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Chapter 8
Satellites (moons), Rings,
and Plutoids
Reading assignment: Chapter 8
Jovian planets satellites
•There are:
•Six large satellites, similar in size to our Moon
•12 medium-sized - 400 to 1500km
•Many small satellites
•Jupiter – 67 satellites
•Saturn – 62 satellites
•Uranus – 27 satellites
•Neptune – 13 satellites
Jupiter’s Satellites
(Galilean satellites or moons)
•Io - Jupiter’s satellite with active
volcanoes!
•Europa - Jupiter’s satellite covered
with layer of frozen water – strong
indications of an ocean of liquid water
beneath
•Ganymede and Callisto - similar in
size to our moon, just a bit larger.
•Ganymede is the largest satellite in the
solar system. It may have an ocean of
liquid water under the frozen surface
Four largest Jupiter satellites - Galilean moons
Jupiter
Saturn
Medium and large
satellites in the
Jovian planets

Uranus

Neptune

All these satellites have
sufficient mass: Selfgravity force them to be
spherical
Some are now or were in
the past geologically active.
Most of them have
substantial amounts of ice.
The largest satellite
in the Solar system
Second largest
Jupiter’s “Galilean Moons”
An Unusual Family
Moon
Io
Europa
Ganymede
Callisto
What makes Jupiter’s Galilean satellites unusual?
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Io has several active volcanoes.
Europa may have an ocean of liquid water under
its icy crust.
Ganymede (And perhaps Callisto) may also have
sub-surface oceans?
How can we account for the unusual
features?
At that distance from the Sun ( ~5 AU),
shouldn’t they be completely frozen and
showing no activity?
Io’s Volcanoes
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Volcanoes were discovered in
images from Voyager’s
spacecraft
So far about 80 active volcanoes
have been identified using data
mainly from Voyager and
Galileo spacecrafts
Volcanic eruptions mainly
composed of sulfur and sulfur
dioxide
Volcanic plumes about 150 km
high and 300 km wide (large
because of low gravity)
Variety of volcanic hot spots
Large lava lakes made of liquid
sulfur
It is the most volcanic active
body in the Solar System
Io has a rocky mantle and an
iron/iron sulfide core
Io: Two images separated by 15 years
(The left one taken by Voyager and the last one on the right by the Galileo
spacecraft)
The plumes of Io’s volcanoes, the material ejected from the satellite, the Io torus
and the low frequency radio emission
The gasses ejected from the volcanic activity are ionized by the UV radiation from the Sun and form a
torus of ionized material around the orbits of Io. This ionized material are accelerated by Io and interact
with the magnetic field of the planet and triggers the low frequency radio emission
Io orbital period is about 42 hours. Jupiter rotational period is about 10 hours. Because of the fast
rotation, the magnetic field of Jupiter sweep pass Io and induce an electric field in the satellite. This
electric field accelerate the electrons. The electrons go down toward the planet in spirals along the
magnetic field. Some are reflected before the hit the atmosphere and start spiraling back away from the
planet and emit the low radio frequency.
Io torus
Electrons spiraling down in Io flux tube
What causes the volcanic activities in Io?
The elliptical orbit of Io
Tidal Heating
Io is squished and stretched as it orbits
Jupiter . This process releases heat and
rises the internal temperature
Why is its orbit so
elliptical?
The Jovian Moons
Elliptical orbit: Orbital resonance between the
orbital periods of Io, Europa and Ganymede
The 3 closest satellites line up every 7 Earth days (resonance)
Tugging in the same direction distorts the orbit from a circle to an ellipse
1 orbit of Ganymede = 2 orbits of Europa = 4 orbits of Io
Smooth Europa
 Icy surface covering a
large rocky core:
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Surface is very smooth &
young.
Fractured into ice rafts &
floes a few kilometers
across,
Repaved by water or
geysers through the cracks
in the ice.
Just a few impact craters:
Young surface
Europa
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
Surface is ice covered
Extensive & complex network of
cracks in icy crust. The darker color
of the cracks is due to the salts
dissolved in the water coming from
below the crust
 Indications of internal geologic
activity
Europa ( a 200 km square area)
Europa


“Salty” water oceans below thick layer of
ice? (twice as much water as Earth!!)
Mostly salty water, some magnesium
sulfate, sulfurs (red color)
Tidal stresses crack Europa’s surface ice. Similar to icebergs,
large chunks of ice that have been broken and reassembled
Does Europa have liquid water?
What lies beneath Europa’s
frozen crust surface?
One possibility:
 100-200 km of convective ice
above a rocky core
The most probably scenario based
on measurement of Europa’s
magnetic field:
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Thin ice crust a few km thick over
a 100 km deep water ocean.
(Diameter of Europa=3138 km)
The liquid conductive layer distort
the magnetic field around Europa.
That distortion was detected by the
Galileo spacecraft.
Europa
How Europa can maintain liquid water?
•Tidal heating due to the elliptical orbit of
Europa around the large mass Jupiter
•Thermal vents may bring the heat from
the core
•Heat may keep the interior temperature
above freezing point
Possibility of life?
• The existence of liquid water does not
imply the emergence of life. The salty
water is a hostile environment. But we
have seen on Earth that life can be present
in environment that were considered
hostile
Titan
Saturn’s largest satellite
Properties:
 Mass: ~0.02 Earthmass
 Radius: 0.4 REarth
 Density: ~1.9
g/cm³ (1900 kg/m³)
 Icy mantle over a
rocky core.
This is the only
satellite (moon) in the
solar system that has a
heavy atmosphere

