller ones (depending on the velocity and energy of the collision). A glancing collision would see one of the objects being totally destroyed and much of its mass absorbed by the other. What was left would form a smaller object or objects that may or may not go into orbit about the remaining large object. There are thousands of collisional outcomes which may or may not occur, but some collisions are dynamically impossible due to the constraints of the physical laws of nature.