A day longer than a year
Mercury rotates on its axis very slowly: one full rotation takes 58.6 Earth days, while one orbit around the Sun takes only 88 days. As a result, a solar day on Mercury—from one local noon to the next—lasts 176 Earth days, which is twice the length of its own year.
This is a consequence of a 3:2 spin-orbit resonance, a stable state created by tidal forces from the Sun.
An almost non-existent atmosphere
Mercury has only an extremely thin "exosphere" composed of atoms stripped from its surface by solar wind and meteoroid impacts. The lack of an atmosphere means there is nothing to retain heat: the dayside reaches up to 430°C, while the nightside drops to -180°C.
An interesting paradox: despite being close to the Sun, impact craters near Mercury's poles never receive direct sunlight, and radar has detected permanent water ice within them.
A giant iron core
About 85% of Mercury's radius consists of a metal core—an unusually large proportion. The most widely supported hypothesis suggests that a massive collision in the early stages of the solar system stripped away most of its rocky mantle, leaving behind a planet that is almost entirely core.
Further reading
References
- [1]Planetary Fact Sheet — NASA NSSDC (2024)
Image: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington — Wikimedia Commons, Public domain.