Titan
Titan is the only satellite in the solar system to have a permanent atmosphere. It has a
methane-ammonia atmosphere
It was recently visited by Cassini (at the present in orbit around Saturn) and the Huygens
probe. Rocky surface and evidence of erosion by liquid/slush.
The atmosphere is
thick, the surface
of Titan cannot be
seen in the visual
part of the
spectrum.
On the right, an IR
image taken by
Cassini’s
telescopes
Titan’s Atmosphere
Composition:
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~80% N2 (nitrogen)
~3% CH4 (methane)
Argon
Hydrocarbons:
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Ethane = C2H6
Acetylene = C2H2
Ethylene = C2H4
Propane = C3H8
Clouds of methane & N2 ices
Thick atmosphere
Titan’s Atmosphere
(Deduced from Voyager 1 spacecraft observation)
The Huygens probe
The Huygens lander was carried by
the Cassini spacecraft mission .
The image to the right is an actual
image from the surface of Titan
returned by the Huygens probe.
Colors are close to true colors.
Rocks are probably icy “rocks”
The methane/ethane lakes in Titan.
Radar images taken by Cassini
Lakes on Titan (radar maps)
Titan’s Liquid Lakes
Cassini radar have been
able to image several
smooth regions that have
been identified as lakes of
liquid methane
Titan, a reflection of sunlight in a methane lake.
(Image taken by Cassini spacecraft)
Titan interior
Model of the interior deduced from gravitational field measurements
during the numerous Cassini flybys.
A layer of liquid water under the ice makes it interesting . Possibility of
life?
Saturn’s satellite Mimas and Star Wars’ Death Star
Image returned by Voyager
spacecraft in 1980 (Fly-by)
Star Wars movie released in
1977
Triton
Triton - Neptune’s largest
moon
•It has a retrograde orbit . It orbits in
direction opposite to Neptune
rotation
•Probably a captured object from the
Kuiper belt. Pluto is a Kuiper belt
object
•Voyager 2 detected geysers of
nitrogen gas rising several km high
• The gas jets of nitrogen may come
from liquid nitrogen heated by some
internal source of heat
•A very thin atmosphere of nitrogen
•Temperature about 37 K
Rings
• All of the Jovian planets
have rings
• The most spectacular are
Saturn’s rings
•They are very thin, just a
few km
• Rings are not solid
objects
• They are comprised of
many small solid particles
• All the particles are in
orbit around the planet
• Water ice is the primary
constituent
Why do rings form?
The tidal forces of the large planet break
apart a close enough moon or satellite
Rings

Rings consist of billions of
small particles or moonlets
orbiting close to their planet
 The size of particle ranges
from the size of grain of
sand to house-sized
boulders
Rings
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The orbits of the particles that make
up the rings follow Kepler’s laws
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inner particles revolve faster than those
farther out
ring are not rotating as a solid body,
rather individual moonlets are revolving
around the planet
 If ring particles are widely spaced,
they move independently
 If the particles are close to each
other, there is a gravitationally
interaction between them
 The gravitational attraction
(resonance) of the satellites (or
moons) clear gaps in rings
Saturn and its main rings (False colors)
The Cassini
division is
caused by a
resonance with
the orbital
period of
Mimas, one of
Saturn’s
satellites
The Cassini
division is easily
visible with a
moderate size
telescope. It can
be seen with any
of the 8” or
bigger
telescopes at the
UF Teaching
observatory
Saturn’s ring in false colors to enhance the
composition
A recent image of Saturn and its rings taken by the
Cassini spacecraft (October 2013)
An image of Saturn’s rings taken by Cassini
spacecraft when Saturn eclipsed the Sun in 2006
It shows a set of faint rings outside the main rings
(The pale blue dot at 9-10 o’clock is the Earth)
Origin of Rings
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Breakup of shattered satellite
Remains of particles that were
unable to come together and form
satellite
Gravity plays important role
 differential force of gravity -tidal forces
 tear bodies apart
 inhibit loose particles from
coming together
The differential force of gravity: Tidal effect
DFg
DFg
The Roche Limit

Roche Limit - the closest distance an object can come to another
large mass object without being pulled apart by tidal forces
Roche limit
Roche Limit

