-----Original Message-----
From: Larry Klaes <[EMAIL PROTECTED]>
To: Icepick (E-mail) <[EMAIL PROTECTED]>
Date: Monday, June 18, 2001 11:50 AM
Subject: Could this water bulge detection work for determining sub-ice crust
ocean on Europa?
>
>Satellites measure bulging earth to map water resources
>
>http://www.spaceref.com/news/viewpr.html?pid=5174
>
>Just as a sponge expands when absorbing water, so too does the Earth
>bulge slightly where aquifers, underground areas of permeable materials,
>absorb unusually large amounts of water from stream runoff or heavy rains.
>Scientists using satellite data have been able to measure these bulges on
>the land surface and believe they can use the technique to study the
>location and size of aquifers in remote regions.
Yep, it could -- or a modified version of it could, and has in fact been on
Europa Orbiter's list of required instruments from the start. The
difference is that it would measure TIDAL bulging in the ice crust. Since
Europa's distance from Jupiter varies slightly over its orbit, the height of
the tiddal bulge produced in its surface by Jupiter naturally increases and
decreases too -- indeed, this produces Europa's tidal heating (although the
degree of flexing, and thus the frictional heat it produces, is only about
1/10 as much as for Io). If Europa has no liquid-water ocean underlying its
ice crust, its surface flexing over each orbit is probably only about a
meter -- whereas, if it has a liquid-water layer underneath that is pulled
toward the Jupiter-facing side and forces the overlying ice layer to flex
upwards, the degree of tidal flexing could be as much as 30 meters, and a
laser altimeter on the probe can easily measure the difference.
The purpose of this instrument was to simply settle the question of whether
Europa has an ocean if the ice crust turned out to be too thick for the
radar sounder to penetrate. That question has now been pretty firmly
answered in the affirmative by Galileo's induced magnetic field
measurements, but the instrument is still worthwhile as further
confirmation -- and to locate individual areas where the crust may be
thinner (or, if the ocean is actually subdivided into several large isolated
basins, to locate them. Galileo's magnetic measurements, however, do also
suggest that Europa's ocean is big enough to be completely unified over the
satellite.)
One further note: there's a positive feedback effect here. The thinner the
ice crust was and the thicker the ocean wsa to start with, the greater the
tidal flexing of the ice crust would be -- and so it would generate more
frictional heat to sustain the existence of the ocean after Europa cooled
down after its origin. This is one reason why theorists were completely
uncertain about the odds of a Europan ocean until Galileo got there; the
chances of an ocean still existing today depended overwhelmingly on how
thick the ocean was during Europa's earliest days, which of course was
completely uncertain. This also means that individual patches of melting
inside the ice crust -- whether they take the form of pockets of liquid
water or of warmer and more ductile ice -- tend to amplify themselves by
producing more tidal flexing (and thus more frictional heat) in their
immediate vicinity, and so they may tend to grow quite dramatically in size
after a small origin.
==
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