World 01 · terrestrial planet
Mercury
The swift, cratered world nearest the Sun.
Mercury is the Solar System's smallest planet, a rocky world with extreme day-night temperature swings and only a tenuous exosphere.
Editorial geometry, not a surface photograph. The hard terminator emphasizes Mercury's extreme light-and-dark cycle.
- Order from Sun
- 1
- World type
- Terrestrial
Mean radius
2,439.4km
4,881 km equatorial diameter
Solar day
4222h 36m
Prograde rotation
Temperature reference
167°C
Global surface reference, not local weather
Solar edge
A small world under an enormous Sun
Mercury's story is not simply heat. It is exposure: long days, almost no atmosphere, and a surface that cannot redistribute energy the way Earth does.
Light
Daylight arrives without a protective sky
Mercury has a tenuous exosphere rather than a substantial atmosphere. Sunlight reaches the surface with little scattering, so the boundary between illumination and darkness is visually and thermally severe.
The displayed mean temperature is a global reference. Individual locations can move far above or below it depending on sunlight and local time.
Time
A solar day outlasts two Mercury years
Mercury moves around the Sun quickly, but turns slowly. Its spin-orbit relationship produces a solar day far longer than its orbital year.
A familiar clock therefore fails twice: sunrise advances slowly while the planet completes repeated trips around the Sun.
Personal comparison
Put the numbers against your own body
Gravity, day length and sunlight delay become more understandable when related to an Earth reference.
Your Earth reference
Light on your feet, exposed to the Sun
A scale would read much less than on Earth, but lower gravity does not soften radiation, vacuum or the enormous day-night temperature range.
Use a value from 0 to 1,000 kilograms.
Mercury scale equivalent
—kg
This is an Earth-style scale equivalent. Your mass does not change, and a giant-planet value refers to a defined atmospheric level rather than a place to stand.
Exploration record
How we learned to read this planet
Mission summaries are tied to dated official records and are not presented as live telemetry.
- 01
Completed orbiter
MESSENGER
The first spacecraft to orbit Mercury mapped the planet, studied its composition and magnetic field, and verified water-ice-dominated polar deposits.
Reference signals
The conditions that define this world
A very thin exosphere rather than a substantial atmosphere.
Surface
Rock, craters and no weather blanket
Mercury is a solid terrestrial world. Its surface record remains sharply preserved because there is no dense atmosphere producing Earth-like weather and erosion.
Companions
No moons, no rings
The current catalog records no recognized moons and no planetary ring system.
Rotation
Slow spin, fast orbit
The contrast between a short year and a very long solar day is the defining time-scale relationship on Mercury.
- Axial tilt
- 2°
- Recognized moons
- 0 · 2026-07-17
- Rings
- Mercury has no ring system.
- Atmosphere
- oxygen · sodium · hydrogen · helium · potassium
Scientific limits
What these values do—and do not—describe
Reference values keep their definition, source and date context. Editorial diagrams are separated from measured planetary data.
Sources and provenance
- NASA Jet Propulsion LaboratoryPlanetary Physical Parameters ReferenceAccessed Jul 17, 2026
- NASA Jet Propulsion LaboratoryApproximate Positions of the Planets ReferenceAccessed Jul 17, 2026
- NASA SciencePlanet Sizes and Locations in Our Solar System ReferenceAccessed Jul 17, 2026
- NASA ScienceTemperatures Across Our Solar System ReferenceAccessed Jul 17, 2026
- NASA ScienceMercury: Facts ReferenceAccessed Jul 17, 2026
- NASA ScienceMESSENGER HistoricalAccessed Jul 18, 2026
NASA mission media
Mercury, through a mission archive
Every preview comes from the reviewed NASA media list for that world. A failed request can only fall back to media curated for Mercury.
Today, MESSENGER Flies by Mercury!
Today, MESSENGER Flies by Mercury!
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of WashingtonNASA media details
MESSENGER Explores Mercury - In Color
MESSENGER Explores Mercury - In Color
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of WashingtonNASA media details
MESSENGER Discovers Volcanoes on Mercury
MESSENGER Discovers Volcanoes on Mercury
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of WashingtonNASA media details