With a total of 79 known moons — including four large moons known as the Galilean satellites — Jupiter almost qualifies as a solar system unto itself. Show Most of the moons of Jupiter are small, with about 60 of the satellites being less than 6.2 miles in diameter. The number of moons changes fairly frequently, with the 79th moon being reported in July 2018. Most of the moons were discovered in the late 1970s and later as a result of several explorations by automated spacecraft, including NASA’s Voyager in 1979 and Galileo in 1995. 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. Jupiter also has the strongest magnetic field of any planet, so anything passing near it, such as an asteroid, is either destroyed by gravitational tides or captured into its orbit. Earth only has one moon because it lacks the strong gravitational field and mass necessary to hold another satellite in orbit. 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. (Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)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. Galileo’s discovery was pivotal point in the history of astronomy as his observation revealed that not all celestial bodies revolved around the Earth. Until that time, Earth was thought to be the center of the universe. Eight satellites — the four Galilean and four smaller moons — are closer to the planet and provide the dust that make up Jupiter's rings. (Image credit: NASA)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. Moving outward from Jupiter is Europa. While 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. It is possible that an extensive ocean beneath the surface harbors life. 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. 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 about the size of Mercury but very low in density. 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. Related: This article was updated on Oct. 1, 2018 by Space.com Senior Writer, Meghan Bartels. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com. Jupiter's four largest moons Io, Callisto, Europa and Ganymede are known as the Galilean moons because the first recorded observation of the moons was by Italian astronomer Galileo Galilei in 1610. The Galilean moons are fascinating worlds. Ganymede, for example, is the largest moon in the Solar System. Io is the most volcanically active world in the Solar System, its volcanoes having been discovered by planetary scientist Linda Morabito during the Voyager mission. The shadow of moon Io projected onto Jupiter. Image credit: Image data: NASA/JPL-Caltech/SwRI/MSSS Image processing by Kevin M. Gill, CC BY 3.0Europa is an icy moon with a subsurface ocean lurking beneath its frozen crust, much like Saturn's moon Enceladus. As a result, it's one of the best places in the Solar System to search for conditions that could support life. Callisto is the third largest moon in the Solar System, after Saturn's moon Titan, and may be considered by some to be the lifeless, characterless member of the four Galilean moons. However, there is evidence that Callisto too hosts a subsurface ocean. Could it be another contender for life-supporting conditions beyond Earth? Images of Jupiter's moon Ganymede captured on 26 December 2019 showing infrared mapping of its North Pole. Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAMHow did the Galilean moons form?It is thought that Jupiter's largest moons Io, Europa, Ganymede and Callisto most probably formed out of material leftover from the formation of Jupiter itself. Like the rest of the Solar System planets, Jupiter formed out of the disc of dust and gas surrounding the young Sun. Once this gas and dust had condensed to form Jupiter, the leftover material coalesced and grew over time to form the Galilean moons. This makes Jupiter's largest moons likely as old of the rest of the Solar System: about 4.5 billion years old. A Hubble Space Telescope image of Europa showing suspected plumes of water vapour erupting at the 7 o'clock mark. These plumes could be evidence of a subsurface ocean below Europa's icy crust. Credits: NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science CenterObserving the Galilean moonsThe Galilean moons are also a favourite target of astronomers because it is possible to view them around Jupiter, provided you have a big enough telescope. If you know when and where to look, you may be able to see transits of the moons passing Jupiter, spot their shadows cast on the gas giant's surface, or even an occultation. Find out more about observing the Galilean moons. Facts about the Galilean moonsA mosaic of the Galileo probe’s highest resolution images of Io, captured on 3 July 1999. Credit: NASA/JPL/University of Arizona
