Solar System

Mercury

Terrestrial planet · The swift, sun-scorched world · Known since antiquity

The smallest planet and the closest to the Sun, Mercury races around its orbit while turning on its axis only slowly — so its day is far longer than its year.

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Texture: Solar System Scope (CC BY 4.0)

Key facts

Distance from Sun 0.39AU 57.9 million km
Radius 2,440km 0.38× Earth
Mean temperature 167°C −173 to 427 °C

Time & motion

A day (one spin) 58.6 Earth days One day lasts about two-thirds of its year.
A year (one orbit) 88 Earth days A year here is under three Earth months.
Surface gravity vs Earth
3.7 m/s² 0.38 g
0Earth = 1 g2.5×

Mass & physical characteristics

Mass 3.30 × 10²³kg 0.055× Earth
Density 5.43g/cm³ Water = 1.0 g/cm³
Escape velocity 4.3km/s Speed to break free
Orbital velocity 47.4km/s Speed around the Sun
Axial tilt 0.03° Almost perfectly upright — virtually no seasons.

Atmosphere & magnetosphere

Atmosphere

Almost none — a trace exosphere of hydrogen, helium, oxygen, sodium and potassium. Effectively a vacuum, blasted away by the Sun.

Surface pressure < 10⁻¹⁴ bar Relative to Earth's 1 bar

Magnetic field

Weak global field, about 1% the strength of Earth's.

Moons & rings

Known moons 0 No known moons.

Make-up

Composition

Rocky world with a large iron core and almost no atmosphere.

Notable features

  • Caloris Basin — a ~1,550 km impact basin ringed by mile-high mountains.

Fun facts

  • Mercury has ice in permanently shadowed polar craters, despite being the closest planet to the Sun.
  • Its axial tilt is near zero — the smallest of any planet — so Mercury has virtually no seasons.
  • Surface temperatures swing from about −180 °C at night to 430 °C in daylight.
  • One solar day on Mercury (sunrise to sunrise) lasts about 176 Earth days — two Mercury years.
  • Its oversized iron core makes up roughly 60% of the planet's mass.

Exploration

Mariner 10 1974–75 First flybys; mapped about 45% of the surface.
MESSENGER 2011–15 First orbiter; found polar ice and mapped composition.
BepiColombo 2018– ESA/JAXA mission arriving to orbit Mercury.