Mercury (planet)
Mercury is the first planet from the Sun and the smallest in the Solar System. In English, it is named after the ancient Roman god Mercurius (Mercury), god of commerce and communication, and the messenger of the gods. Mercury is classified as a terrestrial planet, with roughly the same surface gravity as Mars. The surface of Mercury is heavily cratered, as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia, has a diameter of 1,550 km (960 mi) and one-third the diameter of the planet (4,880 km or 3,030 mi). Similarly to the Earth's Moon, Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants.
"First planet" redirects here. For other systems of numbering planets, see Planet § History and etymology. For other uses, see Mercury (disambiguation).Designations
Mercurian /mərˈkjʊəriən/,[1]
Mercurial /mərˈkjʊəriəl/[2]
.svg){kind=small}
0.466697 AU (69.82 million km)
0.307499 AU (46.00 million km)
0.387098 AU (57.91 million km)
0.205630[4]
- 87.9691 d
- 0.240846 yr
- 0.5 Mercury synodic days
115.88 d[4]
47.36 km/s[4]
174.796°
- 7.005° to ecliptic
- 3.38° to Sun's equator
- 6.35° to invariable plane[5]
48.331°
29.124°
None
0.0009[4]
- 7.48×107 km2[6]
- 0.147 Earths
- 6.083×1010 km3[6]
- 0.056 Earths
- 3.3011×1023 kg[8]
- 0.055 Earths
5.427 g/cm3[6]
0.346±0.014[9]
4.25 km/s[6]
176 d[10]
- 58.646 d
- 1407.5 h[6]
10.892 km/h (3.026 m/s)
- 18h 44m 2s
- 281.01°[4]
61.45°[4]
437 K (164 °C) (blackbody temperature)[13]
−2.48 to +7.25[15]
−0.4[16]
4.5–13″[4]
trace (≲ 0.5 nPa)
Mercury's sidereal year (88.0 Earth days) and sidereal day (58.65 Earth days) are in a 3:2 ratio. This relationship is called spin–orbit resonance, and sidereal here means "relative to the stars". Consequently, one solar day (sunrise to sunrise) on Mercury lasts for around 176 Earth days: twice the planet's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all the time until the next sunrise after another 88 Earth days.
Combined with its high orbital eccentricity, the planet's surface has widely varying sunlight intensity and temperature, with the equatorial regions ranging from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. Due to the very small axial tilt, the planet's poles are permanently shadowed. This strongly suggests that water ice could be present in the craters. Above the planet's surface is an extremely tenuous exosphere and a faint magnetic field that is strong enough to deflect solar winds. Mercury has no natural satellite.
As of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Like other planets in the Solar System, Mercury was formed approximately 4.5 billion years ago. Its mantle is highly homogeneous, which suggests that Mercury had a magma ocean early in its history, like the Moon. According to current models, Mercury may have a solid silicate crust and mantle overlying a solid outer core, a deeper liquid core layer, and a solid inner core. There are many competing hypotheses about Mercury's origins and development, some of which incorporate collision with planetesimals and rock vaporization.
Nomenclature
The ancients knew Mercury by different names depending on whether it was an evening star or a morning star. By about 350 BC, the ancient Greeks had realized the two stars were one.[19] They knew the planet as Στίλβων Stilbōn, meaning "twinkling", and Ἑρμής Hermēs, for its fleeting motion,[20] a name that is retained in modern Greek (Ερμής Ermis).[21] The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin Mercurius), whom they equated with the Greek Hermes, because it moves across the sky faster than any other planet.[19][22] The astronomical symbol for Mercury is a stylized version of Hermes' caduceus; a Christian cross was added in the 16th century:.svg){kind=small}
.[23][24]
Transit of Mercury. Mercury is visible as a black dot below and to the left of center. The dark area above the center of the solar disk is a sunspot.
Elongation is the angle between the Sun and the planet, with Earth as the reference point. Mercury appears close to the Sun.
Water ice (yellow) at Mercury's north polar region
