Interiors 10/18/10
Saturn
The of the Gas Giants
Mass: ~ 1/3 of mass of Jupiter
Radius: ~ 16 % smaller than Jupiter
Av. density:
0.69 g/cm3
Would float in
water!
Please insert figure 43.3
Rotates about as fast as Jupiter, but is twice as
oblate No large core of heavy elements
Mostly hydrogen and helium; liquid hydrogen core
Saturn radiates ~ 1.8 times the energy received from the sun
Probably heated by liquid helium droplets falling towards center
Storms on Saturn
Saturn, though it
appears calmer,
has storms as
well
Higher wind
speeds than
Jupiter
Storms are
deeper in its
atmosphere
Uranus and Neptune
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10/18/10
The Discovery of Uranus
In 1781 a new planet
was discovered by W.
Herschel
Originally thought to
be a comet!
Herschel named it
Georgium Sidus
(Georges Star) after
King George III
Name changed to
Uranus to stay
consistent with the
mythological names of
the other planets
A New Method of Discovery
It was noted that
Uranus was not
following its
calculated orbit
Another planet must
be influencing its
orbit
Two scientists
calculated where the
new planet should be
Astronomers looked
at this location, and
found Neptune!
Galileo saw Neptune
while looking at
Jupiters moons, but
didnt realize what it
was
The Atmospheres of
Uranus and Neptune
Interior of Uranus
The atmospheres of both Uranus
and Neptune are rich in
hydrogen and helium
Both have larger amounts of
methane, giving them their blue
color
Methane crystals scatter blue
light, and methane gas absorbs
red light
Please insert figure 44.3
Both planets are very cold
Uranus: 80K
Neptune: 75K
Densities:
Uranus: 1.3 kg/liter
Neptune: 1.6 kg/liter
Their interiors are probably ordinary
water mixed with methane and
ammonia, surrounding a core of rock
and iron-rich material
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Storms
Uranuss Axial Tilt
High winds lead to storms
on Neptune
Neptune has the Great Dark
Spot, which disappeared
recently
Uranus is tipped almost 90
degrees to the ecliptic plane
Possible that a collision early in
its history tipped the axis, and
splashed out material for its
moons
Odd Magnetic Fields
Both Uranus and
Neptune have magnetic
fields
Stronger than Earths
Uranus: 47x
Neptune: 25x
Probably generated by
currents in the liquid
water in their interiors
Tipped in odd
directions!
Not centered on the
center of the planet, as
with Earth, Jupiter and
Saturn
Titan
This inclination means that for
half of Uranus orbit, one
hemisphere is in uninterrupted
daylight, and the other
hemisphere is in darkness!
About the size of Jupiters
moon Ganymede
Rocky core, but also large
amount of ice
Thick atmosphere
resembling early Earths
-> Potential life
supporting place
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Titans Atmosphere
Titans Surface
Explored in detail by the Cassini
spacecraft and the Huygens
probe, which landed on Titans
surface in 2005
Huygens discovered outflow channels,
possibly of liquid methane
Atmosphere consists mostly of
nitrogen, methane and ethane
and
grapefruit
sized rocks
on the
surface.
Surface pressure: 50% greater
than air pressure on Earth
Surface temperature: 94 K (-290 oF)
methane and ethane can condense and lead to
rain of methane ethane
Methane and is gradually converted to
ethane in the Atmosphere
Methane must be constantly replenished, probably
through breakdown of ammonia (NH3).
Neptunes Moon Triton
Uranuss Moon Miranda
Triton is larger than
Pluto and almost as
big as Europa
Orbits backwards
relative to Neptunes
rotation
Miranda appears to have
been shattered by an impact
Likely a captured icy
planetesimal
The leading hypothesis for
Mirandas appearance is
that the complex pattern
arose as the result of strong
tectonic activity that broke
the surface into plates
Has an atmosphere
that freezes out on
the night side of the
planet
Odd surface features
Long cracks or faults riddle
its surface
Huge 20 km cliffs
Rolling hills adjacent to
wrinkled terrain
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Ring Systems
Ringlets
A close examination of
Saturns rings shows
that they are composed
of tiny ringlets
Galileo first saw the rings of Saturn,
but didnt know what they were
Huygens observed that the rings
were detached from Saturn
Maxwell realized that the rings were
not solid, and were made up of small
particles all following Keplers
Laws!
Saturn has three main rings
Outermost ring is the A ring
Middle ring is the B ring
Inner ring is the C, or crepe ring,
and is very dark
The separation between the A and B
ring is called the Cassini division
All of the Gas Giants
Have Ring Systems
These might be caused
by gravitational
influences of very small
moons, creating waves
in the main rings (spiral
density waves)
Larger gaps (Cassini and
Enke divisions, for
example) are caused by
orbital resonances with
some of Saturns moons
Shepherd Satellites
The thin rings
of the gas giants
are maintained
by shepherd
satellites
The
gravitational
pull of these
small moons
keeps ring
particles in line!
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The Origin of Planetary Rings
If a body held together only
by gravity gets too close to a
planet, tidal forces pull it
apart
Planetary Rings
The Roche limit is
where the tidal
forces of the
planet are too
strong for a moon
to survive; this is
where rings are
formed.
This distance is called the
Roche Limit
Solid bodies (chunks of rock
or ice, or even the space
station) are safe, as they are
held together by forces other
than gravity
The fragments of the brokenup satellite go into orbit
around the planet, forming a
ring
Roche Limit stage 1
Consider an orbiting mass of fluid held together by
gravity, here viewed from above the orbital plane. Far
from the Roche limit the mass is practically spherical.
Credit: Wikipedia
Roche Limit stage 2
Closer to the Roche limit the body is deformed by
tidal forces.
Credit: Wikipedia
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Roche Limit stage 3
Within the Roche limit the mass's own gravity can no
longer withstand the tidal forces, and the body
disintegrates.
Roche Limit stage 4
Particles closer to the planet move more quickly than
particles farther away, as represented by the red
arrows.
Credit: Wikipedia
Roche Limit stage 5
Credit: Wikipedia
How does tidal force work?
Differential Forces
The varying orbital speed of the material
eventually causes it to form a ring.
Credit: Wikipedia
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Planetary Rings
All observed ring systems are within this limit.
Conservation of Angular Momentum
|L|= msv d
v
d
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