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Title: A pre-Caloris synchronous rotation for Mercury
Authors: Mark A. Wieczorek, Alexandre C. M. Correia, Mathieu Le Feuvre, Jacques Laskar, Nicolas Rambaux

The planet Mercury is locked in a spin-orbit resonance where it rotates three times about its spin axis for every two orbits about the Sun. The current explanation for this unique state assumes that the initial rotation of this planet was prograde and rapid, and that tidal torques decelerated the planetary spin to this resonance. When core-mantle boundary friction is accounted for, capture into the 3/2 resonance occurs with a 26% probability, but the most probable outcome is capture into one of the higher-order resonances. Here we show that if the initial rotation of Mercury were retrograde, this planet would be captured into synchronous rotation with a 68% probability. Strong spatial variations of the impact cratering rate would have existed at this time, and these are shown to be consistent with the distribution of pre-Calorian impact basins observed by Mariner 10 and MESSENGER. Escape from this highly stable resonance is made possible by the momentum imparted by large basin-forming impact events, and capture into the 3/2 resonance occurs subsequently under favourable conditions.

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Mercury's wobbly orbit could be due to huge 'basin-forming' impact from asteroid

Scientists used to believe that Mercury permanently turned 'one face' towards the Sun - with one side scorchingly hot and the other cold. A Mercury 'day' was believed to be the same as the planet's year.
But radar scans of the planet revealed that this wasn't the case - the planet wobbles round three times for every two circuits it makes round the sun.
Why the planet had escaped the 'tidal locking' of the Sun's gravitational pull - the same force that makes our moon face one way towards the Earth - had been a puzzle. Now scientists have fingered a likely culprit - a huge asteroid.

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Mercury may once have orbited the Sun in a synchronous rotation, according to new calculations that suggest a collision with a large asteroid may have knocked Mercury into its unusual orbit.
The rotation of rocky planets in our Solar System is determined by many factors. These include gravitational forces, friction between the mantle and crust, the initial spin state after the planet has formed and the 'obliquity' angle between the planet's equator and the plane of its orbit around the Sun.

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Asteroid Crash May Explain Mercury's Strange Spin

Researchers suggest that Mercury was once tidally locked, initially spinning in the opposite direction to its orbit.

"Mercury once had a spin rate synchronous with the sun, like the moon with the Earth" study co-author Alexandre Correia, a planetary scientist at the University of Aveiro in Portugal.

Computer models suggest that a giant impact from an asteroid then knocked it into its current strange configuration. The space rock would have been at least 70 kilometers wide and 550 trillion metric tons in mass, or 1/600,000 the mass of Mercury, Correia said.

Evidence of this collision might include Caloris Basin, Mercury's largest impact crater, which matches the predicted size, age and location of the impact, the researchers said.

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