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The Jovian Moons
and Rings
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The Jovian Planets
Jupiter’s Moons
Jupiter’s Moons
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Jupiter’s family of 67 moons can be divided into
3 groups:
1. Outer moons, eccentric orbits, many
retrograde, dark surfaces, captured asteroids.
2. 4 inner moons orbit very close to Jupiter and
are probably fragmented moonlets (form and
shape Jupiter’s ring).
3. 4 Galilean moons, nearly circular orbits,
smallest is 5,000 times more massive than
the largest of the other moons.
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The Jovian Planets
Jupiter’s Moons
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Io, the Galilean moon closest to Jupiter, has
active volcanic sulfuric geysers.
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Creates many surface layers
But does not build high volcanoes (lava too fluid)
Io’s heat is produced by tidal forces caused
by Europa and its eccentric orbit around
Jupiter.
Io is surrounded by a halo of sodium atoms,
which itself is embedded in a sodium torus
that surrounds Jupiter.
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Courtesy: NASA
Io during eclipse
The Jovian Planets
Jupiter’s Moons
Volcanoes on Io
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How? Should have cooler interior than
Mercury and Mars (smaller object).
Io’s elliptical orbit forced by resonance with Europa
and Ganymede causes differential tidal heating.
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Io is tidally distorted more when closer to Jupiter than farther
away.
This constant flexing heats the interior.
Spewed material from volcanoes forms torus of
sodium(?) around Jupiter (Io Torus).
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The Jovian Planets
Jupiter’s Moons
Resonances
 Previous examples: spin-orbit
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New examples: spin-orbit
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Moon (1:1) around Earth, Mercury (3:2) around Sun
All other major satellites to parent planet (1:1)
New examples: orbit-orbit
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Io-Europa (2:1)
Europa-Ganymede (2:1)
Later, in Saturn’s rings: Mimas-Cassini Division (2:1)
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The Jovian Planets
Jupiter’s Moons
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Europa’s surface is ice; its moderate density
indicates a rocky world covered by an ocean
of frozen water.
Europa also experiences some tidal heating
which has resurfaced it.
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Ice rafts and lenticulae (100-m ice mounds)
This tidal heating of Europa also suggests
that an interior liquid ocean of water may
exist.
Europa is the smallest of the Galilean moons
(and is smaller than Earth’s Moon).
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The Jovian Planets
Jupiter’s Moons
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Ganymede has a surface that appears similar
to our moon.
The surface is composed mostly of ice.
With fewer craters than Callisto, some
resurfacing has occurred.
Ganymede is the largest moon in the solar
system.
It also generates its own magnetic field
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How? There may be a layer of salty-water buried 150
km beneath the surface
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The Jovian Planets
Jupiter’s Moons
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Callisto also has a surface that appears
similar to our moon.
The surface is composed mostly of ice.
There may be a water ocean below the
surface.
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Callisto is very heavily cratered implying that
it is tectonically inactive.
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Radioactive heating may contribute
There is a detectable magnetic field
May be the oldest surface in the solar system
Callisto’s interior is appears to be
undifferentiated.
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Closer to Jupiter
Further to Jupiter
Dense
Less Dense
Younger
Io
Older
Europa
Ganymede
Callisto
Courtesy: NASA
Size of Earth’s Moon
Galilean
Moon
Stats
Surface age determined
by crater counts
The Jovian Planets
Saturn’s Moons
Moons of Saturn
 Saturn has 62 moons, second only to
Jupiter in number.
 Major moons include (from largest to
smallest): Titan (second largest moon in
the Solar System), Rhea, Iapetus, Dione,
Tethys, Enceladus and Mimas.
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Some Moons of Saturn
Rhea
Mimas
Courtesy: NASA
Enceladus
Close-up of surface
Enceladus
Enceladus has a very shiny
surface (albedo = 0.9) and
has just been discovered to
have a “significant”
atmosphere (which must be
Courtesy: NASA
replenished)
False-color image of
anti-Saturn hemisphere
Dione
Tethys
Courtesy: NASA
Iapetus (the two toned moon)
Phoebe
enlarged
Courtesy: NASA
Phoebe
Hyperion (next page too)
Hyperion
Courtesy: NASA
The Jovian Planets
Saturn’s Moons
Titan
 Titan may be the most interesting moon in
the solar system because it has an
atmosphere (How?).
