rocky
Mercury
The swift innermost world
Mercury is a small, rocky world with a heavily cratered surface that resembles our own Moon. It has almost no atmosphere, which means daytime temperatures soar while nights turn extremely cold. A single Mercurian year takes only about 88 Earth days.
Key facts
- Diameter
- 4,879 km
- Radius
- 2,439.7 km
- Mass
- 3.301e+23 kg
- Gravity (Earth = 1)
- 0.378
- Density
- 5.427 g/cm³
- Distance from Sun
- 57,910,000 km
- Orbital period
- 87.97 Earth days
- Rotation period
- 1407.60 hours
- Average temperature
- 167°C
- Known moons
- 0
Values are approximate and based on cited sources.
Atmosphere
- Trace oxygen
- Trace sodium
- Trace hydrogen
- Trace helium
Surface
A rocky, cratered surface with vast plains formed by ancient volcanic activity.
Internal structure
A large iron core relative to the planet's size, wrapped in a thin silicate mantle and crust.
Moons
Mercury has no known moons.
Exploration missions
- • Mariner 10
- • MESSENGER
- • BepiColombo
Sources
- NASA Science — Mercury— last verified 2026-01-15
- NASA Mercury Facts— last verified 2026-01-15