Mercury

The closest planet to the Sun in the Solar System, a small rocky planet with virtually no atmosphere and a day-night temperature difference reaching 600°C.

Mercury (수성)

Overview

Mercury (水星, Mercury; Korean: 수성) is the planet closest to the Sun in the Solar System and the smallest of the eight planets. It lies an average of about 57.9 million km (0.387 astronomical units) from the Sun, and its orbital period is about 88 days—less than a quarter of an Earth year. It is a rocky planet with virtually no atmosphere, possessing an extreme environment in which surface temperatures rise to about 430°C during the day and fall to -180°C at night. (Note that "수성" is a homophone of Suseong District in Daegu Metropolitan City and of "수성(水性)," meaning water-based paint, so distinction by context is necessary.)

Main Content

Physical Characteristics

Equatorial radius: about 2,440 km (0.383 times that of Earth, about 1.4 times larger than the Moon)
Mass: 3.301×10²³ kg (about 0.055 times that of Earth)
Average density: 5.427 g/cm³ — second highest among the Solar System planets after Earth
Surface gravity: about 0.38 times that of Earth
Moons: none (along with Venus, the only two planets without moons)

By volume, about 55% of Mercury is filled by a giant metallic core made of iron-nickel. This is the largest core proportion of any Solar System planet, and by radius the core occupies about three-quarters of the planet's entire diameter. Such an abnormally large core is explained by hypotheses including that the outer silicate crust was stripped away by a giant impact early in its formation, or that the metallic components of the primordial solar nebula were strongly concentrated.

Orbit and Rotation

Mercury's orbit has an eccentricity of 0.2056, the most elongated of the eight planets, and its orbital inclination is also large at about 7 degrees. Its distance varies from 46 million km at perihelion to 69.8 million km at aphelion. Its rotation period is 58.646 days, forming a 3:2 spin-orbit resonance with its orbital period (87.97 days). That is, Mercury rotates exactly three times while it orbits the Sun twice. Because of this, a Mercury day (solar day) is about 176 Earth days, twice as long as its year (88 days).

The excess in Mercury's perihelion precession could not be explained by Newtonian mechanics alone; when Einstein's general theory of relativity predicted it precisely in 1915, it became decisive evidence for verifying the theory.

Surface and Geology

Mercury's surface is covered with numerous impact craters, very much like the Moon. The largest impact structure is the Caloris Basin, about 1,550 km in diameter; immediately after the impact, "chaotic terrain" formed on the opposite side, where the terrain became jumbled. In addition, enormous fault scarps called "rupes," created as the planet cooled and contracted, extend for hundreds of km, suggesting that Mercury's radius has shrunk by more than about 7 km since its formation. The MESSENGER probe also discovered unique depression terrain called "hollows," formed as volatile materials sublimated.

Atmosphere and Magnetosphere

Mercury is the only terrestrial planet with virtually no atmosphere. However, an extremely thin exosphere exists, made of atoms released by the solar wind and micrometeorite impacts, in which oxygen, sodium, hydrogen, helium, potassium, and the like are detected. It has an intrinsic magnetic field only about 1% as strong as Earth's, which blocks the solar wind and forms a magnetosphere. In permanently shadowed craters in the polar regions, water ice and organic-rich material have been found, interpreted as material delivered by comet and asteroid impacts that has been preserved for billions of years.

Exploration History

1974–1975: NASA's Mariner 10 made the first close flyby of Mercury, photographing about 45% of the surface
Launched 2004, 2011–2015: NASA's MESSENGER became the first spacecraft to enter Mercury's orbit, completing a map of the entire surface and discovering polar water ice, hollow terrain, and more
Launched 2018: ESA and JAXA's joint mission BepiColombo has launched and is in the middle of a seven-year voyage

Latest Developments

In 2024–2025, BepiColombo carried out successive Mercury flybys, performing deceleration maneuvers for orbit insertion. Precise measurements of the magnetosphere and plasma environment were made during the flybys in September and December 2024 and January 2025, and full-scale observation is scheduled to begin after entering Mercury's orbit at the end of 2026.

In the scientific community, in 2024 the possibility that a diamond layer tens of km thick exists at Mercury's core-mantle boundary was raised and drew attention. According to high-pressure experiments and computer modeling, carbon inside Mercury may have formed a graphite crust as it cooled during its magma ocean phase and then crystallized into diamond at greater depths. This is regarded as a new clue for explaining the mechanism by which Mercury generates its magnetic field and its internal thermal evolution.

In addition, through reanalysis of MESSENGER data, phenomena such as the solar wind pushing sodium on Mercury's surface to create a tail, and the way Mercury's exosphere fluctuates with the solar activity cycle, were quantitatively elucidated in 2024–2025 research. Meanwhile, observations of Mercury's atmosphere using ground-based observations and the James Webb Space Telescope are also being attempted.