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Transcript
Chapter 6
The Outer Solar System
© 2010 Pearson Education, Inc.
Are jovian planets all alike?
© 2010 Pearson Education, Inc.
Jovian Planet Composition
• Jupiter and Saturn
– Mostly H and He gas
• Uranus and Neptune
– Mostly hydrogen compounds: water (H2O),
methane (CH4), ammonia (NH3)
– Some H, He, and rock
© 2010 Pearson Education, Inc.
Density Differences
• Uranus and Neptune
are denser than Saturn
because they have less
H/He, proportionately.
© 2010 Pearson Education, Inc.
Density Differences
• Uranus and Neptune
are denser than Saturn
because they have less
H/He, proportionately.
© 2010 Pearson Education, Inc.
Rotation and Shape
• Jovian planets are
not quite spherical
because of their
rapid rotation.
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Interiors of Jovian Planets
• No solid surface
• Layers under high pressure and temperatures
• Cores (~10 Earth masses) made of hydrogen
compounds, metals, and rock
• The layers are different for the different
planets. WHY?
© 2010 Pearson Education, Inc.
What are jovian planets like on
the inside?
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Inside Jupiter
• High pressures inside
Jupiter cause phase of
hydrogen to change
with depth.
• Hydrogen acts like a
metal at great depths
because its electrons
move freely.
© 2010 Pearson Education, Inc.
Inside Jupiter
• Core is thought to be
made of rock, metals,
and hydrogen
compounds.
• Core is about same
size as Earth but 10
times as massive.
© 2010 Pearson Education, Inc.
Comparing Jovian Interiors
• Models suggest cores of jovian planets have similar
composition.
• Lower pressures inside Uranus and Neptune mean no
metallic hydrogen.
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Jupiter’s Internal Heat
• Jupiter radiates
twice as much
energy as it receives
from the Sun.
• Energy probably
comes from slow
contraction of
interior (releasing
potential energy).
© 2010 Pearson Education, Inc.
Internal Heat of Other Planets
• Saturn also radiates twice as much energy
as it receives from the Sun.
• Energy probably comes from differentiation
(helium rain).
• Neptune emits nearly twice as much energy
as it receives, but the source of that energy
remains mysterious.
© 2010 Pearson Education, Inc.
What is the weather like on
jovian planets?
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Jupiter’s Atmosphere
• Hydrogen
compounds in
Jupiter form clouds.
• Different cloud
layers correspond to
freezing points of
different hydrogen
compounds.
© 2010 Pearson Education, Inc.
Jovian Planet Atmospheres
• Other jovian planets
have cloud layers
similar to Jupiter’s.
• Different
compounds make
clouds of different
colors.
© 2010 Pearson Education, Inc.
Jupiter’s
Colors
• Ammonium sulfide clouds (NH4SH) reflect red/brown.
• Ammonia, the highest, coldest layer, reflects white.
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Saturn’s
Colors
• Saturn’s layers are similar, but deeper in and
farther from the Sun (more subdued).
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Methane on Uranus and Neptune
• Methane gas of Neptune and Uranus absorbs red light
but transmits blue light.
• Blue light reflects off methane clouds, making those
planes look blue.
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Weather on Jovian Planets
• All the jovian planets have strong winds and
storms.
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Jupiter’s
Great
Red Spot
• Is a storm twice as wide as Earth
• Has existed for at least three centuries
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Jovian Ring Systems
• All four jovian planets have ring systems.
• Others have smaller, darker ring particles than Saturn.
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Why do the jovian planets have
rings?
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Why do the jovian planets have
rings?
• They formed from dust created in impacts
on moons orbiting those planets.
How do we know?
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How do we know?
• Rings aren’t leftover from planet formation
because the particles are too small to have
survived for so long.
• There must be a continuous replacement of
tiny particles.
• The most likely source is impacts with
jovian moons.
© 2010 Pearson Education, Inc.
Ring Formation
• Jovian planets all have rings because they
possess many small moons close in.
• Impacts on these moons are random.
• Saturn’s incredible rings may be an “accident”
of our time.
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What are Saturn’s rings like?
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What are Saturn’s rings like?
• They are made up of numerous, tiny
individual particles.
• They orbit around Saturn’s equator.
• They are very thin.
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Artist’s Conception of Rings Close-Up
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Spacecraft View of Ring Gaps
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What kinds of moons orbit the
jovian planets?
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Sizes of Moons
• Small moons (< 300 km)
– No geological activity
• Medium-sized moons (300–1500 km)
– Geological activity in past
• Large moons (> 1500 km)
– Ongoing geological activity
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Medium and
Large Moons
• Enough self-gravity to
be spherical
• Have substantial
amounts of ice
• Formed in orbit
around jovian planets
• Circular orbits in same
direction as planet
rotation
© 2010 Pearson Education, Inc.
