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Mercury (planet) facts for kids

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Mercury Astronomical symbol of mercury
Orbital characteristics
Epoch J2000
Aphelion 69,816,900 km
0.466 697 AU
Perihelion 46,001,200 km
0.307 499 AU
57,909,100 km
0.387 098 AU
Eccentricity 0.205 630
87.969 1 d
(0.240 846 a)
115.88 d
47.87 km/s
Inclination 7.005° to Ecliptic
3.38° to Sun’s equator
6.34° to Invariable plane
Known satellites None
Physical characteristics
Mean radius
2,439.7 ± 1.0 km
0.3829 Earths
Flattening < 0.0006
7.48×107 km²
0.147 Earths
Volume 6.083×1010 km³
0.054 Earths
Mass 3.3022×1023 kg
0.055 Earths
Mean density
5.427 g/cm³
3.7 m/s²
0.38 g
4.25 km/s
58.646 day
1407.5 h
Equatorial rotation velocity
10.892 km/h (3.026 m/s)
2.11′ ± 0.1′
North pole right ascension
18 h 44 min 2 s
North pole declination

0.119 (bond)

0.106 (geom.)
Surface temp. min mean max
0°N, 0°W 100 K 340 K 700 K
85°N, 0°W 80 K 200 K 380 K
up to −1.9
Angular diameter
4.5" – 13"
Composition by volume 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 and innermost planet in the Solar System. Its orbital period around the Sun of 87.97 days is the shortest of all the planets in the Solar System. Because Mercury was the fastest planet as it moved around the Sun, it is named after the Roman deity Mercury, the messenger of the gods.

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. Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years before exhausting its fuel and crashing into the planet's surface on April 30, 2015. The BepiColombo spacecraft is planned to arrive at Mercury in 2025.


The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639, Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.

A rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory. The next occultation of Mercury by Venus will be on December 3, 2133.


Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, and possibly a solid inner core.

It 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, only a little bit less than Earth’s. For it to have such a high density, its core must be large and rich in iron. Mercury's core has a higher iron content than that of any other major planets in the Solar System. Geologists estimate that Mercury's core occupies about 55% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that it has a molten core.

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

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.

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.

Surface geology

On the Edge of Mercury (16442218401)
On the Edge of Mercury

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."

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 ended 3.8 billion years ago. During this period of intense crater formation, Mercury received impacts over its entire surface, facilitated by the lack of any atmosphere to slow impactors down. During this time Mercury was volcanically active; basins such as the Caloris Basin were filled by magma, producing 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. 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 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".

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, Beethoven Basin which has 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.


Enhanced-color image of Munch, Sander and Poe craters amid volcanic plains (orange) near Caloris Basin

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. These inter-crater plains appear to have obliterated many earlier craters, and show a general presence of smaller craters below about 30 km in diameter.

Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to the lunar maria, which are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. Notably, they fill a wide ring surrounding the Caloris Basin.

One unusual feature of Mercury's surface is the numerous compression folds, or rupes, that crisscross the plains. As Mercury's interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults. The scarps can reach lengths of 1000 km and heights of 3 km. These features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent. The Lunar Reconnaissance Orbiter discovered that similar small thrust faults exist on the Moon.

Surface conditions & atmosphere

Colors of the Innermost Planet
These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up Mercury's surface, made by using images from the color base map imaging campaign during MESSENGER's primary mission

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).

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 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 much too small for its gravity to retain any significant atmosphere over long periods of time, but it does have a very tenuous atmosphere containing hydrogen, helium, oxygen, sodium and potassium. The atmosphere is not stable - atoms are continuously lost and replenished, from a variety of sources. The hydrogen and helium atoms 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 vapour is probably also present, water being brought to Mercury by comets impacting on its surface ..

Magnetic field and magnetosphere

The MESSENGER spacecraft noted that Mercury's magnetic field is responsible for several magnetic "tornadoes"

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. 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 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 night-side.

During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes" – twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space – that were up to 800 km wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via magnetic reconnection This also occurs in Earth's magnetic field.

Orbit and rotation

Animation of Mercury's and Earth's revolution around the Sun

Mercury has the most eccentric orbit of all the planets, with the planet's distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. It takes 88 days to complete the orbit. transits of Mercury across the face of the Sun occurs about every seven years on average.

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 be retrograde. Four days after perihelion, the Sun's normal apparent motion resumes.

It was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times, in the same way that the same side of the Moon always faces the Earth. However, radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbit makes this resonance stable.

