The Outer Solar System

1
The Outer Solar System
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The Gas Giants
The outer planets have no solid surface, and are
much bigger. The primary gases are hydrogen and
helium (like the Sun), although Uranus and
Neptune have substantial (15 Earth mass) rock/ice
cores.
3
Jupiter King of the Planets
Mass 0.001 solar (300 earths), Radius 11.2
Earths, density 1.3 x water Distance 5.2AU
Orbital Period 11.8 years Rotation period 9.9
hours.
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Hydrostatic Equilibrium Pressure balance
Hydrostatic equilibrium governs the structure
of all gaseous bodies (planets or stars). The
inside has higher temperature and density because
of the weight of the overlying material.
5
Giant Interiors
Jupiter and Saturn have similar structures. Both
are still collapsing slowly, and the
gravitational energy released makes them shine
more heat out than they get from the Sun. It is
carried out by convection. Helium is also slowly
settling faster than hydrogen.
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Atmospheres of Jupiter and Saturn
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Colorful Clouds
The Great Red Spot is a cyclone the size of the
Earth that has lasted at least 300 years.
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Red Spot Movie
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Banded Structure of Clouds
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Jupiters Magnetosphere Bigger than the Sun
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Making Magnetic Fields
Magnetic field arises when there is a conducting,
convecting medium in a rapidly rotating body.
This makes a dynamo the same mechanism in the
Sun, the Earth and other contexts.
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Auroral Zones
The high energy particles come down the field
lines and hit the atmosphere near the poles,
causing the gases to glow. Just like on the
Earth, this makes an aurora in a ring-like zone.
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Saturn
Mass 95 Earths, Radius 9.4 Earths, density
0.7 x water (floats) Distance 9.5 AU Orbital
Period 29.4 years Rotation period 10.6 hours.
Although it is impossible to think of Saturn
without its rings, they are of no planetary
consequence, and are temporary. All the other
outer planets also have rings systems (but not as
nice).
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The Cassini Mission to Saturn
Already passed Jupiter, will reach Saturn in July
2004. We will learn much more about the planet,
rings, and moons.
The Huygens probe will drop into Titans
atmosphere, hopefully reaching and analysing the
surface.
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Uranus
Mass 14.5 Earths, Radius 4.0 Earths, density
1.3 x water Distance 19.2 AU Orbital Period
84 years Rotation period 17.2 hours.
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The Interior of Uranus (and Neptune)
Uranus and Neptune have relatively thin hydrogen
layers on top they may have just begun to
accrete large amounts of gas when the solar
nebula dissipated, interrupting their formation.
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A Very Tilted Pole
The seasons on Uranus are extreme. Half the
planet shares the fate of the pole in not seeing
the Sun for half the Uranus year (40 years).
Nonetheless, the temperature is fairly uniform
around the planet the gases redistribute the
heat.
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Neptune
Mass 17 Earths, Radius 3.9 Earths, density
1.76 x water Distance 30 AU Orbital Period
163 years Rotation period 16.1 hours.
Voyager showed it is more like Jupiter than
Uranus in appearance. Recently, we have developed
the ability to see its storms from Earth, using
adaptive optics in infrared.
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Clouds and Storms on Neptune
High clouds are made of methane ice crystals. The
heat flow is greater than expected, giving more
storms. The Great Dark spot was an upwelling, but
has already disappeared.
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Pluto
Discovered in 1930 by Clyde Tombaugh. Charon
discovered at USNO in 1978.
Pluto radius 1145km, mass 0.002 Earths,
density 2.1x water Charon radius 600 km,
mass 0.1-0.2 Plutos
Pluto was closer than Neptune in the late 1990s,
but never crosses Neptunes actual path (23
orbital resonance, tilted).
Adaptive Optics image
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Pluto and Charon

Last decade Charons orientation carried it
through a number of eclipses, giving us good
sizes and even maps for the 2 bodies.

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What Pluto probably looks like
NASA is considering a Pluto mission now.
Neptunes moon Triton is likely a good model for
Pluto now (Triton is twice its size) eventually
Plutos atmosphere will freeze out onto its
surface as it recedes from the Sun. Triton has
ice volcanoes erupting methane.