Dr. Henry Throop / University of Colorado
Astronomy 1110
June 21, 2000
Lecture 13: Minions of the Giant: The Galilean Satellites

Announcements
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Observatory: tonight, 9-11 PM
Exams
HW #3: due tomorrow 

Today is summer solstice!  Day = 5:30 AM .. 8:30 PM = 15 hr day, 9 hr night 

Satellites of the Jovian Planets

Introduction
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Each about size of our moon
If Jupiter wasn't in the way, we could see with our naked eye

If you go thru calculations, you can figure out that they are much
colder than our moon -- and it's like exam problem: at 5 AU, they
should be cold, icy, dead.

But they're not.

Look at orbits: in resonance
Resonance: any integer ratio between orbital or rotational periods;
e.g., 1:2 rotation, 1:10 orbital, 2:3 orbital:rotational, etc.

Resonances often add or subtract energy in a system, and slightly
change the orbital positions and times of planets or moons.

Example: pushing a swing at the right (or wrong) time adds (or
subtracts) energy from the swing.

Think of a resonance: singing in shower, hit a low note, and whole
shower resonantes.  It's because you're adding energy into the walls of
your shower, and the note your singing -- its period -- is maybe twice,
or one third, or etc -- the natural period of the shower walls.

Like pushing a swing.
Demo: hang a planet from ceiling and give it a push.
  Push it every time, or every two times: 

Consider them to be TP's
  o Same geological processes
  o Not much in the way of atmospheres -- no gravity
  o Thus, no erosion.  Lots of other processes (volcanism, tectonics, impacts -- and more?)

Heat sources
  o Sun (e.g., Earth atmo)
  o Differentiation (e.g., Jupiter core)
  o Radioacticity (e.g., Earth core)
  o Tidal heating (e.g., Io, Europa, Ganymede)
  o Impacts

Slideshow
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Io: 
  One of the most exciting moon in SS
  Tidal forces squeezing it -- like bending a paperclip, it gets hot.
    If you bend paperclip long enough, it melts & gets volcanoes going on it, too!
  Lots of rocks.  Just like Mercury is the densest planet, & closest to sun,
    so is Io densest and closest to Jupiter.  Lost its volatiles, too -- too hot for
    water.

Europa:
  Liquid ocean?
  All water
  Can't see liquid water on surface.  But, see _evidence_ for it:
    Frozen solid ice on top
    Looks like it recently cracked through
    Count the craters!  Don't see any of them!  Therefore, this was all liquid recently.
  Also 
  
Ganymede:
  `Grooved terrain' -- big mystery.  Doesn't look like anything on Earth

  Q: If it's cold, what could have heated it?

Callisto:
  Getting colder, since moving slower and this tidal forcing is
  weaker.  No resonance.  Looks just like our moon
  Icier than ours -- like Pluto, it has an 

  On our moon, atmosphere will boil away, and also surface ice sublimates.
  On Callisto, atmosphere boils away, but cold enough that surface ice stays there

I Eat Green Cows

Saturnian moons:
  More `normal' overall
  Mostly like our moon -- a bit icy, some different colors

  Mimas -- looks almost shattered
  Miranda -- looks like it did shatter
  Our earth -- we saw video yesterday -- probably did shatter
  Check out HW #3 for more details

Rings
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Take pile of sand & throw it into air.  What happens with it?

  Not gas.  Some people think they are, but gravity just doesn't work
  that way.  Gas either gets attracted to planet, or it doesn't.

  On Earth, makes a little cloud
  But will eventually go into orbit

  Lots of resonances.  Maybe resonances heat particles a bit, but
  mostly they change their orbits.

  Tidal forces: 
    talked about SL/9 breakup
    Rings happen in same way

  Most of the moons are _outside_ Roche radius, and rings _inside_ Roche radius.

  Demo of ring tilt.

  Demo: chalk in a bag: collisions over & over make tiny, tiny particles

  Demo of Keplerian shear -- get some nice rocks.

  Get a fan!  And slide projector.  And make ring dust all over the room!

  Or, breathe on it -- solar wind.

  Clumps of explosions -- like F ring.

  Concl: rings are probably pretty young

Collab q's
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Which of these is a resonance?
  Ring A orbits 2x every time satellite B orbits 5x.(Mimas gap)
  Planet A orbits 2x and rotates 3x in a given time. (Mercury)
  Planet A orbits Sun 2x every time Planet B orbits 3x.(Pluto/Neptune)
  Satellites A, B, C orbit Jupiter 1x, 2x, and 4x in same time. (GLL Satellites)
  All of the above

As a planetary scientist, you see new pictures of Jupiter's moon Europa
that make it appear that the moon is (or was recently) warm, with
liquid water near its surface.  What heating could cause this water to
be melted?

  Impact craters
  Tidal forces & resonances
  Small craters & micrometeorites hitting its surface or atmosphere
  Differentiation

Saturn has the biggest ring system.  Why?

Observing
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Props
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Moon ball
Chalk
Chalk bag & bison ball
Bag of Rocks
Fan
Asteroid on a string