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Moons, Pluto, and Rings
22 February 2005
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Ring and Satellite Systems (1)
The rings and moons in the outer solar
system are different in composition from
objects in the inner solar system
Most contain dark, organic compounds mixed
with ice and rock
The presence of
dark material
implies that they
reflect very little
light
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Ring and Satellite Systems (2)
Most satellites in the outer solar system are in
direct or regular orbits
They revolve about their planet in a west-toeast direction and in the plane of the planet’s
equator
A number of them have irregular orbits
They orbit in a retrograde (east-to-west)
direction, or else have orbits with high
eccentricity or high inclination
These satellites are usually smaller, located
relatively far from their planet, probably formed
far away and subsequently captured by the
planet they now orbit
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Jupiter's Moons
To date, Jupiter has 63 known satellites
The largest four are Callisto, Ganymede,
Europa, and Io
Europa and Io are the size
of our Moon
Ganymede and Callisto
are bigger than Mercury
The rest are much
smaller
More moons will likely
be found in the future
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The Galilean Moons
Callisto, Ganymede, Europa, Io
They were first seen by Galileo (the
astronomer)
Studied by the Galileo space probe and
by Hubble Space Telescope
The combined data about the moons
has shown important similarities to the
terrestrial planets
The differences between the moons
seem to be mostly due to distance from
Jupiter
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Io
Europa
Ganymede
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Callisto: Cratered World (1)
This is the outermost of
the Galilean satellites
2 million km from Jupiter
Noon-time surface
temperature: 130 K
(140°C below freezing!)
Its diameter: 4820 km,
about the same as Mercury’s
Its mass: about 1/3 of Mercury’s mass
So it’s 1/3 as dense as Mercury
It has far less rocky and metallic materials
than do the terrestrial planets
It is composed largely of ice
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Callisto: Cratered World (2)
Callisto seems not to have fully differentiated
(separated into layers of different density materials)
The details of gravitational pull on the Galileo spacecraft
suggest that Callisto lacks a dense core
This is surprising to astronomers!
All big icy moons are expected to be differentiated (they
should have differentiated more easily than rocky ones)
Yet Callisto appears to have
been frozen solid before the
differentiation was complete
Callisto is covered with impact craters
Although they look similar to craters
on the Moon, they formed very
differently
Callisto lacks the interior forces
to drive geological change —
it’s geologically dead
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Ganymede: Largest Moon (1)
It is the largest satellite in
the solar system
It is cratered, but less so
than Callisto
Crater counting suggests
that ¾ of the surface may
have formed more recently
than the lunar maria
The lighter areas are younger than the darker ones
Ganymede is geologically very different from Callisto
It is a differentiated world, like terrestrial planets
a core about the size of our Moon
a mantle and crust of ice “floating” above the core
a magnetic field, a signature of a partly molten interior
It’s geologically active, powered by internal heat
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Ganymede: Largest Moon (2)
It has a diameter of 5262 km
Slightly larger than Mercury
Why is Ganymede very different from Callisto?
Possible explanation
Their small difference in size and internal heating may
have led to the significant differences between the two
moons now
The gravity of Jupiter may be
responsible for Ganymede’s
continuing geological activity
Ganymede is close enough to
Jupiter that tidal forces from
Jupiter may have occasionally
heated Ganymede’s interior
This could have triggered major
convulsions on its crust
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Ganymede: Old Dark Terrain
Old: it is
covered with
craters
Dark: ice
covered with
dust from
meteoroid
impacts
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Ganymede: New Bright Terrain
New: suggested by
fewer craters
Bright: due to
fracturing of
the icy surface
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Europa: Moon with Ocean (1)
Its surface is covered with a thick layer of water ice
For the most part, it is smooth, but is crisscrossed with
cracks and low ridges often stretching for thousands of
kilometers across icy plains
Under the ice, there may be liquid water or slush
If so, tidal heating may be responsible for keeping the
water in liquid form
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Actual color
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Enhanced
color
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Evidence for Warm Oceans on Europa
Galileo images appear to confirm the existence of a
global ocean on Europa
It has ridges and multiple-line features that may have
formed when thick layers of ice were broken up into giant
icebergs and ice flows and then refrozen in place
It also has smooth areas where water may have flowed up
and refrozen
If it has liquid water, could life exist on Europa?
Enhanced color
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Io: Volcanic Moon
It is the most volcanically active
world in the solar system
Io has an elliptical orbit, causing it
to twist back and forth relative to
Jupiter and experience tidal forces
This twisting and flexing is the likely
source of Io’s internal heating that
drives its massive volcanism
Io’s interior heat may also have
produced a differentiated interior
Io
Jupiter
Io probably has
an iron core
surrounded by
a molten rocky
mantle
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Volcanism on Io
Io has no impact craters
They must have been
eradicated by its volcanic
activity
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)
Io’s volcanoes can be very
long lived
Some have been observed
for 20 years
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Lava fountains
Loki volcano erupts
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Pele Volcano
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Saturn’s Moons
To date, Saturn has 33 known
satellites
The largest is Titan
Almost as big as Ganymede
The only satellite with a substantial
atmosphere
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Some of Saturn’s Moons
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Titan: Moon with Atmosphere (1)
It’s the second largest moon in
the solar system
It’s the only moon in the solar
system with a substantial
atmosphere
The thick atmosphere makes its
surface impossible to see
Why does Titan have an atmosphere?
