Composition Of Outer Satellite Ices, What are Jupiter's moons made of? Options

[ljk4-1]

Does the military (or a military) have the capability of communicating
through thick ice when their submarines are, say, under the Arctic ice
pack, or is it all hush-hush?

I recall a line from the novel version of Tom Clancy's The Hunt for
Red October how whatever the military designs in secret the public
sees about twenty years later.

[helvick]

I recall a line from the novel version of Tom Clancy's The Hunt for
Red October how whatever the military designs in secret the public
sees about twenty years later.

Naval surface to subsurface radio comms using ELF (Extremely Low Frequency) transmission was around and not very secret since the late 60's. It can\could penetrate up to few hundred feet of water and possibly ice but it's technical limitations are severe.
The US version used a carrier frequency of 76hz. Data rate was around 3 minutes per character (!). The effective transmitter antenna length needed to be somewhere in the 200km range although the physical installations used were in the 10's of km range and took advantage of local geology to be efficient enough to work.
The biggest problem with it is that it was a one way communication channel - that probably had something to do with the 200km transmitter antenna - and was only ever really used as a "bell ringer" to alert subs so they could then manoeuver to a location where two way comms was possible.
Some relevant info here - can't vouch for the site but the pdf docs look fairly official.
It was fully decomissioned a couple of years back presumably because the neutrino comms system had finally come on stream allowing high speed DTE\DTW* communications.

*Direct Through Earth\Direct Through Water. **

** This is a joke.

[tty]

Radio/radar through water is heavily dependent on whether the water is fresh or not. Salt water is a fairly good conductor which means that radio waves are quickly absorbed (the decline is more or less exponential), normally the penetration depth under such circumstances are only a small fraction of the wavelength. This is the reason for ELF. Even a small fraction of a 76-Hz wave is quite a long way. Ordinary LF ("Long-wave") can be used for communicating with submarines, but only if they are close to the surface ("periscope depth").
As for the arctic pack ice I think that ordinary LF would probably pentrate to a sub that was just under it if you used a strong transmitter.

In principle things work the same for ice, but penetration is better than for water. That's why the MARSIS scientist feel fairly sure that there is no water layer under the polar ice. It would presumably be at least somewhat salty and thus show up as a more or less impenetrable layer.

As for Europa a strong radar might be able to penetrate the ice if it is not too thick, but almost certainly not an underlying ocean (which would be salty). That would require seismic measurements. i. e. set up a network of seismometers and then set of a lot of bangs, or you might perhaps use a rover/hopper with some kind of a "thumper".

tty

[dvandorn]

I'm not so sure you would need any kind of artificial seismic inputs (thumps, bangs, or impacts) to study Europa seismically. I once read an analysis of Europa's position in the Jovian tidal sequence that predicts Europan surface movement on the order of hundreds of meters per orbit. Whether the surface is a thin or a thick layer of ice over a subsurface ocean, it *must* flex to some degree under the tidal stresses.

That flexing would provide sufficient seismic energy that a set of passive seismometers ought to be able to get a lot of internal structure information, I would think...

-the other Doug

[tty]

Yes, but you need a minimum of three seismometers operating simultaneously, and even then there would be a great deal of ambiguity in the data. You can get much more precise results when You know exactly when and where the energy release is.

tty

[ljk4-1]

Did Galileo ever detect any surface shifts? Are comparisons between
the Voyager images of Europa and Galileo years later possible?

[Rob Pinnegar]

Yes, but you need a minimum of three seismometers operating simultaneously...

For triangulation, yes. But a single geophone would be a damned sight better than nothing. My guess is that it would produce something useful.

[Guest_BruceMoomaw_*]

There's been a HUGE amount of discussion at science working groups over how much a single seismometer (including a short-lived one) could tell us about Europa. The consensus at this point is that it would be worthwhile -- at least to tell us the overall seismic activity background to design a better seismometer later, and maybe for additional information as well. Nicholas Makris says that measuring the speed at which seismic waves of different frequencies arrive could probably by itself tell us how thick the ice crust is.