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Full-color image of Mercury from first MESSENGER flyby.

Mercury is the smallest planet in the Solar System and is the closest one to the Sun, orbiting it at 58 million kilometers away.


Mercury as the deity Mercurius or Hermes in Guido Bonatti's Liber astronomiae, 1550.

The ancient Greeks called the planet as Στίλβων (Stilbon), meaning "glowing", Ἑρμάων (Ermaon) and Ἑρμής (Hermhes),[1] a planetary name that is retained in modern Greek (Ερμής; Ermis).[2] The Romans dedicated the planet after the swift-footed Roman messenger god Mercury (Latin Mercurius), which they believed was the same Greek Hermes, because it moves across the sky faster than any other planet.[3][4] The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.[5]

The Roman-Egyptian astronomer Ptolemy wrote about the possibility of planetary transits of Mercury across the face of the Sun in his publication Planetary Hypotheses. He believed that zero transits had been observed either because planets such as Mercury were too small to see, or because the transits were not that often occuring.[6]

In ancient China, Mercury was known as "the Hour Star" (Chen-xing or Hanzi 辰星). It was associated with the direction north and the phase of water in the Five Phases system of metaphysics.[7] Modern east Asian cultures refer to the planet literally as the "water star" (水星), based on the Five elements of Chinese mythology.[8][9][10]


Mercury has a diameter of 4,879 kilometers and has a very little atmosphere (all of it would not blow up a balloon to its maximum size), which mostly consists of Sodium. The surface of Mercury resembles Earth's moon, due to it covered with impact craters. The craters were made by asteroids flying through space; craters are made where the meteorite hits. Earth has a blanket of air around it; Mercury barely has one. The blanket is what helps keep Earth from getting too hot or cold. Because it is so close to the sun, Mercury can be very hot, but at night, Mercury gets very cold, at less than -100 degrees Celsius.

Surface features

Surface features on Mercury are named after famous writers, artists, and musicians, with the exception of the crater Hun Kal, located at 20 degrees West of the planet; the name "Hun Kal" being "20" in the Mayan language. There is also the Caloris Basin ("Caloris" being Latin for heat), a large plain on Mercury which is believed to be the impact site of a collision between Mercury and a protoplanet. On the antipode of the Caloris Basin lies jagged plain. This is believed to be formed when shockwaves from the impact that formed the Caloris Basin travelled around and through the planet, meeting and cumulating forces on the antipode of the impact site.

Internal structure, impacts, vulcanoids, and density

Diagram of Mercury's internal structure and magnetic field.

Mercury's mantle is very thin, and it has a relatively large core made mostly of iron and around 70% of the planet's diameter. This is likely due to a collision with another body that stripped away most of Mercury's mantle and leaving most of the planet's core intact. The debris resulting from the collision would likely still be orbiting the Sun between it and Mercury's orbit, forming a hypothetical population of red-hot asteroids known as the vulcanoids.[11][12][13][14] However, none have been detected so far, and searches by SOHO and STEREO have ruled out any vulcanoids larger than 5.7 kilometers in diameter.[15]

Mercury is expected to consist of approximately 70% iron and 30% silicates.[16] Because of this large ratio between denser iron and more diffuse silicates, the planet has a very high average density of 5.427 grams per cubic centimeter.[17] This makes Mercury the second-densest large body in the Solar System, and among the "big three" of Solar System bodies in terms of density; the other two being Venus (third densest on average) and the Earth (on average the densest).


Mariner 10 was the first spacecraft to visit Mercury. It flew by in 1974 and 1975. Not even half of Mercury was seen then. After that, nothing was sent to Mercury for more than 30 years. NASA's MESSENGER spacecraft flew by Mercury in 2008 and 2009. In March 2011, it began to orbit Mercury. In October 2018, the Icons-flag-eu.png European and Icons-flag-jp.png Japanese space agencies launched BepiColombo, a set of two probes sent to the planet. It is estimated to arrive at Mercury on 5 December 2025.


  1. Στίλβων, Ἑρμάων, Ἑρμῆς. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.
  2. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value). See also the Greek article about the planet.
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  6. Goldstein, Bernard R. (1996). "The Pre-telescopic Treatment of the Phases and Apparent Size of Venus". Journal for the History of Astronomy 27: 1. Bibcode 1996JHA....27....1G.
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