11/28/2011
The Jovian Moons
The Jovian Moons
• The Jovian planets are home to an
enormous number of moons.
• More than 150 moons between the four
planets.
– Jupiter has the most with 64
– Saturn has at least 62
• We will categorize the moons by size
– Larger than 1500 km in diameter
– 300-1500 km
– Less than 300 km
The Jovian Moons
The Jovian Moons
• These moons resemble terrestrial planets in
a number of ways
–
–
–
–
Spherical with solid surfaces
Varying geologies
Atmospheres (some)
Hot interiors and magnetic fields (some)
• Two are larger than Mercury (Ganymede
and Titan), four larger than Pluto (Io,
Europa, Callisto and Triton).
The Jovian Moons
• Differ from terrestrial planets in their
composition.
– Formed in the outer solar system
• Composition accounts for the geological
activity of some of the smaller (mediumsized) moons.
The Jovian Moons
• The Jovian planet systems (i.e. the planets
and their moons) resemble tiny solar
systems in their formation and dynamics.
• The medium and large Jovian moons likely
formed in accretion disks surrounding the
planets
– Explains their circular orbits near the equatorial
planes of their respective planets as well as their
orbital direction.
• Nearly all moons are also phase-locked to
their planets.
The Jovian Moons
• The medium and large moons are the most
interesting, but the small moons are the
most numerous.
• Many are likely captured asteroids and
comets
– Do not lie in the equatorial planes of their planets
for the most part
• Not much beyond appearance is known.
– Most are irregular, potato-shaped bodies.
Small Jovian Moons
The Galilean Moons
• The Galilean moons are the four largest
moons of Jupiter.
– Named for Galileo, who was the first to observe
them in 1609.
• The four moons are Io, Europa, Ganymede,
and Callisto
• Would be considered planets or dwarf
planets if they orbited the Sun instead of
Jupiter.
The Galilean Moons
Io
Io
• Io is the Galilean moon closest to Jupiter
• It is the most geologically active body in the
solar system.
– NO impact craters are visible on its surface.
• Surface is covered entirely by volcanoes and
volcanic debris
– Probably has tectonic activity, but it is buried by
volcanic debris
Io
• Volcanic outgassing provides Io with a very
thin atmosphere
– Mostly composed of sulfur dioxide, sulfur, and
sodium
– Most gases condense on the surface as ices,
giving the moon its distinctive colors.
Io’s Volcanoes
Io
• Why is Io so geologically active?
– About the same size as our geologically-dead
moon.
• The result of its elliptical orbit.
– Tidal forces from Jupiter periodically stretch and
compress the moon, heating its interior.
– Called tidal heating.
Tidal Heating
Orbital Resonance
Europa
• Europa is the smallest of the Galilean moons
and the second closest to Jupiter
• Europa is unique because its surface is
covered almost entirely by water ice(!).
• Few impact craters on surface
– Tidal heating similar to Io
• Interior likely consists of a metallic
core/rocky mantle and either water or warm,
convecting ice.
Europa
Europa
Europa
• Evidence strongly points toward Europa
having a liquid interior.
• Europa has a relatively significant magnetic
field
– Changes as Jupiter rotates
• Explainable if Europa has an interior of
conductive fluids (e.g. salt water)
– Salty deposits on the surface lend support to this
theory.
Ganymede
• Ganymede is the largest moon in the solar
system and the third Galilean moon.
• Surface has dual personality
– Dark, old regions and light, young regions
– Sometimes sharp boundaries between the two
• Young portions of the surface are thought to
be the result of liquid water welling up to the
surface
– Paves over old craters.
Ganymede
Ganymede
Ganymede
• Is Ganymede’s interior liquid like Europa’s?
– Magnetic field supports the idea
• One problem: tidal heating may not be
enough to keep its interior liquid
– May have retained some internal heat.
– Radioactive decay can supply some heat.
Callisto
• Callisto is the last of the Gallilean moons and
shows the oldest surface.
• Bright patches on surface are “clean” ice
blasted out by impacts.
• Dark, sooty material covers low-lying areas
– Maybe the debris left behind by sublimating ice.
• Interesting fact: Callisto never differentiated
– No significant source of internal heat
• May still contain a subsurface ocean.
Callisto
Other Moons: Titan
• Titan is the second largest moon in the solar
system and the largest of Saturn’s moons.
• Titan is unique in that it has a thick
atmosphere
– Completely hides the surface at most
wavelengths
• Atmosphere is mostly nitrogen with
hydrogen compounds mixed in
Titan
Titan
• Titan’s atmosphere is composed mainly of
greenhouse gases, but it is still very cold at
only 93 K
• This is so cold that methane and ethane are
able to condense out of the atmosphere and
fall as rain, leading to the creation of
methane and ethane rivers and lakes on the
surface.
Saturn’s Medium-Sized Moons
• Saturn has six medium-sized moons
– Tethys, Rhea, Dione, Iapetus, Mimas, and
Enceladus
• All of these moons except Mimas show
evidence of past geological activity.
– Enceladus shows evidence of ongoing geological
activity, probably the result of tidal heating
Saturn’s Medium-Sized Moons
Enceladus
Uranus’ Medium-Sized Moons
• Uranus has no large moons, but has five
medium-sized moons.
– Miranda, Ariel, Umbriel, Titania, and Oberon
• Largely composed of ice
• Ariel and Umbriel are nearly identical in size,
but Ariel shows evidence of volcanism and
tectonics whereas Umbriel is heavily
cratered.
• Miranda, the smallest moon, shows the
tremendous tectonic features and few
craters.
Miranda
Neptune’s Large Moon: Triton
• Triton is Neptune’s only large moon.
– Contains over 99% of the mass of all Neptune’s
satellites.
• Triton is unique because it has a retrograde
orbit that is highly inclined.
– Evidence that it is likely not a natural moon of
Neptune, but a captured object.
• Evidence of volcanic and tectonic activity as
well as a thin atmosphere.
Triton
Emerging Trends
• One thing should have stuck out about these
moons:
– A LOT of geological activity despite their small
sizes.
– Why?
• Composition
– Ices melt at much lower temperatures than rock,
meaning that less interior heat is needed to
trigger geological activity
Next Time…
Rings and Minor Bodies
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