Transcript chapter9

Comparative
Planetology of the
Outer Planets
Astronomy: The Solar System and Beyond
5th edition
Michael Seeds
Comparative
Planetology of the
Outer Planets
There is something
fascinating about science.
One gets such wholesale returns
of conjecture out of such
a trifling investment of fact.
- MARK TWAIN
Life on the Mississippi
Comparative
Planetology of the
Outer Planets
• Mark Twain was poking gentle fun at
science, but he was right.
– The exciting thing about science isn’t the so-called
facts, the observations in which scientists have
greatest confidence.
– Rather, the excitement lies in the understanding that
scientists get by rubbing a few facts together.
Comparative
Planetology of the
Outer Planets
• Science can take you to strange new
worlds.
• The outer planets are certainly strange.
• However, you can combine the available
observations with known principles of
comparative planetology and understand
them.
Comparative
Planetology of the
Outer Planets
• The Jovian worlds can be studied from
Earth, but much of what astronomers
know has been radioed back to Earth from
space probes.
– Pioneer 10 and Pioneer 11 explored Jupiter and
Saturn in the mid-1970s, but their instruments were
not highly sensitive.
– The Voyager 1 and Voyager 2 spacecraft were
launched in 1977 on a mission to visit all four Jovian
worlds, which concluded when Voyager 2 flew past
Neptune in 1989.
Comparative
Planetology of the
Outer Planets
– The Galileo spacecraft orbited Jupiter from 1995 to 2003.
– The Cassini probe to Saturn went into orbit in 2004.
– Throughout this discussion of the Jovian worlds, you will
find images and data returned by these robotic explorers.
Comparative
Planetology of the
Outer Planets
• Today’s astronomers have a tremendous
library of photos, measurements, and
facts about the Jovian planets.
– Your task is to discover the relationships that
explain how these worlds got to be the way they
are.
Comparative
Planetology of the
A Travel Guide to the Outer Planets Outer Planets
• If you travel much, you know that some
cities make you feel at home and some
do not.
• You are about to visit five worlds that are
truly unearthly.
Comparative
Planetology of the
The Outer Planets Outer Planets
• The outermost planets in our solar system
are Jupiter, Saturn, Uranus, Neptune, and
Pluto.
– The first four are often called the ‘Jovian planets,’
meaning that they are like Jupiter.
– However, they are all individuals with their own
separate personalities.
Comparative
Planetology of the
The Outer Planets Outer Planets
• The figure compares the five outer worlds.
• One striking feature is diameter.
Comparative
Planetology of the
The Outer Planets Outer Planets
– Jupiter is the largest, over 11 times Earth’s diameter.
– Saturn is a bit smaller.
– Uranus and Neptune are quite a bit smaller than Jupiter,
but still quite large compared to Earth.
Comparative
Planetology of the
The Outer Planets Outer Planets
–
–
–
–
In contrast, Pluto is hardly visible in the figure.
It is a tiny world and doesn’t seem to fit in with the rest.
You will discover it is, indeed, an oddball.
In fact, some astronomers argue that it is not a planet.
Comparative
Planetology of the
The Outer Planets Outer Planets
• The other feature you will notice
immediately is Saturn’s rings.
– They are bright and beautiful and composed of billions
of ice particles, each orbiting around the planet.
Comparative
Planetology of the
The Outer Planets Outer Planets
– Jupiter, Uranus, and Neptune also have rings.
– However, they are not easily detected from Earth and
are not visible in the figure.
– Nevertheless, as you study these worlds, you will be
able to compare
four different
sets of
planetary rings.
Comparative
Planetology of the
Atmospheres and Interiors Outer Planets
• The four Jovian worlds have hydrogen-rich
atmospheres filled with clouds.
– On Jupiter and Saturn, the clouds form belts and zones
that circle the planets, like the stripes on a child’s ball.
– This is called belt-zone circulation.
– There are traces of
belts and zones on
Uranus and Neptune
too, but they are not
very distinct.
Comparative
Planetology of the
Atmospheres and Interiors Outer Planets
• The atmospheres of the Jovian planets are
not very deep.
– Jupiter’s atmosphere makes up only about one
percent of its radius.
– Below that atmosphere, Jupiter and Saturn are
composed of liquid hydrogen.
– So, the older term for these planets, the gas giants,
would be more accurate as the liquid giants.
– Only near their centers do these worlds have solid
cores of dense material with the composition of rock
and metal.
Comparative
Planetology of the
Atmospheres and Interiors Outer Planets
• Uranus and Neptune are sometimes called
the ice giants, because they are rich in
water in both liquid and solid forms.
– They too have denser material in their central cores.
Comparative
Planetology of the
Atmospheres and Interiors Outer Planets
• The outer planets are low-density worlds
rich in hydrogen.
– Jupiter and Saturn are mostly liquid hydrogen, and
even Uranus and Neptune contain a much larger
proportion of hydrogen than does Earth.
– These worlds are hydrogen-rich, low-density worlds
because they formed in the outer solar nebula where
water vapor could freeze to form tremendous
amounts of tiny ice particles.
– Once these planets grew massive enough, they could
draw in the hydrogen gas directly by gravitational
collapse.
Satellite Systems
Comparative
Planetology of the
Outer Planets
• You can’t really land your spaceship on
the Jovian worlds, but you might be able
to land on one of their moons.
– All the Jovian worlds have large satellite systems.
– Even little Pluto has three moons.
Satellite Systems
Comparative
Planetology of the
Outer Planets
• You will study how each planet’s moons
affect both each other and the planet’s
ring system.
– Moons have gravitational interactions with each
other that affect their orbits.
– They also interact with and shape the rings that
circle their parent world.
Satellite Systems
Comparative
Planetology of the
Outer Planets
• The second thing to notice is that a few
moons are geologically active whereas
others show signs of past activity.
– You have learned that cratered surfaces are old.
– So, when you see a section of a moon’s surface that
has few craters, you can assume that the moon must
have been geologically active since the end of the
heavy bombardment.
– However, geological activity also depends on heat.
– So, you should also consider sources of internal heat.
Building Scientific Arguments
Comparative
Planetology of the
Outer Planets
• Why should you expect the outer planets
to be low-density worlds?
– You need to think about how the planets formed from
the solar nebula.
– You have learned that the inner planets could not
incorporate ice when they formed, because it was too
hot near the sun.
– However, in the outer solar nebula, the growing
planets could accumulate lots of ice.
Building Scientific Arguments
Comparative
Planetology of the
Outer Planets
• Eventually, they grew massive enough to
grow by gravitational collapse.
• That pulled in hydrogen and helium gas.
– This gas makes the outer planets low-density
worlds.
Building Scientific Arguments
Comparative
Planetology of the
Outer Planets
• The outer planets may be unearthly, but
they are understandable.
• For example, extend your argument.
– Why do you expect the outer planets to have rings
and moons?
Jupiter
Comparative
Planetology of the
Outer Planets
• Jupiter is the most massive of all the Jovian
planets, containing 71 percent of all the
planetary matter in our solar system.
– This high mass accentuates some processes that are
less obvious or nearly absent on the other Jovian
worlds.
Jupiter
Comparative
Planetology of the
Outer Planets
• Just as you used Earth as the basis for
comparison in your study of the terrestrial
planets, you can examine Jupiter in detail—to
use as a standard in your comparative study of
the other Jovian planets.
Comparative
Planetology of the
The Interior Outer Planets
• The density of Jupiter is only a third greater
than that of water.
– For comparison, Earth is over 4 times denser than
Jupiter.
– Theoretical models of Jupiter conclude that it is
composed mostly of
hydrogen and helium.
