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CHAPTER 9
The Jovian Planets
CHAPTER OUTLINE
9-1 Jupiter
1. Copernicus deduced that Jupiter was larger than Venus, using the two planets’ relative distances and brightnesses.
2. Galileo observed the angular sizes of Venus and Jupiter and using their relative distances determined that Jupiter is larger.
Jupiter as Seen from Earth
1. Jupiter is 318 times more massive than the Earth. It has more than twice the mass of all the
other planets, their moons, and the asteroids.
2. Jupiter’s diameter is 11 times that of Earth, and thus its volume is 1,400 times Earth’s.
3. Jupiter’s density is 1.3 g/cm3, only 1/4 of Earth’s. This low density means Jupiter is composed
of a higher percentage of light elements.
4. Jupiter is 5.2 AU from the Sun and takes about 12 years to complete one orbit around the Sun.
5. Jovian planets have much greater rotation rates than do terrestrial planets. Jupiter spins on its
axis very quickly, once every 9h55m.
6. On Jupiter cloud bands near the equator rotate slightly faster (9h50m) than bands near the poles
(9h56m). We say that Jupiter exhibits differential rotation—the rotation of an object in which
different parts have different periods of rotation.
7. As a result of its fast rotation and low density, Jupiter is very oblate. Jupiter’s equatorial diameter is 6.5% greater than its polar diameter.
Jupiter as Seen from Space
1. In late 1973 Pioneer 10 flew within 130,000 km of Jupiter. A year later Pioneer 11 came within 50,000 km of the planet’s surface. The two craft sent back great amounts of data.
2. Improved images of Jupiter were returned by the Voyager 1 and 2 spacecraft that flew by Jupiter in 1979. Finally, the spacecraft Galileo orbited Jupiter and its moons during 1995–2003.
3. Unlike terrestrial planets, surface features have little effect on Jupiter’s upper atmosphere, allowing weather patterns to last for long periods.
4. Jupiter’s weather patterns last for long periods because “surface” features have little effect on
Jupiter’s upper atmosphere.
5. Jupiter’s Great Red Spot, first seen in the mid-1600s, has lasted for over 300 years. It is 40,000
km long and 15,000 km across, larger than the 13,000-km diameter Earth.
6. The red spot is a storm system of rising high-pressure gas whose cloud tops are colder and
about 8 km higher than the surrounding regions. It rotates counterclockwise with a period of 6
days.
7. The banded appearance of Jupiter is due to its differential rotation. The standard interpretation
of the bands was based on our experience with Earth’s atmosphere: the light-colored bands mark
the tops of low-P, low-T, high-altitude regions while the dark-colored bands mark the tops of
high-P, high-T, low-altitude regions.
8. Observations from Cassini, en route to Saturn, suggest that this interpretation may be wrong.
Almost without exception, individual storm cells of upward-moving bright-white clouds exist in
the dark-colored bands. This suggests that these bands are the regions of net upward-moving gas
motion (the opposite of the standard interpretation).
9. Data from Voyager showed that at the boundaries of each band, the wind velocities are different and in opposite directions. Data from Galileo support the idea that lighting storms beneath the
upper cloud cover are the energy source for Jupiter’s weather patterns.
The Composition of Jupiter’s Atmosphere
1. The Galileo probe showed that Jupiter is (by number) about 90% hydrogen, 10% helium, with
small amounts of water (H2O), methane (CH4), and ammonia (NH3). This is similar to the composition of the Sun.
2. The Galileo probe found that Jupiter has the same helium content as the Sun’s outer layers but
only 10% of its neon concentration. It is possible there is a helium rain in Jupiter’s atmosphere,
with neon dissolving in it.
3. The concentration of deuterium was found to be similar to that of the Sun but very different
from that of comets or of Earth’s oceans. This minimizes the possible effect of comets on the
composition of Jupiter’s atmosphere.
4. The Galileo probe also found that the concentrations of argon, krypton, and xenon are 2 to 3
times higher than that of the Sun. Such concentrations require very cold temperatures. This suggests that the material in Jupiter’s atmosphere must have originated at a much colder place than
Jupiter occupies today.
5. The colors seen in Jupiter’s upper atmosphere are likely due to chemical reactions induced by
sunlight and/or lightning in its atmosphere. Another possibility is that impurities (such as sulfur
or phosphorus) in the cloud droplets of water, ammonia, and ammonia sulfides result in the colors.
Jupiter’s Interior
1. Jupiter’s gaseous atmosphere is a few thousand miles thick.
2. As one goes deeper in Jupiter’s atmosphere, gaseous hydrogen becomes liquid hydrogen.
3. At around 15,000 km below the clouds, it is theorized that the high pressure and temperature
result in electrons moving easily from one atom to another, making the hydrogen a good electrical
conductor; because it conducts electricity like a metal, we call it liquid metallic hydrogen. Most
of the planet is made up of this state of matter.
