Transcript Rings and Moons of the Giant Planets Lecture 16: The Planets Homework • Read Chapter 11: “Jovian Planet Systems” • Homework: Mastering Astronomy due Friday at 6pm.

Rings and Moons
of the Giant Planets
Lecture 16:
The Planets
Homework
• Read Chapter 11:
“Jovian Planet Systems”
• Homework: Mastering Astronomy
due Friday at 6pm.
Observing Project #2
Motions of Stars
• At an exact time you specify, i.e. 7:00pm, go to a
location where stars are visible. Sketch the
foreground objects (buildings, trees, whatever) and
the location of ~5 stars (placed as best you can).
• About a week later, at the same time and location,
sketch the new positions of the stars relative to the
foreground objects. Repeat each week for a total
of four sketches.
Lecture Outline
1. Saturn’s rings: Made of icy particles
2. “Four Galilean Moons” of Jupiter
Io: Volcanic
Europa: Ice crust; Ocean below
Ganymede: Icy, craters
Callisto: More ice, craters
3. Moons of Saturn:
Titan: Methane atmosphere, lakes
Enceladus: Water Geysers!
Rings: All Four Giant Planets Have Them
Jupiter
Saturn’s
rings
labeled
A-F
Neptune
Saturn
Uranus
Saturn’s Rings: Names
Saturn’s Rings: Names
Saturn’s Rings: Faint Ones
Saturn’s Rings:
What are they made of ?
Clue: View the Rings from the back side.
Observe the scattering of sun light off the back-lit rings..
Saturn’s Rings:
View from Back Side: Not Lit by Sun
The Rings scatter light forward:
Like the silver-lining (edges) of clouds.
(Big rocks would be dark on the dark side.)
The rings must be composed of tiny particles.
Saturn’s
rings
labeled
A-F
Saturn’s Rings: Viewed Edge-On
Ring Thickness: Much smaller than tiny moons.
Saturn’s
rings
labeled
A-F
Saturn’s Rings:
What are they made of ?
Spectroscopy of Rings: Water Ice and Rock !.
Saturn’s Rings:
What are they made of ?
• Small, Dirty icy particles
• Ring particles orbit around Saturn
• Flat ring (few meters thick)
• Stray orbits cause collisions between particles:
Saturn’s Rings:
What are they made of ?
• Small, Dirty icy particles
• Ring particles orbit around Saturn
• Flat ring (few meters thick)
Rings of Saturn: Made of Icy Rocks
Saturn’s Rings
10 sec delay to start.
Saturn’s B-Ring and A-Ring
The ring’s outside edge is influenced by meddling moon Mimas, which orbits the planet once for every two circuits the icy ring particles complete.These
periodic gravity perturbations are thought to compress the ring particles into clumps, while maintaining the ring’s well-defined outer edge.Beyond
the B-ring lies the Huygens gap, the widest dark void visible in this image, punctuated only by the bright Huygens ringlet.
The 4800 km-wide Cassini Division separates the B-ring
from the outermost A-ring, but itself is marked out with faint, concentric strands of ring material.
Encke gap
The Moon “Pan”
orbits in the middle.
Image of a gap moon that creates
ripples ahead of itself.
Where do gaps
in the rings
come from?
Gap moons
The motion of moon Pan through the A ring's Encke Gap.
Shepherd moons
change and sculpt the rings
The moons Prometheus
and Pandora shepherd
the F ring of Saturn.
Shepherding moons
Sculpt the Rings
The moons Prometheus
and Pandora shepherd
the F ring of Saturn.
Moons of Jupiter:
Satellites of Ice and Rock
• What kinds of moons orbit the jovian planets?
• What makes Jupiter’s Galilean moons unusual?
• What makes Saturn’s largest moon Titan different
from other moons?
• Why are small icy moons more geologically active
than small rocky planets?
Many moons …
• Jupiter has over 60 moons
• Saturn has over 30 moons
Medium &
Large Moons
• Enough self-gravity
to be spherical
• They are (or were)
geologically active
• Have substantial
amounts of ice
Medium &
Large Moons
• Formed in orbit around
jovian planets
• Circular, equatorial orbits
in same direction as
planet rotation
Small moons
• Far more numerous than the medium and large moons
• Not enough gravity to be spherical: “potato-shaped”
Small moons
• Captured asteroids,
so orbits do not
follow patterns
• Orbits can be tilted,
elliptical, and even
backwards!
What makes Jupiter’s
Galilean moons unusual?
Io, Europa, Ganymede, Callisto
Earth’s Moon for scale
What makes Jupiter’s
Galilean moons unusual?
