Mercury is the smallest planet and the closest to the Sun - a cratered, airless world that looks a lot like our Moon. With almost no atmosphere to trap heat, it swings between scorching days and freezing nights more than any other planet.
A single Mercury day (sunrise to sunrise) lasts 176 Earth days - two of its years.
Its temperature swings ~600 °C between day and night.
Despite being closest to the Sun, it is not the hottest planet - Venus is.
Close means fast
The closer a planet is to the Sun, the stronger the pull of gravity - and the faster it must move to stay in orbit. Mercury races around the Sun in just 88 days; Neptune takes 165 years.
Physical characteristics
Diameter
4,879 km
Mass
3.300e+23 kg
Surface gravity
3.7 m/s²
Density
5.43 g/cm³
Rotation period
1,407.6 h
Axial tilt
0.03°
Mean temperature
167 °C
Temperature range
-173 °C to 427 °C
Geometric albedo
0.14
Known moons
0
Atmosphere and composition
Atmosphere
A vanishingly thin exosphere of oxygen, sodium, hydrogen, helium and potassium - atoms knocked off the surface that rarely ever collide.
Composition
An outsized iron core - about 85% of the planet’s radius - wrapped in a thin silicate mantle and crust.
Interior structure
Solid inner coremodelled
A solid iron centre, suggested by MESSENGER spin and gravity data.
Molten outer coremodelled
Enormous for the planet’s size - 83% of the radius is metal.
Mantlemodelled
A comparatively thin rocky shell.
Crustmodelled
Ancient, cratered, and laced with shrinkage scarps.
How we know: MESSENGER measured Mercury’s libration (a tiny rotational wobble) and gravity field; both demand a huge, partly molten metallic core.
The first spacecraft to visit Mercury, and the first ever to use one planet’s gravity (Venus) to slingshot to another. Its three flybys in 1974-75 mapped about 45% of Mercury’s surface.
MESSENGERNASA · Orbiter · 2004–2015 · completed
The first spacecraft to orbit Mercury (2011-2015), MESSENGER mapped the entire planet and confirmed something remarkable: water ice hiding in permanently shadowed craters at the poles of the Sun’s closest planet.
BepiColomboESA/JAXA · Orbiter · 2018 · en route
BepiColombo carries two orbiters stacked together - ESA’s Mercury Planetary Orbiter and JAXA’s Mio - on a long spiral inward using nine planetary flybys. Mercury orbit insertion is expected in late 2026, when the pair will separate to study the planet and its magnetosphere.