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Chapter 11:
Moons,
Rings and
Pluto
Ring and Satellite Systems
General properties.
– Composition different from objects in the inner solar
system
 Most
contain dark, organic compounds mixed with ice and
rock
 Dark materials imply small reflectivity
– Most satellites in regular orbits
 West-to-east
direction
 In the plane of the planet’s equator
 Likely to have formed at about the same time as the planet
– Irregular satellites
 Retrograde
(east-to-west), or else have orbits of high
eccentricity, or high inclination
 Usually smaller satellites, located relatively far from their
planet
 Formed subsequently or captured
Jupiter's Moons
52 known satellites (as of about 2 years ago)
– Four large moons (Callisto, Ganymede, Europa, Io)
 Europa,
Io are the size of our moon
 Ganymede, Callisto – the largest – are bigger than mercury
– Many smaller moons.

Note: moon and satellite mean the same thing
in astronomy.
Large "Galilean" Moons
– Callisto, Ganymede, Europa, Io
– first seen by Galileo (the
astronomer)
– studied by the Galileo space probe
– Hubble Space Telescope
observations
– combined data has found important
similarities to the terrestrial planets
– differences between moons mostly
due to distance from Jupiter
Io
Europa
Ganymede
Callisto: Crater
Packed
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Outermost of the Galilean satellites.
2 million km from Jupiter.
Noontime surface temperature: 130 K
(140oC below freezing!)
Diameter 4820 km ~ same as Mercury
Mass ~ 1/3 Mercury’s mass
Not as dense as Mercury
Composed largely of ice.
Not fully differentiated.
– From details of gravitational pull on
Galileo spacecraft.
– Surprising!
– Yet appears to be “frozen”
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Covered with craters
Absence of interior forces – geologically
dead.
Craters on Callisto
Ganymede: Largest, Most Varied
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Largest satellite
Cratered, but less so than
Callisto
¾ of the surface seem to have
formed “recently”
– 1 billion years old – rather
than 4.4
Differentiated
– Rock and metals sank to form
a core about the size of the
Moon
– Mantle + crust floating above
core.
– Magnetic Field
– partly molten interior
Ganymede
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Ganymede is the
largest moon in the
solar system
– Diameter = 5262 km
– Slightly larger than
Mercury
Ganymede should
have little tidal
heating due to its
distance from Jupiter
– But, Ganymede
shows evidence for
surface alteration
Ganymede must have
had more geological
activity in the past
– Did it once have a
more eccentric orbit?
Old Dark Terrain
 Old:
it is
covered with
craters
 Dark:
ice
covered with
dust from
meteoroid
impacts
New Bright
Terrain
 New
due to fewer
craters
 Bright
due to
fracturing of the
icy surface
Craters – varying degradation
Geologically active.
– Younger
terrain result
of tectonic
and volcanic
forces.
– Extensive
Mountain
ranges
formed by
compression
of the crust.
– Some
indication of
large scale
crustal
movements.
Why is Ganymede different from Callisto?
Small difference in
– size.
– Internal heating.
 Gravity of Jupiter
– Ganymede close enough to Jupiter to have
tidal force effects – episodically heating the
crust.
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Europa: Ice-Covered Ocean
real color
enhanced color
Europa
Europa’s surface is covered with a layer of ice
 Under the ice is water or warm fluid ice
 Water flows up to the top continually
resurfacing Europa
 Tidal heating produces the internal energy
– It also has differentiated Europa into an iron
core, a rocky mantle and an icy crust
 Tidal flex may also crack the surface
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Evidence for Warm Oceans on
Europa
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Europa has ice rafts where
the surface has been
broken up and
reassembled
Galileo has imaged faults
where the ice has pulled
apart and water as flowed
up
Europa also has smooth
areas where water has
flowed up and re-frozen
On Earth simple life forms
evolved under water at
warm deep ocean vents
– Could something similar
have happened on Europa?