Comparative
Planetology of the
The Interior Outer Planets
• In the interior, the pressure compresses
the hydrogen into a liquid.
• At the center, a ‘rocky core’ contains
heavier elements, such as iron, nickel,
and silicon.
Comparative
Planetology of the
The Interior Outer Planets
• With a temperature four times hotter than
the surface of the sun and a pressure 50
million times Earth’s air pressure at sea
level, this material is unlike any rock on
Earth.
– The term ‘rocky core’ refers to the chemical
composition and not to the consistency of the
material.
Comparative
Planetology of the
The Interior Outer Planets
• Careful measurements of the heat
flowing out of Jupiter reveal that it emits
about twice as much energy as it absorbs
from the sun.
– This energy appears to be heat left over from the
formation of the planet.
– Jupiter probably grew very hot when it formed, and
some of this heat remains trapped in its interior.
Comparative
Planetology of the
The Interior Outer Planets
• Although Jupiter is mostly liquid hydrogen,
there is no ocean surface on which you
might imagine sailing a Jovian boat.
– The base of the atmosphere is so hot and the
pressure is so high that there is no real distinction
between liquid and gas.
– As you fell deeper through the atmosphere, you
would see the gas density increasing around you until
you were sinking through a liquid.
– However, you would never splash into a liquid
surface.
Comparative
Planetology of the
The Interior Outer Planets
• Under high pressure, liquid hydrogen
becomes liquid metallic hydrogen, which
is a very good conductor of electricity.
– Laboratory measurements of the properties of this
material show that the transition from liquid
hydrogen to liquid metallic hydrogen must occur at a
relatively shallow depth in Jupiter—about 10 percent
of the way from the surface to the center.
Comparative
Planetology of the
The Interior Outer Planets
• This mass of conducting liquid, stirred by
convection currents and spun by the
planet’s rapid rotation, drives the
dynamo effect and generates a powerful
magnetic field.
– Jupiter’s field is over 10 times stronger than
Earth’s.
Comparative
Planetology of the
The Interior Outer Planets
• A planet’s magnetic field deflects the solar
wind and dominates a volume of space
around the planet called the
magnetosphere.
– Jupiter’s magnetosphere is 100 times larger than
Earth’s.
– If you could see it in the sky, it would be 6 times larger
than the full moon.
Comparative
Planetology of the
The Interior Outer Planets
• One consequence of Jupiter’s magnetic
field is the formation of auroras.
– As in the case of Earth,
interactions between
Jupiter’s magnetic field
and the solar wind
generate powerful
electric currents.
Comparative
Planetology of the
The Interior Outer Planets
– These currents flow down from the edge of the
magnetosphere into the atmosphere, in two rings around
the planet’s magnetic poles.
– These are visible at ultraviolet wavelengths as rings of
auroras larger in diameter than Earth.
Comparative
Planetology of the
The Interior Outer Planets
• The magnetic field around Jupiter traps
charged particles from the solar wind in
large, doughnut-shaped radiation belts
that surround the planet just as Earth is
surrounded by its own radiation belts.
– As Jupiter’s magnetic field is stronger, its trapped
radiation is a billion times more intense than Earth’s.
– The spacecraft that flew through these regions
received over 4,000 times the lethal radiation dose
for a human.
Comparative
Planetology of the
The Interior Outer Planets
• However unearthly Jupiter may be, it
obeys the same natural laws as Earth.
– So, it shares some of the same features.
– Thus, Jupiter has a magnetic field and auroras too.
• However, some parts of Jupiter—like its
atmosphere—still seem decidedly
unearthly.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• Jupiter is almost entirely a liquid planet.
• However, its outermost layer—hardly more
than a skin—consists of a turbulent layer of
gases and clouds.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• The processes you will find there are
repeated in slightly different ways on the
other Jovian worlds.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• You will notice four important ideas about
Jupiter’s atmosphere.
• One, the atmosphere is hydrogen-rich,
and the clouds are confined to a shallow
layer.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• Two, the cloud layers lie at certain
temperatures within the atmosphere where
ammonia (NH3), ammonium hydrosulfide
(NH4SH), and
water (H2O)
can condense.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• Three, the belt-zone circulation is related
to the high- and low-pressure areas you
find on Earth.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• Finally, the major spots, although they are only
circulating storms, can remain stable for decades
or even centuries.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• Astronomers watched
Jupiter’s atmosphere
in the summer of 1994
as fragments of a
comet slammed into
the planet at high
velocity.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
– Over a period of six days, 20some objects 0.5 km or so in
diameter hit with the energy of
millions of megatons of TNT.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
– Each impact created a fireball of
hot gases and left dark smudges
that remained visible for months
afterward.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• From models of the complex physics of the
impacts, two things are clear.
– First, the violent impacts had little effect on the
long-term circulation patterns in Jupiter’s atmosphere.
– Second, the fragments of the comet were very fragile
and didn’t penetrate down to the cloud layers before
they exploded.
– In the long run, such impacts on Jupiter probably occur
roughly once every century or two.
Comparative
Planetology of the
Jupiter’s Complex Atmosphere Outer Planets
• Major impacts on Earth occur less often
because Earth is smaller.
• However, they are inevitable.
– All the planets in the solar system are occasionally
struck by these objects.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Another interesting feature of Jupiter does
not lie within its cloudy atmosphere but
orbits above its equator.
• Jupiter, like the other three Jovian planets,
has a ring.
– The ring was not discovered until 1979, when the
Voyager 1 spacecraft sent back photos.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Less than 1 percent as bright as Saturn’s
rings, the ghostly ring was a puzzle.
– What is it made of?
– Why is it there?
– A few simple observations helped astronomers
answer these questions.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Saturn’s rings are made of bright ice
chunks.
• However, the particles in Jupiter’s ring are
very dark and reddish.
– You can guess that the ring is rocky rather than icy.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Astronomers conclude that the ring
particles are mostly microscopic.
– Photos show that it is very bright when illuminated
from behind.
– It is scattering light forward.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Efficient forward scattering occurs when
particles have diameters roughly the same
as the wavelength of light—a few
millionths of a meter.
– Large particles do not scatter light forward.
– So, a ring filled with basketball-size particles would
look dark when illuminated from behind.
• Jupiter’s ring is made of particles about
the size of those in cigarette smoke.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• There is a reason these particles are
so close to Jupiter.
– They orbit inside the Roche limit, the distance
from a planet within which a moon’s own
gravity cannot hold it together.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Large moons are held together by their
own gravity.
– Their surfaces comprise chunks of rock and dust.
– Their solid crusts are fractured deeply by impacts.
• As long as they orbit relatively far from
their planets, their gravity is much greater
than the tidal forces caused by the planet.
– Thus, they can hold themselves together.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• However, if one of these large fragmented
moons is pushed inside the Roche limit,
tidal forces will tear the moon apart.
– The International Space Station can orbit inside
Earth’s Roche limit because it is held together by
bolts and welds, not its own gravity.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Similarly, small moons that are not held
together by their own gravity—little moons
that are one solid piece of rock—can
survive inside the Roche limit as long as
they are so small that tidal forces do not
tear them apart.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• If a planet and its moon have similar
densities, the Roche limit is 2.44
planetary radii.
– Jupiter’s rings—and those of Saturn, Uranus,
and Neptune—lie inside this limit.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Now, you can understand the dust near
Jupiter.
– If a dust speck gets knocked loose from a larger rock
inside the Roche limit, the rock’s gravity cannot hold
onto the dust speck.
