Mercury (planet) facts

Mercury Astronomical symbol of mercury
How to say it how to say: /ˈmɜrkjəri/
Reference date J2000
Longest distance from the Sun 69,816,900 km
0.466 697 AU
Shortest distance from the Sun 46,001,200 km
0.307 499 AU
Longest distance from the center of its orbital path
("semi-major axis")
57,909,100 km
0.387 098 AU
How long it takes to complete an orbit 87.969 1 d
(0.240 846 a)
How long an orbit seems to take
(from the central body)
115.88 d
Average speed 47.87 km/s
Mean anomaly 174.796°
Angle above the reference plane
7.005° to Ecliptic
3.38° to Sun’s equator
6.34° to Invariable plane
Natural things which orbit it None
Size and other qualities
Average radius 2,439.7 ± 1.0 km
0.3829 Earths
Surface area 7.48×107 km²
0.147 Earths
Volume 6.083×1010 km³
0.054 Earths
Mass 3.3022×1023 kg
0.055 Earths
Average density 5.427 g/cm³
Surface gravity 3.7 m/s²
0.38 g
Escape velocity 4.25 km/s
Turning speed 10.892 km/h (3.026 m/s)
Angle at which it turns
(in relation to its orbit)
2.11′ ± 0.1′
Angle above the celestial equator
How much light it reflects

0.119 (bond)

0.106 (geom.)
Surface temp. Min. Avg. Max.
0°N, 0°W 100 K 340 K 700 K
85°N, 0°W 80 K 200 K 380 K
Seeming brightness
("apparent magnitude")
up to −1.9
Make-up 42% Molecular oxygen
29.0% sodium
22.0% hydrogen
6.0% helium
0.5% potassium
Trace amounts of argon, nitrogen, carbon dioxide, water vapor, xenon, krypton, & neon

Mercury is the smallest planet in the Solar System. It is the closest planet to the sun. It makes one trip around the Sun once every 87.969 days. Mercury is bright when it is visible from Earth, ranging from −2.0 to 5.5 in apparent magnitude. It cannot be easily seen as it is usually too close to the Sun. Because Mercury is normally lost in the glare of the Sun (except during a solar eclipse), Mercury can only be seen in the morning or evening twilight.

Compared to what is known about the other planets in the Solar System, little is known about Mercury. Telescopes on the Earth show only a small, bright crescent. The first of two spacecraft to visit the planet was Mariner 10, which mapped only about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which finished mapping the planet in March 2013.

Mercury looks a lot like Earth's Moon. It has many craters with areas of smooth plains, no moons around it and no atmosphere as we know it. However, Mercury does have an extremely thin atmosphere, known as an exosphere. Unlike Earth's moon, Mercury has a large iron core, which gives off a magnetic field about 1% as strong as that of the Earth. It is a very dense planet due to the large size of its core. Surface temperatures can be anywhere from about 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Known sightings of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers thought that Mercury was two different objects: one able to be seen only at sunrise, which they called Apollo; the other that was only able to be seen at sunset, which they called Hermes. The English name for the planet is from the Romans, who named it after the Roman god Mercury, which they thought to be the same as the Greek god Hermes. The symbol for Mercury is based on Hermes' staff.

Even though Mercury is the closest planet to the Sun, it is not the warmest. This is because it has no greenhouse effect, so any heat that the Sun gives to it quickly escapes into space.

Inside Mercury

Mercury is one of four inner planets in the Solar System, and has a rocky body like the Earth. It is the smallest planet in the Solar System, with a radius of 2,439.7 km (1,516.0 mi) Mercury is even smaller than some of the largest moons in the solar system, such as Ganymede and Titan. However, it has a greater mass than the largest moons in the solar system. Mercury is made of about 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only a little bit less than Earth’s.

Surface geology

Mercury in color c1000 700 430
First high-resolution image of Mercury transmitted by MESSENGER (false color)

Mercury’s surface is overall very similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. Since our knowledge of Mercury's geology has been based on the 1975 Mariner flyby and terrestrial observations, it is the least understood of the terrestrial planets. As data from the recent MESSENGER flyby is processed this knowledge will increase. For example, an unusual crater with radiating troughs has been discovered which scientists are calling "the spider."

Albedo features refer to areas of markedly different reflectivity, as seen by telescopic observation. Mercury also possesses Dorsa (also called "wrinkle-ridges"), Moon-like highlands, Montes (mountains), Planitiae, or plains, Rupes (escarpments), and Valles (valleys).

Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the late heavy bombardment that came to an end 3.8 billion years ago. During this period of intense crater formation, the planet received impacts over its entire surface, facilitated by the lack of any atmosphere to slow impactors down. During this time the planet was volcanically active; basins such as the Caloris Basin were filled by magma from within the planet, which produced smooth plains similar to the maria found on the Moon.

