The swift, scorched innermost world
Mercury
Mercury is the smallest planet in the Solar System and the closest to the Sun, circling it at an average of about 58 million kilometres. It races around its orbit in just 88 Earth days — faster than any other planet — which is why the ancient Romans named it after their fleet-footed messenger god.
Seen up close, Mercury looks a lot like our Moon: grey, airless and covered in craters. Underneath that familiar face, though, it hides one of the strangest interiors of any rocky world, an unusual rhythm of days and years, and ice tucked into shadows only a few dozen million kilometres from the Sun.
Key facts
- Diameter
- 4,879 km0.38× Earth
- Mass
- 3.30 × 10²³ kg0.06× Earth
- Gravity
- 3.7 m/s²0.38 g
- Mean distance from Sun
- 0.39 AU≈ 58 million km
- Length of year
- 88.0 dEarth days or years
- Length of day
- 175.9 dSunrise to sunrise
- Mean temperature
- 167°CAt the surface
- Known moons
- None
- Axial tilt
- 0.034°
1 astronomical unit (AU) is Earth's average distance from the Sun, about 149.6 million km. Values follow the NASA Planetary Fact Sheet; many are averages and vary with how and where they are measured.
Physical characteristics
With a diameter of 4,879 km, Mercury is only about 40% wider than Earth's Moon and smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Yet it is remarkably heavy for its size: its average density of 5,429 kg/m³ is second only to Earth's.
That density comes from an enormous metallic core that extends to roughly 85% of the planet's radius, leaving only a thin rocky mantle and crust on top. Surface gravity is 3.7 m/s² — about 38% of Earth's — so a person who weighs 70 kg on Earth would feel as if they weighed about 27 kg on Mercury.
Atmosphere and climate
Mercury has no atmosphere in the everyday sense. Instead it has an extremely thin exosphere of atoms — mainly oxygen, sodium, hydrogen, helium and potassium — knocked off the surface by the solar wind and tiny meteorite impacts, then quickly lost to space.
Without an atmosphere to hold heat, Mercury has the most extreme temperature swing of any planet: daytime surfaces reach about 430°C, hot enough to melt tin and lead, while the night side plunges to around −180°C. Even so, Mercury is not the hottest planet — that title belongs to Venus, whose thick atmosphere traps heat.
Composition: Trace exosphere — O, Na, H, He
Surface and interior
A battered surface
Billions of years of impacts have left Mercury densely cratered. Its largest known feature, the Caloris Basin, is an impact scar about 1,550 km across — wider than the distance from London to Rome — ringed by mountains and fractured plains.
A planet that shrank
Long cliffs called lobate scarps run for hundreds of kilometres across the surface. They formed as Mercury's interior cooled and contracted, wrinkling the crust like the skin of a drying apple. Measurements from NASA's MESSENGER spacecraft show the planet's radius has shrunk by several kilometres over its history.
Ice near the Sun
Because Mercury's axis is almost perfectly upright, the floors of some polar craters never see sunlight. MESSENGER confirmed that these permanently shadowed craters hold water ice, likely delivered by comets and asteroids.
A weak magnetic field
Mercury is the only rocky planet besides Earth with a global magnetic field. It is only about 1% as strong as Earth's, but its existence suggests that part of the planet's iron core is still molten.
Moons and rings
Mercury has no moons and no rings. Its closeness to the Sun makes it very hard for a small world to hold onto a satellite: the Sun's gravity would pull a moon away or destabilise its orbit.
Orbit and rotation
Mercury's orbit is the most elongated of all the planets. Its distance from the Sun varies from about 46 million km at its closest to 70 million km at its farthest.
The planet is locked in a 3:2 spin–orbit resonance: it rotates exactly three times for every two trips around the Sun. One rotation relative to the stars takes 58.6 Earth days, but because Mercury also moves quickly along its orbit, a full solar day — sunrise to sunrise — lasts about 176 Earth days, or two Mercury years. Its axial tilt is tiny, about 0.03°, so it has no seasons of the kind Earth does.
Exploration and missions
Mercury is hard to reach: a spacecraft falling towards the Sun speeds up so much that it needs complex gravity-assist flybys to slow down enough to enter orbit. Only three missions have visited.
- 1974–1975
Mariner 10 · NASA
Made three flybys and mapped about 45% of the surface, discovering Mercury's magnetic field.
- 2011–2015
MESSENGER · NASA
The first spacecraft to orbit Mercury. It mapped the entire planet, confirmed polar ice and ended its mission by impacting the surface in April 2015.
- 2018–
BepiColombo · ESA / JAXA
A two-orbiter mission launched in 2018 that has made several Mercury flybys and is scheduled to enter orbit around the planet in late 2026.
Surprising facts about Mercury
- A single solar day on Mercury lasts two of its own years.
- Despite being closest to the Sun, Mercury is cooler on average than Venus.
- Water ice survives in craters that never see sunlight.
- The planet has shrunk as its huge iron core cooled.
- Averaged over time, Mercury is the closest planet to Earth — and to every other planet — because it never strays far from the Sun.
Mercury compared with Earth
| Property | Mercury | Earth |
|---|---|---|
| Diameter | 4,879 km | 12,756 km |
| Mass (Earth = 1) | 0.055 | 1.000 |
| Gravity | 0.38 g | 1.00 g |
| Day | 175.9 d | 24.0 h |
| Year | 88.0 d | 365.2 d |
| Mean temperature | 167°C | 15°C |
| Moons | 0 | 1 |