Why do planets and moons spin faster than others?
It mostly is because of the density, but other factors are at work too. Venus and Mercury are a bit special. Venus has a slow retrograde rotation (spins in the opposite direction), most likely due to a huge impact event some time in its early history. Mercury on the other hand is different. In some cases where the orbit is eccentric and the tidal effect is relatively weak, the smaller body may end up in an orbital resonance, rather than tidally locked. Mercury is such a case as it is locked in its orbit around the Sun in a 3:2 resonance (it spins once around its axis for every two rotations around the Sun). When bodies orbit close enough to their parent planet, they can get tidally locked. This simply means one spin around its axis is equal to the rotation around its parent planet, also called a synchronous orbit. When this happens, the tidally locked body slows down its rotation, transferring some of its angular momentum to its host. Triton is the same, only it has a retrograde spin b