Small
Moons
• They are captured asteroids or comets, so their
orbits do not follow usual patterns.
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Why are Jupiter’s Galilean
moons so geologically active?
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Io’s Volcanic Activity
• Io is the most volcanically active body in the
solar system, but why?
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Tidal Heating
Io is squished and
stretched as it orbits
Jupiter.
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But why is its
orbit so
elliptical?
Orbital
Resonances
Every 7 days,
these three
moons line up.
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The tugs add up over
time, making all
three orbits elliptical.
Europa’s Ocean: Waterworld?
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Tidal stresses crack Europa’s
surface ice.
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Europa’s interior also warmed by tidal heating.
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Ganymede
• Largest moon in
the solar system
• Clear evidence of
geological activity
• Tidal heating plus
heat from radioactive decay?
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Callisto
• “Classic” cratered
iceball
• No tidal heating,
no orbital
resonances
• But it has a
magnetic field!?
© 2010 Pearson Education, Inc.
What is remarkable about Titan
and other major moons of the
outer solar system?
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Titan’s Atmosphere
• Titan is the only
moon in the solar
system to have a
thick atmosphere.
• It consists mostly of
nitrogen with some
argon, methane, and
ethane.
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Titan’s Surface
• Huygens probe provided first look at Titan’s surface in
early 2005.
• It found liquid methane and “rocks” made of ice.
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Medium Moons of Saturn
• Almost all of them show evidence of past
volcanism and/or tectonics.
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Medium Moons of Saturn
• Ice fountains of
Enceladus suggest it may have
a subsurface ocean.
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Medium Moons of Uranus
• They have varying
amounts of
geological activity.
• Miranda has large
tectonic features and
few craters (possibly
indicating an
episode of tidal
heating in past).
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Neptune’s Moon Triton
• Similar to Pluto, but larger
• Evidence of past geological activity
© 2010 Pearson Education, Inc.
Asteroids, Comets, and
the Impact Threat
© 2010 Pearson Education, Inc.
Asteroid Orbits
• Most asteroids orbit in
the asteroid belt
between Mars and
Jupiter.
• Trojan asteroids
follow Jupiter’s orbit.
• Orbits of near-Earth
asteroids cross
Earth’s orbit.
© 2010 Pearson Education, Inc.
Origin of Asteroid Belt
• Rocky planetesimals
between Mars and
Jupiter did not
accrete into a planet.
• Jupiter’s gravity,
through influence of
orbital resonances,
stirred up asteroid
orbits and prevented
their accretion into a
planet.
© 2010 Pearson Education, Inc.
Asteroid
Facts
• Asteroids are rocky leftovers of planet formation.
• The largest is Ceres, diameter ~1000 kilometers.
• 150,000 in catalogs, and probably over a million with
diameter >1 kilometer.
• Small asteroids are more common than large asteroids.
• All the asteroids in the solar system wouldn’t add up to
even a small terrestrial planet.
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Asteroids are cratered and not round.
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What are comets like?
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Comet Facts
• Formed beyond the frost line, comets are
icy counterparts to asteroids.
• Nucleus of comet is a “dirty snowball.”
• Most comets do not have tails.
• Most comets remain perpetually frozen in
the outer solar system.
• Only comets that enter the inner solar
system grow tails.
© 2010 Pearson Education, Inc.
How did they get there?
• Kuiper belt comets formed in the Kuiper belt: flat
plane, aligned with the plane of planetary orbits,
orbiting in the same direction as the planets
• Oort cloud comets were once closer to the Sun,
but they were kicked out there by gravitational
interactions with jovian planets: spherical
distribution, orbits in any direction
© 2010 Pearson Education, Inc.
Anatomy of a Comet
• A coma is the
atmosphere that
comes from a
comet’s heated
nucleus.
• A plasma tail is gas
escaping from coma,
pushed by the solar
wind.
• A dust tail is pushed
by photons.
© 2010 Pearson Education, Inc.
Only a tiny number of
comets enter the inner
solar system. Most
stay far from the Sun.
Oort cloud:
on random orbits
extending to about
50,000 AU
Kuiper belt:
on orderly orbits
from 30–100 AU in
disk of solar system
© 2010 Pearson Education, Inc.
Have we ever witnessed a major
impact?
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Comet SL9 caused a string of violent impacts on Jupiter in
1994, reminding us that catastrophic collisions still happen.
Tidal forces tore it apart during a previous encounter with
Jupiter.
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Did an impact kill the dinosaurs?