• Io has volcanoes
• Europa has an ocean under its ice
• Ganymede & Callisto may also have
sub-surface oceans
…but they’re so small???
Shouldn’t they be cold and geologically dead?
(like our moon and Mercury)
Jupiter’s moon:
Io
Prometheus Plume: ~75 km
Pillan Patera Plume: 140 km
Jupiter’s Moon, Io
Are there any impact craters?
Io is the most volcanically active world in the solar system.
No single impact crater left. WHY?
Galileo spacecraft caught this volcanic eruption in
action on Jupiter's moon Io on Nov. 25, 1999
Tidal Heating
Io is squished and
stretched as it orbits
Jupiter
But why is its
orbit so elliptical?
Orbital
Resonances
Every 7 days,
these 3 moons
line up.
The tugs add up over
time, making all 3
orbits elliptical.
Which moon has the strongest
tidal heating?
(A) Io
(B) Europa
(C) Ganymede
(D) Callisto
Europa – A Water World
with a Sub-Surface Ocean?
Europ
a
Density: 3 g/cm3 (same as rock); High reflectivity (60%);
Spectrum: absorption lines of water ice
Internal structure: metal-rich core, rocky mantle,
100 – 150 km thick crust composed of water/ice
Large Impacts
Few large impact craters: Suggests 60 Myr surface age.
A couple of multi-ringed impacts: Penetrated 20 km thick ice!
Ridged Plains
Agenor linea: bright white band cutting through
darker area. Young (few craters)
Colored regions higher resolution (135 x 60 km)
Conclusion: Europa is tectonically active!
Fast spreading band on Europa
Delphi crack having the
shape of a cycloid
Europa: best candidate
for life beyond Earth?
Ganymede
• Largest moon in the solar system (2xEarth moon)
• Clear evidence of (very old) geological activity
• Tidal heating expected - less than Io, Europa
Ganymede: tectonic features, but lots of craters
Bright grooved terrain and old, heavily cratered areas
Evidence of shear motion
G
a
n
y
m
e
d
e
Enki Catena Crater Chain On Ganymede Galileo Image
Callisto
• “Classic” cratered
iceball
• No tidal heating - no
orbital resonance
• Furthest from Jupiter
among all four
Galilean moons
Io: Fe/FeS core,
rocky mantle
Europa: Fe/FeS core, rocky
mantle, ~100 km water/ice
crust
Ganymede: Fe/FeS
core, rocky mantle, ice
crust
Callisto: ice/rock
mixture, icy crust.
Jupiter’s big
Moons
• Like a mini solar system
• Decreasing geological activity from
innermost to outmost satellite
(decreasing importance of tidal heating)
• Increasing amount of ice with increasing
distance from Jupiter
• Europa and Io are geological active now
What makes Titan different
from other moons?
TITAN: Voyager encounters
Atmosphere: very thick
90% Nitrogen
few% Methane, Argon
Surface Pressure: 1.5 atm
Surface Temp: 94 K cold!
Bulk Density: 1.88 g/cm3
(rock+ lots of ice)
Gravity << Earth’s
Voyager images
of Titan
Titan up close
Landed July 2004
Art by Craig Attebery
Saturn’s Moon: Titan
Lakes (radar-smooth regions)
Saturn’s Moon: Titan
Lakes (radar-smooth regions)
Saturn’s Moon: Titan
Lakes (radar-smooth regions)
Saturn’s Moon: Titan
Lakes: Imaged by the Cassini Spacecraft
Saturn’s Moon: Titan
Lakes (radar-smooth regions)
Saturn’s Moon Titan: Mountains and rivers
Titan:
Fluid transported
cobbles of ice
15 cm
Titan’s smoggy atmosphere
• 90% nitrogen, the rest argon, methane,
ethane, other hydrogen compounds.
• Methane & ethane are greenhouse
gases
• Still cold: 93 K (-180 degrees C)
Enceladus
Surface age?
Tectonics?
Volcanism?
Enceladus
Water Geisers on Enceladus
John Whatmough (extrasolar.net)
Water Geisers on Enceladus
John Whatmough (extrasolar.net)
Water Geisers on Enceladus
Water Geisers on Enceladus
John Whatmough (extrasolar.net)
Geisers on Enceladus
John Whatmough (extrasolar.net)
Water (ice and vapour) ejected from the surface
Cryovolcano
Volcanic activity on Enceladus?
John Whatmough (extrasolar.net)
Why are small icy moons
more geologically active
than small rocky planets?
Why are small icy moons more geologically
active than small rocky planets?
• Hot interiors are necessary for geological
activity
• Ice deforms more easily than solid rock,
so less internal heat is required, and
smaller objects can be geologically active
• Tidal heating is not important for rocky
planets (except Io)