If the density of the planet is
similar to the density of the
satellite (moon)
Then, the Roche limit = 2.446
Rplanet
 A large moon orbiting
inside the Roche limit will
be destroyed.
 Roche Limits for some
planets:
Earth - 18,470 km
(Distance to Moon =385,000 km )
Jupiter - 175,000 km
Saturn - 147,000 km
Uranus - 62,000 km
For comparison, Saturn outer
diameter of A ring is 137,000.
It is inside the Roche limit
(D is Density)
Comparing Jovian Ring Systems
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Compared to Saturn, the ring
system of other Jovian planets:
•
have fewer particles
•
are smaller in extent
•
have darker particles
The rings of Uranus were
discovered in 1977 when the planet
passed in front of a star and the
rings dimmed the light from the star
The rings of Jupiter and Neptune
were discovered by the Voyager
spacecrafts
Other unsolved mysteries regarding
rings:
•
Uranus’ rings are eccentric and
slightly tilted from its
equatorial plane.
•
Neptune has partial rings.
The Roche limit of the Jovian planets
(Distance of the rings from the planets, in planets radius)
Pluto, a dwarf planet
•Discovered in 1930 by Clyde Tombaugh (at Lowell
observatory)
•Charon, the first satellite discovered was found in
1978. Image taken with ground-based telescope
•Pluto has a total of 5 satellites
• Pluto is located about 40 times the Earth’s distance
from the Sun (40AU)
•The New Horizons mission launched in January
2006 arrived in July 14, 2015 (Fly-by).
•Similar in mass and size to Neptune’s large moon
Triton
•Probably formed in the Kuiper belt (comet birth
place)
•The Kuiper belt is located outside the orbit of
Neptune, at distances between 30-50 AU
• Pluto has a highly inclined orbital plane
•Orbital period 248 years
•Average density 2000 kg/m³
•Pluto has only 20% the mass of our Moon
•Mass about 0.0022 mass of Earth
•Diameter 1160 km, 0.18 diameter of Earth
•In 2003 it was renamed as a dwarf planet by
Hubble ST image of Pluto
A recent Hubble telescope image of Pluto and its 5 satellites
(Satellite P5 was discovered in 2012)
Modeled “images” of Pluto obtained by processing 24 images
taken by the Hubble Space Telescope. The process produced a
mathematical description of its surface
Recent images of Pluto from the New Horizon spacecraft
The New Horizons mission arrived at
Pluto on July 14, 2015 (Fly by). It
took about 10 years to make the trip.
It is the first spacecraft to reach Pluto.
It has been returning the first detailed
images of Pluto.
The parts of the surface of Pluto
shows few impact craters
Tombaugh Regio (“The Heart”)
(Named after Clyde Tombaugh,
Pluto discover).
New Horizon image of Sputnik Planum
Sputnik Planum
shows no impact
craters.
Its surface must
be younger than
10 millions years
old
Region showing
impact craters
Details of Pluto surface.
Images from New Horizon
This structures cause by
convection of nitrogen
ice
Pluto haze layers and foggy hazes
Blue skies. The sunlight
scattered in the atmosphere
of Pluto produces a halo of
bluish color
The image was taken when
the spacecraft passed in front
of Pluto and the Sun was
being occulted by Pluto
Haze layers
in the
atmosphere
above Pluto
surface.
Composition
of
atmosphere:
nitrogen,
carbon
monoxide,
methane
An image of Charon from the New Horizon spacecraft
Darker,
reddish color
in North pole
Probably the
South pole
may be
similar
The Kuiper belt
Kuiper belt
• Located outside the orbit
of Neptune
•A region of the solar
system located between 3050 AU from the Sun
• Bodies in the Kuiper belt
are composed of “ices”,
mainly methane, ammonia
and water ices.
• Pluto, Eris, Makemake
and Haumea are examples
of Kuiper belt objects
•Some short period comet
(Periods<200 years) are
also object that belong to
the Kuiper belt.
Kuiper Belt Objects (plutoids) compared
Eris
The discovery of Eris in 2005 showed that
Pluto was not unique. These objects, along
with Pluto and others, seem to be the largest of
the Kuiper Belt objects, also known as transNeptunian objects or plutoids. Several
hundred have been found. It is estimated that
several thousands may form the Kuiper belt
Other Kuiper belt objects not shown in the
picture are Haumea and Makemake
A possible ninth planet
The object that was
nicknamed “Ninth Planet” is
about 10 times the mass of
the Earth.
The distance from the Sun is
about 20 times farther than
Neptune. It is estimated than
it will take 10,000 to 20,000
years to orbit the Sun
Caltech astronomers Konstantin Batygin (right) and Mike Brown
found evidence of the ninth planet by modeling the orbits of several
objects located beyond the Kuiper belt. In order to account for the
distorted and high eccentric orbits of these 6 objects, it is required
the existence of an object with 10 times the mass of the Earth and
located in the modeled orbit.
The results were published in the January 20, 2016 issue of the
Astronomical Journal