– The billions of dust specks in the ring can’t pull
themselves together to make a new moon because of
Jupiter’s tidal forces.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• The ring particles are not old.
– The pressure of sunlight and Jupiter’s powerful
magnetic field alter the orbits of the particles. They
gradually spiral into the planet.
– Images show faint ring material extending down
toward the cloud tops. This is evidently dust specks
spiraling into the planet.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
– Dust is also lost from the ring as
electromagnetic effects force it out of the
plane of the ring to form a low-density halo
above and below the ring.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Another reason the ring particles can’t be
old is that the intense radiation around
Jupiter will grind the dust specks down to
nothing in a century or so.
– Thus, the rings you see today can’t be material left
over from the formation of Jupiter.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Due to the constant loss of material, the
rings of Jupiter must be continuously
resupplied with new dust.
– Small moons that orbit near the outer edge of the
rings lose dust particles when they are hit by
micrometeorites.
– Observations made by the Galileo spacecraft show
that the main ring is densest at its outer edge where
the small moon Adrastea orbits, and that another
small moon, Metis, orbits inside the ring.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
– Galileo images reveal much fainter rings
called the gossamer rings, extending twice as
far from the planet as the main ring.
– These gossamer rings are most dense at the
orbits of two small moons, Amalthea and
Thebe, again providing evidence that the dust
is being blasted into space by impacts on the
inner moons.
Comparative
Planetology of the
Jupiter’s Ring Outer Planets
• Besides supplying the rings with particles,
it is possible that the moons help confine
the ring particles and keep them from
spreading outward.
– This is an important process in planetary rings.
– You will explore it in detail when you study Saturn’s
rings.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Jupiter has four large moons and at least
two dozen smaller moons.
• Larger telescopes and modern techniques
are rapidly finding more small moons
orbiting the Jovian planets.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Most of the small moons are probably
captured asteroids.
• In contrast, the four largest moons—called
Galilean moons after their discoverer,
Galileo—are clearly related to each other
and probably formed with Jupiter.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Astronomers know these worlds
surprisingly well because the two Voyager
spacecraft and the Galileo spacecraft have
studied them in detail.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• The outermost Galilean moon, Callisto, is
half as big as Earth’s moon.
• However, it has a low density—of only 1.79
g/cm3.
– So, it must consist roughly of half rock and half ice.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Observations of its gravitational field by
the Galileo spacecraft reveal that it does
have a dense core and a lower-density
exterior.
– So, it presumably differentiated.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Also, the way it interacts with Jupiter’s
magnetic field leads astronomers to
suspect it has a mineral-rich ocean of liquid
water 100 km below its icy crust.
– However, photos of its surface show thousands of
impact craters and little
sign of geological
activity.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Next inward from Callisto is
Ganymede, the largest moon in the
solar system.
– In fact, Ganymede is 8 percent larger than
Mercury.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• With a density of 1.9 g/cm3, Ganymede
must contain a mix of rock and ice.
– The Galileo spacecraft has detected a weak magnetic
field, which suggests a differentiated metallic core.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
– The icy crust is marked by old, cratered dark areas, and
younger, brighter regions of grooved terrain believed to be
systems of faults in the brittle crust.
– Sets of grooves overlap other sets of grooves, which
suggests extended episodes of geological activity.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• The density of the next moon inward,
Europa, is 3 g/cm3, high enough to suggest
that it is mostly rock with a thin icy crust.
– The visible surface is very clean ice, contains very few
craters, and has long scars suggestive of cracks in the
icy crust.
– In other places,
mountainlike
folds cross the
surface.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• The pattern of folds and cracks suggests
that the icy crust breaks as the moon is
flexed by tides.
• This evidence and Europa’s gravitational
influence on the Galileo spacecraft reveal
that a liquid-water ocean perhaps 200 km
deep lies below the 100-km-thick crust.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• One wonders what might swim through
such a lightless ocean.
• The lack of craters tells you that Europa is
an active world where craters are quickly
erased.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Images from spacecraft reveal that Io, the
innermost of the Galilean moons, has over
100 volcanoes active on its surface.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• The active volcanoes throw sulfur-rich gas
and ash high above the surface.
• The ash falls back to bury the surface at a
rate of a few millimeters a year.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• That explains why you see no impact
craters on Io.
– They are covered up as fast as they form.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Io’s density is 3.55 g/cm3, showing
that it is not ice but rather rock and
metal.
– Its gravitational influence on the passing Galileo
spacecraft shows that it has a large metallic core, a
rocky mantle, and a low-density crust.
– Although it is small, it is bursting with energy.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• The activity in the Galilean moons
must be driven by energy flowing
outward.
– Io is too small to have remained hot from its
formation.
– The violently active volcanism of Io is apparently
caused by tidal heating.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Io’s orbit is slightly elliptical.
• As it moves closer to and then farther from
Jupiter, tides caused by the planetary
gravitational field flex the moon, and
friction heats its interior.
• That heat flowing outward causes
volcanism.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• Europa is not as active as Io.
– However, it too must have a heat source, presumably
tidal heating.
• Ganymede is no longer active.
– However, when it was younger, it had internal heat
that broke the
crust to produce
the grooved
terrain.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• In fact, the three moons are linked together
in an orbital resonance.
– Io orbits Jupiter four times whereas Europa orbits twice
and Ganymede orbits once.
– In a resonance, moons exert periodic gravitational tugs
on each other that can make their orbits more stable.
– A small moon can’t drift outward or inward if it is in
resonance with a bigger moon that keeps tugging it
back into place.
Comparative
Planetology of the
Jupiter’s Family of Moons Outer Planets
• The tugs on Io keep its orbit slightly
elliptical, causing the tidal heating that
makes Io active.
– Distant Callisto is not caught in the orbital resonance
and appears never to have been strongly active.
Comparative
Planetology of the
The History of Jupiter Outer Planets
• Can you put all the evidence together and
tell the story of Jupiter?
• Creating such a logical argument of
evidence and hypotheses is the ultimate
goal of planetary astronomy.
Comparative
Planetology of the
The History of Jupiter Outer Planets
• Jupiter formed far enough from the sun to
incorporate large numbers of icy
planetesimals, and it must have grown
rapidly.
– Once it was a dozen times more massive than Earth,
it could grow by gravitational collapse.
– Thus, it grew rich in hydrogen and helium from the
solar nebula.
– Its present composition is quite sunlike and
resembles the composition of the solar nebula.
Comparative
Planetology of the
The History of Jupiter Outer Planets
– The location of
Jupiter’s point
on the graph
shows that its
gravity is strong
enough to hold
onto all its gases.
Comparative
Planetology of the
The History of Jupiter Outer Planets
• Jupiter’s large family of moons may be
mostly captured asteroids, and it may still
encounter a wandering asteroid or comet
now and then.
– Some of these are deflected, some captured into
orbit, and some—like the comet that struck Jupiter in
1994—actually fall into the planet.
– Dust blasted off of the inner moons by
micrometeorites settles into the equatorial plane to
form Jupiter’s rings.
Comparative
Planetology of the
The History of Jupiter Outer Planets
• The four Galilean moons are large and
seem to have formed in a disk of gas and
dust around the forming planet.
– The innermost, Io, is densest.
– The densities of the others decrease as you move
away from Jupiter—similar to the way the densities of
the planets fall as you move away from the sun.
Comparative
Planetology of the
The History of Jupiter Outer Planets
• Perhaps the inner moons incorporated
less ice because they formed closer to the
heat of the growing planet.
• Also, tidal heating has been important, and
the intense heating of the inner moons
could have driven off much of their ices.
