What is Mercury?

Overview

Mercury, the closest planet to the Sun, is the smallest in our solar system—a rocky, cratered world with extreme temperature swings.

  • Diameter: 4,880 km
  • Distance from Sun: 58 million km
  • Mass: 3.3 x 10²³ kg
  • Orbit Period: 88 days
  • Rotation Period: 58.65 days
Mercury from MESSENGER

Image credit: NASA/JHUAPL/Carnegie Institution

Details

Atmosphere: Virtually none—just a thin exosphere of solar wind particles like oxygen and sodium.

Moons: None; Mercury's small size and proximity to the Sun make moons unlikely.

Surface: Covered in craters and vast plains, with a massive impact basin called Caloris Planitia.

Exploration: Visited by Mariner 10 and MESSENGER, which mapped its surface and detected ice in shadowed craters.

Fun Fact: Mercury's day (rotation) is two-thirds of its year (orbit), a unique 3:2 resonance.