The Galilean Moons of Jupiter Content from the guide to life, the universe and everything

The Galilean Moons of Jupiter

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Io, the moon nearest Jupiter.
Or sweet Europa's mantle blew unclasp'd,
From off her shoulder backward borne;
From one hand droop'd a crocus;
The other grasp'd the mild bull's golden horn.

–  Alfred, Lord Tennyson  –  The Palace of Art.

The Medicean Planets

Jupiter's four largest moons were discovered by Galileo Galilei (1564 - 1642) in January 1610, after he used his telescope to study the heavens instead of spying on his neighbours1. The discovery was the first time a moon was found to be orbiting a planet other than Earth; even though we now know that much-closer Mars has two attendant moons, they are much smaller and weren't discovered until the 19th Century.

I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world up to our own times, their positions, and the observations made during the last two months about their movements and their changes of magnitude; and I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day... On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavens through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude... When on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night. I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter.
–   Galileo Galilei, author of Sidereus Nuncius ('Starry Messenger') March 1610

Galileo called his discoveries the 'Medicean planets', after the important Medici family, who were Galileo's patrons. He referred to the moons as Jupiter I, II, III and IV, and that system remained in place until the mid-19th Century as more satellites were being discovered, and the numerical system was deemed too confusing.

At the time Galileo was studying the heavens through his telescope, most people believed in the geocentric system favoured by Aristotle, Plato and Ptolemy. Geocentrism is the belief that the Earth is fixed at the centre of the Universe with all the other celestial bodies revolving around it. It was a perfectly safe belief as it was the one favoured by the Roman Catholic Church. On the other hand, it was quite unsafe to publicly express your views if they clashed with those of the Church, which had the power to put people on trial and order their executions if they did not recant their beliefs. Galileo believed in the heliocentric theory – where the Sun is the central body and everything else in the Solar System orbits it – and specifically that the Earth moves, and portrayed the Pope as an idiot in his book for not agreeing with him. For this he spent the rest of his life under house arrest.

German astronomer and astrologer Simon Marius (1573 - 1624) claimed to have observed Jupiter's main moons in November 1609, two months earlier than Galileo, but he didn't publish his find so Galileo takes the kudos. Indeed, Marius was denounced as a plagiarist2 by Galileo. Marius did, however, supply the familiar names which they carry today, based on a conversation he had with the great astronomer Johannes Kepler after they met at Ratisbon Fair in Germany in October 1613:

Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto... This fancy, and the particular names given, were suggested to me by Kepler, Imperial Astronomer. [...] So if, as a jest, and in memory of our friendship then begun, I hail him as joint father of these four stars, again I shall not be doing wrong.

The four main Jovian3 moons were subsequently named Ganymede, Europa, Io and Callisto, after the chief Roman god's lovers, and collectively dubbed 'the Galilean Moons' in Galileo's honour.


Galileo's discovery of Jupiter's main moons provided strong evidence for heliocentrism, which was also suspected by the likes of Nicolaus Copernicus (1473 - 1543), Johannes Kepler (1571 - 1630) and Giordano Bruno (1548 - 16004). While Galileo had complete faith that the Bible was a sure map to Heaven, he did not believe that it was ever intended to be an accurate map of the heavens. For that, only careful observations of the natural world would do – and he saw no conflict in this.

Table of Eclipses

Galileo worked out that the occultation of Jupiter's moons happened at wholly predictable times and produced a reference table of the approximately 1,000 occurrences that took place each year. In 1675-6 the Danish astronomer Ole Christensen Rømer5 calculated a reasonably accurate estimate of the speed of light after he adjusted Galileo's table of eclipses, making allowances for the distance between the Earth and Jupiter, which varies considerably dependant upon their relative positions in their orbits.

What Size is the Earth?