Io
Io: Volcano World
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Io is the most volcanically
active world in the solar
system
Io has an elliptical orbit, so
the tidal forces on it vary
with time
These changing forces
squeeze and flex Io
producing heat
The hot interior produces
massive volcanism
The interior heat has also
produced a differentiated
interior
– Io has an iron core
surrounded by a molten,
rocky mantle
Io
Jupiter
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Io has no impact craters
– They have been
eradicated by lava
Volcanoes produce
plumes of material that
extend up to 280 km
above the surface
The colors on Io come
from sulfur (yellow,
black, red) and from
sulfur dioxide (SO2,
white)
Volcanoes can be very
long lived
– Some have been
observed for 20 years
Volcanism on Io
Loki volcano erupts
Pele Volcano
Io’s Plasma
Torus
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Io’s volcanoes put lots of
ions into its orbit
– Ions are atoms that have
lost an electron giving
them a net electrical
charge
The ions are effected by
Jupiter’s magnetic field
producing a plasma torus
As Jupiter rotates its
changing magnetic field
produces an electrical
current through the torus
and interior
Saturn’s Moons and Rings
 30
known satellites (before
Cassini)
–Titan: largest of Saturn’s
satellites
Almost as big as
Ganymede
Only satellite with
substantial atmosphere
 Rings of Saturn
Titan
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The second largest moon in the solar
system
– Hint: on the test, don’t be fooled by the name!
Only moon with a significant atmosphere
 Thick atmosphere makes the surface
impossible to see
 Why does Titan have an atmosphere?
– Titan is large enough to have a strong
gravitational field
– Titan is cold enough so that the gas in the
atmosphere is slow moving
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Titan: Cloud World
Dense atmosphere:
pressure 1.5 times Earth's
mostly nitrogen plus 6% argon and
a few percent methane.
trace amounts of organic
compounds (i.e. ethane, hydrogen
cyanide, carbon dioxide) and water
water is formed when methane in
Titan's upper atmosphere is exposed
to sunlight.
chemical activity despite low surface
temperature, 94 K (-290 F).
like the smog found over large
cities, but much thicker.
Conditions like Earth early in its history
when life was first getting started.
May have the necessary building
blocks for life!
Huygens
Probe
Lands on
Titan
Uranus System
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Ring and Satellite tilted at
98o just like the planet
itself.
11 rings
– Composed of very dark
particles
– Discovered 1977
– Narrow ribbons of
material with broad
gaps
 very different from
the rings of Saturn
20 known satellites –
none really large
Neptune System
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8 known satellites
– 6 regular close to the planet
– 2 irregular farther out
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Triton (remember, Neptune carries a trident; in
mythology, Triton is Neptune’s son)
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–
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large moon in retrograde orbit,
has an atmosphere,
active volcanism,
Bears some resemblance to Pluto
Triton:
Ice World
Triton:
Ice World
orbit is retrograde:
didn’t form with Neptune!
•Perhaps a captured Kuiper Belt
object
•Capture may have shattered
another Neptunian moon
Capture scenario accounts for:
•Triton's orbit
•unusual orbit of Nereid
•provides energy to melt and
differentiate Triton's interior
Historical connection to Pluto?
•similar bulk properties
•Pluto has eccentric Neptunecrossing orbit
Is Triton a captured comet?
What about Pluto?
Ice volcanoes on
Triton:
plume rising 8 km
above the surface and
extending 140 km
"downwind"
Triton: eruptions of
volatile gases like
nitrogen or methane
driven by seasonal
heating from the Sun.
Earth, Venus, Mars:
rocky magma driven by
internal heat.
Io: sulfur compounds
driven by tidal
interactions with
Jupiter.
plume
Triton's Atmosphere
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Triton’s Atmosphere
Triton has a very thin nitrogen atmosphere
(1.6 X 10-5 atmospheres of pressure)
Triton is very cold (37 K) and thus nitrogen is
mostly frozen on the surface
A little bit of nitrogen evaporates to produce
the atmosphere
Motions of the atmosphere (wind) seem to
effect the plumes
Tenuous Clouds
Summary: Six Large Moons
 The
six large moons of the gas giants
resemble the terrestrial planets of
the inner solar system
–They can have volcanoes, atmospheres,
and evidence of resurfacing
 In general they are cold and have
rocky interiors and icy exteriors
 Some produce internal energy
through tidal heating
 Europa and Titan may have the
conditions for life to exist
Pluto
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Discovered through systematic search.
– At P. Lowell observatory in 1930.
– Named Pluto after the roman god of
the underworld (also PL are the
initials of Percival Lowell).
Highest inclination to the ecliptic (17o).
Largest eccentricity ~ 0.248.
Average distance ~40 AU = 5.9 billion
km.
– Perihelion closer than Neptune
Orbital period ~ 248.6 earth years.
Rotation : ~ 6.4 days on its side.
Pluto's diameter 2240 km
Largest satellite: Charon
– Charon’s orbit is locked to Pluto:
Charon revolves and rotates in the
same time as Pluto rotates.
