Table Of ContentFirst Edition, 2012
ISBN 978-81-323-1034-1
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Table of Contents
Chapter 1 - Moons of Jupiter
Chapter 2 - Europa
Chapter 3 - Callisto
Chapter 4 - Ganymede
Chapter 5 - Io
Chapter- 1
Moons of Jupiter
Jupiter and its four largest moons (montage)
Jupiter has 63 confirmed moons, giving it the largest retinue of moons with "reasonably secure"
orbits of any planet in the Solar System. The most massive of them, the four Galilean moons,
were discovered in 1610 by Galileo Galilei and were the first objects found to orbit a body that
was neither Earth nor the Sun. From the end of the 19th century, dozens of much smaller Jovian
moons have been discovered and have received the names of lovers, conquests, or daughters of
the Roman god Jupiter, or his Greek predecessor, Zeus. The Galilean moons are by far the
largest objects in orbit around Jupiter, with the remaining 59 moons and the rings together
comprising just 0.003 percent of the total orbiting mass.
Eight of Jupiter's moons are regular satellites, with prograde and nearly circular orbits that are
not greatly inclined with respect to Jupiter's equatorial plane. The Galilean satellites are
spheroidal in shape, and so would be considered dwarf planets if they were in direct orbit about
the Sun. The other four regular satellites are much smaller and closer to Jupiter; these serve as
sources of the dust that makes up Jupiter's rings.
Jupiter's other 55 moons are irregular satellites, whose prograde and retrograde orbits are much
farther from Jupiter and have high inclinations and eccentricities. These moons were probably
captured by Jupiter from solar orbits. There are 13 recently discovered irregular satellites that
have not yet been named, plus a 14th whose orbit has not yet been established.
The relative masses of the Jovian moons. Those smaller than Europa are invisible at this scale, and taken
together would only just be visible at 50× magnification.
Characteristics
The moons' physical and orbital characteristics vary widely. The four Galileans are all over
3,100 kilometres (1,900 mi) in diameter; the largest Galilean, Ganymede, is the ninth largest
object in the Solar System, after the Sun and seven of the planets (Ganymede being larger than
Mercury). All other Jovian moons are less than 250 kilometres (160 mi) in diameter, with most
barely exceeding 5 kilometres (3.1 mi). Orbital shapes range from nearly perfectly circular to
highly eccentric and inclined, and many revolve in the direction opposite to Jupiter's spin
(retrograde motion). Orbital periods range from seven hours (taking less time than Jupiter does to
spin around its axis), to some three thousand times more (almost three Earth years).
Origin and evolution
Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of
accreting gas and solid debris analogous to a protoplanetary disk. They may be the remnants of a
score of Galilean-mass satellites that formed early in Jupiter's history.
Simulations suggest that, while the disk had a relatively low mass at any given moment, over
time a substantial fraction (several tens of a percent) of the mass of Jupiter captured from the
Solar nebula was processed through it. However, the disk mass of only 2% that of Jupiter is
required to explain the existing satellites. Thus there may have been several generations of
Galilean-mass satellites in Jupiter's early history. Each generation of moons would have spiraled
into Jupiter, due to drag from the disk, with new moons then forming from the new debris
captured from the Solar nebula. By the time the present (possibly fifth) generation formed, the
disk had thinned out to the point that it no longer greatly interfered with the moons' orbits. The
current Galilean moons were still affected, falling into and being partially protected by an orbital
resonance which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it
would have migrated inward at a faster rate than Europa or Io.
The outer, irregular moons are thought to have originated from passing asteroids while the
protolunar disk was still massive enough to absorb much of their momentum and thus capture
them into orbit. Many broke up by the stresses of capture, or afterward by collisions with other
small bodies, producing the families we see today.
Discovery
The Galilean moons. From left to right, in order of increasing distance from Jupiter: Io, Europa,
Ganymede, Callisto
Jupiter and the Galilean moons through a 10" (25 cm) Meade LX200 telescope
The Galilean moons and their orbits around Jupiter
The first claimed observation of one of Jupiter's moons is that of the Chinese astronomer Gan De
around 364 BC. However, the first certain observations of Jupiter's satellites were those of
Galileo Galilei in 1609. By March 1610, he had sighted the four massive Galilean moons with
his 30x magnification telescope: Ganymede, Callisto, Io, and Europa. No additional satellites
were discovered until E.E. Barnard observed Amalthea in 1892. With the aid of telescopic
photography, further discoveries followed quickly over the course of the twentieth century.
Himalia was discovered in 1904, Elara in 1905, Pasiphaë in 1908, Sinope in 1914, Lysithea and
Carme in 1938, Ananke in 1951, and Leda in 1974. By the time Voyager space probes reached
Jupiter around 1979, 13 moons had been discovered, while Themisto was observed in 1975, but
due to insufficient initial observation data, it was lost until 2000. The Voyager missions
discovered an additional three inner moons in 1979: Metis, Adrastea, and Thebe.
