Download Question 2 (9-3 thru 9-4 PPT Questions)

Courtesy of NASA/JPL/Space Science Institute
Chapter 9
9-3 thru 9-4
The Jovian Planets
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.
Figure 9.20: Saturn seasonal
progression
Figure 9.21: Saturn's orbit around Earth
Courtesy of NASA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: R.G. French (Wellesley College), J. Cuzzi
(NASA/Ames), L. Dones (SWRI), J. Lissauer (NASA/Ames)
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.
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
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
carbon-based molecules.
Figure 9.26a: Cassini image of Enceladus
Courtesy of NASA/JPL/Space Science Institute
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.
Figure 9.27a: Titan in color-enhanced UV
Courtesy of NASA/JPL/Space Science Institute
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.
Figure 9.28b: A composite Huygens image shows many different flows into
a river channel.
Courtesy of ESA/NASA/JPL/University of Arizona
Figure 9.28a: Objects on surface of Titan
Courtesy of ESA/NASA/JPL/University of Arizona
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.
Figure 9.29: Saturn's rings
Courtesy of NASA/JPL/Space Science Institute
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 close-orbiting, 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 was plotted on star
charts as early as 1690,
Uranus’s slow orbital motion
caused it to go unnoticed until
Herschel discovered it in 1781.
2. Uranus’s 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’s diameter was made in
1977 during an occultation of a star by the planet.
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’s 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’s 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.)
Question 1 (9-3 thru 9-4 PPT Questions)
Why don’t we see the rings of the planets Jupiter,
Uranus, and Neptune? What might be the reason
behind your answer?
Uranus’s Orientation and Motion
1. Uranus’s equatorial plane is tilted 98° to its plane of revolution.
This results in a retrograde rotation, 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.
Figure 9.35: Tilted axis of Uranus
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’s 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).
Courtesy of NASA/JPL-Caltech
6. Five moons were known
before Voyager; we now
know of 27 moons. All are
low-density, icy worlds.
– The innermost, Miranda
appears as if it were torn apart
by a great collision and then
reassembled.
7. Two of Uranus’s moons are
shepherd moons. Material in
Uranus’ rings is very sparse;
all of it together is less than
the material in Cassini’s
division!
Figure 9.38: Miranda
Question 2 (9-3 thru 9-4 PPT Questions)
Speculate what possibly could have happened to
Miranda.
9-4 Neptune
1. 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).
2. Unlike the nearly featureless Uranus, Neptune
exhibits weather patterns in its atmosphere.
It has parallel bands around it and its Great Dark Spot
is similar in appearance to Jupiter’s Great Red Spot.
Figure DP10.02: Neptune
Courtesy of NASA/JPL-Caltech
3. 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.
4. The wispy white clouds seen on Neptune are
thought to be crystals of methane.
5. 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.
6. Neptune’s magnetic field rotates with a period of
16h7m, which is taken as the planet’s basic
rotation rate.
7. Neptune’s temperature is remarkably uniform at
–216°C and its axis is tilted less than 30° to its
orbit.
8. 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); 11 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.
Figure 9.42: Triton
Courtesy of NASA/JPL-Caltech
Courtesy of NASA/JPL-Caltech
7. Stellar occultations
observed in 1984
revealed that
Neptune has rings.
They are “lumpy,”
perhaps as a result
of undiscovered
moons orbiting with
them.
Figure 9.43: Rings of Neptune
Question 3 (9-3 thru 9-4 PPT Questions)
List some unique features about Neptune’s moons
Triton and Nereid.