Jupiters Moons

In 1608 Hans Lippershey, a Dutch eyeglass maker, attached a lens to each end of a hollow tube, and thus created the first telescope. Galileo Galilei, the Italian astronomer born in Pisa in 1564, made his first telescope in 1609 from Lippershey's model. In 1610 Galileo discovered four of Jupiter's moons. Shown in the lower color insert on page C-4, these moons, Io, Europa, Ganymede, and Callisto, are still called the Galilean satellites, or Galilean moons. These were the first bodies in the solar system that could clearly be demonstrated not to be orbiting Earth, and in fact their discovery marked the beginning of the end of the wildly incorrect theory that all heavenly objects orbit the Earth.

Galileo himself sent a telescope to Johannes Kepler, the prominent German mathematician and astronomer. When Kepler saw the moons for himself he promptly coined the word satellite from a Latin term meaning "hangers-on to a prominent man." Thirteen of Jupiter's moons were known before the Voyager space mission, and since Voyager, teams of scientists, including a team at the University of Hawaii, have been searching for new moons.The Hawaii team discovered 11 new moons in 2001, and more are expected. Thirty-nine moons were known in 2002, and by the beginning of 2004, 62 moons had been identified. The satellites were detected using the world's two largest digital telescope cameras at the Subaru (8.3 meter diameter) and Canada-France-Hawaii (3.6 meter diameter) telescopes atop Mauna Kea in Hawaii.

The Galilean satellites, Io, Europa, Ganymede, and Callisto, make a kind of miniature solar system orbiting Jupiter (see the upper color insert on page C-4). Their densities fall with distance from Jupiter, just as the densities of the planets fall with distance from the Sun. Their density progression is consistent with the moons having formed at the same time as the planet, as are their circular orbits in the plain of Jupiter's equator. Io is volcanically active and covered with silicate minerals and sulfur, while the other three Galilean satellites are ice covered. This portrait of the Galilean satellites was constructed out of separate images of each moon, but they are shown at approximately the same scale to compare their sizes. The four moons are shown in order of increasing distance from Jupiter from left to right.

Unlike the Galilean satellites, based on their composition and shape many of Jupiter's moons are probably captured asteroids (see figure on page 61). Moons that were formed of the material that makes up Jupiter, and at the same time as Jupiter, should have a density consistent with their distance from Jupiter, and they should be round from the heat of formation. Irregular or unusually dense moons are probably captured asteroids, as are moons that orbit in a retrograde sense, opposite to natural moons and the planets' direction around the Sun. All of Jupiter's moons keep one face toward Jupiter at all times as they rotate and revolve, just as the Moon does with Earth.This is known as synchronous rotation and is described in the sidebar "What Are Synchronous Orbits and Synchronous Rotation?" on page 64.

All Planets: Number of Moons v. Planetary Gravity u

70 r


Mercury i


^ Saturn ^ Uranus


J Neptune .Earth

10 15

Gravity (m/sec2)

Jupiter J

All the moons are within the huge magnetosphere of Jupiter, which deflects the solar wind around them.Thus, the surfaces of the moons have no solar gardening (alteration of the surface by solar wind and micrometeorites), though the inner moons are bombarded by ions streaming along Jupiter's own magnetic field.

There is a wonderful connection between coming to understand the orbits of Jupiter's moons and making early measurements of the speed of light. After Galileo discovered his four moons of Jupiter, he challenged others to make careful measurements of their periods. Tables of their movements were developed in 1665 by Giovanni Borelli, the Italian mathematician, and in 1668 by Giovanni Cassini, the Italian astronomer.

The orbital periods of Jupiter's four moons were found to be 1.769 days, 3.551 days, 7.155 days, and 16.689 days. These seemed to be very constant and predictable, just like all other heavenly motions. Based on these figures, it was possible to predict within minutes the times of eclipses and passages (when the planet moves behind and in front of Jupiter) that would occur in future observations. Previous measurements of the moons' movements had been made when Jupiter was in opposition, that is, on the opposite side of the Earth from the Sun, as shown in the figure on page 62. Opposition makes Jupiter brighter and easier to see, since the Sun shines directly upon

The number of moons per planet is partly a function of the size and therefore the gravity field of the planet.

Jupiter from the point of view of the Earth throughout Earth's night. Now knowing when the moons' passages should occur based on their orbital periods, astronomers began to make observations of the moons at times when measurements were more difficult, such as when Jupiter was nearly in conjunction with the Sun. At conjunction, the Sun is nearly between the Earth and Jupiter, and it is only possible to observe Jupiter just after sunset or before daybreak.

The eclipses and passages of Jupiter's moons at conjunction, which had been predicted so precisely when Jupiter was in opposition, were consistently later than their predicted occurrence. All four moons seemed to be late by the same amount of time.The early astronomers

Opposition and conjunction are the two cases when the Earth, the Sun, and the body in question form a line in space.

Opposition and Conjunction

Opposition and Conjunction

measured the discrepancy as 22 minutes, but more recent measurements have shown that the moons' movements are slightly less than 17 minutes late. At the subsequent opposition six months later, all the moons were found to be back on their predicted schedule. Cassini recognized these discrepancies, and, at first, he attributed them to a finite speed of light. He wrote in his notes that the discrepancies were due to light taking a certain amount of time to reach us from the moon, and that it takes 10 or 11 minutes for light to travel across the radius of Earth's orbit. This explanation for the data discrepancies is exactly right, but being a scientist of his time, Cassini could not quite believe his own reasoning and tried to find other explanations.

While mulling over this data in 1675 on a visit to Paris, the Danish astronomer Ole Christensen R0mer also thought of an explanation based on the idea that sight is not instantaneous. If light travels at a finite speed, when an observer sees distant things he or she is really seeing how those distant things were at some time in the past. The farther away the observer from an object, the greater the time delay. Applying this hypothesis to the observations of Jupiter's moons, R0mer realized that when Jupiter was in conjunction with the Sun, light from Jupiter and its moons had to travel an extra distance to equal the diameter of the Earth's orbit. Using this distance of two radii of the Earth's orbit (that is, two astronomical units) and the time of delay, R0mer made the first reasonable calculation of the speed of light (about 1AU / 8.5 minutes = 3.0 x 108 m/sec).

The moons of Jupiter are listed in the table on page 66 in order from the moon orbiting nearest Jupiter (Metis) to the farthest moons now known.The Greeks knew Jupiter originally as the god Zeus, and many of the moons of Jupiter are named for characters from Greek and Roman mythology. Metis orbits in an almost perfectly circular path 80,062 miles (128,100 km) from Jupiter, and the farthest moon now known, S/2003 J23, orbits at 15 million miles (24 million km) from the planet. Some of these moons have been well studied and a lot can be written about them, but others are hardly known, often because they are so small. Good reflectance spectra and good images are difficult to obtain from tiny, distant bodies. When poorly known moons are written about here, their compositions and dimensions should be taken as only rough estimates. Only 22 of Jupiter's moons are larger than 10 miles (15 km) in diameter. The other 41 moons now known are often truly tiny objects, less than three miles (2 km)

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