– Thus, two processes may be responsible for the
compositions of the Galilean moons.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Why is Jupiter so big?
– You can analyze this question by constructing a
logical argument that relates the formation of Jupiter
to the solar nebula theory.
• Jupiter is rich in hydrogen and helium.
• However, Earth is relatively poor in these
elements.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• While the solar nebula existed, Earth grew
by the accretion of solid, rocky
planetesimals.
– However, it never became massive enough to capture
gas directly from the solar nebula.
– That is, it never grew by gravitational collapse.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• However, Jupiter grew so rapidly from icy
planetesimals in the outer solar nebula
that it was eventually able to grow by
gravitational collapse.
– By the time the solar nebula cleared away and ended
planet building, Jupiter had captured large amounts of
hydrogen and helium and was quite massive.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Often, the present nature of a world can
be traced back to the way it formed.
• Can you create a logical argument to
explain the nature of the Galilean moons?
Comparative
Planetology of the
Saturn Outer Planets
• Saturn has played second fiddle to its own
rings since Galileo first viewed it through a
telescope in 1610.
– The rings are dramatic, strikingly beautiful, and easily
seen through even a small telescope.
– However, Saturn itself, only slightly smaller than
Jupiter, is a fascinating planet.
Comparative
Planetology of the
Saturn Outer Planets
• Although Saturn lies roughly 10 AU from
the sun, astronomers know a surprising
amount about it.
– The beautiful rings are easily visible through the
telescopes of modern amateur astronomers.
– Large Earth-based telescopes have explored the
planet’s atmosphere, rings, and moons.
– The two Voyager spacecraft flew past Saturn in 1979,
transmitting back to Earth detailed measurements
and images.
Comparative
Planetology of the
Saturn Outer Planets
– The Cassini spacecraft went into orbit around
Saturn in 2004 on an extended exploration of the
planet, its rings, and its moons.
Comparative
Planetology of the
Saturn the Planet Outer Planets
• Seen from Earth, Saturn shows only faint
evidence of belt-zone circulation.
– However, images from Voyager, Cassini, and the
Hubble Space Telescope show that belts and zones
are present and that the associated winds blow up to
three times faster than
on Jupiter.
Comparative
Planetology of the
Saturn the Planet Outer Planets
– The belts and zones on Saturn are less visible
because they occur deeper in the cold
atmosphere,
below a layer
of methane
haze.
Comparative
Planetology of the
Saturn the Planet Outer Planets
• Saturn is less dense than water: it would
float.
– Thus, it is, like Jupiter, rich in hydrogen and helium.
– In fact, its density is so low that it must have a
relatively small core
of heavy elements.
Comparative
Planetology of the
Saturn the Planet Outer Planets
• The shape of a Jovian planet can tell you
about the interior.
– As all the Jovian planets are mostly liquid and rotating
rapidly, they are slightly flattened.
Comparative
Planetology of the
Saturn the Planet Outer Planets
• A planet’s oblateness is the fraction by
which its equatorial diameter exceeds its
polar diameter.
• Saturn is the most oblate of the planets.
– Thus, it is mostly
liquid.
Comparative
Planetology of the
Saturn the Planet Outer Planets
• A world with a large rocky core and mantle
would not be flattened much by rotation.
• However, an all-liquid planet would flatten
significantly.
– Thus, the oblateness of a Jovian planet, combined
with its average density, can help astronomers model
the interior.
Comparative
Planetology of the
Saturn the Planet Outer Planets
• Models of Saturn predict that it must have
a small core of heavy elements and less
liquid metallic hydrogen than Jupiter.
– This is because Saturn’s internal pressure is lower.
– Perhaps this is why Saturn’s magnetic field is 20
times weaker than Jupiter’s.
– Like Jupiter, Saturn has a
hot interior and radiates
more energy than it
receives from the sun.
Comparative
Planetology of the
Saturn’s Rings Outer Planets
• Saturn may look a bit bland, but it
makes up for that in the splendor of
its rings.
Comparative
Planetology of the
Saturn’s Rings Outer Planets
• The beautiful rings are made up of
billions of ice particles orbiting the
planet in the plane of its equator.
Comparative
Planetology of the
Saturn’s Rings Outer Planets
• The rings are also fascinating for what they
can tell us about the worlds of the outer
solar system.
• One, the rings can’t be as old as Saturn.
– They must be replenished now and then by impacts
on Saturn’s icy moons or by the disruption of a small
moon that wanders too close to the planet.
Comparative
Planetology of the
Saturn’s Rings Outer Planets
• Two, the gravitational effects of small
moons can confine some rings in narrow
strands or keep the
edges of rings sharp.
Comparative
Planetology of the
Saturn’s Rings Outer Planets
• Moons can also
produce waves in
the rings that
are visible as
tightly wound
ringlets.
Comparative
Planetology of the
Saturn’s Rings Outer Planets
• Finally, the ring particles are confined in a
thin plane spread among small moons and
confined by gravitational interactions with
larger moons.
– The rings of Saturn, and the rings of the other Jovian
worlds, are created by and controlled by the planet’s
moons.
– Without the moons, there would be no rings.
Comparative
Planetology of the
Saturn’s Rings Outer Planets
• Observations made by the Cassini
spacecraft show that the ring particles
have compositions that resemble that of
Saturn’s distant icy moon Phoebe.
– A large impact on
Phoebe may be part
of the complex history
of Saturn’s rings.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• Saturn has more than 30 known
moons.
– Many are small but all contain mixtures of ice
and rock.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• Most of the moons were discovered
by Earth-based observations.
– However, astronomers know them well because the
two Voyager spacecraft flew past Saturn in 1980 and
1981. Also, the Cassini spacecraft went into orbit
around Saturn in 2004 to begin an extended study of
the planet, its rings, and its moons.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• The largest of Saturn’s moons is Titan, a
few percent larger than Mercury.
• Its density suggests that it must contain a
rocky core under a thick mantle of ices.
• Unlike Mercury, Titan has an atmosphere.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• Titan is so cold its gas molecules
do not travel fast enough to
escape.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• The atmosphere is mostly nitrogen
with traces of argon and methane.
• Sunlight converts some of the
methane into organic molecules.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• These organic molecules collect into small
particles of organic goo that fill the
atmosphere with orange smog.
– These particles settle slowly downward to coat the
surface.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• The surface of Titan is a cold -179°C
(-290°F).
– So, its surface is mainly composed of water ices and
frozen molecules like methane.
• Some evidence suggests that liquid
methane may be present too, at least
occasionally.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• The Cassini spacecraft dropped the
Huygens probe into Titan’s atmosphere.
• As it descended, it photographed dark
drainage channels.
– This suggests that liquid methane
falls as rain, washes dark organic
molecules off of the higher terrain,
and drains into flat lowlands.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• However, such methane downpours may
be rare.
– No direct evidence of liquid methane was detected at
the time the probe parachuted to the surface.
– Infrared images suggest the presence of methane
volcanoes that replenish the methane in the
atmosphere.
– So, Titan must have some internal heat to power the
activity.
– Certainly, Titan is an unearthly world.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• The remaining moons of Saturn are small
and icy, have no atmospheres, and are
heavily cratered.
• Most have ancient surfaces.
– For example, Tethys is less than a third the diameter of
Earth’s moon, and its heavily
cratered crust seems quite old.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• A valley 3 km deep trails three-fourths of
the way around the satellite.
– Such cracks are found on a few other satellites.
– These appear to have formed long ago, when the
interiors of the icy moons froze and expanded.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• These cracks may resemble Mercury’s
lobate scarps, but they are the result of
expansion, not contraction.
