The Outer Jovian Planets
Jupiter
 A. Jupiter is named after the Roman king of the gods. It is more than twice
as massive as all the other planets in the solar system combined.
B. Jupiter is a giant ball of hydrogen-based gasses, whose density is only
slightly greater than water. Its surface features many alternating bands of
clouds that spin in opposite directions. White bands are known as zones, dark
bands are known as belts. They are caused by material rising from, and
failing to, the interior of the planet. The risings and failings are due to
convection currents within the planet. The Coriolis effect on the surface
causes the winds to spin in opposite directions. Organic chemicals cause the
color of the belts.
C. The Great Red Spot on Jupiter is caused by winds spinning in opposite
directions on the surface - essentially a permanent hurricane.
D. Jupiter has no surface per se. As one descends through the clouds, they
become increasingly dense. At a certain point, they are so dense, they are
essentially liquid.
E. Jupiter gives off twice as much energy as it receives from the sun. This
energy is probably residual heat from the formation of the planet.
F. Jupiter has 16 named moons. The four largest can be seen easily from Earth,
and are known as the Galilean satellites.
- Io is particularly interesting, as it is extremely volcanically active.
The enormous gravitational field of Jupiter pulls and distorts Io to such an
extent that internal friction heats up the moon. The internal heat manifests
itself in volcanic activity, which produces sulfur, making the surface of the
moon look reddish-orange.
- Europa, the next moon out, contains an icy crust covering a mantle of
liquid water. Some planetary scientists think that Europa might be able to
harbor life. Its surface has recently been found to be covered with a great deal
of Epsom salts.
- Ganymede, the next moon, is the largest moon in the solar system,
and is in fact larger than the planet Mercury.
- Callisto, the last Galilean moon, is similar in composition to
Ganymede, but not quite as large. The moons of Jupiter formed in such a way as
to mimic the formation of the solar system. The small rocky moons formed towards
the interior of the planetary nebula and the larger, lighter moons formed
further out.
G. Jupiter, as well as all of the Jovian planets, has rings. However, its
rings are not as large or as visible as those of Saturn. The rings were not
discovered until spacecraft were able to image the planet from behind so as to
see the rings in relief.
H. Jupiter exhibits differential rotation, in which the equator of the
planet spins faster than the north and south poles. Only a planet made
essentially out of gas could do this. All of the Jovian planets exhibit this
feature.
SATURN
 A. Saturn is named after the Roman god of the harvest. It is known for its
complex ring system, as well as for its moon, Titan. Saturn is almost as large
as Jupiter, but has less than one-third the mass. In a large enough ocean,
Saturn would float, as it has an average density of less than one.
B. Saturn lacks the striking banded cloud patterns of Jupiter. The reason is
that it is just too cold. Saturn is nearly twice as far away from the sun as
Jupiter. The exterior of Saturn is made up of frozen ammonia clouds. The
interior is otherwise similar to Jupiter's.
C. Saturn has a large and complex system of rings. They may
be from several different sources. First, the rings lie at about 2.44 the
planetary radii of Saturn. At that distance, it as been calculated that any moon
made of similar substance to the planet will be pulled apart by the
gravitational attraction of the planet. This distance is known as the Roche
Limit. Any satellite within this limit will be pulled apart. Additionally, the
planet may have captured satellites, and broken them apart. There is a large gap
in the rings of Saturn called the Cassini Division. The gaps in the rings are
most probably due to shepherding satellites that orbit in resonance with the
rings. The rings are made of myriad individual particles of rock and ice. They
are 50,000 miles across and 200 yards deep.
D. Saturn has 22 moons, the most interesting of which is Titan. Titan is one
of two moons in the solar system with its own atmosphere. Some planetary
scientists believe that Titan is one of the most likely places in the solar
system for life to be found.
URANUS
 A. Uranus is named after the Roman god of' the sky. Uranus was discovered in
1781 by Sir William Herschel by careful observation of the heavens. He saw an
object in his telescope which was disk-shaped and too large to be a star. After
several months it became apparent to astronomers that this was a planet because
it moved against the background stars but didn't grow a tail like a comet did.
B. Uranus is a relatively featureless, pale blue planet. It lacks the cloud
bands that are distinctive on the surface of Saturn and Jupiter. Part of the
reason for this is it has a rotation about its axis that is almost in the plane
of its orbit. In other words, whereas most planets, including Earth, spin almost
directly up and down with respect to their orbital path, Uranus has been pushed
over on its side and is spinning with its north or south pole occasionally
pointing directly at the sun. This rotational pattern keeps Uranus from having a
dynamic weather pattern. Half of the planet has almost constant heating for half
the year, whereas most planets have the entire planet covered with partial
heating over very short periods of rotation. This odd rotation was probably
caused by the impact of a large object early in the formation of the planet.
C. Most of the moons of Uranus are named for characters in Shakespearean
plays. There are moons named after Juliet and Puck, as well as
other famous characters. Two of the moons are named after characters in Pope's
Rape of the Lock, Umbrial and Belinda.
NEPTUNE
 A. Neptune is named after the Roman god of the sea. The two largest moons of
Neptune are Triton and Nereid. Triton is the Roman name for
mermen, and Nereid the name for mermaids, thus keeping with the oceanic theme.
Neptune was discovered simultaneously by English and French astronomers.
B. Neptune is approximately the same size and composition of Uranus, but has
a radically different atmosphere. Because it spins vertically on its axis,
rather than on its side, it has a more dynamic surface, with the fastest winds
in the solar system at well over a thousand miles per hour.
C. Neptune was once seen to have a large blue spot, discovered by the
Viking Space Probe, much like the Great Red Spot on Jupiter. However, recent
pictures from the Hubble Space Telescope show that this spot has disappeared.
D. Neptune's moon, Triton, joins Saturn's moon, Titan, as the only
other moon in the solar system with its own atmosphere. It is the largest of
Neptune's many moons.
PLUTO
 A. Pluto, named after the Roman god of the underworld, is not a Jovian
Planet. American astronomer Clyde Tombaugh discovered the planet in 1930.
Pluto's large moon is named after Charon, the ferryman over Styx, the
river of the dead. Charon is very large compared to Pluto, and in fact it
could be said that Pluto and Charon orbit one another, rather than Charon
orbiting Pluto. Charon was not discovered until 1977.
B. Not much is known about Pluto because of its small size and vast distance
from the sun. It is presumed to be made of mostly frozen gasses and water,
similar to most comets, and is thus probably a remnant left over from the
formation of the solar system. In addition, because Pluto is very cold, the
water on it would have a consistency similar to that of steel on Earth.
C. Pluto has a very elliptical orbit, which is inclined to the plane of
orbit of the other planets by approximately 17 degrees. This trajectory takes it
well away from the path of the other planets. Because of its highly elliptical
orbit, Pluto is occasionally closer to the sun than Neptune.
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