LECTURE 3: JUPITER'S GALILEAN MOONS
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus (the Greek equivalent of the Roman god Jupiter): Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance.
As a result of improvements Galileo Galilei made to the telescope, he was able to see celestial bodies more distinctly than ever before in human history. As a result, sometime between December 1609 and January 1610 Galileo Galilei discovered what came to be known as the Galilean moons [1] although a Chinese historian of astronomy, Xi Zezong, claimed that the Chinese astronomer Gan De observed one of Jupiter's moons in 362 BC, nearly 2 millennia earlier than Galileo.[2]
On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter’s moons. At the time, he only saw three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from January 8 to March 2, 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.[1]
Galileo’s discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the incontrovertible discovery of celestial bodies orbiting something other than the Earth dealt a serious blow to the then-accepted Ptolemaic world system, which held that the Earth was at the center of the universe and all other celestial bodies revolved around it. That Jupiter has four moons while Earth has only one further undercut the near-universal belief that the Earth was the center of the universe both in position and in importance. Galileo's Sidereus Nuncius (Starry Messenger), which announced celestial observations through his telescope, does not explicitly mention Copernican heliocentrism, a theory that placed the Sun at the center of the universe. Nevertheless, Galileo believed in the Copernican theory.[1]
Galileo also developed a method of determining longitude based on the timing of the orbits of the Galilean moons.
In 1605, Galileo had been employed as a math tutor for Cosimo de’ Medici. In 1609, Cosimo became Grand Duke Cosimo II of Tuscany. Galileo, seeking patronage from his now-wealthy former student and his student's powerful family, used the discovery of Jupiter’s moons to gain it.[1] On February 13, 1610, Galileo wrote to the Grand Duke’s secretary:
God graced me with being able, through such a singular sign, to reveal to my Lord my devotion and the desire I have that his glorious name live as equal among the stars, and since it is up to me, the first discoverer, to name these new planets, I wish, in imitation of the great sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name of the Most Serene Grand Duke.[1]
Galileo asked whether he should name the moons the "Cosmian Stars," after Cosimo alone, or the "Medician Stars," which would honor all four brothers in the Medici clan. The secretary replied that the latter name would be best.[1]
On March 12, 1610, Galileo wrote his dedicatory letter to the Duke of Tuscany, and the next day sent a copy to the Grand Duke, hoping to obtain the Grand Duke’s support as quickly as possibly. On March 19, he sent the telescope he had used to first view Jupiter’s moons to the Grand Duke, along with an official copy of Sidereus Nuncius (The Starry Messenger) that, following the secretary's advice, named the four moons the Medician Stars.[1] In his dedicatory introduction, Galileo wrote:
[S]carcely have the immortal graces of your soul begun to shine forth on earth than bright stars offer themselves in the heavens which, like tongues, will speak of and celebrate your most excellent virtues for all time. Behold, therefore, four stars reserved for your illustrious name … which … make their journeys and orbits with a marvelous speed around the star of Jupiter … like children of the same family … Indeed, it appears the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others.[1]
Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars"), in honour of Cosimo II de' Medici (1590–1621), grand-duke of Tuscany from 1609, whose patronage he wanted to secure. At the grand-duke's suggestion, Galileo changed the name to Medicea Sidera ("the Medician stars"), honouring all four Medici brothers (Cosimo, Francesco, Carlo, and Lorenzo). The discovery was announced in the Sidereus Nuncius ("Starry Messenger"), published in Venice in March 1610, less than two months after the first observations.
Other names put forward include 'Principharus, Victipharus, Cosmipharus and Ferdinandipharus', for each of the four Medici brothers, proposed by Giovanni Batista Hodierna, a disciple of Galileo and author of the first ephemerides (Medicaeorum Ephemerides, 1656) Johannes Hevelius called the moons the 'Circulatores Jovis' or 'Jovis Committees', and Jacques Ozanam called them 'Gardes' or 'Satellites' (from the Latin satelles, satellitis, meaning "escorts").
The names that eventually prevailed were chosen by Simon Marius, who claimed to have discovered the moons at the same time as Galileo: he named them after lovers of the god Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.