The French Académie Royale de Sciences proposed to obtain a true dimension for the Earth to allow angular measurements to be turned into accurate distances on the Earth's curved surface. They engaged French astronomer Jean-Felix Picard (1620 - 1682) who surveyed a baseline on the Paris meridian using triangulation between Malvoisine, near Paris, northward to Sourdon near Amiens, France. Each of the cardinal points was verified using eclipses of Jupiter's main moons. After careful calculation Picard established that one degree of latitude equated to just over 69 miles (110km) and his computation of the radius of the Earth was the most accurate so far (he was only 28km out).

The Mason-Dixon Line

In 1763 English surveyors Charles Mason and Jeremiah Dixon, both experienced astronomers who had previously collaborated on the 1761 transit of Venus, began their survey to resolve a border dispute between the Calvert family of Maryland and the Penn family of Pennsylvania. The base position near the eastern end of what was to become the Mason-Dixon line was established with observations of Jupiter's main moons. The Mason-Dixon line came to symbolise the split between northern and southern states that led to the American Civil War.

The Galilean Moons

There is interesting interplay between three of the Galilean moons, Io, Europa and Ganymede. The ratio they are locked into is 1:2:4 – Io orbits Jupiter four times to Europa's twice and Ganymede's once. This causes gravitational effects of tidal distortion, and could explain why the moons are warmer internally than they ought to be.

Io - Jupiter I

Slightly larger than our own Moon at 3,642km diameter, Io (pronounced EYE-oh) orbits Jupiter once every 42 hours at a distance of 420,000km in a slightly elliptical orbit. We now know that fiery Io is the most volcanic body in our Solar System; its surface bubbles like the cheese topping of a pizza while it is cooking. This is because at its closest point to Jupiter, tidal forces stretch Io into an ellipsoid. At its further point from the planet 21 hours later, the gravitational pull has eased off somewhat and the moon attempts to return to a sphere. This constant stress and strain causes friction within the moon, which heats it up causing the volcanic activity that has been observed.

Io's volcanism was not suspected by NASA scientists, as it should have ended several billion years ago, the same as most of the other moons in the Solar System, including our own. But the discovery was made by one of the Voyager 1 (see 'Visiting the Jovian System' section later) navigation team members, Linda Morabito of JPL Pasadena, working through one Friday night. Initial examination of the images from the Voyager 1 encounter with Io had not revealed anything unusual, other than that its surface bore almost no meteorite craters. Morabito was examining the rejected photos when she noticed a 'bulge' on the edge of Io's disc. The bulge was present in most of the photos. Deducing that the only thing it could be was the emissions plume from a volcano, Morabito had to wait until staff returned to work after the weekend to report her discovery. The resurfacing of Io was found to be an almost complete moon-wide phenomenon because the team had photos from the earlier Pioneer probes to compare. Some of the volcanoes have been named, for example, one which was photographed in the act of erupting was named Pele.

In legend Io was a princess whose beauty attracted Jupiter. The god pursued and won his prize, but when his wife Juno (Greek name Hera) discovered his deceit, Jupiter changed Io into a white heifer which he then gave to his wife as a gift. Suspicious Juno handed the cow over to Argus, a hundred-eyed monster. Io was rescued by Mercury, who told her how to find Jupiter to request transformation back to her original form. Jupiter agreed and fell in love with Io all over again. They had a son, Epaphus, who was the ancestor of Hercules, one of the greatest mythological heroes of all time. Jupiter was also Hercules' father, but that's another story. For the rest of her life Io was plagued by a stinging insect which had been sent by Juno to annoy her.

