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Why is a solar day on Mercury longer than its year?

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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.

A solar day on Mercury is longer than its year because of the unique way the planet's rotation and revolution periods combine. While Mercury completes one orbit around the Sun (a year) in approximately 88 Earth days, its rotation period with respect to the stars (a sidereal day) is 58.6 Earth days.12

Because Mercury is simultaneously orbiting the Sun while it rotates, these two motions interact to determine the length of the solar day—the time it takes for the Sun to return to the same position in the sky. Specifically, Mercury’s rotation and revolution periods combine such that the Sun takes three Mercurian sidereal days, or 176 Earth days, to complete one full circuit.12

This specific spin-orbit relationship is the result of tidal forces exerted by the Sun, which slowed the planet's initial, faster rotation. Rather than becoming locked in a synchronous rotation (where one side would perpetually face the Sun), Mercury became trapped in a stable resonance where it rotates exactly three times for every two times it orbits the Sun.13