Given: Law of Conservation of Angular Momentum.
- Reverse spinning with dense atmosphere (92 times > Earth & CO2 dominant sulphur based).
- Surface same degree of aging all over.
- Theoretical large impact is not a sufficient answer.
Assuming any object large enough to alter a planets rotation or even orbit would likely destroy most of its shape, yet Venus has retained a spherical property with a seemingly flat, even terrain indicating no volcanoes,and few if any visible meteor impacts. It would be fragmented and dispersed for billions of years. Even the question of what meteor, comet, asteroid composition could survive traveling that close to the sun's temperature, radiation, electromagnetic energy, solar flares, or gravity to equal a mass reactionary change as to alter it's spin.