Europa - Jupiter II

I want to go ice fishing through the thick ice of Jupiter's moon, Europa, see what's down there. It's an ocean; we have pretty good evidence that life began in our own oceans. We've got an ocean there, rendered liquid the entire life of that moon. I don't know if that can happen within the next 10 years, but it's not out of our reach as a nation that has sustained a space program for this long. I can tell you that the knowledge of what is there under the ice of Europa and below the surface of Mars – if there's any evidence, confirmable evidence, that there was once life there, or that there's life there now – that that would signal a change in the human condition that we might not even be able to imagine, realizing that we are not alone in the universe.
– Astrophysicist Neil de Grasse Tyson

Europa (pronounced yu-ROH-puh) is mostly rocky with an iron core, but it has a surface covered in ice. There may be liquid water underneath the ice, because the interior is hotter than the surface, but we don't know for sure. Europa orbits Jupiter in just 3½ days, at a distance of 670,900km (416,900 miles). Its diameter is 3,122km (1,940 miles), which means it is slightly smaller than Earth's moon. Europa shows the same side to Jupiter all the time, and travels in an elliptical orbit. The possibility of life on Europa is discussed in its own h2g2 Entry.

The legend of Europa is about the Phoenician princess of Tyre, daughter of King Agenor of Sidon. Attracted to a magnificent white bull (Jupiter in disguise), Europa decorated his horns with a garland of flowers and could not resist straddling him for a ride. The bull then carried her off across the waters to the continent which still bears her name, Europe. According to some interpretations, their third son became King Minos of Crete.

Ganymede - Jupiter III

Ganymede (pronounced GAN-E-meed) is the largest natural satellite in our Solar System; at 5,268km diameter it's bigger even than Mercury6, the nearest planet to the Sun, and not much smaller than Mars. If Ganymede revolved around the Sun instead of Jupiter, it would hold planetary status. Of course, when viewed with Jupiter, Ganymede looks comparatively tiny! Ganymede has its own internally-generated magnetic field, and a thin oxygen atmosphere. There is a region on Ganymede which has been named Marius Regio after the German astronomer Simon Marius, whose idea for the main Jovian moons' naming has been perpetuated.

According to the myth, Ganymede was a young shepherd lad whom Jupiter fancied. To attract his attention, Jupiter metamorphosed into an eagle. When Ganymede tried to protect his flock by chasing off the fabulous bird of prey, the eagle swooped, grasped the young man by his shoulders, then carried him off to Mount Olympus. Once there, he made Ganymede the cup bearer to all the gods and his personal slave. When the goddess Juno found out that Ganymede was her husband's lover, she was so enraged that Jupiter was forced to hide Ganymede in the heavens as the stars which make up the constellation Crater 'the cup'.

Callisto - Jupiter IV

Callisto (pronounced kah-LIS-toh) is the second largest moon, with a diameter of 4,800km. It is the outermost of the four Galilean moons, and is as different from its sibling Io as chalk is from cheese. While Io is constantly changing due to being remoulded by gravitational forces, Callisto's surface is abundantly cratered from impacts of many millions of years' worth of bombardments from space. Callisto has a thin carbon dioxide atmosphere and a weak magnetic field.

The myth involving Callisto connects it to the constellation Ursa Major 'the great bear'. Callisto was the virgin daughter of King Lycaon of Arcadia who was in the employ of Diana, the goddess of the hunt. Unfortunately she caught the eye of Jupiter, who disguised himself as Diana to get close to Callisto. When she relaxed, Jupiter seized the opportunity and seduced her. Callisto became pregnant and she tried to hide her pregnancy from Diana, who only discovered the truth when Callisto stripped off to share a bath with her. Diana disavowed Callisto, which left her at the mercy of the goddess Juno, Jupiter's jealous wife.

There are two different versions of the ending of the story. In the first, Jupiter changed Callisto into a bear to keep her safe, and the baby boy was raised by Atlas' daughter Maia. One day, when the child Arcas was grown, he was out hunting when the bear rushed to embrace him. Thinking he was being attacked, Arcas tried to kill the bear, not knowing it was really his mother. Callisto could not harm her own son, so she appealed to Jupiter on their behalf and he placed them both in the heavens, Callisto as the constellation Ursa Major and Arcas as Ursa Minor 'the little bear'.