Possible reasons:
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: Moon with Atmosphere (2)
Its atmosphere
has a pressure 1.6 times Earth's
is comprised of mostly nitrogen, plus 6%
argon and a few percent methane
has trace amounts of organic compounds (such as
carbon monoxide, ethane, and hydrogen cyanide) and
water
has multiple layers of clouds
the bottom layer is probably composed of methane
the top layer includes a dark reddish haze or smog, which
hides Titan’s surface from our view
Its surface has a temperature of about 90 K
This means that on Titan’s surface methane may exist
in liquid or solid form, and there may even be seas or
lakes of methane, as well as methane ice
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After separating
from Cassini, the
Huygens probe
landed on Titan
in Jan. 2005
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Some of Huygens’ Images
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Uranus System
To date, it has 27 known
satellites
none are really large
Its rings and satellites are
tilted at 98o just like the
planet itself
It has 11 rings
composed of very dark
particles
discovered 1977
consist of narrow ribbons of
material with broad gaps
very different from the
rings of Saturn
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Neptune’s Moons
To date, it has 13 known
satellites
6 are regular, close to the planet
2 are irregular, farther out
Its largest moon is Triton (in mythology,
Triton is Neptune’s son)
the only large moon in the solar system that circles its
planet in a direction opposite to the planet's rotation (a
retrograde orbit)
may once have been an independent
object that Neptune captured
has an atmosphere and
active volcanism
bears some resemblance to Pluto
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Triton: Icy World
Its surface has the
coldest temperature
(between 35 and 40K)
of any of the worlds
our robot spacecrafts have visited
Its surface material is made of frozen water,
nitrogen, methane, and
carbon monoxide
Triton has a very thin
nitrogen atmosphere, with
a pressure 16 millionths of
our atmospheric pressure
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Ice volcanoes on
Triton: plume rising 8 km
above the surface and
extending 140 km
"downwind"
plume
On Triton: eruptions of
volatile gasses like
nitrogen or methane
driven by seasonal
heating from the Sun
On Earth, Venus, Mars:
rocky magma driven by
internal heat
On Io: sulfur compounds
driven by tidal
interactions with Jupiter
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Pluto (1)
HST Picture
Discovered through systematic search
at P. Lowell observatory in 1930
Highest inclination to the ecliptic (17°)
Largest eccentricity ~ 0.248
Average distance from the Sun ~40 AU
Perihelion closer than Neptune
Orbital period ~248.6 earth years
Rotation: ~6.4 days on its side
Pluto's diameter 2240 km
Only 1 known satellite: Charon
Charon
Charon’s orbit is locked to Pluto,
revolving and rotating at the
same time as Pluto rotates
Pluto
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Pluto (2)
The only planet not
yet visited by spacecraft
Very faint from Earth
observation requires the best telescopes
Diameter is ~2190 km (60% of the Moon)
Density is ~2.1 g/cm3
Mixture of rocky material and water ice
Similar to Neptune’s moon Triton
Has a highly reflective surface
frozen methane, carbon monoxide, & nitrogen
Surface temperature between 50 and 60K
Has a tenuous atmosphere
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Quaoar – New planet?
Its orbit is more circular than Pluto's
It is closer to the ecliptic
7.9° inclinatiion compared to Pluto's 17°
Its diameter is
1280 km
compared to
Pluto's 2240 km
Possibly Pluto and
Quaoar are both
Kuiper-belt objects
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Pluto (3)
Pluto is not like the terrestrial or jovian
planets
Pluto, Quaoar, Charon, and possibly Triton,
are examples of Kuiper-belt objects
The Kuiper belt is a disk-shaped region of
space beyond Neptune’s orbit
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Planetary Rings (1)
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 very different
from each other in
mass, structure, and
composition
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Planetary Rings (2)
Saturn’s rings
made up of icy
particles spread out into several
vast, flat rings, with a great deal
of fine structure
Neptune’s & Uranus’ rings
made up of dark particles,
confined to a few narrow rings,
with broad empty gaps
Jupiter’s rings
transient dust bands, constantly
renewed by erosion of dust grains
from small satellites
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Planetary Rings (3)
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What causes Rings?
Each ring is a collection of vast
numbers of objects and particles
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
Two basic theories of how the rings came to be
Breakup hypothesis: remains of a shattered satellite
Another hypothesis: particles that were unable to fuse
into a single body
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Ring Causes Continued
In either theory, the gravitation of the planet
plays an important role
Tidal forces for orbits close to the planet can
tear bodies apart, or inhibit loose particles to
come together
The rings of Saturn and Uranus are close to the
planet
In the breakup theory, a satellite, or a passing
comet, may have come too close and been torn
apart under tidal forces, or through some
collision
Some scientists believe that some of the rings
are young and must therefore be the result of
a breakup
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Rings of Saturn
Consists of many rings and
subrings: A, B, C
B Ring: brightest, most closely
packed particles
A and C rings: translucent
Total mass of B estimated to be similar to that of an icy
satellite 250 km in a diameter
A & B separated by a wide gap called Cassini division
Rings are broad and very thin
Width of main ring ~70000 km, thickness ~20 m
Ring particles composed mainly of water ice
Particles range from grains the size of sand up to
house-sized boulders
A handful of narrow rings ~100-km wide, in addition
to the main rings
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Rings of Saturn
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Rings of Uranus and
Neptune
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
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Satellite-Ring Interactions
Each ring has an intricate structure, as
discovered by Voyager
Structures due to mainly gravitational effects
of satellites
Without satellites, the rings would be flat and
featureless
There could even be no rings at all…
Gaps in Saturn’s A-ring result from gravitational
resonances with smaller inner satellites,
especially Mimas
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