4. Jupiter’s core, if it exists, is very small, contributing only 1% of the planet’s mass.
5. Jupiter’s magnetic field is quite strong— nearly 20,000 times stronger than Earth’s. Jupiter’s
magnetic field is generated by its large mass of liquid metallic hydrogen and its rapid rotation
rate.
6. Jupiter’s magnetic field deflects the solar wind around the planet as well as trapping charged
particles of the wind in belts.
7. Jupiter’s magnetosphere—the volume of space in which the motion of charged particles is
controlled by the magnetic field of the planet rather than by the solar wind—extends 15 million
km from Jupiter and envelopes most of its satellites.
8. Jupiter’s field accelerates the charged particles in its magnetosphere to such high speeds that
their temperature can reach 400 million K (25 times larger than that at the Sun’s core). However,
the density of this plasma is very low for nuclear reactions to take place. The synchrotron radiation emitted by these particles is observed at radio wavelengths.
Energy from Jupiter
1. Jupiter emits more energy (about twice as much) than it receives from the Sun.
2. There is no reason to support the idea that chemical reactions or radioactivity within Jupiter
can be the source of this excess energy.
3. Jupiter would have to be 80 times more massive to support nuclear fusion; thus it cannot act
like a miniature star.
4. Jupiter may still be shrinking and producing heat in the process but this is not enough to explain the observations.
5. It is now thought that Jupiter’s excess energy is left over from its formation; because of its
great size, Jupiter is cooling very slowly.
Jupiter’s Moons
1. Jupiter’s family of 63 moons can be divided into 3 groups:
(a) 4 inner moons orbit very close to Jupiter and are probably fragmented moonlets.
(b) 4 Galilean satellites, orbiting in nearly circular orbits (Io, Europa, Ganymede, Callisto); Europa, the smallest, is 7000 times more massive than the largest of the non-Galilean moons.
(c) the majority of the remaining moons orbit in a different direction from the 4 inner moons,
have eccentric orbits, dark surfaces, and are probably captured asteroids
2. Io, the Galilean moon closest to Jupiter, has active “volcanoes” (geysers). Voyager images
suggested that Io’s lava flows were mostly molten sulfur but Galileo observed flows at 1800 K
(much higher than sulfur’s vaporization temperature of 700 K). Thus, lava flows probably consist
of rock formed by a large amount of melting of Io’s mantle.
3. Io’s energy is produced by tidal forces caused by its eccentric orbit around Jupiter.
4. Io is surrounded by a halo of sodium atoms. Other elements observed on Io: sulfur, oxygen,
potassium, and chlorine.
5. Io’s density is about 3.5 g/cm3; this indicates that Io is composed mostly of rock.
6. Europa’s surface is ice; its moderate density indicates a rocky world covered by an ocean of
frozen water.
7. Galileo data suggest that Europa has a magnetic field that reverses every 5.5 hours. It is possible that under the ice there is a layer of conductive liquid, such as liquid salt water.
8. Europa also experiences some tidal heating.
9. Ganymede—larger than Mercury —is the largest moon in the solar system.
10. Ganymede exhibits a less active, darker surface than Io or Europa.
11. Galileo data suggest that Ganymede has a small iron or iron/sulfur core, surrounded by a
rocky mantle and a shell of ice at the surface. They also suggest that Ganymede generates its own
field, most likely due to a thick layer of liquid, salty water under its crust.
12. Callisto, the outermost Galilean moon, shows more cratering, has the least active surface, and
experiences little tidal heating.
13. Callisto has the largest known impact crater—Valhalla— in the solar system. No signs of the
impact exist in the region opposite to this crater, suggesting that a liquid layer exists under Callisto’s crust acting as a shock absorber. This idea is supported by Galileo magnetometer data.
14. Callisto seems to have a relatively uniform mixture of ice (40%) and rock (60%), with the
percentage of rock increasing toward the center.
Summary: The Galilean Moons
1. The farther a Galilean moon is from Jupiter, the less active its surface, the lower its average
density and the greater the proportion of water.
2. The Galilean moons formed slowly, over 100,000 to 1 million years, in a disk where the temperature remained low enough for ice to exist naturally.
Jupiter’s Rings
1. Voyager I discovered thin rings around Jupiter. The rings are made of very tiny particles.
2. The rings are close to Jupiter, extending to only about 0.8 planetary radius from Jupiter’s surface.
3. The rings are thought to be replenished from the small moonlets within or near it.
9-2 Saturn
Size, Mass, and Density
1. Except for its obvious rings, Saturn is similar to Jupiter.
2. Saturn’s density (0.7 that of water) is half that of Jupiter, due to a less dense core and a lower
percentage of liquid metallic hydrogen.