– Mercury’s metallic core shrank as it cooled, forming
compression features on its surface.
– The icy interior of Tethys expanded as it froze,
causing the crust to stretch and crack.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• Enceladus, a bit smaller than Tethys,
shows signs of recent activity.
– Some parts of its surface contain 1,000 times fewer
craters than other regions, showing that these lightly
cratered regions must be younger.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• Observations made by the Cassini
spacecraft show that Enceladus is spewing
water vapor from vents in its southern
hemisphere.
– The water shoots high enough above the surface that
some escapes to form crystals that replenish the
particles in the diffuse E ring around Saturn.
– Clearly, the little moon still has internal heat to drive
geological activity.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• Like nearly all moons in the solar system,
Saturn’s moons are tidally locked to their
planet, rotating to keep the same side
facing the planet.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• Sometimes, the leading side of these
moons—the side facing forward in the
orbit—is modified by debris.
– For example, Iapetus has a cratered trailing side
about as dark as dingy snow, but its leading side is
as dark as fresh asphalt.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• One hypothesis is that the dark material is
carbon-rich dust from meteorite impacts on
the next moon out, Phoebe.
Comparative
Planetology of the
The Moons of Saturn Outer Planets
• Iapetus is also marked by a strange
ridge up to 13 km (8 mi) high along
its equator.
– The ridge may be evidence
that the little moon spun
rapidly when it was young.
Comparative
Planetology of the
The History of Saturn Outer Planets
• Can you put all the evidence and
hypotheses together to describe the
history of Saturn?
• Doing so is a real test of your
understanding.
Comparative
Planetology of the
The History of Saturn Outer Planets
• Saturn formed in the outer solar nebula,
where ice particles were stable and may
have contained more trapped gases.
– The protoplanet grew rapidly and became massive
enough to attract hydrogen and helium by
gravitational collapse.
– The heavier elements sank to the middle to form a
small core, and the hydrogen formed a liquid mantle
containing liquid metallic hydrogen.
Comparative
Planetology of the
The History of Saturn Outer Planets
• The outward flow of heat from the interior
is believed to drive convection inside the
planet that produces its magnetic field.
– As Saturn is smaller than Jupiter, the internal
pressure is less, the planet contains less liquid
metallic hydrogen, and
its magnetic field is
weaker.
Comparative
Planetology of the
The History of Saturn Outer Planets
• The rings can’t be primordial.
– That is, they can’t be material left over from the
formation of the planet.
– Such ices would have been vaporized and driven
away by the heat of the protoplanet.
• Rather, you can suppose that the ring
material is debris from occasional
collisions between comets and Saturn’s
icy moons.
Comparative
Planetology of the
The History of Saturn Outer Planets
• Some of Saturn’s moons are probably
captured asteroids that wandered too
close, but some of the moons probably
formed with Saturn.
• Many have ancient
surfaces.
Comparative
Planetology of the
The History of Saturn Outer Planets
• Titan may have formed with Saturn,
or it may be a very large icy
planetesimal captured into orbit
around Saturn.
– You will see more evidence for this capture
hypothesis when you explore farther from the sun.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Why do the belts and zones on Saturn
look so bland?
– To answer this question, you need to compare and
contrast the two largest Jovian planets.
– Jupiter has colorful bands.
– So, in order to understand why Saturn does not, you
need to make an argument that takes into account the
important differences between the two planets.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• In the atmosphere of Jupiter, the dark belts
form in regions where gas sinks, and
zones form where gas rises.
– The rising gas cools and condenses to form icy
crystals of ammonia, which are visible as bright
clouds.
– Clouds of ammonia hydrosulfide and water form
deeper—below the ammonia clouds—and are not as
visible.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Saturn is twice as far from the sun as
Jupiter, so sunlight is four times dimmer.
– The atmosphere is colder, and gas currents do not
have to rise as far to reach cold levels and form
clouds.
– That means the clouds are deeper in Saturn’s
atmosphere than in Jupiter’s atmosphere.
– As the clouds are deeper, they are not as brightly
illuminated by sunlight and look dimmer.
– Also, a layer of methane-ice-crystal haze high above
the ammonia clouds makes the clouds even less
distinct.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Now, build a new argument comparing
the ring systems.
– How is Saturn’s ring system similar to and
different from Jupiter’s ring system?
Comparative
Planetology of the
Uranus Outer Planets
• Now that you are familiar with the gas
giants in our solar system, you will be able
to appreciate how weird the ice giants,
Uranus and Neptune, are.
• Uranus, especially, seems to have
forgotten how to behave like a planet.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• Uranus is only a third the diameter of
Jupiter, only a twentieth as massive, and,
because it is four times farther from the
sun, its atmosphere is over 100 degrees
colder than Jupiter’s.
– As it is smaller than Jupiter, its internal pressure is
lower, and it does not contain liquid metallic
hydrogen.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• Models of Uranus—based in part on its
density and oblateness—suggest that it
has a small core of heavy elements and a
deep mantle of partly frozen water
containing rocky material and dissolved
ammonia and methane.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• Circulation in this electrically conducting
mantle may generate the planet’s peculiar
magnetic field, which is highly inclined to
its axis of rotation.
• Above the mantle lies
the deep hydrogen
and helium of the
planet’s atmosphere.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• Uranus rotates on its side, with its equator
inclined 98° to its orbit.
– With an orbital period of 84 years, each of its four
seasons lasts 21
years and is
extreme, because
the sun shines
almost directly on
each of its poles.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• This peculiar rotation may have been
produced if Uranus collided with a very
large planetesimal late in its formation.
– When Voyager 2
flew past in 1986,
the planet’s south
pole was pointed
almost directly at
the sun.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• Voyager 2 photos show a nearly
featureless blue-green world.
– The atmosphere is mostly hydrogen and helium
(12 percent).
– However, traces of methane absorb red light and thus
make the atmosphere look blue.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• There is no belt-zone circulation visible in
the Voyager photographs.
– However, extreme computer
enhancement does reveal a
few clouds and bands
around the south pole.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• In the decades since Voyager 2 flew past
Uranus, spring has come to the northern
hemisphere of Uranus and fall to the
southern hemisphere.
• Images made by the Hubble Space
Telescope and modern Earth-based
telescopes reveal changing clouds and
cloud bands in both hemispheres.
– Further studies may reveal the secrets of Uranus’s
cycle of seasons.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• Laboratory experiments show that, at the
temperatures and pressures present
inside the atmospheres of Uranus and
Neptune, methane can decompose, and
the released carbon can form crystals of
diamond perhaps the size of pebbles.
– A continuous hailstorm of diamonds falling into a
planet’s interior would release energy and could help
warm the planet.
– This process has not been observed, but it serves to
warn you that other worlds are truly un-Earthly and
may harbor processes you can hardly imagine.
Comparative
Planetology of the
Uranus the Planet Outer Planets
• Observations show that Uranus is
radiating about the same amount of
energy that it receives from the sun.
• It has little heat flowing out of its interior.
– This may account for its limited atmospheric activity.
Comparative
Planetology of the
The Rings of Uranus Outer Planets
• The rings of Uranus are similar to those of
Neptune.
• So, it is useful to consider the two ring
systems together.
– They are dark, faint, and confined by shepherd
satellites.
– They are not easily visible from Earth.
Comparative
Planetology of the
The Rings of Uranus Outer Planets
• The first hint that the planets had rings
came during occultations, the passage of
the planet in front of a star.
• Most of what astronomers know about
these ring systems comes from the
observations
of the
Voyager 2
spacecraft.