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Mass Extinctions
• Fossil record shows occasional large dips in
the diversity of species: mass extinctions.
• Most recent was 65 million years ago,
ending the reign of the dinosaurs.
© 2010 Pearson Education, Inc.
Iridium: Evidence of an Impact
• Iridium is very rare in Earth surface rocks
but often found in meteorites.
• Luis and Walter Alvarez found a worldwide
layer containing iridium, laid down 65
million years ago, probably by a meteorite
impact.
• Dinosaur fossils all lie below this layer.
© 2010 Pearson Education, Inc.
Iridium Layer
No dinosaur fossils
in upper rock layers
Thin layer
containing the rare
element iridium
Dinosaur fossils in
lower rock layers
© 2010 Pearson Education, Inc.
Consequences of an Impact
• Meteorite 10 kilometers in size would send
large amounts of debris into atmosphere.
• Debris would reduce sunlight reaching
Earth’s surface.
• Resulting climate change may have caused
mass extinction.
© 2010 Pearson Education, Inc.
Likely Impact Site
• Geologists have
found a large
subsurface crater
about 65 million
years old in Mexico.
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Meteor Crater, Arizona: 50,000 years ago (50-meter object)
© 2010 Pearson Education, Inc.
Frequency of Impacts
• Small impacts
happen almost daily.
• Impacts large
enough to cause
mass extinctions
happen many
millions of years
apart.
© 2010 Pearson Education, Inc.
Facts about Impacts
• Asteroids and comets have hit Earth.
• A major impact is only a matter of time: not IF but
WHEN.
• Major impacts are very rare.
• Extinction level events happen millions of years
apart.
• Major damage happen tens to hundreds of years
apart.
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Is the impact threat a real danger
or media hype?
© 2010 Pearson Education, Inc.
The asteroid with our name on it
• We haven’t seen it yet.
• Deflection is more probable with years of
advance warning.
• Control is critical: Breaking a big asteroid
into a bunch of little asteroids is unlikely to
help.
• We get less advance warning of a killer
comet….
© 2010 Pearson Education, Inc.
What have we learned?
• Are jovian planets all alike?
– Jupiter and Saturn are mostly H and He gas.
– Uranus and Neptune are mostly H compounds.
• What are jovian planets like on the inside?
– Layered interiors with very high pressure and cores
made of rock, metals, and hydrogen compounds
– Very high pressure in Jupiter and Saturn can produce
metallic hydrogen.
© 2010 Pearson Education, Inc.
What have we learned?
• What is the weather like on jovian planets?
– Multiple cloud layers determine colors of
jovian planets.
– All have strong storms and winds.
• Do jovian planets have magnetospheres
like Earth’s?
– All have substantial magnetospheres.
– Jupiter’s is the largest by far.
© 2010 Pearson Education, Inc.
What have we learned?
• What kinds of moons orbit the jovian planets?
– Moons come in many sizes.
– The level of geological activity depends on a moon’s
size.
• Why are Jupiter’s Galilean moons so
geologically active?
– Tidal heating drives geological activity, leading to
Io’s volcanoes and ice geology on other moons.
© 2010 Pearson Education, Inc.
What have we learned?
• What is special about Titan and other major
moons of the solar system?
– Titan is only moon with thick atmosphere.
– Many other major moons show signs of geological
activity.
• Why are small icy moons more geologically
active than small rocky planets?
– Ice melts and deforms at lower temperatures, enabling
tidal heating to drive activity.
© 2010 Pearson Education, Inc.
What have we learned?
• What are Saturn’s rings like?
– They are made up of countless individual ice
particles.
– They are extremely thin with many gaps.
• How do other jovian ring systems compare to
Saturn’s?
– The other jovian planets have much fainter ring
systems with smaller, darker, less numerous
particles.
• Why do the jovian planets have rings?
– Ring particles are probably debris from moons.
© 2010 Pearson Education, Inc.
What have we learned?
• Have we ever witnessed a major impact?
– The most recent major impact happened in
1994, when fragments of comet SL9 hit
Jupiter.
• Did an impact kill the dinosaurs?
– Iridium layer just above dinosaur fossils
suggests that an impact caused mass
extinction 65 million years ago.
– A large crater of that age has been found in
Mexico.
© 2010 Pearson Education, Inc.
What have we learned?
• Is the impact threat a real danger or media
hype?
– Large impacts do happen, but they are rare.
– They cause major extinctions about every 100
million years.
• How do the jovian planets affect impact
rates and life on Earth?
– Jovian planets sometimes deflect comets
toward Earth but send many more out to Oort
cloud.
© 2010 Pearson Education, Inc.