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Chapter
14
Prepared by:
Jennifer West
Comparative
Department of PhysicsPlanetology
and Astronomy
University of Manitoba
of the Jovian Planets
Saturn, Uranus, Neptune
their satellite systems
&
Dwarf planets: Trans-Neptunians
1
• Voyager 2 flew past each of the outer
planets in the 1970s and 1980s.
• The Galileo spacecraft circled Jupiter dozens
of times in the late 1990s.
• The Cassini/Huygens orbiter and probe
arrived at Saturn in 2004.
• Throughout this discussion, you will find images
and data returned by these robotic explorers.
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Saturn
• The Roman god Saturn was celebrated in
a weeklong Saturnalia at the time of the
winter solstice in late December
• Romans took over that tradition from
Mesopotamia (god Marduk’s festival)
• Early Christians took over the Roman
holiday to celebrate Xmas.
3
Saturn
• Saturn is most famous for its beautiful rings.
• These are easily visible through the telescopes of
modern amateur astronomers.
• Amateurs recently discovered a storm on Saturn
4
Saturn
• Saturn’s orbital radius is almost twice that of Jupiter
(9.5 AU) and Saturn takes almost 30 years to orbit the
Sun.
• Earth is shown
for comaprison
of sizes
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Saturn
• Interestingly, Saturn’s density is
ρ = 0.69 g/cm3,
One often hears that Saturn would float on
water
Actually, that’s not true, the lake would fall toward
and wrap around Saturn..
Low density means it’s made mostly of hydrogen
+ helium gas
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Saturn
• The two Voyager spacecraft flew past
Saturn in 1979.
• The Cassini spacecraft went into orbit around
Saturn in 2004 on an extended exploration of
the planet, its rings, and its moons.
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Saturn the Planet
• Saturn shows only faint belt-zone
circulation.
• However, Voyager, Hubble Space Telescope,
and Cassini images show that belts and
zones are present and that the associated
winds blow up to three times faster than on
Jupiter.
8
Saturn the Planet
• Belts and zones on Saturn are less visible
because they occur deeper in the cold
atmosphere – below a layer of methane
haze (CH4)
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Saturn the Planet
• The low density of Saturn suggests that it
is, like Jupiter, rich in hydrogen and
helium.
• Photos show that Saturn is the most oblate of
the planets.
• That evidence shows that its interior is mostly
liquid with a small core of heavy elements.
10
Saturn the Planet
• As its internal pressure is lower, Saturn
has less liquid metallic hydrogen than
Jupiter.
• Perhaps this is why its magnetic field is 20
times weaker than Jupiter’s.
• Like Jupiter, Saturn radiates more energy
than it receives from the Sun.
• Models predict that it has a very hot interior.
• Scientists suspect that He rains down toward
the center of Saturn in the form of droplets.
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Saturn’s Rings
• There are several important points to note
about the icy rings of Saturn.
#1 Cassini division at 2:1 resonance with moon Mimas
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Saturn’s Rings
• #2. The rings are made up of billions of ice
particles, each in its own orbit around the
planet.
• Particles you observe now can’t be as old as
Saturn. They must be replenished now and
then by impacts on Saturn’s moons or other
processes.
• The same is true of the rings around the other
Jovian planets.
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Saturn’s Rings
• #3. The gravitational effects of small
moons can confine some rings in narrow
strands or keep the edges of rings sharp.
• Moons can also produce waves in the rings
that are visible as tightly wound ringlets.
Daphnis,
8 km moon
Opens the
Encke gap
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Saturn’s Rings
• #4. The ring particles are confined in a very
thin plane (10 m only!!!) spread among small
moons and confined by gravitational
interactions with larger moons.
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Saturn’s Rings & moons
• The rings of Saturn & other Jovian worlds are
created by and controlled by the planet’s moons.
• Without the moons, there would be no rings.
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Saturn’s Family of Moons
• Saturn currently (2012) has 62 known moons.
• They contain mixtures of ice and rock. Many are
small. Many are probably captured objects.
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Saturn’s Family of Moons: Titan
• The largest of Saturn’s moons is Titan.
• It is a bit larger than Mercury.
• Its density suggests that it must contain a
rocky core under a thick mantle of ices.
