Sun & planets · Space Codex
Mercury
A small, dense world whose iron core fills 85% of its radius.
Smallest planet · iron-heavy and airless
At a glance
Key points
With no real atmosphere to hold heat, its surface swings from 430 °C by day to −180 °C at night.
- One solar day lasts two Mercury years
- Caloris Basin is about 1,550 km wide
- Water ice survives in permanently shadowed polar craters
Interior
An iron core about 2,074 km in radius, possibly partly molten, leaves only about 400 km for the rocky mantle and crust. No other planet is so metal-rich for its size, which hints that a giant impact may have stripped away much of its original rock.
Surface
Caloris Basin, about 1,550 km across, is one of the largest impact scars in the Solar System. Lobate scarps, cliffs up to about 1.6 km high and hundreds of kilometres long, formed as the cooling core shrank and the crust buckled. Inside permanently shadowed polar craters, MESSENGER confirmed deposits of water ice.
Days and nights
Mercury spins three times for every two orbits, so one sunrise-to-sunrise day lasts 176 Earth days, two full Mercury years. Near perihelion the Sun's motion across the sky briefly reverses for observers in some places.
Exploration
Mariner 10 flew past three times in 1974–75. MESSENGER orbited from 2011 to 2015 and mapped the whole planet. BepiColombo enters Mercury orbit on 21 November 2026, and its two orbiters separate in December.
What we still don't know
How a planet this small keeps a global magnetic field, why the core is so large, and how volatile elements such as sulfur and sodium survived so close to the Sun.
Missions
- 1973–1975
Mariner 10
NASACompletedFirst spacecraft to fly past Mercury, three times in 1974–75.
- 2004–2015
MESSENGER
NASACompletedFirst Mercury orbiter (2011–2015); confirmed water ice in polar craters.
- 2018–
Mercury orbit insertion 21 Nov 2026; the two orbiters separate 9–10 Dec 2026 and science begins Apr 2027.
First-hand · our 3D model
How we render it
Mercury is drawn as bare rock. It gets no atmosphere shell, because its exosphere is far too thin to see, so the limb stays hard and the night side falls to near-black. Its spin axis is tilted only 0.03°, so it turns almost perfectly upright, and the single key light always comes from the true direction of the Sun.
Sizes and distances in the model are not to scale, so every world can be seen up close. Colour maps are derived from NASA mission imagery via Solar System Scope (CC BY 4.0). The first frame uses light startup maps; higher-resolution maps stream in after the scene is on screen, decoded off the main thread so the page stays responsive.
Related entries
- BepiColomboESA–JAXA Mercury mission
Sources
- JPL Solar System Dynamics: planetary physical parameters (opens in a new tab)
- NASA NSSDC Mercury Fact Sheet (opens in a new tab)
- NASA NSSDC Planetary Fact Sheet (opens in a new tab)
- NASA Science: Mercury facts (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.