Io (moon)

True-color image taken by the Galileo orbiter
Galileo spacecraft true-color image of Io. The dark spot just left of the center is the erupting volcano Prometheus. The whitish plains on either side of it are coated with volcanically deposited sulfur dioxide frost, whereas the yellower regions contain a higher proportion of sulfur.
Discovered byGalileo Galilei
Discovery date8 January 1610[1]
Jupiter I
Orbital characteristics
Periapsis420000 km (0.002807 AU)
Apoapsis423400 km (0.002830 AU)
Mean orbit radius
421700 km (0.002819 AU)
1.769137786 d (152853.5047 s, 42.45930686 h)
17.334 km/s
Inclination0.05° (to Jupiter's equator)
2.213° (to the ecliptic)
Satellite ofJupiter
Physical characteristics
Dimensions3,660.0 × 3,637.4 × 3,630.6 km[3]
Mean radius
1821.6±0.5 km (0.286 Earths)[4]
41910000 km2 (0.082 Earths)
Volume2.53×1010 km3 (0.023 Earths)
Mass(8.931938±0.000018)×1022 kg (0.015 Earths)[4]
Mean density
3.528±0.006 g/cm3[4]
1.796 m/s2 (0.183 g)
0.3755±0.0045[5] (estimate)
2.558 km/s
Equatorial rotation velocity
271 km/h
Surface temp.minmeanmax
Surface90 K110 K130 K[7]
5.02 (opposition)[6]
Surface pressure
5 to 40 nbar
Composition by volume90% sulfur dioxide

Io (Jupiter I) is the innermost of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon, has the highest density of all the moons, and has the least amount of water of any known astronomical object in the Solar System. It was discovered in 1610 and was named after the mythological character Io, a priestess of Hera who became one of Zeus' lovers.

With over 400 active volcanoes, Io is the most geologically active object in the Solar System.[8][9] This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (300 mi) above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Io's silicate crust. Some of these peaks are taller than Mount Everest.[10] Unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron-sulfide core. Most of Io's surface is composed of extensive plains coated with sulfur and sulfur-dioxide frost.

Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur. Numerous extensive lava flows, several more than 500 km (300 mi) in length, also mark the surface. The materials produced by this volcanism make up Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere. Io's volcanic ejecta also produce a large plasma torus around Jupiter.

Io played a significant role in the development of astronomy in the 17th and 18th centuries. It was discovered in January 1610 by Galileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of the Copernican model of the Solar System, the development of Kepler's laws of motion, and the first measurement of the speed of light. From Earth, Io remained just a point of light until the late 19th and early 20th centuries, when it became possible to resolve its large-scale surface features, such as the dark red polar and bright equatorial regions. In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. The Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between Io and Jupiter's magnetosphere and the existence of a belt of high-energy radiation centered on Io's orbit. Io receives about 3,600 rem (36 Sv) of ionizing radiation per day.[11]

Further observations have been made by Cassini–Huygens in 2000, New Horizons in 2007, and Juno in 2017 and 2018, as well as from Earth-based telescopes and the Hubble Space Telescope.


Size comparison between Io (lower left), the Moon (upper left) and Earth

Although Simon Marius is not credited with the sole discovery of the Galilean satellites, his names for the moons were adopted. In his 1614 publication Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici, he proposed several alternative names for the innermost of the large moons of Jupiter, including "The Mercury of Jupiter" and "The First of the Jovian Planets".[12] Based on a suggestion from Johannes Kepler in October 1613, he also devised a naming scheme whereby each moon was named for a lover of the Greek mythological Zeus or his Roman equivalent, Jupiter. He named the innermost large moon of Jupiter after the Greek mythological figure Io.[12][13] Marius' names were not widely adopted until centuries later (mid-20th century).[14] In much of the earlier astronomical literature, Io was generally referred to by its Roman numeral designation (a system introduced by Galileo) as "Jupiter I",[15] or as "the first satellite of Jupiter".[16][17]

