Schedule for Remaining Classes
•  M - May 4: Review of basic concepts and
topics for final exam
•  W - May 6: Sample final exam exercise
with answers
•  Monday – May 11: FINAL EXAM
7:30 – 9:30 am in this lecture room
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Habbal Astro 110-01 Ch. 8 Lecture 36
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Final Exam, Monday May 11
•  2-hour final exam to cover all chapters after midterm IV
–  About 100 multiple choice questions from
Chapters 14, 15, 16, 7 and 8
–  Questions from Basic concepts (to be
reviewed in class Monday, May 5) [worth up
to 5 extra points to be added to FINAL
COURSE grade]
•  Each student can propose 5 questions for final
exam, by email to me. I will choose from them
as long as they make sense. You have until May
4 to do that.
•  If anyone can’t make May 11 exam please see
me asap
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Habbal Astro 110-01 Ch. 8 Lecture 36
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Chapter 8
Jovian Planet Systems
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Some historical facts
Galileo was the first to
observe Saturn with a
telescope in 1610; he
noted its odd
appearance but was
confused by it.
“I do not know what to say
in a case so surprising,
so unlooked for, and so
novel” said Galileo
when Saturn’s rings
apparently vanished in
1612
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Galileo's drawing of Saturn, 1610
Galileo's drawing of Saturn, 1616
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Other historical observational attempts
Riccioli, 1648
Galileo, 1610
Gassendi, 1634
Huygens, 1655
Fontana, 1646
Cassini, 1676
It was not until 1659 that Christiaan Huygens correctly
inferred the geometry of the rings
“It is surrounded by a thin flat ring, nowhere touching,
and inclined to the ecliptic” (1656).
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and Huygens was right …
Saturn’s rings backlit by the Sun.
(Credit: Cassini Imaging Team,
SSI, JPL, ESA, NASA)
Saturn's rings and shadow from
the largest moon Titan. NASA
/HST
Early observations of Saturn were complicated by the
fact that Earth passes through the plane of Saturn's
rings every few years as Saturn moves in its orbit.
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The Jovian Planets or Gas
Giants
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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
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Jovian Planet Formation
•  Beyond the frost line, planetesimals
could accumulate ICE.
•  Hydrogen compounds are more
abundant than rock/metal so jovian
planets got bigger and acquired H/He
atmospheres.
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Jovian Planet Formation
•  The jovian cores are very similar:
~ mass of 10 Earths
•  The jovian differences are in the
amount of H/He gas accumulated.
Why did that amount differ?
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Differences in Jovian Planet
Formation
•  TIMING: The planet that forms earliest
captures the most hydrogen and helium
gas. Capture ceases after the first solar
wind blew the leftover gas away.
•  LOCATION: The planet that forms in a
denser part of the nebula forms its core
first.
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The special Saturn
•  Saturn has the lowest density of any planet in our
solar system. In fact its density is so low that it would
float if it was placed in water!
•  Visually, Saturn is flattened at the poles, due to the
quick rotation on its axis.
•  The winds in its atmosphere reach speeds up to 1700
kilometers per hour!
•  Saturn is known for its extensive ring system which is
formed by a thousand individual rings.
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Density Differences
•  Uranus and Neptune
are denser than Saturn
because they have less
H/He, proportionately.
•  But that explanation
doesn’t work for
Jupiter.
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Sizes of Jovian Planets
•  Adding mass to
a jovian planet
compresses the
underlying gas
layers.
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Sizes of Jovian Planets
•  Greater compression
is why Jupiter is not
much larger than Saturn
even though it is three
times more massive.
•  Jovian planets with even
more mass can be
smaller than Jupiter.
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What are jovian planets like
on the inside?
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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
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Inside Jupiter
•  High pressure
inside of Jupiter
causes the phase
of hydrogen to
change with
depth.
•  Hydrogen acts like
a metal (metallic
hydrogen) at great
depths because its
electrons move
freely.
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Inside Jupiter
•  The core is
thought to be
made of rock,
metals, and
hydrogen
compounds.
•  The core is about
the same size as
Earth but 10 times
as massive.
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Comparing Jovian Interiors
•  Models suggest that cores of jovian planets have
similar composition.
•  Lower pressures inside Uranus and Neptune
mean no metallic hydrogen.
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Jupiter’s Magnetosphere
Aurora on Jupiter
•  Jupiter’s strong magnetic field gives it an enormous
magnetosphere.
•  Gases escaping Io feed the donut-shaped Io torus.
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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.
•  Other jovian planets
have similar cloud
layers.
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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 are deeper in
and farther from the Sun — more subdued.
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Methane on Uranus and
Neptune
•  Methane gas on
Neptune and Uranus
absorbs red light but
transmits blue light.
•  Blue light reflects off
methane clouds,
making those planets
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
•  A storm twice as wide as Earth
•  Has existed for at least 3 centuries
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The swirling spots on this photo are
storms on Saturn
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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 (300–1,500 km)
—  Geological activity in past
•  Large moons (> 1,500 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
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Small Moons
•  Far more
numerous than
the medium
and large
moons
•  Not enough
gravity to be
spherical:
“potato-shaped”
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Why are Jupiter’s Galilean
moons geologically active?
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Io’s Volcanic Activity
•  Io is the most volcanically active body in the solar
system
•  Volcanic eruptions continue to change Io’s surface.
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Why? Tidal Heating
Io is squished and
stretched as it orbits
Jupiter.
