Download Lecture #27: Saturn The Main Point

Lecture #27: Saturn
The Main Point
• Saturn:
– General properties.
Saturn is a large Jovian-class planet with a
composition very much like the Sun, and is
encircled by a beautiful and elaborate
system of rings and other small satellites.
– Atmosphere.
Atmosphere
– Interior.
– Origin and evolution.
• Reading: Chapters 7.1 (Saturn) and 11.1.
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The Jovian Planets
Basic Properties of Saturn
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Average Distance from Sun: 1.43 billion km (a=9.54 AU).
Orbital period: 29.46 years; eccentricity: 0.056.
Period of Spin around axis: ~10 hours (varies with lat.)
Tilt of Saturn's spin axis: 26.7° (seasons and tilted rings).
Mass: 5.7x10
5 7x1026 kg = 94 ME ; Radius: 60,270
60 270 km = 9
9.4
4 RE
Density = 0.7 g/cm3 (mostly H, He, small "rocky" core?)
No "surface" like the terrestrial planets: only clouds visible.
Gravity = 10.5 m/sec2 (1.07 times Earth's).
Cloud-top temperature: 95 K (-178°C).
Atmosphere: Mostly H, He (plus CH4, NH3, H2O, NH4HS, ...)
Moons: 60+ known as of ’08 (many discovered since 2000!)
Rings! Billions of icy moonlets all orbiting together.
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Observations
Space Missions
• Saturn is the most distant planet that can be seen
without a telescope (if you know where to look...)
• Through a small telescope, Saturn's yellow clouds
and spectacular rings can be seen
seen.
• Features in Saturn's atmosphere are much more
subtle than in Jupiter's.
• 3 robotic spacecraft have flown past Saturn.
• Cassini started orbiting Saturn in 2004.
Mission
Pioneer 11
Voyager 1
Voyager 2
Dates
1979
1980
1981
Goals and Results
First Saturn flyby; studied magnetic field, clouds.
Flyby; high resolution imaging and other studies.
Flyby; high resolution imaging and other studies.
Cassini 1997-2008 Orbiter; imaging, spectroscopy, magnetic fields,
satellite radar mapping;
Huygens entry probe landed on Titan 1/14/05.
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Atmospheric Circulation
Features in Saturn's Atmosphere
• Saturn has stronger
equatorial winds than Jupiter.
• Wind patterns & "shear"
different than Jupiter's.
• Zones, belts, and storms like
Jupiter.
• But smaller, lower contrasts,
and shorter lifetimes.
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max > 900 mph!
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Atmospheric Structure
Atmospheric Composition
• Determined by spectroscopy from
telescopes and spacecraft.
• Similar to Jupiter, but less H2O, more C–H.
Name
Hydrogen
Helium
Methane
Ammonia
Deuterated Hydrogen
Deuterated Methane
Ethane
Phosphine
Acetylene
Propane
Symbol
H2
He
CH4
NH3
HD
CH3D
C2H6
PH3
C2H2
C3H8
% Volume
94
6
8 x 10-4
2 x 10-4
5 x 10-5
2 x 10-5
5 x 10-6
1 x 10-6
2 x 10-8
1 x 10-10
}
Thicker haze
layer may explain
lower contrast of
cloud features on
Saturn.
The composition
of Saturn is also
very close to the
composition of the
Sun
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Minor trace gases
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H2O?
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T (K)
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Saturn's Interior
Saturn's Interior
• Hydrogen and helium at immense pressures
• Estimated using same methods as Jupiter...
• Shape of Saturn particularly revealing:
and temperatures.
– Saturn is very oblate (a flattened sphere).
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Like for Jupiter, Saturn's
interior structure is
inferred from models &
extrapolation of data from
upper cloud layers.
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Saturn's Magnetic Field
Saturn: Internal vs. External Heat Sources
• Examine the energy balance (heat in vs. heat out).
• For Saturn: Outgoing ≈ 1.7 × Incoming.
• The ratio of outgoing to incoming is about the same as it is for
Jupiter. But because Saturn only receives about 25% as much
sunlight as Jupiter, the heat source is unlikely to be related to
the solar input.
• Saturn must have a strong internal heat source:
– Saturn too small to generate much heat from planetary contraction.
– Primordial and radioactive heat not sufficient.
– Heat released from interior phase changes! Helium condenses at high
altitudes and rains down (differentiation). Atmosphere depleted in He.
• Internal heat has a strong influence on atmospheric circulation.
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Satellites
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• Saturn's metallic hydrogen interior and fast rotation
generate substantial magnetic field.
• Saturn has less metallic H
than Jupiter so the field is
weaker.
• But Saturn's field is still
about 1000x Earth's field.
• Saturn's field is aligned
with its spin axis, which
supports the idea that the
field is generated in the
deep interior.
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Rings
Huygens discovered Saturn's large satellite
Titan (2575 km radius) in 1655.
– Titan has a thick and complex atmosphere!
17 other smaller rocky/icy moons found
between 1671 and 1990.
Many more tiny outer moons discovered since
2000. More details in Lecture 30...
• Saturn's rings are made
of millions of housesized chunks of "dirty
ice", all sharing similar
orbits.
• Origin? Unknown!
More detail in Lecture 31...
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Summary
• Saturn is a gas giant planet composed almost
entirely of H and He in solar abundances.
• Saturn's atmosphere has belts, zones, & storms, but
smaller
ll andd att lower
l
contrast
t t than
th Jupiter's.
J it '
• Saturn's interior consists of liquid and metallic
hydrogen, at enormous temperatures and pressures.
• Saturn has ~50 known satellites, including one with
a thick, organic-rich atmosphere (Titan).
• Saturn has a complex and immense ring system.
Power for the instruments
on Cassini comes from a
small nuclear reactor
(~ 72 lbs of 238Pu)
The Cassini
Mission to
Saturn
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Next Lecture...
• Uranus:
– General properties.
– Atmosphere.
– Interior.
– Origin and evolution.
• Reading: 7.1 (Uranus), 11.1.
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