Io is the innermost of the four Galilean moons of the planet Jupiter.
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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.
An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe.
Water is a transparent fluid which forms the world's streams, lakes, oceans and rain and is the major constituent of the fluids of organisms.
The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly.
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It was discovered in 1610 and was named after the mythological character Io, a priestess of Hera who became one of Zeus' lovers.
Hera is the wife and one of three sisters of Zeus in the Olympian pantheon of Greek mythology and religion.
Zeus is the sky and thunder god in ancient Greek religion, who ruled as king of the gods of Mount Olympus.
With over 400 active volcanoes, Io is the most geologically active object in the Solar System.
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
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.
An ion is an atom or a molecule in which the total number of electrons is not equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge.
Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km above the surface.
Sulfur or sulphur is a chemical element with symbol S and atomic number 16.
Sulfur dioxide is the chemical compound with the formula SO2.
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.
In chemistry, a silicate is any member of a family of anions consisting of silicon and oxygen, usually with the general formula [SiO−4−x]n, where 0 ≤ x < 2.
Some of these peaks are taller than Mount Everest.
Mount Everest, also known in Nepali as Sagarmāthā and in Tibetan as Chomolungma, is Earth's highest mountain.
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.
Volcanism is the phenomenon of eruption of molten rock onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called a vent.
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 in length, also mark the surface.
The materials produced by this volcanism make up Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere.
A magnetosphere is a region of space surrounding an astronomical object in which charged particles are manipulated or affected by that object's magnetic field.
An atmosphere is a layer of gases surrounding a planet or other material body, that is held in place by the gravity of that body.
Io's volcanic ejecta also produce a large plasma torus around Jupiter.
In geometry, a torus is a surface of revolution generated by revolving a circle in three-dimensional space about an axis coplanar with the circle.
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.
Galileo Galilei was an Italian astronomer, physicist and engineer, sometimes described as a polymath.
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.
Earth is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to harbor life.
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.
An impact crater is an approximately circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body.