Mercury: temperature, surface and a large iron core

A small world with an enormous core

Mercury orbits the Sun faster than any other planet, but its day-night cycle is very slow. Its craters resemble the Moon's, and its tenuous exosphere retains almost no heat. Proximity to the Sun thus coexists with extremely cold nights.

Mercury: key facts

Type

Rocky planet

Mean radius

2,439.7 km

Mean distance from the Sun

≈ 0.39 AU · 58 million km

Orbital period

≈ 88 Earth days

Rotation

58.65 Earth days

Temperature

−180…+430 °C

Mass

3,30 × 10²³ kg

Mean density

5 429 kg/m³

Values are rounded. The distance describes the orbit, not the current distance from Earth. For giant planets, temperature refers to a reference atmospheric level: they have no solid ground. Mean radius differs from equatorial radius.

Atmosphere and appearance

Almost no atmosphere, only a tenuous exosphere. It cannot retain heat.

What is inside?

Scientific model. Layer colors are illustrative; the thin crust is enlarged for clarity.

The cutaway is based on physical models and indirect measurements. It is not a photograph of the interior; in 3D, you can select each layer separately.

Natural satellites

Mercury has no known natural satellites or ring system.

History of discovery and exploration

Known since ancient times

Mercury can be seen without a telescope near the morning or evening horizon. Its individual discoverer is unknown; detailed maps arrived only in the space age.

1. March 29, 1974

   The first close flyby

   Mariner 10 was the first spacecraft to explore Mercury up close. Its images revealed a heavily cratered surface.

2. March 18, 2011

   The first orbiter

   MESSENGER entered orbit around Mercury. Until 2015, it studied the planet's chemical composition, magnetic field and surface.

3. 2016

   Terrain in three dimensions

   MESSENGER data produced the first global elevation model, from deep depressions to high terrain.

4. 2018

   A new international mission

   BepiColombo launched as a joint ESA and JAXA mission carrying two scientific orbiters.

Mariner 10 was the first spacecraft to see the planet up close. MESSENGER produced a global map.

Craters and scarps as a geological record

Compare Mercury’s impact craters by their surroundings as well as diameter. If one crater disrupts the rim of another, the relative sequence becomes clearer: the intersecting structure formed later. This does not yield a precise age in years without additional assumptions. Preserved terrain helps reconstruct successive changes, but one photograph cannot turn the planet’s entire history into a fully dated chronology.

Long scarps record crustal deformation as the planet cooled and contracted. They can cross older features, linking impact history with internal geology. Choose lighting that produces visible shadows and follow a scarp through several areas. If it almost disappears in another mosaic, the Sun’s position during imaging may explain the difference rather than an enormous structure having vanished between observations.

How ice can survive near the Sun

Polar ice survival depends on local temperature, not merely the planet’s average distance from the Sun. A crater floor in permanent shadow receives a very different energy input from an exposed daytime plain. A hot sunlit surface and a cold trap therefore do not contradict each other. The familiar daytime maximum cannot be applied simultaneously to the entire planet, every latitude and every terrain feature.

The ice interpretation becomes stronger when reflected signals, material composition and illumination models agree. One bright point in a processed image does not establish frozen water. On a polar map, locate the crater and shadowed area before comparing measurement locations. Distinguish exposed ice from material beneath a darker protective layer as well; these settings can produce different appearances while both retaining frozen deposits.

Unusual hollows and enhanced colours

MESSENGER discovered small depressions known as hollows. Unlike ordinary impact craters, they raise questions about the loss of volatile components from surface material. Their formation mechanism remains under investigation. Not every bright pit should be labelled an ancient volcano or evidence of water: resemblance is insufficient. Geological interpretation needs shape, context and composition measurements, rather than an impressive name alone.

Multicolour Mercury mosaics often exaggerate subtle reflectance differences to distinguish materials. A blue map region need not look blue to a person standing there. Compare an enhanced version with a more natural view of the same area. One helps identify differences, while the other conveys overall appearance; neither should silently substitute for another instrument’s measurements or a precise chemical analysis.

Open questions about the innermost planet

Mercury’s large metallic core raises questions about how its unusual interior developed. The visible surface alone cannot select a unique formation scenario. Testing models requires mass, gravity-field measurements, rock composition and rotation data. A cutaway presents the core as a simple coloured region, but that visual clarity does not mean its exact boundary and every physical property are known with equal confidence.

Observing Mercury from Earth is constrained by geometry: it appears close to the Sun and is not available on every dark night. Check a current chart and the local horizon rather than expecting it high at midnight. Before searching with optics, ensure the Sun has completely set and cannot accidentally enter the instrument. Finding the tiny planetary disk does not promise the surface detail visible in spacecraft images.

New measurements are expected from the BepiColombo mission, whose two orbiters will operate at different distances from the planet. MPO is designed to study the surface, composition and interior in detail, while Mio focuses primarily on the surrounding plasma environment. Comparing their data will help connect processes inside the planet with changes in its external magnetic field. The main observations from separate science orbits remain a forthcoming stage of the investigation.