Impact basins and craters

Caloris basin labeled
Mercury’s Caloris Basin is one of the largest impact features in the Solar System.

Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity.

The largest known craters are Caloris Basin, with a diameter of 1550 km, and the Skinakas Basin with an outer-ring diameter of 2,300 km. The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around the planet, converging at the basin’s antipode (180 degrees away). The resulting high stresses fractured the surface. Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode.

Overall about 15 impact basins have been identified on the imaged part of Mercury. Other notable basins include the 400 km wide, multi-ring, Tolstoj Basin which has an ejecta blanket extending up to 500 km from its rim, and its floor has been filled by smooth plains materials. Beethoven Basin also has a similar-sized ejecta blanket and a 625 km diameter rim. Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including Solar wind and micrometeorite impacts.


There are two geologically distinct plains regions on Mercury. Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces, predating the heavily cratered terrain. The inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter. It is not clear whether they are of volcanic or impact origin. The inter-crater plains are distributed roughly uniformly over the entire surface of the planet.

Mercury's 'Weird Terrain'
The so-called “Weird Terrain” was formed by the Caloris Basin impact at its antipodal point.

Smooth plains are widespread flat areas which fill depressions of various sizes and bear a strong resemblance to the lunar maria. Notably, they fill a wide ring surrounding the Caloris Basin. An appreciable difference between these plains and lunar maria is that the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic features, their localisation and lobate-shaped colour units strongly support a volcanic origin. All the Mercurian smooth plains formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket. The floor of the Caloris Basin is also filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they are volcanic lavas induced by the impact, or a large sheet of impact melt.

One unusual feature of the planet’s surface is the numerous compression folds, or rupes, which crisscross the plains. It is thought that as the planet’s interior cooled, it contracted and its surface began to deform. The folds can be seen on top of other features, such as craters and smoother plains, indicating that they are more recent. Mercury’s surface is also flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.

Surface conditions & atmosphere

The mean surface temperature of Mercury is 442.5 K (169.4 °C; 336.8 °F), but it ranges from 100 K (−173 °C; −280 °F) to 700 K (427 °C; 800 °F), due to the absence of an atmosphere. On the dark side of the planet, temperatures average 110 K (−163 °C; −262 °F). The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1370Wm−2).

Merc fig2sm
Radar image of Mercury's north pole

Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. The floors of some deep craters near the poles are never exposed to direct sunlight, and temperatures there remain far lower than the global average. Water ice strongly reflects radar, and observations by the 70m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles. While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.

The icy regions are believed to be covered to a depth of only a few meters, and contain about 1014–1015 kg of ice. By comparison, the Antarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars’ south polar cap contains about 1016 kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.

Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, it does have a tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium and potassium. This atmosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen and helium atoms probably come from the solar wind, diffusing into Mercury’s magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury’s crust is another source of helium, as well as sodium and potassium. Water vapor is probably present, being brought to Mercury by comets striking its surface.

Sodium and potassium were discovered in the atmosphere during the 1980s, and are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts. Due to the ability of these materials to diffuse sunlight, Earth-based observers can readily detect their composition in the atmosphere. Studies indicate that, at times, Sodium emissions are localized at points that correspond to the planet's magnetic dipoles. This would indicate some interaction between the magnetosphere and the planet's surface.

Magnetic field and magnetosphere

Mercury Magnetic Field NASA
Graph showing relative strength of Mercury's magnetic field.

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about 300 nT. Like that of Earth, Mercury's magnetic field is dipolar in nature. Unlike Earth, however, Mercury's poles are nearly aligned with the planet's spin axis. Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.

It is likely that this magnetic field is generated by way of a Dynamo effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.

Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within the Earth, is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface. Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles were detected in the planet's magnetotail, which indicates a dynamic quality to the planet's magnetosphere.

Orbit and rotation

ThePlanets Orbits Mercury PolarView
Orbit of Mercury (yellow).

Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. It takes 88 days to complete an orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the same semi-major axis. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet’s rotation around its axis, result in complex variations of the surface temperature.

Mercury’s orbit is inclined by 7° to the plane of Earth’s orbit (the ecliptic), as shown in the diagram on the left. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on average.

ThePlanets Orbits Mercury EclipticView
Orbit of Mercury as seen from the ascending node (bottom) and from 10° above (top).

Functionally, Mercury’s axial tilt is nonexistent, with measurements as low as 0.027°. This is significantly smaller than that of Jupiter, which boasts the second smallest axial tilt of all planets at 3.1 degrees. This means an observer at Mercury’s equator during local noon would never see the Sun more than 1/30 of one degree north or south of the zenith. Conversely, at the poles the Sun never rises more than 2.1′ above the horizon.

At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four days prior to perihelion, Mercury’s angular orbital velocity exactly equals its angular rotational velocity so that the Sun’s apparent motion ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.


Size comparison with other Solar System objects

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