Rick Jones wrote:
That presumes Europa started thawed and then froze right - with the
ice expanding inwards as well as outwards? Otherwise, since water
expands when it freezes, if it started frozen and had its core thawed
by tidal (?) forces there might not be as much pressure.


I wasn't thinking of it generating pressure by squeezing the water
(which is pretty much incompressible anyway, but simply from the weight
of ice on top of the water... the gravity is far lower than that of
Earth (about 1/6 ours, like the Moon) but that means if you have a ice
shell six miles thick over you it will generate the same amount of
pressure as it you were around one mile deep in the ocean. Military subs
tend to operate at a maximum depth of 1,000 -1,300 feet, so that means
if the ice crust on Europa is over around a mile thick, you are going to
have a very hard time building a habitat under it or venturing outside
of it in any sort of diving suit.
Unfortunately, no one knows just how thick the ice is, with estimates
ranging from around 700 feet to around 19 miles:
http://en.wikipedia.org/wiki/Europa_(moon)
If the lower estimate is correct, you could swim around in it in
conventional scuba gear, as it would be like if you were around 100 feet
deep on Earth. If the upper estimate is right, then it would mean that
you could only move around in it in some sort of deep submergence
vehicle, as it would be equivalent to being three miles deep in the
ocean on Earth.
Since the truth probably lies between these two extremes, it's pretty
unlikely you are going to be doing much swimming in the cold Europian
seas unless you are wearing some sort of a atmospheric diving suit with
propellers on it, and probably even that wouldn't be tough enough.
Which is just as well, as Europian Sono-Squids, though not as dangerous
as Titanic Titanian Methane Muskies, are still nothing to be trifled
with - particularly given they have a average IQ approaching that of
Einstein, and a hunger as rapacious as that of a six-year-old at the
movies. They would look upon human divers in hard suits entering their
realm as simply witless snack food that had delivered itself into their
hands like so many salted-in-the-shell peanuts and gnaw on them as they
discussed the finer nuances of the thirteen body problem. :-)

Pat