3. The composition of Saturn’s atmosphere is similar to Jupiter’s: 96% hydrogen, 3% helium, 1%
of heavier metals.
Saturn’s Motions
1. Saturn orbits the Sun at an average distance of 9.6 AU; its distance from the Earth varies from
8.5 AU to 10.5 AU.
2. Saturn has an orbital period of 29.5 years.
3. Saturn is tilted 27° with respect to its orbital plane, so over time its rings appear in different
orientations when viewed from Earth.
4. Like Jupiter, Saturn shows differential rotation. Its equatorial rotation rate is 10h39m.
5. Saturn is even more oblate than Jupiter, with its equatorial diameter 10% greater than its polar
diameter.
Pioneer, Voyager, and Cassini
1. Pioneer 11 passed Saturn in 1979, followed by Voyager 1 in 1980 and Voyager 2 in 1981.
Knowledge gained from these probes was used to guide scientists in decisions concerning probes
that followed.
2. Saturn’s magnetic field is only 5% as strong as Jupiter’s because Saturn’s liquid metallic hydrogen only extends about half way to its cloud tops.
3. As for the case of Jupiter, Saturn’s interior structure is inferred from models and extrapolation
of data from the outer layers.
4. Saturn’s clouds are less colorful than Jupiter’s because the colder temperatures at Saturn’s distance from the Sun inhibit chemical reactions that give Jupiter’s atmosphere its varied colors, and
a layer of methane haze above the cloud tops on Saturn blurs out color differences.
5. Saturn has atmospheric features similar to Jupiter’s, but Saturn’s winds reach speeds 3 to 4
times faster.
6. On December 25, 2004 the Cassini spacecraft dropped it Huygens probe into the atmosphere of
Saturn’s largest moon, Titan.
Saturn’s Excess Energy
1. Saturn radiates more energy than it absorbs. It also has less helium in its upper atmosphere than
Jupiter has, by a factor of two (by mass).
2. The leading hypothesis in explaining both observations is that the cooling of Saturn’s atmosphere causes helium to condense to liquid for and rain downward. As the helium droplets fall,
they lose gravitational energy, which is converted to thermal energy.
Enceladus and Titan
1. Saturn has 47 moons, most of which consist of dirty ice. Major moons include Titan, Mimas,
Enceladus, Dione.
2. Enceladus is covered in water ice and its interior may be liquid today. Active volcanism exists
on this object; Cassini images show plumes of water vapor and ice water particles.
3. The atmosphere of Enceladus also includes carbon dioxide, methane, and other simple carbonbased molecules.
4. Titan may be the most interesting moon in the solar system because it has an atmosphere,
which is composed mostly of nitrogen with a few percent of methane and argon. There are also
traces of water and organic compounds.
5. When sunlight breaks down methane in Titan’s upper atmosphere, organic molecules are
formed; these molecules then slowly drift down to the surface. This raises the question of whether
life might have formed on Titan’s surface.
6. Huygens data show bright highlands, deep channels, and dark lowlands that look like dried lake
or river beds on Titan’s surface. All the existing data suggests that Titan resembles Earth, with
clouds, rain and seas.
7. Titan is the second largest moon (after Ganymede) in the solar system with a diameter of 5,150
km.
8. Titan’s atmosphere is denser and 10 times more massive than Earth’s because its surface temperature of –180°C is low enough to keep gas molecules from escaping.
Planetary Rings
1. Saturn’s rings are very thin, a few tens of meters across.
2. The rings are not solid sheets but are made up of small particles of water ice or rocky particles
coated with ice.
3. Each ring particle revolves around Saturn according to Kepler’s laws.
4. Three distinct ring bands are visible from Earth, and named (outer to inner) A, B, C.
5. The largest division between the rings is known as Cassini’s division. This space is caused
primarily by the gravity of Mimas and the synchronous relationship between the orbital periods of
Mimas and of any particle in the Cassini division.
6. Other features of the rings are explained by the presence of small shepherd moons.
The Origin of Rings
1. The origin of Saturn’s rings is not well understood but is thought to be the result of a closeorbiting, icy moon that was shattered by a collision with a passing asteroid. Another possibility is
that an object from the outer solar system came too close to Saturn and was torn apart by the
planet’s gravity.
2. Tidal forces are greater on a moon in orbit close to a planet than they are on a moon in an orbit
farther out.
3. The Roche limit is the minimum radius at which a satellite (held together by gravitational
forces) may orbit without being broken apart by tidal forces.
4. Saturn’s rings are inside Saturn’s Roche limit, so no moons can form from the particles in the
rings.
5. If all ring particles were to be collected to form a small moon, its mass would be about
1/20,000 the mass of our Moon.