Also two smaller satellites found.
HST Picture
Charon
Pluto
Pluto Basics
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Not visited by spacecraft,
– very faint,
– observation requires best
telescopes
Diameter ~ 2190 km (60% of the
Moon)
Density ~ 2.1 g/cm3
Mixture of rocky material and water
ice
Similar to Triton (Neptune)
Highly reflective surface –
– frozen methane, carbon monoxide,
nitrogen
Surface temperature 50 K/ 60 K
Tenuous atmosphere.
Quaoar – new planet?
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orbit more
circular than
Pluto's
closer to the
ecliptic – 7.9
degree
inclination
compared to
Pluto's 17
degrees.
diameter 1280
km vs. Pluto's
2240 km
possibly Pluto
and Quaoar are
both Kuiper belt
objects
Pluto's Orbit
11.3.4 The Nature of Pluto
 Pluto
is not like the Terrestrial or Jovian
planets.
 Pluto, Quaoar, Xena, Charon, and possibly
Triton, are examples of Kuiper belt
objects.
Rings
 All
four giant planets have rings
 Each ring is a system of billions of small
particles (moonlets).
 Each ring displays complicated structure
related to the interaction between the
rings and satellites.
 The four ring systems are quite different.
Ring Basics
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Saturn
– Made up of icy particles spread out
into several vast, flat rings, with a
great deal of fine structure.
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Neptune/Uranus
– Made up of dark particles, confined
to a few narrow rings, with broad
empty gaps.
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Jupiter
– Rings are transient dust bands,
constantly renewed by erosion of
dust grains from small satellites
What causes Rings?
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Ring = collection of vast numbers of particles
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–
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Each particle obeys Kepler’s laws.
Inner particles revolve faster
Ring does not rotate as a solid body.
Better to consider the revolution of individual moonlets.
Particles within the ring are close to one another.
– Exert mutual gravitational influence, even collide in low speed
collisions.
– Gives rise to waves that move across the rings.
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Two basic theories
– Breakup theory, remains of a shattered satellite.
– Make up of particles that did not fuse into a single body
Ring Causes Continued
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Gravitation of the planet
– Tidal forces for orbits close to the planet, can tear
bodies apart, or inhibit loose particles to come
together.
 Rings
of Saturn, Uranus are close to the planet…
Breakup – a satellite, or a passing comet
may have come too close and torn apart
under tidal forces, or through some collision.
 It is believed that some of the rings are
young, and must therefore be the result of a
breakup.
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Rings of Saturn
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Many rings and sub-rings: A, B, C
B-Ring : Brightest, most closely packed particles
A/C-rings : translucent.
Total mass of B ~ that of icy satellite 250 km in a
diameter.
A & B separated by a wide gap called Cassini division.
Rings are broad and very thin.
Main ring ~ 70000 km, thickness ~ 20 m.
Composed of water ice.
Particles range from grains the size of sand up to
house-sized boulders
A handful of narrow rings ~ 100 km, in addition to the
main rings.
Rings of Uranus
and Neptune
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Narrow and black
Almost invisible from Earth
Nine rings discovered (1977) during
observation of a star – occultation
First seen by Voyager (1986)
Outermost and most massive called
Epsilon
– 100 km wide, ~ 100 m thick,
51000 km from the planet.
Other rings much smaller : 10 km
wide.
Particles are very dark; black carbon
and hydrocarbon compounds.
Rings of Neptune are similar but even
more tenuous.
Satellite-Ring Interactions
Rings have intricate structure as discovered by
Voyager.
 Structures due to mainly gravitational effects of
satellites.
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– Without satellites, the rings would be flat and
featureless.
– There could even be no rings at all…
– Gaps in Saturn A-ring result from gravitational
resonances with smaller inner satellites. (Mimas)
Uranus' moon Miranda: innermost
and smallest of the five major
satellites, just 480 kilometers (about
300 miles) in diameter.
Two major strikingly different types of
terrain:
old, heavily cratered, rolling terrain
with relatively uniform reflectivity.
young, complex terrain characterized
by sets of bright and dark bands,
scarps and ridges (ovoid regions at
right and left and the distinctive
chevron feature below and right of
center).
likely due to upwelling of partially
melted ices
Discussion Question
 Astronomers
wish to search for life in the
ocean believed to lie beneath the ice of
Europa. How should we approach this
exploration to avoid possible cross
contamination of Earth and Europa with
organisms (DNA) from each other?
April 11, 2006
Astronomy 2010
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