Comparative
Planetology of the
The Rings of Uranus Outer Planets
• The rings of
Uranus are
very dark,
contain little
dust, and are
confined by
small moons.
Comparative
Planetology of the
The Rings of Uranus Outer Planets
• The rings of
Uranus cannot
survive for long
periods.
– So, they need to
be resupplied
with material
from impacts
on moons.
Comparative
Planetology of the
The Rings of Uranus Outer Planets
• Planetary rings have a close
relationship with moons.
– Due to collisions among ring particles, the rings tend
to spread outward, almost like an expanding gas.
– If a planet had no moons, its rings would spread out
into a more and more tenuous sheet until they were
gone.
Comparative
Planetology of the
The Rings of Uranus Outer Planets
• The spreading rings can be confined by
the gravitational interaction with small
moons, which get pushed outward very
slowly.
– Through orbital resonances, those small moons can
be anchored by larger, more distant moons that are
so massive they do not get pushed outward by any
significant amount.
– In this way, a system of moons can confine and
preserve a system of planetary rings.
Comparative
Planetology of the
The Rings of Uranus Outer Planets
• Planetary rings are beautiful blossoms
created when debris falls into a system of
small moons and becomes trapped in the
most stable orbits among the moons.
• Without moons, there could be no rings.
Comparative
Planetology of the
The Uranian Moons Outer Planets
• Until recently, astronomers could see only
five moons orbiting Uranus.
• However, Voyager 2 discovered 10 small
moons in 1986.
• More have been found in images recorded
by new, giant telescopes on Earth.
– Each of the Jovian planets probably has many small,
undiscovered moons.
Comparative
Planetology of the
The Uranian Moons Outer Planets
• The five major satellites were
photographed by Voyager 2.
• Oberon and Titania, the two outer moons,
are about 45 percent the diameter of
Earth’s moon.
– They have old, cratered surfaces with faults hundreds
of kilometers long.
– Some regions of these icy moons have been
resurfaced by liquid water ‘lava’ that covered old
craters and froze.
Comparative
Planetology of the
The Uranian Moons Outer Planets
• Umbrial, the next moon inward, is about a
third the diameter of Earth’s moon.
– It shows no sign of activity on its ancient, cratered
surface.
• Ariel, the fourth moon inward, is slightly
smaller than Umbrial.
– Its cratered crust is marked by
broad, smooth-floored valleys
that may have been cut by
flowing ice (glaciers).
Comparative
Planetology of the
The Uranian Moons Outer Planets
• Miranda, the innermost moon, is only 14 percent
the diameter of Earth’s moon.
– Its surface is marked by oval patterns of grooves called
ovoids.
– These are believed to
have been caused
by internal heat that
produced convection
in the icy mantle.
Comparative
Planetology of the
The Uranian Moons Outer Planets
– Rising currents of ice have deformed the crust and
created the ovoids.
– From craters on the ovoids, astronomers conclude
that the entire surface is
old and the moon is
no longer active.
– Perhaps it was warmed
by tidal heating long ago.
Comparative
Planetology of the
A History of Uranus Outer Planets
• The challenge of comparative
planetology is to tell the story of a
planet.
• Uranus is the most difficult to explain.
– It is not only far away and hard to study, but it is
peculiar in a number of ways.
Comparative
Planetology of the
A History of Uranus Outer Planets
• Uranus must have grown slowly in the
outer solar nebula, where the material was
less dense and where orbital motions were
slower and collisions less frequent.
– However, it did not grow massive enough to capture
large amounts of gas from the nebula, as did Jupiter
and Saturn.
Comparative
Planetology of the
A History of Uranus Outer Planets
• Uranus is rich in water and ice rather
than in hydrogen and helium.
– You might like to call it an ice giant.
– However, it contains lots of rock, liquid, and
gas too.
Comparative
Planetology of the
A History of Uranus Outer Planets
• Perhaps, as it was forming, Uranus was
struck by a large planetesimal and given
its highly inclined rotation.
• Also, it is likely that impacts between the
icy heads of comets and moons may
produce the debris that replenishes
planetary rings.
Comparative
Planetology of the
A History of Uranus Outer Planets
• The highly inclined magnetic field of
Uranus may be produced by convection in
its electrically conducting mantle.
• With very little heat flowing out of the
interior, this convection must be limited.
Comparative
Planetology of the
A History of Uranus Outer Planets
• The lack of interior heat in Uranus is not
well understood.
• However, some astronomers suspect that
the impact that gave Uranus its odd
rotation also stirred it so thoroughly that it
lost much of its heat.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Why are the rings of Uranus so
narrow?
– Unlike the rings of Jupiter and Saturn, the rings of
Uranus are quite narrow, like hoops of wire.
– You would expect collisions among ring particles to
gradually spread the rings out into thin sheets.
– So, something must be confining the narrow rings.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• In fact, two small moons
have been found orbiting
just inside and outside the
ε ring.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• If a ring particle drifts away from the ring,
the corresponding moon’s gravity will boost
it back into the ring.
– Moons too small to
detect are thought
to shepherd the
– other rings.
– Thus, the Uranian
rings resemble
Saturn’s narrow
F ring.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• This explains the narrowness of the rings.
• However, it seems odd that moons just
happen to be in the right place to keep the
rings narrow.
• Revise your argument.
– How did that happen?
Comparative
Planetology of the
Neptune Outer Planets
• Through a telescope, Neptune looks
like a tiny blue dot with no visible
cloud features.
• In 1989, Voyager 2 flew past and
revealed some of Neptune’s secrets.
Comparative
Planetology of the
Planet Neptune Outer Planets
• Only 4 percent smaller in diameter than
Uranus, Neptune has a similar interior.
– A small core of heavy elements lies within a slushy
mantle of water, ices, and minerals (rock) below a
hydrogen-rich
atmosphere.
Comparative
Planetology of the
Planet Neptune Outer Planets
• However, Neptune looks quite
different.
• It is dramatically
blue and has
active cloud
formations.
Comparative
Planetology of the
Planet Neptune Outer Planets
• The dark-blue tint to the atmosphere
is understandable because it contains
3 percent methane, compared with
only 2 percent in Uranus.
– The methane absorbs red photons better than blue,
giving Neptune a blue tint.
Comparative
Planetology of the
Planet Neptune Outer Planets
• Also, its atmosphere scatters shorterwavelength (blue) photons better than red
photons, just as does Earth’s atmosphere.
– This also helps make Neptune look blue.
• Finally, Uranus and Neptune are quite cold.
– Their temperatures may prevent the formation of the
molecules that give the clouds on Jupiter and Saturn
their colors.
Comparative
Planetology of the
Planet Neptune Outer Planets
• Atmospheric circulation on Neptune is
much more dramatic than on Uranus.
– When Voyager 2 flew by Neptune in 1989, the largest
feature was the Great Dark Spot.
– Roughly the size of Earth, the
spot seemed to be an
atmospheric circulation much
like Jupiter’s Great Red Spot.
Comparative
Planetology of the
Planet Neptune Outer Planets
– Smaller spots were visible in Neptune’s
atmosphere.
– Photos showed they were circulating like
hurricanes.
Comparative
Planetology of the
Planet Neptune Outer Planets
• Recently, the Hubble Space Telescope
photographed Neptune and found that the
Great Dark Spot is gone and new cloud
formations have appeared.
• Evidently, the weather on Neptune is
changeable.
Comparative
Planetology of the
Planet Neptune Outer Planets
• The atmospheric activity on Neptune is
apparently driven by heat flowing from the
interior and the dim sunlight.
– The heat causes convection in the atmosphere.
