Mercury

Planet· orbits the Sun

Mercury is the smallest planet and the closest to the Sun. Airless and heavily cratered, it swings through extreme temperatures and has the most eccentric orbit of the eight planets.

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Key facts

Mass

3.30 × 10²³ kg

0.0553 M⊕

Radius

2,440 km

38% of Earth's radius

Surface gravity

3.7 m/s²

38% of Earth's gravity

Day length

58.6 days

Year

87.97 Earth days

Distance from Sun

0.387 AU

Moons

None

Science

Physical

Mass: 3.30 × 10²³ kg · 0.0553 M⊕
Radius: 2,440 km · 38% of Earth's radius
Density: 5,427 kg/m³ · 5.4× water; Computed from mass and radius
Surface gravity: 3.7 m/s² · 38% of Earth's gravity; Computed from mass and radius
Escape velocity: 4.25 km/s · 38% of Earth's escape velocity; Computed from mass and radius
Axial tilt: 0.03 °

Orbital

Distance from the Sun: 0.403 AU; Computed from the position at the selected time
Semi-major axis: 0.387 AU; Computed from the cached state vector
Eccentricity: 0.2056; Computed from the cached state vector
Inclination: 28.55 °; Computed from the cached state vector
Orbital period: 88.0 days; Computed from the cached state vector
Perihelion: 0.307 AU; Computed from the orbital elements
Aphelion: 0.467 AU; Computed from the orbital elements
Mean orbital velocity: 47.9 km/s; Computed from the orbital elements

Rotation

Rotation period: 58.6 days

Atmosphere

Composition: Tenuous exosphere (O, Na, H, He, K)
Surface pressure: 1.0 × 10-10 Pa

Temperature

Minimum: 100 K (-173 °C)
Mean: 440 K (167 °C)
Maximum: 700 K (427 °C)

Composition

Bulk composition: Large iron core (~85% of the radius), thin silicate mantle and crust

Radiation & temperature

Insolation: 6.67 S⊕ · 6.67× the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Bond albedo: 0.09 · reflects 9% of incoming sunlight
Equilibrium temperature: 437 K (164 °C); Computed from albedo + distance from the Sun (airless black body)
Measured mean temperature: 440 K (167 °C)
Surface magnetic field: 300 nT · 1% of Earth's magnetic field

Discovery & history

First known: Known since antiquity
Name: The Roman messenger god, for its swift motion across the sky.

Mercury has been known since at least Sumerian times, recorded over 3,000 years ago. Because it never strays far from the Sun it is hard to see, and ancient Greeks at first thought the morning and evening apparitions were two different objects. It was named for the fleet-footed messenger of the gods. Its true nature — a cratered, airless world — was revealed only by Mariner 10 in 1974–75 and mapped fully by MESSENGER after 2011.