Mercury attains inferior conjunction (nearest approach to Earth) every 116 Earth days on average, but this interval can range from 105 days to 129 days due to the planet's eccentric orbit. Mercury is on average the closest planet to the Earth: it is closest to Earth 46% of the time; Venus is closest 36% of the time, while Mars is closest just 18% of the time.


Atlas-Centaur launch with Mariner 10
The launch of the Atlas-Centaur carrying the Mariner X spacecraft on November 3, 1973. This mission was for the exploration of the planets Venus and Mercury

Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun's glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight.

Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.

Mercury is technically brightest as seen from Earth when it is at a full phase. Although Mercury is farthest from Earth when it is full, the greater illuminated area that is visible more than compensates for the distance. Mercury can be easily seen from the tropics and subtropics more than from higher latitudes. Mercury is more often and easily visible from the Southern Hemisphere than from the Northern.

An alternate method for viewing Mercury involves observing the planet during daylight hours when conditions are clear. This allows the planet to be found easily, even when using telescopes. Care must be taken to ensure the instrument isn't pointed directly towards the Sun because of the risk for eye damage.

Mariner 10 gravitational slingshot
Mariner 10 was the first spacecraft to make use of a "gravitational slingshot" maneuver, using Venus to bend its flight path

The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures that prevent its pointing too close to the Sun. Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail.

Reaching Mercury from Earth poses significant technical challenges, because it orbits so much closer to the Sun than Earth. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of its surface from 1974 to 1975. The second is the MESSENGER spacecraft, which after three Mercury flybys between 2008 and 2009, attained orbit around Mercury on March 17, 2011, to study and map the rest of the planet.

Mariner 10

Mariner 10
Mariner 10, the first probe to visit Mercury

The first spacecraft to visit Mercury was NASA's Mariner 10 (1974–1975). The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational "slingshot" effect and the first NASA mission to visit multiple planets. Mariner 10 provided the first close-up images of Mercury's surface, which immediately showed its heavily cratered nature, and revealed many other types of geological features, such as the giant scarps that were later attributed to the effect of the planet shrinking slightly as its iron core cools. Unfortunately, the same face of the planet was lit at each of Mariner 10 close approaches. This made close observation of both sides of the planet impossible, and resulted in the mapping of less than 45% of the planet's surface.

This photo identifies various parts of the spacecraft and the science instruments

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km (203 mi) of the surface. At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury's rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet's magnetic field is much like Earth's, which deflects the solar wind around the planet.

On March 24, 1975, just eight days after its final close approach, Mariner 10 ran out of fuel. Because its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut down. Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.


MESSENGER - spacecraft at mercury - atmercury lg
Artist depiction of the MESSENGER spacecraft in orbit around Mercury

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004. It made a fly-by of Earth in August 2005, and of Venus in October 2006 and June 2007 to place it onto the correct trajectory to reach an orbit around Mercury. A first fly-by of Mercury occurred on January 14, 2008, a second on October 6, 2008, and a third on September 29, 2009. Most of the hemisphere not imaged by Mariner 10 was mapped during these fly-bys. The probe successfully entered an orbit around the planet on March 18, 2011. The first orbital image of Mercury was obtained on March 29, 2011. The probe finished a one-year mapping mission, and then entered a one-year extended mission into 2013.

MESSENGER - installation of solar panels
MESSENGER - installation of solar panels

The mission was designed to clear up six key issues: Mercury's high density, its geological history, the nature of its magnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe carried imaging devices that gathered much-higher-resolution images of much more of Mercury than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles.

MESSENGER final maneuver was on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015. The spacecraft's impact with Mercury occurred near 3:26 PM EDT on April 30, 2015, leaving a crater estimated to be 16 m (52 ft) in diameter.

BepiColombo spacecraft model
BepiColombo spacecraft model


The European Space Agency and the Japanese Space Agency developed and launched a joint mission called BepiColombo, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere. Launched on October 20, 2018, BepiColombo is expected to reach Mercury in 2025. It will release a magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for one terrestrial year. The mapper probe carries an array of spectrometers similar to those on MESSENGER, and will study the planet at many different wavelengths including infrared, ultraviolet, X-ray and gamma ray.

Planetary Feature names

Names for features on Mercury come from a variety of sources. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or fossae are named for works of architecture. Montes are named for the word "hot" in a variety of languages. Plains or planitiae are named for Mercury in various languages. Escarpments or rupēs are named for ships of scientific expeditions. Valleys or valles are named for radio telescope facilities.


Size comparison with other Solar System objects

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