– The planet’s rapid rotation converts the heat into highspeed winds, high-level
white clouds of methane
ice crystals, and rotating
storms that are seen
as spots.
Comparative
Planetology of the
Planet Neptune Outer Planets
• Neptune may have more activity than
Uranus because it has more heat flowing
out of its interior.
Comparative
Planetology of the
Planet Neptune Outer Planets
• Like Uranus, Neptune has a highly inclined
magnetic field that must be linked to
circulation in the interior.
– In both cases, astronomers suspect that ammonia
dissolved in the liquid water mantle makes the mantle
a good electrical conductor and that convection in the
water, coupled with the rotation of the planet, drives
the dynamo effect and generates the magnetic field.
Comparative
Planetology of the
The Rings of Neptune Outer Planets
• Neptune’s rings are faint and very
hard to detect from Earth.
• However, they illustrate some
interesting processes of comparative
planetology.
Comparative
Planetology of the
The Rings of Neptune Outer Planets
• Neptune’s rings—named after the
astronomers involved in the discovery of the
planet—are similar to those
of Uranus, but
contain more
dust.
Comparative
Planetology of the
The Rings of Neptune Outer Planets
• There is also a new way that rings can
interact with moons.
– One of Neptune’s
moons is producing
short arcs in the
outermost ring.
Comparative
Planetology of the
The Rings of Neptune Outer Planets
• Neptune’s rings resemble the rings of
Uranus in one important way.
– They can’t be primordial: they can’t have lasted since
the formation of Neptune.
– Evidently, impacts on moons occasionally scatter
debris through the satellite system, and some of it
falls into the rings in the most stable places among
the orbits of the moons.
– Planetary rings are constantly being remade.
Comparative
Planetology of the
The Moons of Neptune Outer Planets
• Neptune has two moons that were
discovered using Earth-based telescopes
before Voyager 2 flew past in 1989.
– The passing spacecraft discovered six more very
small moons.
– Since then, a few more small moons have been found
by astronomers using large Earth-based telescopes.
– Neptune probably has even more small moons yet to
be found.
Comparative
Planetology of the
The Moons of Neptune Outer Planets
• The two largest moons have peculiar
orbits.
– Nereid, about a tenth the size of Earth’s moon,
follows a large, elliptical orbit—taking nearly an
Earth year to circle Neptune once.
– Triton, nearly 80 percent the size of Earth’s moon,
orbits Neptune backward—clockwise as seen from
the north.
– These odd orbits suggest that the system was
disturbed long ago in an interaction with some other
body such as a massive planetesimal.
Comparative
Planetology of the
The Moons of Neptune Outer Planets
• Although Triton is smaller than Earth’s
moon, it has an atmosphere of nitrogen
and methane about 105 times less dense
than Earth’s.
• Triton can keep this gas because it is so
far from the sun that it is very cold, only 37
K (-393°F).
Comparative
Planetology of the
The Moons of Neptune Outer Planets
• In fact, a significant part of Triton is ice,
and deposits of nitrogen frost are visible at
the southern pole, which has been turned
toward sunlight for the last 30 years.
– This nitrogen appears to be
vaporizing in the sunlight
and is probably refreezing
in the darkness at the
north pole.
Comparative
Planetology of the
The Moons of Neptune Outer Planets
• Many features on Triton suggest it has had
an active past.
– It has few craters, but it does have long faults that
appear to have formed when the icy crust broke.
– Some approximately round
basins about 400 km in
diameter appear to have
been flooded time after
time by liquids from the
interior.
Comparative
Planetology of the
The Moons of Neptune Outer Planets
– Most exciting of all are the dark smudges visible in the
southern polar cap.
– Analysis of the photos reveals that these are deposits
produced when liquid nitrogen in the crust, warmed by the
sun, erupts through vents and spews up to 8 km high into
the atmosphere.
Comparative
Planetology of the
The Moons of Neptune Outer Planets
– Methane in the gas is converted by sunlight into dark
deposits that fall to the surface, leaving the smudges.
– These geysers make Triton one of only three worlds
in our solar system observed to be active, that is,
apart from Earth and Io.
Comparative
Planetology of the
The Moons of Neptune Outer Planets
• By counting craters on Triton, planetary
scientists conclude that the surface has
been active as recently as a million years
ago and may still be active.
Comparative
Planetology of the
The Moons of Neptune Outer Planets
• The energy source for this volcanism
could come from radioactive decay.
– The moon is two-thirds rock and, although such a
small world would not be able to generate sufficient
radioactive decay to keep molten rock flowing to its
surface, frigid Triton may have enough heat for waterammonia volcanism.
– A mixture of water and ammonia could melt at very
low temperatures and erupt to resurface parts of the
moon.
Comparative
Planetology of the
The History of Neptune Outer Planets
• This history of Neptune is quite short
because many of its characteristics are
understandable in terms of comparative
planetology.
• On the other hand, some of its
peculiarities are difficult to understand
because astronomers have little data.
Comparative
Planetology of the
The History of Neptune Outer Planets
• Neptune must have formed much as
Uranus did.
– It grew slowly in the outer solar nebula and never
becoming massive enough to trap large amounts of
hydrogen and helium.
– It developed a core of heavy elements, a mantle of
slushy ices and rock, and a
deep hydrogen-rich
atmosphere.
Comparative
Planetology of the
The History of Neptune Outer Planets
• Neptune has more internal heat flowing
outward than Uranus.
– Part of that may be generated by radioactive decay in
its core.
– Some of the energy may be released by denser
material falling inward, including—as in the case of
Uranus—diamond crystals formed by the disruption of
methane.
– The heat flowing outward toward Neptune’s surface
could drive convection, produce a magnetic field, and
help create atmospheric circulation.
Comparative
Planetology of the
The History of Neptune Outer Planets
• The moons of Neptune suggest some
cataclysmic encounter long ago that put
Nereid into a long-period elliptical orbit and
pushed Triton into a retrograde orbit.
– Certainly, impacts on the satellites could provide the
debris that is trapped among the smaller moons to
form the rings.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Why is Neptune blue but its clouds
white?
– To solve this problem, you
must build a scientific
argument that follows a
process step by step.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• When you look at Neptune, the light you
receive is sunlight that is reflected from
various layers of Neptune on its way to
your eyes.
– As sunlight contains photons of all visible
wavelengths, it looks white to human eyes.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• However, sunlight entering Neptune’s
atmosphere must pass through hydrogen
gas that contains a small amount of
methane.
– Whereas hydrogen is transparent, methane is a good
absorber of longer wavelengths.
– So, red photons are more likely to be absorbed than
blue photons.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Once the light is scattered from deeper
layers, it must run this methane gauntlet
again to emerge from the atmosphere.
– Again, red photons are more likely to be absorbed.
– The light that finally emerges from Neptune and
eventually reaches your eyes is poor in longer
wavelengths and thus looks blue.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• The methane-ice-crystal clouds lie at high
altitudes.
– So, sunlight does not have to penetrate very far into
Neptune’s atmosphere to reflect off the clouds.
– Consequently, it loses many fewer of its red photons.
– The clouds look white.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• This discussion shows how a careful, stepby-step analysis of a natural process can
help you better understand how nature
works.
• Now, build another step-by-step argument.
– Where does the energy come from to power Triton’s
surface geysers?
Comparative
Planetology of the
Pluto Outer Planets
• Pluto is a small icy world clearly different
from the Jovian worlds yet also different
from the terrestrial planets.
– Since its
discovery in
1930, it has
been a
mystery on
the edge of
the solar
system.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• You may have learned in school that
there are nine planets in our solar
system.
• However, some astronomers aren’t so
sure.
