Download Moons, Pluto, and Rings

Moons,
Pluto,
&
Rings
19 July 2005
AST 2010: Chapter 11
1
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 of these rings
and moons contain
dark, organic
compounds mixed with ice and rock
The presence of dark material implies that the
objects 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
They were 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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Galilean Moons
Io
They were first seen by Galileo,
using his telescopes
most recently studied by the Galileo
spacecraft and by the Hubble Space
Telescope
Europa
The combined data about the
moons have shown important
similarities to the terrestrial planets
Ganymede
The differences between the moons
seem to be mostly due to distance
from Jupiter
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Callisto
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Major Satellites in Solar System
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Callisto: Cratered World (1)
This is the outermost of the
Galilean satellites
2 million km from Jupiter
Its noon-time surface
temperature
is 130 K
(140°C below
freezing!)
Its diameter is 4820 km,
about the same as Mercury’s
Its mass is about 1/3 of Mercury’s
So it’s 1/3 as dense as Mercury
This means that Callisto has far less rocky and
metallic materials than do the terrestrial planets
It must instead be composed largely of ice
Movie from a mosaic of images taken by the Voyager
spacecraft
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
It 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 thought
to be younger than the darker ones
Ganymede is geologically very different from Callisto
It is a differentiated world, like terrestrial planets, with
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 is 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 reasons
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
It is close enough to Jupiter that tidal forces from
Jupiter may have occasionally heated its interior
This could have triggered major convulsions on its
crust
Movie from a mosaic of images taken by the Voyager
spacecraft
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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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 probably has
an iron core
surrounded by
a molten rocky
mantle
Io, the movie
19 July 2005
Jupiter
Io
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Volcanism on Io
Io has no impact craters
They must have been
eradicated by its volcanic
activity
Many of its volcanic eruptions
produce plumes of material
that extend hundreds of
km out into space
The colors on Io come from
sulfur (yellow, black, red)
and sulfur dioxide (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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Infrared Images of Io
Images taken by the
Keck telescope plus
adaptive optics
Galileo image
Bright spots indicate
hot lava erupted by
volcanoes
Io in infrared, the movie
Without
adaptive
optics
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Saturn’s Moons
To date, Saturn has 46 known
satellites
The largest is Titan
almost as big as Ganymede
Titan is the only satellite among the jovian
moons that has 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 massive enough to have strong gravity
that can retain its atmospheric gases
Titan is cold enough so that the gases in the
atmosphere are slow moving and hence do not
easily escape into space
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Titan: Moon with Atmosphere (2)
Titan’s 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 probably composed of methane
the top layer including 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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Cassini Spacecraft
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After
separating
from Cassini,
the Huygens
probe landed
on Titan
in Jan. 2005
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Some of Cassini’s Images
Cassini-Huygens mission’s
website
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Uranus System
To date, it has 27 known satellites
none are really large
Its rings and satellites are
tilted at 98° just like the
planet itself
It has 11 rings
composed of very dark
particles
discovered in 1977
consisting 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)
It is the only large moon in the solar system that
circles its planet in a direction opposite to the
planet's rotation (a retrograde orbit)
It may once have been an independent object that
Neptune captured
Triton
has an atmosphere and active
volcanism
bears some resemblance to Pluto
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Triton: Icy World (1)
It has an atmosphere and
active volcanism
Its surface has the coldest
temperature (between 35
and 40 K) of any of the
worlds our robot spacecraft have visited
Its surface material is made of frozen water, nitrogen,
methane, and carbon monoxide
It has a very thin nitrogen
atmosphere, with a pressure of
only 16 millionths of our
atmospheric pressure
Triton resembles Pluto in size,
composition, and temperature
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Triton: Icy World (2)
On Triton, volcanic
eruptions produce plumes
of nitrogen gas and are
driven by seasonal heating
from the Sun
The plumes can rise 10 km
above the surface
On Earth, Venus, Mars, rocky magma is driven by
internal heat
On Io, sulfur compounds are driven by tidal
interactions with Jupiter
Triton movie
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Pluto (1)
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
It has only 1 known satellite,
Charon
Charon’s orbit is locked to Pluto,
revolving and rotating at the
same time as Pluto rotates
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Charon
Pluto
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Pluto (2)
It is the only planet not yet
visited by spacecraft
It appears very faint from
Earth
Its observation requires the best telescopes
Its diameter is ~2190 km (60% of the Moon)
Its density is ~2.1 g/cm3
It is composed of a mixture of rocky material and
water ice
similar to Neptune’s moon Triton
It has a highly reflective surface
frozen methane, carbon monoxide, & nitrogen
Its surface temperature is between 50 and 60 K
It has a tenuous atmosphere
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Pluto (3)
Pluto is not like the terrestrial or jovian
planets
Pluto, Quaoar, Charon, and Triton are probably
examples of Kuiper-belt objects
The Kuiper belt is a disk-shaped region of space
beyond Neptune’s orbit
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Quaoar: New planet?
Its orbit is more circular than Pluto's
It is closer to the ecliptic
7.9° inclination 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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Quaoar
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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)
The dotted
vertical line is
the limit inside
which the
gravitational
(tidal) force of
a planet can
break up a
satellite
In other words,
only really
small bodies
survive inside
the tidal
stability limit
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What Causes Rings? (1)
Each ring is a collection of vast
numbers of objects and particles
Each particle obeys Kepler’s laws
The inner particles revolve faster than those farther out
The ring does not rotate as a solid body
Particles within the ring are close to one another
The particles exert mutual gravitational influence and
occasionally even touch in low speed collisions
Two basic theories on how the rings formed have been
proposed
Breakup hypothesis: they are the remains of a shattered
satellite
Another hypothesis: the rings consist of particles that
were unable to fuse into a single body
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What Causes Rings? (2)
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
planets
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 Jovian Planets
Jupiter’s rings
Saturn’s rings
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Uranus’ rings
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Neptune’s rings
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Satellite-Ring Interactions
Each ring has an intricate structure, as
discovered by Voyager
The rings’ structures are due mainly to the
gravitational effects of the satellites
Without the 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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