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How did tidal forces affect Mercury’s rotation?

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Messenger: Mercury — Mercury Mercury as seen by the Messenger probe, January 14, 2008. This image shows half of the hemisphere missed by Mariner 10 in 1974–75 and was snapped by Messenger's Wide Angle Camera when it was about 27,000 km (17,000 miles) from the planet.

Mercury’s rotation has been significantly shaped by the Sun’s gravitational influence. While the planet likely spun faster during its formation, it was gradually slowed by tidal forces—deformations raised within the planet by the Sun’s gravity.1

Rather than slowing to a state of synchronous rotation (where the rotation period matches the orbital period), Mercury became trapped in a stable 3:2 spin-orbit resonance. This means the planet rotates exactly three times for every two times it orbits the Sun, resulting in a rotation period of 58.6 Earth days.1

This specific state is maintained because Mercury’s high orbital eccentricity allows for restoring torques on the planet's permanent axial asymmetry. These torques keep the longest equatorial axis aligned with the Sun at perihelion. The trapping of this spin rate was further enhanced by tidal friction between the planet's solid mantle and its molten core.12