 It is composed mostly of nitrogen with 1%
methane and a trace of argon.
 When sunlight strikes methane, it can
cause the formation of organic molecules,
which are a known precursor to life.
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Titan
Rhea
Courtesy: NASA
The Jovian Planets
Uranus’s Moons
Uranus’s Moons
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Five moons were known before Voyager (Miranda, Ariel,
Umbriel, Titania, Oberon); now 22 more are known (total
= 27).
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Many moons named for Shakesperian characters.
All the moons appear to be low-density, icy worlds (but
they appear to have had been more active than the
Saturnian satellites of a similar size).
The innermost, Miranda, is perhaps the strangest looking
object in the solar system. It appears as if it were torn
apart by a great collision and then reassembled.
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The Jovian Planets
Neptune’s Moons
Neptune’s Moons
 Before Voyager 2, Neptune was known to have 2
moons; 13 moons are now known.
 Triton, Neptune’s largest moon, is the only major moon
to revolve around a planet in a clockwise (retrograde)
direction.
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Causes significant enough tides on Triton.
Triton is also tilted 23 deg relative to Neptune’s equator
Triton has a very thin atmosphere of N2 and CH4.
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The Jovian Planets
Neptune’s Moons
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Triton has a light-colored surface composed
of water ice with some nitrogen and
methane frost.
Its surface appears young, with few craters
and active geyser-type volcanoes observed
(nitrogen ice and carbon compounds).
Triton’s active volcanism is probably due to
internal heating from tides, heating from the
Sun or internal residual heat.
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The Jovian Planets
Planetary Rings - Saturn
Planetary Rings
 Saturn’s rings are very thin, in some cases
less than 100 meters thick.
 The rings are not solid sheets but are made
up of small particles of water ice or water-ice
mixed with dust.
 Three distinct rings are visible from Earth,
and were named (outer to inner) A, B, and C.
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Saturn from Earth
Voyager approaching Saturn
Voyager leaving Saturn
Courtesy: NASA
Mimas
The Jovian Planets
Planetary Rings - Saturn
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The largest division between rings is known as
the Cassini division.
This space is caused largely by the gravity of
Mimas acting synchronously (2:1 resonance)
on the orbital path of nearby ring particles.
Some other ring features are explained by the
presence of small shepherd moons.
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Close-up of Main Rings
A Ring
Courtesy: NASA
C Ring
Cassini
Division
B Ring
The F ring: Confined by
Shepherd Satellites Pandora
and Prometheus
The A ring
Cassini
Voyager
The Jovian Planets
Planetary Rings - Saturn
The Origin of Rings
 Saturn’s rings are probably about 100 million years old.
 The origin of Saturn’s rings is not well understood, but is thought
to be the result of:
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A close-orbiting, icy moon that shattered in a collision with an
asteroid .
A large comet which got too close to Saturn (much like
Shoemaker-Levy 9 did at Jupiter in 1994).
Rings around the Jovian planets are not billions of years old and
must be replaced or renewed on a much smaller time scale.
Tidal forces are greater on a moon in orbit close to a planet than
they are on a moon in an orbit farther out.
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The Jovian Planets
Saturn’s Rings
Roche limit is the minimum radius at
which a satellite (held together by
gravitational forces) may orbit without
being broken apart by tidal forces.
 Saturn’s rings are inside Saturn’s Roche
limit, so no moons can form from the
particles.
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The Jovian Planets
Planetary Rings - Jupiter
Voyager
from
“behind”
Jupiter
Courtesy: NASA
Jupiter’s Ring
Voyager I discovered a thin ring (system)
around Jupiter.
 The ring is close to Jupiter, extending to
only about 1.8 planetary radii.
 The ring is thought to be replenished from
the small moonlets within or near it.
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The Jovian Planets
Planetary Rings - Uranus and Neptune
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The rings of Uranus and Neptune and are
made of particles which are darker and
smaller than that of Saturn.
The Uranian rings are narrow, a few of which
are clearly confined by shepherding moons.
The Neptunian rings vary in width and are
confined by resonances of some of the
moons.
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The End
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