• Quiz #2 in discussion section this week
• Covers Chapters 18.1-18.6, 10,11,12
• Solutions to Homework #5 on the web
• Honors rough draft due soon
• Homework #6 out soon

• Io, Europa, Ganymede, Callisto
• Seeing moons from Earth
• General properties
• Formation theories
• Io
• Sulfur volcanos
• Plasma torus
• Europa
• Ice [over water?]
• Active surface
• Ganymede
• Ice covered, rock core
• surface not active
• Callisto
• Ice rock mix
• surface not active
• All of Jupiter's moons

• Galileo saw first
• dots moving back and forth every ~2--17 days
• deduced they were orbiting Jupiter
• analogy to Solar System
• Modern telescopes can see light and dark patches
• synchronous rotation about Jupiter
• verified with spacecraft
• Ratio of orbital periods:
• Io:Europa:Ganymede [:Callisto]
• 1:2:4 [:9.434]
• Tidal forces with Jupiter and each other are important to Io, Europa, Ganymede
• As big or bigger than our our Moon/Mercury
• Observing slight orbital deflections (and spacecraft deflections) gives masses
• Observing occultations (and spacecraft pictures) gives sizes
• Average densities (mass/volume)
• Io, Europa >3000 kg/m³
• Ganymede, Callisto < 2000 kg/m³

Note Titan orbits Saturn, Triton orbits Neptune

• Jupiter's moons formed at the same time as Jupiter
• Analogous to planets forming around the Sun
• large cloud
• dense concentration in center
• disk
• clumps in disk
• Lots of water ice at Jupiter's cloud
• Different densities of moons suggest Jupiter's internal heat played a role
• Close moons had water evaporated away
• Tidal heating allowed chemical differentiation
• Io, Europa (close moons)
• denser
• active surfaces (caused by warm interiors)
• Ganymede, Callisto
• less dense, less active
• Callisto not even differentiated
• Jupiter even has equivalent of asteroid belt
• inner small moons Amalthea, Adrastea, Metis, ...
• ring of rock dust (each particle <1mm)

• Io has active volcanos!
• Caused by warm interior bubbling out
• If Io is as big as our Moon, why is Io's interior warm and not the Moon's
• Tidal heating
• Io has no impact craters.  Why?
• Io is funny colored
• Sulfurous compounds
• "volcanos" are probably SO₂ (sulfur dioxide) geysers
• Material ejected from Io
• some electrons missing
• charges effected by magnetic fields
• material caught in Jupiter's magnetic field
• Io plasma torus
• material in plasma torus orbits every ~10 hours
• Io orbits every ~2 days

Io hemispheres

April 1997 vs Sept. 1997

Io plasma torus (one edge only)

• Average density 3020 kg/m³
• mostly rock
• Surface is ice
• No craters
• surface constantly renewing
• Mostly covered with ridges and groves
• Evidence of sub-surface liquid water or soft ice
• broken, re-floated regions
• Triple bands possibly stretched out geyser
• dark material on either side might be erupted

• Ganymede has craters
• surface no longer active
• Bright and dark terrain
• opposite of moon
• dark older
• bright newer
• Bright terrain has groves similar to Europa, but no rafts
• Ganymede has a magnetic field!
• indicates warm liquid metal interior
• heat not from current tidal forces
• may have previously had a eccentric orbit
• tidally heated at perijove
• orbit circularized by other moons
• still warm inside

• Callisto has craters
• surface no longer active
• Biggest crater named Valhalla
• Callisto has no groves or any sign of geologic activity
• other than slumping of Valhalla's rings
• Valhalla crater probably 4 billion years old
• No magnetic field
• Not differentiated

• 16 moons discovered so far
• Smallest is Leda (16 km)
• sizes in Appendix 3
• Outer 4 orbit backwards (retrograde)
• probably captured later