Io is the most volcanically active body in the solar system, hosting hundreds of volcanoes that are kept active by the gravitational interactions between the moon, Jupiter, and fellow Jovian satellites Europa and Ganymede. It has a highly elliptical orbit, and tidal forces generated through its interaction with Jupiter generate a fierce amount of heat that causes molten lava to spew from volcanoes, filling surface impact craters. Jupiter's magnetic field strips material from Io's surface, producing a cloud of radiation around the volcanic moon. As a result of all these processes, Io is an incredibly dynamic world, and perhaps one of the oddest looking moons in the Solar System. According to Linda Morabito, when Voyager scientists got their first glimpse of Io, they described it as looking like 'mouldy pizza'. EuropaThis view of Europa shows long linear cracks crisscrossing across the surface of the icy moon. Credit: NASA/JPL-Caltech/SETI Institut
The smallest of the moons, Europa is an icy world known to have a liquid ocean beneath its 15-25km thick icy crust, which may contain twice as much as all of Earth's oceans combined. This makes it a promising place to search for life, and this is exactly what upcoming missions JUICE - JUpiter ICy moons Explorer - and Europa Clipper will be doing. Images of Europa reveal long, linear fractures and rusty red-brown material that could be salty, sulphuric compounds that have mixed with water ice and been blasted by radiation. Its surface is young, potentially as young as 40 million years old, which is evinced by the very small number of craters that can be seen. Studies using the Hubble Space Telescope have revealed that Europa might actually be venting water from its subsurface ocean out into space, which would be prime targets for future missions to explore Europa, just like the Cassini spacecraft at Saturn's moon Enceladus. GanymedeA view of Ganymede captured by NASA's Juno spacecraft during its flyby on 7 June 2021. Credit: NASA/JPL-Caltech/SwRI
Ganymede is quite a specimen. It's the largest moon in the Solar System and the only moon orbiting the Sun with its own magnetic field. As a result it has aurorae at its north and south poles. It is thought to have a large rocky core covered with layers of ice and water, which show signs of tectonic activity. The surface of Ganymede seems to have two types of terrain: dark cratered areas and lighter terrain sporting cracks and grooves. These grooves can stretch 700 metres high and could be a result of tensional faulting, or it could be that Ganymede has a subsurface ocean that's releasing water into space. CallistoAn image of Jupiter's moon Callisto: the only complete global colour image of the moon obtained by the Galileo spacecraft. It was captured in May 2001. Credit: NASA/JPL/DLR
Callisto is Jupiter's second largest moon and perhaps the dark horse of the Galilean satellites. It seems to be a geologically inactive, dark, cratered world, but planetary scientists believe it could host a liquid ocean beneath its surface. More like thisThat's not to diminish what's going on at the surface, however. Callisto's surface is about 4 billion years old - a result of its geological inertness - and thought to be the oldest, most cratered surface in the Solar System. The bright spots that pepper Callisto's surface might well be water ice, frozen on the peaks of its many craters. Callisto's subsurface ocean could be interacting with a layer of rocks 250km deep, and oxygen has been detected in the moon's exosphere. Could Callisto be a key player in the search for life in the Solar System? The discovery of the Galilean moonsA fresco by Giuseppe Bertini portraying Galileo showing the Doge of Venice how to use a telescope. Credit: Giuseppe BertiniOn the title page of Galileo’s Sidereus Nuncius (Sidereal Messenger) – the first astronomical work based on telescope observations when it was published in 1610 – the author says that the book introduces the reader to "great and surpassingly wondrous sights… observed by Galileo Galilei, Florentine patrician and public mathematician at the University of Padua… in four planets revolving with remarkable speed at differing distances and periods around the star Jupiter. "They have been known to no one up to this day, and the author was the first to discover them. He has decided to call them the Medicean planets." The initial naming of the moons as the Medicean planets was to acknowledge the patronage he received from Grand Duke Cosimo II de’ Medici of Tuscany (1590–1621), whom Galileo had served as mathematics tutor in 1605. Galileo’s first idea was to name the newly discovered moons the Cosmica Sidera (Cosmian Stars) solely in honour of his patron, but ultimately chose a name that honoured all four surviving Medici brothers: Cosimo, Francesco, Carlo and Lorenzo.
