spin-orbit-resonance

Mercury has an oversized iron core, ice at its poles, and a day longer than its year. What MESSENGER found and what BepiColombo is due to do in November 2026.

Tidal locking happens when a body's rotation period equals its orbital period, so one face always points at its partner. The Moon is locked to Earth, Mercury is caught in a 3:2 resonance instead, and many planets around red dwarf stars are probably locked to their stars.