New simulations suggest Venus' extremely slow backward rotation may have been [1]triggered by a high-angle collision with a fast-moving object roughly one-tenth its mass . The impact could have dramatically altered Venus' spin and melted nearly its entire mantle. Universe Today reports:

> Venus' bizarre and extraordinarily slow retrograde rotation on its axis has long puzzled planetary scientists. But in a [2]new paper presented at the recent European Geosciences Union General Assembly in Vienna, the authors argue that their models indicate that a high angle moon-sized, high-velocity impactor likely triggered Venus's strange 248-day rotation. And it probably happened within the first 50 million years of Venus' formation. [...] The team found that an impactor that is about a tenth of Venus' mass hitting the planet at a high angle could drastically show the early young planet's rotation.

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> Depending on the actual impact parameters, we can slow down a rapidly rotating early Venus to rotation rates that are that are compatible with long-term evolution towards a slow rotating planet, says [Cedric Gillmann, the paper's lead author and a planetary scientist at ETH Zurich]. Or even in some cases with large energetic impact that happen with a tangential impact that would even put planets early on in already a retrograde but faster rotation, he says. In the simulations, giant impacts expectedly produce surface magma oceans, the paper's authors note. Their relative depths vary depending on impact properties: from a shallow melt layer in the order of 100km thick to a fully molten mantle, they note. If the surface can radiate heat to space efficiently, the magma ocean cools down quickly, they write.

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> If Gillmann and colleagues are correct, Venus' likely impactor also melted some 99 percent of Venus' mantle. That is, the interior structure that extends between its core and crust. You will get rid of that impact heat pretty efficiently, and after a few hundred million years, you end up seeing an evolution that is very difficult to distinguish from a case where you don't have an impact, says Gillmann. What role the impact may have played in Venus' lack of plate tectonics, however, remains open for debate. But it's known that Venus' lack of a large-scale carbon recycling mechanism likely led to its current runaway greenhouse.



[1] https://www.universetoday.com/articles/venus-strange-rotation-was-likely-triggered-by-a-high-velocity-moon-sized-impactor

[2] https://meetingorganizer.copernicus.org/EGU26/EGU26-10551.html