• Titan is so cold that its gas molecules do not
travel fast enough to escape.
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Saturn’s Family of Moons: Titan
• It has an atmosphere that is about 1.5 times denser than
Earth’s but is composed mostly of N2, with traces of argon
(Ar) and methane (CH4).
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Saturn’s Family of Moons
• Sunlight converts some of the methane
into complex carbon-rich molecules.
• These collect into small particles, filling the
atmosphere with orange smog.
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Titan
• These particles settle
slowly downward to coat
the surface with what has
been described as dark
organic goo, composed
of carbon-rich molecules.
Dunes on Earth (upper)
& Titan (lower)
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Saturn’s Family of Moons: Titan
• Titan’s surface is mainly composed of ices of water
and methane at T= -180°C (93 K).
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Saturn’s Family of Moons
• The Cassini spacecraft dropped the Huygens
probe into Titan’s atmosphere.
• It photographed dark drainage channels – rivers of
liquid methane (CH4)
• It falls as rain
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Saturn’s Family of Moons
• Such methane downpours may be rare, though. No
direct evidence of liquid methane was detected as
the probe descended. But the orbiter found lakes!
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Saturn’s Family of Moons
• Later radar images made by the Cassini
orbiter have detected what appear to be
lakes presumably containing liquid methane:
the first extraterrestrial river discovered
flows into lake Ontario on Titan.
• Infrared images suggest the presence of
methane volcanoes that replenish the methane
in the atmosphere.
• So, Titan must have some internal heat source to
power the activity.
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Saturn’s Family of Moons: Iapetus
Iapetus close-up by
Cassini space probe
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Saturn’s Family of Moons
• Most of the remaining moons of Saturn:
•
•
•
•
Are small and icy,
Have no atmospheres,
Are heavily cratered, and
Have dark, ancient surfaces (are not active)
27
Saturn’s Family of Moons: Pandora (diameter 84km)
• Most of the remaining moons of Saturn:
•
•
•
•
Are small and icy,
Have no atmospheres,
Are heavily cratered, and
Have dark, ancient surfaces.
Pandora
Diameter 84 km
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Saturn’s Family of Moons
• The moon Enceladus, however, shows
signs of recent geological activity
• Some parts of its
surface contain 1000
times fewer craters
than other regions.
It also has
ice volcanoes!
29
Saturn’s Family of Moons: Enceladus
• Infrared observations show that its south
polar region is unusually warm and venting
water and ice geysers, which have been
directly imaged.
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Saturn’s Family of Moons
• Evidently, a reservoir of liquid waters lies only
tens of metres below the surface.
• At some point in its history, the moon must have
been caught in a resonance with another moon
and was warmed by tidal heating.
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Planet Uranus
• Now that you are familiar with the gas
giants in our solar system, you will be able
to appreciate how weird the ice giants –
Uranus and Neptune – are.
Uranus
• Uranus was discovered in 1781
serendipitously (accidentally) by
astronomer William Herschel, a German
expatriate living in England.
• He named it George’s Star – after his patron the
English King George III.
• Continental astronomers – especially the French
– refused to accept a planet named after an
English king. They called it Herschel.
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Planet Uranus
• Years later, the name Uranus was
suggested – the oldest of the Greek gods
(god of the heavens)
Uranus
• Uranus orbits the Sun at an average of 19.2
AU, almost twice as far away as Saturn.
• Once again, we see how greatly spaced the
Jovian planets are compared to the inner
Terrestrial planets, and that the next one is about
1.8-2 times further than its inner neighbour
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Planet Uranus
• Uranus is only a third of the diameter of
Jupiter and only 1/20 as massive.
• Being four times farther from the Sun, its
atmosphere is over 100°C colder than
Jupiter’s. [1 K = 1°C (temp. difference), but
Kelvin scale starts from absolute zero or T = 0 K
= -273°C]
• T (Jupiter) ~ 150 K (110 K w/o internal heat)
• T (Saturn) ~ 70 K
• T (Uranus) ~ 50 K
• T (Neptune) ~ 40 K
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Planet Uranus
• Uranus never grew massive enough to
capture large amounts of gas from the
nebula as Jupiter and Saturn did.