9-3 Uranus
1. Uranus, though barely visible to the naked eye, was unknown to the ancients. It was plotted on
star charts as early as 1690, but Uranus’ slow orbital motion caused it to go unnoticed until Herschel discovered it in 1781.
2. Uranus’ diameter is difficult to determine from Earth because its angular size is very small and
it can’t be seen clearly. The first reliable value for Uranus’ diameter came from a telescope in a
high-altitude balloon.
3. An improved determination of Uranus’ diameter was made in 1977 during an occultation of a
star by the planet. An occultation is the passing of one astronomical object in front of another.
4. Uranus has a diameter of 51,000 km (32,000 mi), 4 times that of Earth.
5. Uranus has a density of 1.27 g/cm3; it might have a very small rocky core or no core at all.
6. Uranus’ atmosphere is similar to that of Jupiter and Saturn: mostly hydrogen and helium with
some methane.
7. Uranus does not have cloud layers, so the methane in its atmosphere, which absorbs red light,
makes the planet appear blue.
8. Occultation data from 1977 showed that Uranus has a system of thin rings that contain very
little material.
9. Uranus’ rings only reflect 5% of the sunlight that hits them so they cannot be seen from Earth.
(Saturn’s rings reflect 80% of incident sunlight.)
 Uranus’ Orientation and Motion
1. Uranus’ equatorial plane is tilted 98° to its plane of revolution. This results in a retrograde rota-
tion, as seen from far above the Sun’s north pole. It also implies extreme seasons since during
each revolution, the planet’s north pole at one time points almost directly to the Sun and at another time faces nearly away from the Sun.
2. Uranus has an orbital period of 84 years.
3. Uranus has a fairly uniform temperature over its surface (about –200°C), indicating that the
atmosphere is continually stirred up.
4. Uranus has cloud bands that rotate differentially—16 hours at the equator and 28 hours at the
poles.
5. Uranus has about the same chemical makeup as Jupiter, about 83% hydrogen and the remainder helium with some methane.
6. Uranus’ magnetic field is comparable to Saturn’s. It probably originates in electric currents
within the planet’s layer of water. The magnetic field’s axis is tilted 59° with respect to its rotation axis. No other planet has such a large angle between the two axes (though Neptune’s at 47° is
close).
7. Five moons were known before Voyager; we now know of 27 moons. All are low-density, icy
worlds. The innermost, Miranda, is perhaps the strangest looking object in the solar system. It
appears as if it were torn apart by a great collision and then reassembled.
8. Two of Uranus’ moons are shepherd moons. Material in Uranus’ rings is very sparse; all of it
together is less than the material in Cassini’s division!
9-4 Neptune
1. Voyager 2 passed by Neptune in 1989 and transmitted 9,000 images back to Earth.
2. Neptune is similar to Uranus, slightly smaller at 49,500 km in diameter. Neptune’s composition
matches that of Uranus. Neptune’s color is also blue (because of methane in its upper atmosphere).
3. Unlike the nearly featureless Uranus, Neptune exhibits weather patterns in its atmosphere. It
has parallel bands around it and its Great Dark Spot (photographed by Voyager 2) is similar in
appearance to Jupiter’s Great Red Spot.
4. Neptune radiates more internal energy than Uranus, although the cause is unknown. This energy drives the weather on Neptune and results in winds that reach speeds of 700 miles/hr.
5. The wispy white clouds seen on Neptune are thought to be crystals of methane.
6. Neptune exhibits the most extreme differential rotation of any of the Jovian planets: 18 hours at
the equator and 12 hours at the poles. However, these differences are confined to the upper few
percent of the atmosphere.
7. Neptune’s magnetic field rotates with a period of 16h7m, which is taken as the planet’s basic
rotation rate.
8. Neptune’s temperature is remarkably uniform at –216°C and its axis is tilted less than 30° to its
orbit.
9. Neptune’s density is greater than Uranus’; this is probably due to a somewhat larger rocky
core.
Neptune’s Moons and Rings
1. Before Voyager Neptune was known to have 2 moons (Triton and Nereid); 13 moons are now
known.
2. Triton, Neptune’s largest moon, is the only major moon to revolve around a planet in a clockwise (retrograde) direction.
3. Nereid has the most eccentric orbit of any moon in the solar system.
4. Triton has a light-colored surface composed of water ice with some nitrogen and methane frost.
Its surface appears young, with active geyser-type volcanoes and very few craters.
5. Triton’s density is about the same as Pluto’s.
6. The leading hypothesis in explaining the properties of both Triton and Nereid is that these
moons were captured by Neptune after the initial formation of the solar system. Triton’s active
volcanism is probably due to internal heating from tidal forces caused by Neptune’s gravity.
7. Stellar occultations observed in 1984 revealed that Neptune has rings. They are “lumpy,” perhaps as a result of undiscovered moons orbiting with them.