Astronomy - The Moons of Jupiter


The Moons of Jupiter

There are 67 known moons of Jupiter. This gives Jupiter the largest number of moons with reasonably stable orbits of any planet in the Solar System.

The most massive of the moons are the four Galilean moons, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun.

Simon Marius January 20, 1573 – January 5, 1625 was a German astronomer. He was born near Nuremberg, but he spent most of his life in the city of Ansbach.

In 1614 Marius published his work describing the planet Jupiter and its moons. Here he claimed to have discovered the planet's four major moons some days before Galileo Galilei.

Regardless of priority, the mythological names by which these satellites are known today (Io, Europa, Ganymede and Callisto) are those given them by Marius.

Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting gas and solid debris analogous to a protoplanetary disk.

They may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history.

The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 63 moons and its rings together comprising just 0.003% of the total orbiting mass.


Ganymede is the largest moon of Jupiter and in the Solar System, and the only moon known to have a magnetic field. It is the seventh satellite outward from Jupiter and third of the Galilean moons. it orbits Jupiter in roughly seven days.

Ganymede has a diameter of 5,268 km (3,273 mi), 8 % larger than the planet Mercury, but its mass is only 45 % that of Mercury. Ganymede is 2 % larger than Saturn's Titan (second-largest moon of the Solar System). At 2.02 times the mass of the Moon, it is the most massive planetary satellite. It is the ninth-largest object in the Solar System, and the largest without a substantial atmosphere.

Ganymede is composed of approximately equal amounts of silicate rock and water ice. It has an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined Its surface is composed of two main types of terrain.

Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder.

The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity due to tidal heating.

Beginning with Pioneer 10, spacecraft have been able to examine Ganymede closely. The Voyager probes refined measurements of its size, whereas the Galileo craft discovered its underground ocean and small magnetic field which is buried within Jupiter's much larger magnetic field and would show only as a local variations of the field lines.


The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. After flybys of all three icy Galilean moons, the probe is planned to enter orbit around Ganymede.


Callisto  is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System.

Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated.

Callisto's rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand nearly still in Callisto's sky.

Callisto is composed of approximately equal amounts of rock and ices, with a density of about 1.83 g/cm3, the lowest density and surface gravity of Jupiter's major moons.

Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds.

Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km.

The surface of Callisto is the oldest and most heavily cratered in the Solar System.

It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts.

Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters  and associated scarps, ridges and deposits.

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere.

Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation.

Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto.

Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.


Io 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 relative water of any known 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 is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites.

Several volcanoes produce plumes of sulphur and sulphur 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.

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-sulphide core. Most of Io's surface is composed of extensive plains coated with sulphur and sulphur-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.

Numerous extensive lava flows, several more than 500 km (300 mi) in length, also mark the surface.

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 cantered on Io's orbit.

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


Europa, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet. It is also the sixth-largest moon in the Solar System.

Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the legendary mother of King Minos of Crete, and lover of Zeus (the Greek equivalent of the Roman god Jupiter).

Slightly smaller than the Moon, Europa is primarily made of silicate rock and has a water-ice crust and probably an iron–nickel core.

It has a tenuous atmosphere composed primarily of oxygen. Its surface is striated by cracks and streaks, whereas craters are relatively rare.

Europa has the smoothest surface of any known solid object in the Solar System. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extra-terrestrial life.

The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below.

Also, sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor.

This may be important in determining if Europa could be habitable.

Europa has been examined by a succession of space probe flybys, the first occurring in the early 1970s.

The Galileo mission, launched in 1989, provides the bulk of current data on Europa.

No spacecraft has yet landed on Europa, but its intriguing characteristics have led to several ambitious exploration proposals.

In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cry geysers.

The European Space Agency's Jupiter Icy Moon Explorer will conduct two flybys of Europa.