– Pluto isn’t Jovian.
– It isn’t terrestrial.
– So, is it a planet?
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• To some extent, the controversy is just an
argument over the meaning of the word
‘planet.’
• However, it is also revealing.
– Indeed, you will be surprised by where your analysis
of Pluto as a planet takes you.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• Pluto is very difficult to observe from Earth.
– Only a bit larger than 0.1 second of arc in diameter,
the little planet is only 65 percent the diameter of
Earth’s moon and shows little surface detail.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• The best photos by the Hubble Space
Telescope reveal large areas of light and
dark terrain, promising that Pluto will reveal
itself to be an interesting world when
spacecraft finally
visit it.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• Most planetary orbits in our solar system
are nearly circular.
• However, Pluto’s is quite elliptical.
• On the average, it is the most distant
planet.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• At some times, it is closer to the sun than
Neptune.
– In fact, from January 21, 1979, to March 14, 1999, Pluto
was closer to the sun than Neptune is.
– However, the planets will never collide, because Pluto’s
orbit is inclined 17°.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• Orbiting so far from the sun, Pluto is cold
enough to freeze most compounds you
think of as gases.
– Spectroscopic observations have found evidence of
carbon monoxide ice.
– It has a thin atmosphere of nitrogen and carbon
monoxide with small amounts of methane.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• Pluto has three moons.
– Two are very small and far from
the planet, but one is quite large.
• Charon, the large moon,
was discovered in 1978 in
a highly inclined orbit with
a period of 6.387 days and
an average orbital radius
of 19,640 km.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• Pluto and Charon are tidally
locked to face each other,
showing that Pluto rotates
at a highly inclined angle.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• Kepler’s third law states that the
mass of the system is 6.5 x 10-9
solar masses, or about 0.2 Earth
masses.
– Most of this mass is Pluto, which is about 12 times
more massive than Charon.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• The mass of a body is important in
astronomy because mass divided by
volume is density.
– The density of Pluto is about 2 g/cm3.
– The density of Charon is just a bit less.
– Pluto and Charon must contain about 35 percent ice
and 65 percent rock.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• So, is Pluto a planet?
– Before you vote, you should know that since 1992
new, large telescopes have discovered roughly a
thousand icy bodies orbiting beyond Neptune.
– This is the Kuiper belt, named after the DutchAmerican astronomer Gerard Kuiper.
– There are probably 70,000 or more objects in the belt.
– They appear to be icy bodies left over from the outer
solar nebula.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• Some of the Kuiper belt objects are nearly
as big as Pluto, and at least one is bigger.
– Two large objects named Sedna and 2004DW are
about 63 percent the size of Pluto.
– Another object called Quaoarh is 50 percent the
diameter of Pluto.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
– The object tentatively dubbed Xena is 1.05 times
larger in diameter than Pluto.
– Another object called Buffy is nearly as big as
Pluto.
– Both follow large orbits inclined over 40° to the
plane of the solar system.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• A surprising fraction of these ice worlds
have moons of their own.
– Rather, you could say that they are binaries with two
icy bodies orbiting each other.
• In that way, they resemble Pluto and its
big moon Charon.
Comparative
Planetology of the
Is Pluto a Planet? Outer Planets
• This leaves astronomers arguing over the
meaning of the word planet.
– Are there thousands of small, icy planets orbiting
beyond Neptune or are the Kuiper belt objects too
small to be planets?
– Is Pluto a planet or is it the king of the Kuiper belt?
• However you prefer to vote, you can be
sure the history of the outer solar system
is more interesting than anyone thought
when Pluto was first discovered.
Comparative
Planetology of the
Pluto and the Plutinos Outer Planets
• This section is about the history of Pluto.
– It will take you back billions of years to watch the
outer planets form.
• You can begin by eliminating an old idea.
– As soon as Pluto was discovered, astronomers
realized that it was much smaller than its Jovian
neighbors.
– Some suggested that it was a moon of Neptune that
had escaped.
– After all, their orbits actually cross.
Comparative
Planetology of the
Pluto and the Plutinos Outer Planets
• The escaped-moon theory has been totally
abandoned.
– It is hard to understand how Pluto could have
escaped from Neptune and reached its present orbit.
– Also, it is difficult to explain how it got a moon if it was
once a moon itself.
Comparative
Planetology of the
Pluto and the Plutinos Outer Planets
• Modern astronomers have a much better
theory.
• It links Pluto to the Kuiper belt objects.
– Over a dozen are known that are caught in a 3:2
resonance with Neptune: they orbit the sun twice
whereas Neptune orbits three times.
– As Pluto is also caught in the same 3:2 resonance,
these Kuiper belt objects have been named Plutinos.
– Many other Kuiper belt objects are caught in other
resonances with Neptune.
Comparative
Planetology of the
Pluto and the Plutinos Outer Planets
• If Pluto is related to the Plutinos, then
Pluto is related to the Kuiper belt
objects.
– They formed in the outer solar nebula.
– How did they get caught in resonances with Neptune?
Comparative
Planetology of the
Pluto and the Plutinos Outer Planets
• Sophisticated models of the formation of
the planets suggest that Uranus and
Neptune may have formed closer to the
sun where the solar nebula was denser.
– Sometime later, gravitational interactions with Jupiter
and Saturn could have gradually shifted the two ice
giants outward.
– As Neptune migrated outward, its orbital resonances
could have swept up icy bodies, like strange fishing
nets pushed in front of a fishing boat.
Comparative
Planetology of the
Pluto and the Plutinos Outer Planets
• Some icy Kuiper belt objects may have
been captured as moons.
– Astronomers have wondered if moons such as Triton
and Charon could have been Kuiper belt objects.
– Could Pluto have started life as a Kuiper belt object
and gotten swept up by a Neptune resonance?
Comparative
Planetology of the
Pluto and the Plutinos Outer Planets
• Now, you can vote: Is Pluto a planet?
• Whatever the final outcome, Pluto and the
Plutinos have rocked modern astronomers
and forced them to reconsider their ideas
on the origin of the outer worlds.
– As more Kuiper belt objects are discovered beyond
the orbit of Neptune, astronomers will see more
clearly how the outer solar nebula formed planets.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• What evidence can you point to that
cataclysmic impacts have occurred
in our solar system?
– To build this argument, you must clearly distinguish
between evidence and theory.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• You would expect objects that formed
together from the solar nebula to rotate
and revolve in the plane of the solar
system.
– So, the high inclination of Pluto and the orbit of
Charon suggest a past collision or, at least, a close
gravitational interaction with a passing body.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• The Pluto-Charon system is not the only
evidence for major impacts in the past.
– The peculiar orbits of Neptune’s moons Triton and
Nereid, the fractured and cratered state of the Jovian
satellites, and the peculiar rotation of Uranus all hint
that impacts and encounters with large planetesimals
or the nuclei of comets have been important in the
history of these worlds.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
– Furthermore, the existence of planetary rings
suggests that impacts have scattered particles into
stable orbits among the small inner moons and
replenished the ring systems.
– Even in the inner solar system, the backward rotation
of Venus, the density of Mercury, and the formation of
Earth’s moon are seen as possible consequences of
major impacts when the solar system was young.
Comparative
Planetology of the
Building Scientific Arguments Outer Planets
• Certainly, astronomers do not imagine that
planets bounce around like billiard balls.
• However, they do recognize that the
planets are not totally isolated and may
have been victims (or beneficiaries) of
large impacts as they formed long ago.
– For example, build a new argument.
– How do the size, orbits, and compositions of Pluto and
Charon suggest they originated in the Kuiper belt?