A Moon-Sized Smash-Up May Explain Venus's Strange Spin
A new study suggests a colossal, high-angle impact early in its history could be responsible for Venus's current slow, retrograde rotation.
Venus's Peculiar Rotation
Unlike most planets in the solar system, Venus rotates incredibly slowly and in the opposite direction of its orbit. This "retrograde" rotation has long puzzled planetary scientists. A study presented at the European Geosciences Union General Assembly 2026 in Vienna now offers a dramatic explanation: a collision with a moon-sized object.
The Violent Birth of a Slow Spinner
Lead author Cedric Gillmann of ETH Zurich explained that computer models indicate an impactor about one-tenth of Venus's mass, striking at a high angle, could have drastically altered the planet's spin within the first 50 million years of its formation.
“Models indicate an impactor with about one-tenth of Venus's mass striking at a high angle could have significantly altered the planet's rotation.”
— Cedric Gillmann, ETH Zurich
The simulations predict these impacts would not only change the rotation but also generate immense energy, creating surface magma oceans. These would range from shallow melt layers to fully molten mantles, though the heat from such an event would dissipate over a few hundred million years.
Long-Term Consequences and the Mystery of Plate Tectonics
Crucially, the study indicates that after the heat dissipated, the planet's long-term evolution would be similar to scenarios without an impact. This leaves the debate over Venus's lack of plate tectonics unresolved.
Why It Matters for Habitability
Stephen Kane of the University of California, Riverside, highlighted the broader importance of this finding.
“Planetary rotation strongly influences climate and habitability, and understanding Venus's rotation history is key to assessing its past habitability.”
— Stephen Kane, University of California, Riverside
Understanding the violent event that set Venus spinning in reverse is therefore a critical piece of the puzzle in determining if Earth's "twin" was ever capable of hosting life.