Space · Planets
Mercury
The innermost planet, smaller than three of Jupiter's moons, with the most extreme temperature swing in the solar system.
Quick facts
Type
Terrestrial
Distance from Sun
0.387 AU
57.9 million km
Diameter
4,879 km
Mass
0.0553 Earth masses
3.30 × 10²³ kg
Surface gravity
3.7 m/s²
Day length
4222.6 Earth hours
Year length
88 Earth days
0.241 Earth years
Mean surface temperature
−173 to 427 °C
Atmosphere
Essentially none (trace Na, O, K, H)
Confirmed moons
0
Naming origin
Roman messenger god
What's there
Mercury is a scorched rock — a Moon-sized world with almost no atmosphere to moderate temperature, so its day side hits 427°C and its shadowed craters hold the coldest places in the inner solar system at −173°C. Permanent ice survives at the poles inside craters whose floors never see sunlight, confirmed by NASA's MESSENGER orbiter (2011–2015) using neutron spectroscopy.
The surface looks Moon-like at first glance — heavy cratering, ancient plains — but Mercury's interior is unusual. A disproportionately large iron core makes up about 60% of the planet's mass, the highest core-to-mantle ratio of any planet. Why is contested: a giant early impact may have stripped away most of the original mantle, or the proto-Mercury may have formed iron-rich. The Caloris Basin, a 1,550-km crater on the day side, is one of the largest impact basins in the solar system; the impact deformed terrain on the opposite hemisphere into a hilly region known as the Weird Terrain.
Who's been there
| Mission | Encounter | Year | Status | Primary objective |
|---|---|---|---|---|
|
Mariner 10 NASA |
Flyby | 1974 | Completed | First Mercury flyby; mapped about 45% of the surface. |
|
MESSENGER NASA |
Orbiter | 2011 | Completed | First Mercury orbiter; complete global mapping; confirmed polar water ice. Impacted Mercury 2015. |
|
BepiColombo ESA/JAXA |
Flyby | 2025 | Active | Joint ESA/JAXA dual-orbiter en route to Mercury orbit insertion in late 2026. |
Moons
Mercury has no moons.
Naming etymology
Mercury takes its name from the Roman messenger god — fast-moving, the wing-footed deity who carried news between gods and mortals. The choice fits the planet's behavior in the sky: Mercury is the fastest of the planets relative to the Sun, completing an orbit in 88 days, and Roman astronomers gave it the name of the swiftest divine courier. The Greek equivalent was Hermes; some early astronomical texts used both names depending on whether the planet was visible at dawn or dusk before the Greeks recognized the two apparitions as a single body.
Methodology & sources
Numerical data drawn from the NASA Planetary Fact Sheets; satellite parameters from JPL Solar System Dynamics — Physical Parameters and the JPL Satellite Discovery Circumstances table. Mission history cross-referenced against NASA's mission catalog and individual mission pages. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Hero rendering is a stylized SVG composed from primary-source visual reference (NASA / JPL imagery) — no photographs are reproduced.