Lecture Notes:
Lecture 25, April 7
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READ SNOW CHAPTER 13, PAGES 272-286
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RESONANCES OF THE GALILEAN MOONS
The perfect 1:2 resonance of Io and Europa and also that of Europa
and Ganymede means there must be powerful gravitational coupling between
these moon. The result is the tidal heating we find evidence for on each
of these three.
The earth's moon spirals outward at the rate of 4 cm/century due
to such tidal coupling. The moon raises tides on the earth's oceans which
are pulled ahead of the moon due to the rotation of the earth. The result
is that the moon is drawn forward in its obit and spirals outward. {The
opposite would occur if the moon were revolving around the earth in a
retrograde direction. In that case the moon would be slowed down and would
spiral into the earth, to be eventually destroyed}.
The gravitational interaction between IO and Jupiter is some 300
times greater than that between the earth and moon, because of the greater
mass of Jupiter. Hence Io should be forced to spiral outward at an even
greater rate. It eventually developed the resonance with Europa; as that
tidally driven outward spiraling continued for both Io and Europa, Europa
moved into a 1:2 resonance with Ganymede, the largest moon in the solar
system. With its large inertial mass, its resisted further outward motion
and has not yet come into a 1:2 resonance with Callisto. Perhaps it will
in the future.
Like a cracked crystal ball, Europa is the smoothest of the
objects in the solar system, a version of an Olympic ice skating rink,
frequently flooded by water. Cracks in its ice layer may allow geysers
to
gush forth.
On February Galileo flew within 363 miles of Europa producing spectacular
evidence for an ocean is in the form of ice bergs that have broken off
an
ice sheet, similar to what one finds in the Antarctic. These bergs, or
ice
rafts, have moved and rotated suggesting tidally driven ocean currents
beneath the ice crust. The crust may be 2 km thick and may overlie an
ocean as great as 60 miles deep (compare to the 3 mile deep earth's
oceans).
see http://www.jpl.nasa.gov/galileo
for the spectacular images produced by
the Galileo spacecraft.
Who knows what alien creatures lurk in its depths. The ice layer would
absorb some of the incoming Van Allen radiation, which actually might
speed up evolutionary changes by inducing mutations. Life exists in the
depths of the earth's oceans where little sunlight enters but volcanic
activity produces the heat it needs.
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