The other version is less pleasant: Callisto's father King Lycaon did not believe his daughter when she claimed that the child's father was the king of the gods, so he invited Jupiter to join them for a meal. The meat Jupiter was offered was the sliced flesh of the infant Arcas, who had been killed earlier on the instructions of the goddess Juno. Jupiter did not relish having his son served up on a silver platter and took his revenge by killing King Lycaon's own sons. Then he gathered up the slices of Arcas, reformed him, then revived him. When he was a normal baby again, the god handed the infant over to Atlas' daughter Maia to raise.


Technological advances like the Hubble Space Telescope provided us with views of crystal clarity, some of which only served to deepen the mysteries of the Jovian system and the Galilean moons in particular. Close-up images of such alien worlds have not shaken the beliefs of the religious bodies; even the head of the Roman Catholic Church, the Pope, acknowledges this. Pope Benedict XVI has said that 'an understanding of the laws of nature could stimulate appreciation of God's work'. There is an observatory at Castel Gandolfo7 in the Alban Hills, 25km (15½ miles) south-east of Rome for the current Pope to enjoy to his heart's content, as other leaders of the papacy have before him.

Visiting the Jovian System

Thanks to technological advancements like space probes and orbiting telescopes, we are able to study the Jovian system more closely than Galileo and his kinsmen could have dreamed of. Pioneer 10 and Pioneer 11 visited the Jovian system between 1973 and 1974. The spacecraft Voyager 1 and Voyager 2 took over 30,000 images of Jupiter and its largest moons in 1979.

Much of what is known about the four largest Jovian moons comes from data gathered by the space probe Galileo, which was launched in October 1989. It orbited Jupiter from 1995 to 2003, sending back important information which is still being analysed. In October 1999 Galileo made its closest approach to Io, (611km/380 miles), revealing 'gigantic lava flows and lava lakes, and towering, collapsing mountains' according to the press release. A member of the Galileo imaging team, Dr Alfred McEwen of the University of Arizona, is quoted as saying: Io makes Dante's Inferno seem like another day in paradise. Finally, after completing 34 orbits, the probe was destroyed by being directed into Jupiter, preventing the possibility of its crashing into one of the attendant moons and thus avoiding contamination from Earth.

Ulysses used Jupiter as an orbit boost on its way to a solar polar orbit. Cassini flew by Jupiter on its way to Saturn, sending back spectacular images of the Jovian system. The New Horizons spacecraft travelled through the Jovian system in 2007, on its way8 to dwarf planet Pluto (ETA 2015) and the Kuiper Belt. Among many other discoveries, New Horizons managed to explore details of the surface of the moon Io, including views inside volcanic eruptions.

Future Visits

It was hoped that the ambitious Jupiter Icy Moons Orbiter (JIMO), due to be launched in 2015, would answer some of the intriguing questions, but it was deemed too expensive and NASA cancelled the project. The replacement Europa Jupiter System Mission (EJSM) is in its early stages with a tentative launch date proposed for 2020.

1Spyglasses were invented for watching out for invading naval fleets. Galileo made a much better instrument from the basic design.2Not for the first time. Marius had taught an Italian student, Baldessar Capra, who was expelled from the university of Padua for plagiarising Galileo's treatise on the compass. Marius is acknowledged as the first to observe the Andromeda 'nebula' (galaxy) with a telescope in 1612.3Belonging to the family of Jupiter.4This was not a natural death. After a lengthy trial, Bruno was sentenced to be roasted alive at the stake by the Catholic Inquisition for his blasphemy and heresy.5Other spellings of his surname are Roemer, Römer, and Romer.6Mercury's diameter is 4,878km.7The Pope's summer residence.8The spacecraft is travelling at 16.5km/s and at the time of writing (November 2009) is 15 AU from the Sun, and still has 16.7 AU to travel. AU means Astronomical Unit, the average distance between the Sun and Earth, about 93 million miles or 150 million km.

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