5/01/2009
But why is its
orbit so
elliptical?
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Orbital
Resonances
The tugs add up over
time, making all
three orbits elliptical.
Every 7 days,
these three
moons line up.
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Europa’s Ocean: Waterworld?
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Tidal Stresses Crack Europa’s
Surface Ice
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Tidal stresses crack Europa’s
surface ice
Tidal flexing
closes crack,
grinds up ice
Tidal flexing
opens crack,
leaving two
ridges
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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
magnetic field !?
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Titan’s Atmosphere
•  Titan is the only
moon in the solar
system which has
a thick
atmosphere.
•  It consists mostly
of nitrogen with
some argon,
methane, and
ethane.
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Titan’s Surface
•  The Huygens probe provided a first look at Titan’s surface
in early 2005.
•  It had liquid methane, “rocks” made of ice.
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Titan’s “Lakes”
•  Radar imaging of Titan’s surface has
revealed dark, smooth regions that may be
lakes of liquid methane.
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Medium Moons of Saturn
•  Almost all show evidence of past volcanism
and/or tectonics.
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Ongoing Activity on Enceladus
•  Fountains of ice
particles and
water vapor from
the surface of
Enceladus
indicate that
geological activity
is ongoing.
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Medium Moons of Uranus
•  Varying amounts
of geological
activity occur.
•  Moon Miranda has
large tectonic
features and few
craters (episode of
tidal heating in
past?).
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Neptune’s Moon Triton
•  Similar to Pluto,
but larger
•  Evidence for past
geological activity
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Why are jovian planet moons
more geologically active than
small rocky planets?
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Rocky Planets vs. Icy Moons
•  Rock melts at higher
temperatures.
•  Only large rocky
planets have enough
heat for activity.
5/01/2009
•  Ice melts at lower
temperatures.
•  Tidal heating can
melt internal ice,
driving activity.
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Summary
•  Different kinds of moons orbit jovian
planets
—  Moons of many sizes
—  Level of geological activity depends on
size
—  Tidal heating drives activity, leading to
Io’s volcanoes and ice geology on other
moons.
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Summary
—  Titan is the only moon with a thick
atmosphere.
—  Many other icy moons show signs of
geological activity.
•  Jovian planet moons are more
geologically active than small rocky
planets
—  Ice melts and deforms at lower
temperatures enabling tidal heating to
drive activity.
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What are Saturn’s rings like?
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Saturn’s rings
•  They are made up of numerous, tiny
individual particles.
•  They orbit over Saturn’s equator.
•  They are very thin.
As seen from Earth
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Spacecraft View of Ring Gaps
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Artist’s Conception of Close-Up
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Gap Moons
•  Some small
moons create
gaps within rings.
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Jovian Ring Systems
•  All four jovian planets have ring systems.
•  Others have smaller, darker ring particles than does
Saturn.
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Origin of rings of jovian planets
•  Rings are not leftover from planet formation
because the particles are too small to have
survived this long.
•  There must be a continuous replacement of
tiny particles.
•  They formed from dust created by impacts on
the numerous small moons orbiting those
planets.
•  Like the ring particles, tiny impacts are
gradually grinding away these small moons
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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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And what about Pluto?
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Pluto’s Orbit
•  Pluto’s orbit is tilted and significantly elliptical.
•  Neptune orbits three times during the time Pluto
orbits twice — resonance prevents a collision.
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Is Pluto a Planet?
•
•
•
•
•
Much smaller than the eight major planets
Not a gas giant like the outer planets
Has an icy composition like a comet
Has a very elliptical, inclined orbit
Pluto has more in common with asteroids and
comets than with the eight major planets.
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Discovering Large Iceballs
•  In summer 2005,
astronomers
discovered Eris,
an iceball even
larger than Pluto.
•  Eris even has a
moon: Dysnomia.
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Other Icy Bodies
•  There are many
icy objects like
Pluto on elliptical,
inclined orbits
beyond Neptune.
•  The largest ones
are comparable in
size to Earth’s
Moon.
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Is Pluto a Planet?
•  In 2006, the International Astronomical
Union decided to call Pluto and objects like it
“dwarf planets.”
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Kuiper Belt Objects
•  These large, icy
objects have
orbits similar to
the smaller
objects in the
Kuiper Belt that
become short
period comets.
5/01/2009
•  So are they very
large comets or
very small
planets?
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What is Pluto like?
•  Pluto is very cold (40 K).
•  Pluto has a thin nitrogen
atmosphere that
refreezes onto the
surface as Pluto’s orbit
takes it farther from the
Sun.
•  Its largest moon Charon
is nearly as large as
Pluto itself (probably
made by a major
impact).
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HST’s view of Pluto and moons
NASA’s New Horizons mission will study Pluto and a
few other Kuiper Belt objects in a planned flyby.
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Summary
•  Jovian planets may lack solid surfaces on
which geology can occur
•  They are interesting and dynamic worlds with
–  rapid winds
–  huge storms
–  Strong magnetic fields
–  Interiors where common materials behave in an
unfamiliar way (for example: metallic hydrogen)
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Summary (cont’d)
•  Many jovian moons are geologically
active because of
–  icy compositions
–  tidal heating
•  Rings we see today are composed of
particles liberated from those moons
quite recently
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Summary (cont’d)
•  Understanding jovian planet systems implies
–  Ice geology
–  Tidal heating
–  Orbital resonances
•  Each new set of circumstances we discover
offers additional opportunities to learn how
our universe works
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