Galileo steadfastly refused to use Marius's names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names. The numbers run from Jupiter outward, thus I, II, III and IV for Io, Europa, Ganymede and Callisto respectively. Galileo used this system in his notebooks but never actually published it.
All four Galilean moons are bright enough that they could, if they were farther away from Jupiter, be sighted from Earth without a telescope. They have apparent magnitudes between 4.6 and 5.6 when Jupiter is in opposition with the Sun, and about one unit of magnitude dimmer when Jupiter is in conjunction. The main difficulty in observing the moons from Earth is their proximity to Jupiter since they are obscured by its brightness. The maximum angular separation of the moons are between 2 and 8 minutes of arc from Jupiter, close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation. The easiest way to observe them is to cover Jupiter with an object, e.g. a tree limb or a power line that is perpendicular to the plane of moons' orbits.
Of the four Galilean moons, Io is the closest to Jupiter and the second smallest. Io is the fourth largest moon in the solar system, is the most volcanically active body in the solar system, has had the largest recorded volcanic eruptions, and has the highest density of all the moons in the solar system. Io is estimated to be 3642.6 kilometers in diameter, and, because of its smaller size, only has 18.3% Earth’s gravity. The name Io is derived from Greek mythology, and although Simon Marius suggested the name soon after its discovery, Io was simply referred to as “Jupiter I,” or “The first satellite of Jupiter,” until mid-20th century. Io does not seem to have many craters in pictures because it has many active volcanoes, 9 of which were observed by Voyager I. Although not proven, recent data from the Galileo orbiter indicates that Io might have its own magnetic field. Io has an extremely thin atmosphere made up mostly of sulfur dioxide (SO2). If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the tank-like bodies of the Soviet Venera landers) to survive the radiation and magnetic fields that originate from Jupiter.
Europa, the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter. Europa is the smoothest object in the solar system. There is a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick. The top of the water is ice and under the ice, the water is liquid. If any life exists in the water, it may be similar to Earth’s deep-sea creatures. Several things could cause the marks on the surface of the moon. Some theories are that Jupiter’s gravity is causing these markings, as one side of Europa is constantly facing Jupiter; volcanic water eruptions splitting the surface of Europa; and even geysers have been considered as a cause. The colour of the markings, reddish-brown, is thought to be caused by sulfur, but scientists can't be sure, as no data collection devices have been sent to Europa. These markings crossing the moon emphasize a flatter, smoother surface. Some have even considered the reddish-brown markings to be bacteria under the ice on Europa. Any bacteria on Europa may be similar to the extremophile bacteria found on Earth because of the intense conditions on this moon. The name Jupiter II was used for a long time instead of Europa. Again, Europa’s name was from Greek mythology, was suggested by Simon Marius, and was not used widely until mid-20th century.
Ganymede, the third Galilean moon and the seventh known satellite from Jupiter, is less shrouded in mystery than Europa. Ganymede is the largest moon in the entire solar system at 5262.4 kilometers in diameter. Ganymede is the only moon in the solar system known to have a magnetic field. Ganymede is composed of silicate rock and water ice, with an ice crust floating over a warmer ice mantle. The metallic core of Ganymede suggests a greater heat at some time in Ganymede's past than had previously been proposed. The surface of Ganymede is a mix of two types of terrain – highly cratered dark regions and younger, but still ancient, regions with a large array of grooves and ridges. Ganymede has a high number of craters, but many are gone or barely visible due to Ganymede’s icy crust forming over them. A small oxygen atmosphere is present on Ganymede. Ganymede was simply called Jupiter III (three) until mid 20th century, when Simon Marius’s name began to be used.
Callisto is the fourth and last Galilean moon, and is the second largest at 4820.6 kilometers in diameter. Callisto is also the least dense of the Galilean moons. Callisto is one of the most heavily cratered satellites in the solar system, and has one major feature, a basin around 3000 km wide called Valhalla, which probably dates back to the formation of Callisto’s crust.
This moon’s surface lies above a layer of ice, which is 150 kilometers thick, and a layer of water, which is ten kilometers thick. Callisto’s crust is approximately four billion years old. Callisto has a small atmosphere of carbon dioxide. Callisto too was called Jupiter IV (four) until mid 20th century, when Marius’s name came into operation.
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