For two decades no additional moons were discovered; but between October 1999 and February
2003, researchers using sensitive ground-based detectors found another 32 moons, most of which
were discovered by a team led by Scott S. Sheppard and David C. Jewitt. These are tiny moons,
in long, eccentric, generally retrograde orbits, and average of 3 km (1.9 mi) in diameter, with the
largest being just 9 km (5.6 mi) across. All of these moons are thought to be captured asteroidal
or perhaps cometary bodies, possibly fragmented into several pieces, but very little is actually
known about them. A number of 14 additional moons were discovered since then, but not yet
confirmed, bringing the total number of observed moons of Jupiter at 63. As of 2008, this is the
most of any planet in the Solar System, but additional undiscovered, tiny moons may exist.
Naming
The Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto) were named by Simon
Marius soon after their discovery in 1610. However, until the 20th century these fell out of favor,
and instead they were referred to in the astronomical literature simply as "Jupiter I", "Jupiter II",
etc., or as "the first satellite of Jupiter", "Jupiter's second satellite", and so on. The names Io,
Europa, Ganymede, and Callisto became popular in the 20th century, while the rest of the
moons, usually numbered in Roman numerals V (5) through XII (12), remained unnamed. By a
popular though unofficial convention, Jupiter V, discovered in 1892, was given the name
Amalthea, first used by the French astronomer Camille Flammarion.
The other moons, in the majority of astronomical literature, were simply labeled by their Roman
numeral (i.e. Jupiter IX) until the 1970s. In 1975, the International Astronomical Union's (IAU)
Task Group for Outer Solar System Nomenclature granted names to satellites V–XIII, and
provided for a formal naming process for future satellites to be discovered. The practice was to
name newly discovered moons of Jupiter after lovers and favorites of the god Jupiter (Zeus), and
since 2004, after their descendants also. All of Jupiter's satellites from XXXIV (Euporie) are
named after daughters of Jupiter or Zeus.
Some asteroids share the same names as moons of Jupiter: 9 Metis, 38 Leda, 52 Europa, 85 Io,
113 Amalthea, 239 Adrastea. Two more asteroids previously shared the names of Jovian moons
until spelling differences were made permanent by the IAU: Ganymede and asteroid 1036
Ganymed; and Callisto and asteroid 204 Kallisto.
Groups
The orbits of Jupiter's irregular satellites, and how they cluster into groups: by semi-major axis (the
horizontal axis in Gm); by orbital inclination (the vertical axis); and orbital eccentricity (the yellow lines).
The relative sizes are indicated by the circles.
Regular satellites
These are split into two groups:
• Inner satellites or Amalthea group: Metis, Adrastea, Amalthea, and Thebe. These orbit
very close to Jupiter; the innermost two orbit in less than a Jovian day. The latter two
are respectively the fifth and seventh largest moons in the Jovian system. Observations
suggest that at least the largest member, Amalthea, did not form on its present orbit,
but farther from the planet, or that it is a captured Solar System body. These moons,
along with a number of as-yet-unseen inner moonlets, replenish and maintain Jupiter's
faint ring system. Metis and Adrastea help to maintain Jupiter's main ring, while
Amalthea and Thebe each maintain their own faint outer rings.
• Main group or Galilean moons: Ganymede, Callisto, Io, and Europa. With radii that are
larger than any of the dwarf planets, they are some of the largest objects in the Solar
System outside the Sun and the eight planets in terms of mass, and Ganymede exceeds
the planet Mercury in diameter. Respectively the first, third, fourth, and sixth largest
natural satellites in the Solar System, they contain almost 99.999% of the total mass in
orbit around Jupiter. Jupiter is almost 5,000 times more massive than the Galilean
moons. The inner moons also participate in a 1:2:4 orbital resonance. Models suggest
that they formed by slow accretion in the low-density Jovian subnebula—a disc of the
gas and dust that existed around Jupiter after its formation—which lasted up to 10
million years in the case of Callisto.
Irregular satellites
Jupiter's outer moons and their highly inclined orbits
The irregular satellites are substantially smaller objects with more distant and eccentric orbits.
They form families with shared similarities in orbit (semi-major axis, inclination, eccentricity)
and composition; it is believed that these are at least partially collisional families that were
created when larger (but still small) parent bodies were shattered by impacts from asteroids
captured by Jupiter's gravitational field. These families bear the names of their largest members.
The identification of satellite families is tentative, but the following are typically listed:
• Prograde satellites:
• Themisto is the innermost irregular moon and not part of a known family.
• The Himalia group is spread over barely 1.4 Gm in semi-major axis, 1.6° in inclination
(27.5 ± 0.8°), and eccentricities between 0.11 and 0.25. It has been suggested that the
group could be a remnant of the break-up of an asteroid from the main asteroid belt.