Features on Io are named after characters and places from the Io myth, as well as deities of fire, volcanoes, the Sun, and thunder from various myths, and characters and places from Dante's Inferno: names appropriate to the volcanic nature of the surface.[18] Since the surface was first seen up close by Voyager 1, the International Astronomical Union has approved 225 names for Io's volcanoes, mountains, plateaus, and large albedo features. The approved feature categories used for Io for different types of volcanic features include patera ("saucer"; volcanic depression), fluctus ("flow"; lava flow), vallis ("valley"; lava channel), and active eruptive center (location where plume activity was the first sign of volcanic activity at a particular volcano). Named mountains, plateaus, layered terrain, and shield volcanoes include the terms mons, mensa ("table"), planum, and tholus ("rotunda"), respectively.[18] Named, bright albedo regions use the term regio. Examples of named features are Prometheus, Pan Mensa, Tvashtar Paterae, and Tsũi Goab Fluctus.[19]

Other Languages
Afrikaans: Io (maan)
Alemannisch: Io (Mond)
العربية: آيو (قمر)
asturianu: Ío (lluna)
azərbaycanca: İo
تۆرکجه: آیو (قمر)
Bân-lâm-gú: Io (oē-chheⁿ)
беларуская: Іо (спадарожнік)
беларуская (тарашкевіца)‎: Іо (спадарожнік Юпітэра)
български: Йо (спътник)
bosanski: Ija (satelit)
brezhoneg: Io (loarenn)
Чӑвашла: Ио (спутник)
čeština: Io (měsíc)
corsu: Io
Cymraeg: Io (lloeren)
dansk: Io (måne)
Deutsch: Io (Mond)
eesti: Io (kuu)
Ελληνικά: Ιώ (δορυφόρος)
español: Ío (satélite)
Esperanto: Ioo (luno)
euskara: Io
فارسی: آیو (قمر)
français: Io (lune)
Gaeilge: Io (gealach)
galego: Ío (lúa)
한국어: 이오 (위성)
հայերեն: Իո (արբանյակ)
हिन्दी: आयो (उपग्रह)
hrvatski: Ija (mjesec)
Bahasa Indonesia: Io
íslenska: Íó (tungl)
italiano: Io (astronomia)
עברית: איו (ירח)
ქართული: იო
қазақша: Ио (серік)
Кыргызча: Ио
latviešu: Jo (pavadonis)
Lëtzebuergesch: Io (Mound)
лезги: Ио
lietuvių: Ijo (palydovas)
magyar: Ió (hold)
македонски: Ија (месечина)
മലയാളം: അയോ
Bahasa Melayu: Io (bulan)
Mìng-dĕ̤ng-ngṳ̄: Io (ôi-sĭng)
Mirandés: Io
монгол: Ио (дагуул)
Nederlands: Io (maan)
日本語: イオ (衛星)
norsk: Io (måne)
norsk nynorsk: Jupitermånen Io
occitan: Io (luna)
پنجابی: آئیو
Plattdüütsch: Io (Maand)
português: Io (satélite)
română: Io (satelit)
русский: Ио (спутник)
Scots: Io (muin)
sicilianu: Io (satèlliti)
සිංහල: අයෝ
Simple English: Io (moon)
سنڌي: آيو
slovenčina: Io (mesiac)
slovenščina: Io (luna)
српски / srpski: Ија (сателит)
srpskohrvatski / српскохрватски: Io (mjesec)
suomi: Io (kuu)
svenska: Io (måne)
Tagalog: Io (buwan)
татарча/tatarça: Ио (иярчен)
тоҷикӣ: Ио (радиф)
Türkçe: İo (uydu)
українська: Іо (супутник)
Tiếng Việt: Io (vệ tinh)
文言: 木衛一
Winaray: Io (bulan)
吴语: 木卫一
粵語: 木衛一
中文: 木卫一