With a total of four large moons known as the Galilean satellites. Jupiter almost qualifies as a solar system unto itself. Not only is Jupiter the largest planet in the solar system, it is also the most massive at more than 300 times the mass of Earth. Its size plays a role in the number of moons orbiting Jupiter because there is a large area of gravitational stability around it to support many moons.
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The moons of Jupiter have orbital periods ranging from seven hours to almost three Earth years. Some of the orbits are nearly circular, while the moons farthest from Jupiter have more irregular orbits. The outer moons orbit in the opposite direction in which Jupiter spins, which is unusual and indicates the moons were asteroids that were sucked into Jupiter’s orbit after the initial system was formed.
In January 1610, Italian astronomer Galileo Galilei discovered four of Jupiter’s moons — now called Io, Europa, Ganymede and Callisto. He originally referred to the individual moons numerically as I, II, III, and IV. The numerical system for naming the moons lasted for a few centuries until scientists determined that simply using numbers as a naming device would be confusing and impractical as more moons were discovered.
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As with most moons and planets, the Galilean moons orbit Jupiter around its equator. This particular cycle is six years long. So every six years we view Jupiter’s equator – and the moons orbiting above its equator – at the most edge-on. During these special times, we can see the moons eclipse and cast shadows on not just giant Jupiter but on each other.
The closest of the Galilean moons to Jupiter is Io, the first moon to be discovered by Galileo. This satellite’s distinctive feature is its volcanoes, making it the only celestial body in the solar system besides Earth to have volcanic activity.
This moon also has sulfur dioxide snowfields, leading to its characterization as a moon of fire and ice. Io has an iron or iron sulfide core and a brown silicate outer layer, which gives it a splotchy orange, yellow, black, red, and white appearance.
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Seen through a telescope from Earth, Io appears reddish orange, while the other moons are neutral in tint. Io’s infrared spectrum shows no evidence of the absorption characteristics of water ice. Scientists expected Io’s surface to look different from those of Jupiter’s other moons, but the Voyager images revealed a landscape even more unusual than anticipated.
The interior contains a dense, iron-rich core, which probably produces a magnetic field. The interactions of Io with Jupiter’s magnetosphere and ionosphere are so complex, however, that it has been difficult to distinguish the satellite’s own field from the current-produced fields in its vicinity.
Moving outward from Jupiter is Europa. It is slightly smaller than Earth’s moon, it is still one of the largest bodies in the solar system but the smallest of the Galilean satellites. Cracks and streaks crisscross the entire icy surface, which is marked with very few craters. Europa has a high degree of reflectivity, making it among the brightest moons in the solar system. At 20 to 180 million years old, the surface is fairly young.
The surface of Europa is totally different from that of Ganymede or Callisto, despite the fact that the infrared spectrum of this object indicates that it, too, is covered with ice. This resurfacing evidently consists of the outflow of water from the interior to form an instant frozen ocean.
This moon possesses both induced and intrinsic magnetic fields. Slightly mottled regions on the surface have been found to contain salt deposits, suggesting evaporation of water from a reservoir below the crust. Europa’s frozen surface is crisscrossed with dark and bright stripes and curvilinear ridges and grooves.
Ganymede also casts the largest shadow on the planet’s cloud tops when it transits in front of Jupiter. Shadow transits are visible at least once a week with ‘double transits’ – two moons casting shadows simultaneously – occurring once or twice a month. Moons also fade away and then reappear over several minutes when they enter and exit Jupiter’s shadow during eclipse.
Ganymede is the third Galilean moon from Jupiter and the largest of the four. This low-density moon is about the size of Mercury but has about half the mass. Its outstanding characteristic is that it is the only moon to have its own magnetic field. The satellite’s iron core is topped off by a thick crust that is mostly ice. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain, which forms intricate patterns across Ganymede.
Hubble Space Telescope observations of how Ganymede’s auroras change when interacting with Jupiter’s magnetic field reveal the likely existence of a subsurface ocean about 100 km (60 miles) thick. The trace components identified in Ganymede’s icy surface include a smaller amount of the same claylike dust found on Callisto and the same traces of solid carbon dioxide, hydrogen peroxide, and sulfur compounds, plus evidence for molecular oxygen and ozone trapped in the ice.
Callisto, the fourth and farthest of the Galilean moons from Jupiter, is the most heavily cratered object in the solar system. The moon’s landscape has essentially remained unchanged since its formation, which has garnered much interest among scientists. It is also experiences the least impact of Jupiter’s magnetic field as its orbit is the farthest from the planet and beyond Jupiter’s primary radiation belt.
The icy surface of this satellite is so dominated by impact craters that there are no smooth plains like the dark maria observed on the Moon. In other words, there seem to be no areas on Callisto where upwelling of material. This record was formed by impacting debris (comet nuclei and asteroidal material) primarily during the first 500 million years after the formation of the solar system.
Other trace surface constituents are hydrogen peroxide, probably produced from the ice by photochemical reactions driven by solar ultraviolet radiation; sulfur and sulfur compounds, probably coming from Io; and organic compounds that may have been delivered by cometary impacts.