Mercury (planet)

Mercury is the closest planet to the Sun and the Solar System’s smallest. This rocky world has only a wisp of an atmosphere, and its surface gravity is a bit weaker than Mars’s. Because it orbits closer to the Sun than any other planet, it never strays far from the Sun in Earth’s sky, showing up as a morning star or an evening star.

Its surface is heavily cratered, much like Earth’s Moon, and is marked by long cliff systems formed by thrust faults, along with bright ray patterns from impact ejecta. The biggest crater, Caloris Planitia, spans 1,550 kilometers (960 miles)—roughly a third of Mercury’s own diameter of 4,880 kilometers (3,030 miles). Traveling to Mercury from Earth requires more change in velocity than reaching any other planet, and the same holds for trips between Mercury and other worlds.

Mercury’s year lasts 88.0 Earth days, and its sidereal day is 58.65 Earth days, locked in a 3:2 spin–orbit resonance. As a result, one full solar day—from sunrise to sunrise—takes about 176 Earth days, or two Mercurian years. This means one side bakes in sunlight for an entire 88‑day year, then spends the next orbit in total darkness until the following sunrise. Above the surface, an extremely thin exosphere and a weak magnetic field barely strong enough to deflect the solar wind exist. The planet’s highly elliptical orbit, combined with this thin atmosphere, causes wild swings in sunlight and temperature: equatorial regions can plunge to −170 °C (−270 °F) at night and soar to 420 °C (790 °F) during the day. Despite being nearest the Sun, Mercury is only the second‑hottest planet—Venus is hotter—because it lacks a thick atmosphere to trap heat. Its poles have almost no tilt, so they lie in permanent shadow, strongly hinting that water ice could survive inside craters there.

Mercury formed about 4.5 billion years ago, like the other planets. Several competing ideas exist about its origins, some involving collisions with planetesimals and rock vaporization; as of the early 2020s, many details of its geological history remain unclear or await data from space probes. Its mantle appears highly uniform, suggesting it once had a magma ocean, similar to the Moon. Current models propose a solid silicate crust and mantle over a solid outer core, a deeper liquid core layer, and a solid inner core. In about seven or eight billion years, when the Sun swells into a red giant, Mercury—along with Venus, and possibly Earth and the Moon—is expected to be destroyed.

Mercury is a classical planet, known and tracked since ancient times as a wandering star. Its English name comes from the Roman god Mercurius, the messenger of the gods and deity of commerce and communication. The first successful flyby was by Mariner 10 in 1974, and later orbiters MESSENGER and BepiColombo have explored it.

**Nomenclature** Historically, people gave Mercury different names depending on whether it appeared as an evening or morning star. By about 350 BC, the ancient Greeks realized the two “stars” were the same object. They called it Στίλβων (Stilbōn, meaning “twinkling”) and Ἑρμής (Hermes) for its swift motion—a name still used in modern Greek (Ερμής, Ermis). The Romans named it after their swift messenger god Mercury (Latin *Mercurius*), equating him with Hermes, because it moves faster across the sky than any other planet; some, however, associated it with Apollo, as Pliny the Elder noted. The astronomical symbol for Mercury is a stylized version of Hermes’ caduceus, with a Christian cross added in the 16th century: ☿.

**Physical characteristics** Mercury is one of four terrestrial planets—a rocky body like Earth. It is the smallest planet, with an equatorial radius of 2,439.7 kilometers (1,516.0 miles). It is also smaller, though more massive, than the largest natural satellites, Ganymede and Titan. The planet is about 70% metallic and 30% silicate material.

**Internal structure** Mercury likely has a solid silicate crust and mantle, overlying a solid metallic outer core, a deeper liquid core layer, and a solid inner core. The iron‑rich core’s exact composition is uncertain but probably includes nickel, silicon, and possibly sulfur and carbon, plus trace elements. Its density is the second highest in the Solar System at 5.427 g/cm³, just below Earth’s 5.515 g/cm³. If gravitational compression is removed, Mercury’s materials are denser than Earth’s: an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³. This high density implies a large, iron‑rich core, because Mercury is much smaller than Earth and its interior is less compressed. The core’s radius is estimated at 2,020 ± 30 kilometers (1,255 ± 19 miles), taking up about 57% of the planet’s volume (compared to 17% for Earth). Research from 2007 suggests the core is molten. The mantle‑crust layer totals about 420 kilometers (260 miles) thick; the crust itself may be around 35 kilometers (22 miles) thick, based on Mariner 10 and MESSENGER data, though an Airy isostasy model gives different projections.