Galileo also recognised that the four objects he had observed through his telescope were the first ever seen to orbit another planet, and the importance of his discovery was not lost on him. But though he may have been the first to name them, Galileo’s claim to having been the first to see them has come under some doubt. It has been suggested that the Chinese astronomer Gan De, who carried out some of the earliest known systematic observations of the planets in the 4th century BC, may have seen Ganymede. It was while he was studying Jupiter during the summer of 365 BC that Gan De recorded what he described as a ‘small reddish star’ next to the planet. The Chinese astronomy historian Xi Zezong (1927-2008) suggested this may have been an early sighting of Ganymede. An infrared image of the Southern Hemisphere of Jupiter’s moon Io, as seen by NASA’s Juno spacecraft on 16 December 2017. Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAMIt is theoretically possible to see the Galilean moons with the naked eye, but it requires near perfect conditions and incredible eyesight. In 1614, around four years after Sidereus Nuncius was published, German astronomer Simon Marius (1753-1625) published his work Mundus Iovialis (1614) in which he described the planet Jupiter and its moons. He laid claim to having discovered them in December 1609. This would mean that Marius had spotted the Jovian moons some time before Galileo who, according to Sidereus Nuncius, had first seen them on 7 January 1610. However, what is certain is that Galileo was the first to publish what he saw. It was on that date that Galileo turned his telescope towards Jupiter and noticed what he described as “three little stars… positioned near (Jupiter) – small but yet very bright” and noting the presence of a fourth ‘little star’ a few days later. Although his first thoughts were that these were ‘fixed stars’, Galileo was sufficiently intrigued by the fact they were “arranged exactly along a straight line and parallel to the ecliptic” to continue his observations, which ultimately revealed their true nature. An illustration showing Jupiter and its four Galilean moons, from left to right Io, Europa, Ganymede, Callisto. Credit: NASA/JPL/DLRAfter announcing the moons in Sidereus Nuncius, independent verification and sightings of the newly discovered Jovian moons came from a number of sources. These included such noteworthy observers as Johannes Kepler in Prague, English astronomer Thomas Harriot and French astronomer Joseph Gaultier de La Vallette. Regardless of who saw them first the mythological names by which these satellites are known today are those given them by Marius (inspired by a suggestion from Johannes Kepler). In 1614, he wrote in Mundus Iovialis, “Io, Europa, the boy Ganymede and Callisto greatly pleased lustful Jupiter.” However, the names didn’t gain favour until the early 20th century, mainly because Galileo refused to use them. In the meantime, they were generally referred to as Jupiter I, II, III and IV according to their closeness to Jupiter. It would be another few centuries until Jupiter’s next moon, Amalthea, was discovered by American astronomer Edward Emerson Barnard using the 36-inch Great Lick Refractor at Lick Observatory on 9 September 1892. Amalthea holds the distinction of being the last planetary satellite to be discovered by direct visual observation. The sixth Jovian moon, Himalia, was revealed on 3 December 1904 using astrophotography, as have all of its many subsequently discovered moons. Photos of Jupiter and its Galilean moonsBelow is a selection of images of Jupiter and its four largest moons captured by astrophotographers and BBC Sky at Night Magazine readers. For advice on photographing Jupiter, read our guide on how to photograph the planets. If you do manage to capture Jupiter and its Galilean moons, be sure to send us your images or share them with us via Facebook, Twitter and Instagram. Jupiter & Galilean Moons by Danny Lee, Kent, UK. Equipment: Skywatcher 150p, EQ5 PRO GOTO, Nikon D40.Callisto, Europa, Jupiter, Io and Ganymede by David Duff, Stockton on Tees, UK. Equipment: Skywatcher Explorer 150P, Microsoft HD Cinema Lifecam.Europa Transits Jupiter by Daniel Mortimer, Leeds, UK. Equipment: 8" reflector, HEQ5 pro, 2x Barlow, 3x Barlow, DFK 21AU04.ASJupiter and Moons by Alan Stewart, Glenrothes, Fife, UK. Equipment: Canon EOS400D, Skywatcher 150PLJupiter with Io by Stuart Powell, Leeds, UK. Equipment: SW ED80 pro, EQ5 with RA and dec motors, SP880 webcam, 2 2x barlow lensesTransit of Europa on November 4th 2011, Imaged from Cornwall by David C Billington, St Agnes, Cornwall, UK. Equipment: 12" US Orion Dobsonion, Watch house Equatorial tracking Platform, SPC880 webcam, 2x Barlow. |