• So, it has much less hydrogen and helium.
• Its internal pressure is enough lower than Jupiter’s
that it should not contain any liquid metallic
hydrogen.
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Planet Uranus
Oblateness and mean density => a small core of heavy
elements and a deep mantle of partly solid water.
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Planet Uranus
• Although referred to as ice, this material
would not be anything like ice on Earth –
given the temperatures and pressures
inside Uranus.
• The internal structure and composition of
Uranus and Neptune are similar
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Planet Uranus
• The mantle also contains rocky material
and dissolved ammonia and methane.
• Circulation in this electrically conducting
mantle may generate the planet’s peculiar
magnetic field, which is highly inclined to its
axis of rotation.
• Above the mantle lies the deep hydrogen and
helium atmosphere.
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Planet Uranus
• Uranus rotates on its side with its equator
inclined 98°to its orbit, meaning that it actually
rotates backwards (backwards and forwards and
sideways) while it appears to “roll” along or across its
orbit.
With an orbital period of 84 years, each of its four
seasons lasts 21 years.
• The winter–summer contrast is extreme.
• During a season when one of its poles is
pointed nearly at the Sun (a solstice), a
inhabitants of Uranus would never see the
Sun rise or set.
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Planet Uranus
• Uranus’s odd rotation may have been
produced when it was struck by a very
large planetesimal late in its formation.
• Alternatively, it could due to tidal interactions
with the other giant planets, as it migrated
outward early in the history of the solar system.
42
Planet Uranus
• Voyager 2 photos show a nearly featureless
ball.
• The atmosphere is mostly hydrogen and helium.
• However, traces of methane absorb red light,
making the atmosphere look
green-blue.
Planet Uranus
• Computer enhancement revealed a few clouds and bands
around
the south pole.
Planet Uranus
• In the decades since Voyager 2 flew past
Uranus, spring has come to the northern
hemisphere of Uranus and autumn to the
southern hemisphere.
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Planet Uranus
• Images made by the Hubble Space
Telescope and modern Earth-based
telescopes reveal changing clouds and
cloud bands in both
hemispheres.
Planet Uranus
• Infrared measurements show that Uranus
is radiating about the same amount of
energy that it receives from the Sun.
• Thus, it has much less heat flowing out of its
interior than Jupiter or Saturn (or Neptune).
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Planet Uranus
• This may account for its limited
atmospheric activity.
• Astronomers are not sure why it differs in this
respect from the other Jovian worlds.
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The Uranian Moons
• The five major moons of Uranus are
smaller than Earth’s Moon and have old,
dark, cratered surfaces.
• A few have deep cracks produced, perhaps,
when the interior froze and expanded.
• In some cases, liquid water “lava” appears to
have erupted and smoothed over some
regions.
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The Uranian Moons: Miranda
Miranda, the innermost moon, has 1/7 of the
diameter of Earth’s moon.
• Its surface is marked
by grooves called ovoids.
It looks like it was broken
apart and reassembled
by a giant collision. But
it could just be ice geology.
The Uranian Moons: Miranda
• The ovoids may have been caused by
internal heat driving convection in the icy
mantle.
• By counting craters on
the ovoids,
astronomers conclude
that the entire surface
is old, and the moon is
no longer active.
The Uranian rings
• The rings of Uranus:
• Are dark and faint,
• Contain little dust,
• Are confined by
shepherd satellites, and
• Must be continuously
resupplied with material
from the moons.
The Uranian rings
• The rings are not easily visible from Earth.
• The first hint that Uranus had rings came from
occultations.
• This is the passage of the planet in front of a star.
53
The Uranian rings
• Most of what astronomers know about the
rings comes from the observations of the
Voyager 2 spacecraft.
• Their composition appears to be water ice
mixed with methane that has been darkened by
exposure to radiation.
• In 2006, astronomers found two new, very faint
rings orbiting far outside the previously known
rings
54
The Uranian rings
• The newly discovered satellite Mab
appears to be the source of particles for the
larger ring.
• The smaller of the new rings is confined
between the orbits of the moons Portia and
Rosalind.
• Note that the International Astronomical Union (IAU)
has declared that the moons of Uranus are to be
named after characters in Shakespeare’s plays.
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