Io (moon)

Io is the innermost and second-smallest of the four Galilean moons of Jupiter. Slightly larger than Earth's Moon, Io is the fourth-largest natural satellite in the Solar System. It has the highest density and strongest surface gravity of any natural satellite, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System.

With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity results from tidal heating from friction generated within Io's interior as it is pulled between Jupiter and two other Galilean moons—Europa and Ganymede. Several volcanoes produce plumes of sulfur and sulfur dioxide as high as 500 km (300 mi) above the surface. Io's surface is also dotted with more than 100 mountains uplifted by extensive compression at the base of Io's silicate crust. Some of these peaks are taller than Mount Everest, the highest point on Earth's surface. Unlike most moons 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 with a frosty coating of sulfur and sulfur dioxide.

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 they also greatly affect the nature and radiation levels of Jupiter's extensive magnetosphere. Io's volcanic ejecta also produces a large, intense plasma torus around Jupiter, creating a hostile radiation environment on and around the moon.

Io was discovered along with the other Galilean moons in 1610 by Galileo Galilei and named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers. The discovery of the Galilean moons played a significant role in the development of astronomy, furthering the adoption of the Copernican model of the Solar System and the development of Kepler's laws of planetary motion. Io in particular was used for the first measurement of the speed of light. 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. Further observations have been made by Cassini–Huygens in 2000, New Horizons in 2007, and Juno since 2017, as well as from Earth-based telescopes and the Hubble Space Telescope.

Nomenclature

Main article: Galilean moons § Names

See also: List of regions on Io, List of volcanic features on Io, and List of mountains on Io

Long ago, a man named Simon Marius suggested names for the moons of Jupiter. He used names from stories about Jupiter, a powerful god. One of these names was Io, after a woman from ancient tales.

Today, places on Io also have names from old stories about gods and heroes, especially those linked to fire and volcanoes because Io has many active volcanoes. Scientists have given names to many of Io's mountains, valleys, and lava flows.

Observational history

Main article: Exploration of Io

The first person to see Io was Galileo Galilei on January 7, 1610. He used a telescope to look at Jupiter and its moons. At first, he could not tell Io apart from another moon called Europa because his telescope was not very strong. The next day, he could see them as two separate moons. This is when Io was discovered.

For many years, Io was just a small dot of light in telescopes. Later, with better telescopes, scientists could see more details on Io’s surface. They noticed that Io looked different from the other moons because it did not have much water ice. Instead, its surface seemed to be made of salts and sulfur.

Spacecraft have visited Io and taken pictures. The first was Pioneer 10 in 1973, followed by Voyager in 1979. These spacecraft showed that Io had many colorful landscapes and tall mountains. They also discovered that Io had active volcanoes, making it the most active volcanic world in our solar system. Later missions like Galileo and Juno have continued to study this fascinating moon.

Pioneer

The first spacecraft to pass by Io were the Pioneer 10 and Pioneer 11 probes.

Voyager

When the twin probes Voyager 1 and Voyager 2 passed by Io in 1979, their more advanced imaging systems allowed for far more detailed images.

Galileo

The Galileo spacecraft arrived at Jupiter in 1995 after a six-year journey from Earth to follow up on the discoveries of the two Voyager probes and the ground-based observations made in the intervening years.

Cassini

In December 2000, the Cassini spacecraft had a distant and brief encounter with the Jovian system en route to Saturn, allowing for joint observations with Galileo.

New Horizons

The New Horizons spacecraft, en route to Pluto and the Kuiper belt, flew by the Jovian system and Io on 28 February 2007.

Juno

The Juno spacecraft was launched in 2011 and entered orbit around Jupiter on 5 July 2016.

Future missions

There are two forthcoming missions planned for the Jovian system. The Jupiter Icy Moon Explorer (JUICE) is a planned European Space Agency mission to the Jovian system that is intended to end up in Ganymede orbit. JUICE launched in April 2023, with arrival at Jupiter planned for July 2031. Europa Clipper is a planned NASA mission to the Jovian system focused on Jupiter's moon Europa.

Orbit and rotation

Io orbits Jupiter at a distance of 421,700 km from Jupiter's center. It is the innermost of the Galilean satellites, located between Thebe and Europa. Io completes one orbit around Jupiter in about 42.5 hours.

Io rotates in sync with its orbit, always showing the same face toward Jupiter. This means one side of Io always faces Jupiter, while the opposite side faces away. The side moving in Io's orbit direction is called the leading hemisphere, and the opposite side is the trailing hemisphere.

See also: Tidal heating

Main articles: Orbital resonance, Galilean satellites, Synchronous rotation

Interaction with Jupiter's magnetosphere

Io helps shape Jupiter's magnetic field. It acts like a big electric generator, creating very strong electric currents. This process adds materials to Jupiter’s magnetic field, making it much larger than it would normally be. These materials come from Io’s volcanoes and include gases and dust such as sulfur, oxygen, and sodium.

A cloud of these particles surrounds Io and stretches far out into space. Some of these particles escape and orbit around Jupiter. Io also creates bright glowing lights, known as aurorae, in both Jupiter’s atmosphere and its own. The magnetic field of Jupiter creates electric currents in Io, which may produce a magnetic field inside Io itself. Recent studies suggest that Io might not have a large ocean of molten rock underneath its surface, even though it has many active volcanoes.

Geology

Io is slightly larger than Earth's Moon. It has a mean radius of 1,821.3 km (1,131.7 mi) and a mass of 8.9319×10²² kg. It is made mostly of rock and iron, with a very high density. Unlike many other moons, Io has very little water.

Io is the most active world in our Solar System, with over 400 active volcanoes. Its surface is young and constantly changing due to volcanic activity. The surface is covered with colorful materials from volcanic eruptions, including sulfur and sulfur dioxide. These materials give Io its distinctive appearance, often compared to a rotten orange or pizza. The lack of craters shows that Io's surface is always being renewed by volcanic activity.

Main article: Tidal heating of Io

Main article: Volcanism on Io

See also: List of volcanic features on Io

Main article: Mountains of Io

See also: List of mountains on Io

Atmosphere

Main article: Atmosphere of Io

Io has a very thin air made mostly of a gas called sulfur dioxide, along with a few other gases. The air changes a lot depending on where you are on Io and the time of day. It is thickest when Io faces Jupiter and thinnest on the side facing away.

The air on Io is constantly being blown away by Jupiter's magnetic field and must be replaced by gases coming from Io's many volcanoes and from the surface when the sun heats it up. When Io passes into Jupiter's shadow, much of the air freezes onto the surface, and then disappears again when Io comes back out into the sunlight.