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I have been wondering what type of communication network could you build to communicate as deep as possible into a gas giant such as Jupiter? Which technologies would you use at what depths?

One could always have satellites orbiting around the body using radio to communicate between them (or even higher frequency like terahertz and optical light when two satellites are within eyeshot of each other)

As you descend through the atmosphere of the gas giants radio would work well up to a certain depth You could probably have hot air balloons with radio antenna talking to each other and sending info up to space or down from space. Once the density gets high enough and you are dealing with a supercritical fluid around you, perhaps E&M Waves are no longer a viable medium of communication.

At these densities you could try to use sound to transmit information (high densities should lead to a fast speed of sound) and have effectively submarines floating around communicating via sound and that might allow you to travel pretty far down

But is there a depth where using sound will fail but there is still some other viable method of communication?

It's also not clear to me at what depths radio actually fails and if there's a spectrum of frequencies (ex: use XXXX hz up to 20,000 ft from space but use YYYY hz at depths exceeding 40,000 ft and use sound beyond that).

Basically I just want to play some imagining of a what a "as deep as possible" network would look like and what methods of communication would be roughly ideal for the various depths.

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  • $\begingroup$ Maybe some other type of radiation like neutron radiation. Neutron lasers are still quite theatrical but I’m not sure about other forms of generation $\endgroup$ Sep 5 at 7:42
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    $\begingroup$ I think it will turn out to be VLF or ELF, in which case the communication would be very slow. $\endgroup$
    – uhoh
    Sep 5 at 10:04
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    $\begingroup$ Wouldn't the atmospheric conditions be like a storm or a hurricane ? I think there would be too much noise for sound based communication (to the surface). $\endgroup$
    – AJN
    Sep 5 at 12:06
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    $\begingroup$ I did my Ph.D. thesis work on a very closely related topic. I don't have the time to answer this question right now but I'll post an answer later this week. $\endgroup$ Sep 5 at 22:38
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    $\begingroup$ @AJN Floating along in a hurricane, not resisting the wind, is quite peaceful. It's only when you resist the air currents, by anchoring yourself to an immobile ground or bashing your way through it with an airplane, that it becomes violent. The further you are from disruptive surfaces the quieter it gets. With no surfaces nearby (in jupiter we are taking hundreds of km!), the less focussed the windshear also gets. Compare turbulence at 40k feet vs 4k feet in a plane. The only real issue in Jupiter's storms would be the lightning, and huge aircurrents pulling you lower or higher than desired. $\endgroup$
    – PcMan
    Sep 14 at 8:06
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At JPL I participated in several studies concerned with telecommunication from depth at Jupiter (and other giant planets). In general, with radio telecom from within Jupiter's atmosphere, the lower you go in frequency, the deeper you can penetrate. But there is a practical limit.

Above about the 100-bar level (the depth in the atmosphere where the pressure is 100 bars) most of the absorption of radio waves is done by ammonia vapor and water vapor. Those molecules have absorption lines from wavelegths of a few tens of cm (not very many there, and they are weak) to the rotational and vibrational lines that are strong and there are oodles of them, at sub-mm and shorter wavelengths. Those strong lines make telecom at short wavelengths impractical at any considerable depth. As pressure increases a given line's linewidth (the band of frequencies over which there is significant absorption) also increases, so each line takes up more of the comm bandwidth, eventually overlapping with others and making large bands unusable. Even far (in frequency) from the line center, a line contributes some absorption, and that contribution increases as pressure increases. At radio wavelengths, orders of magnitude in frequency from the sub-mm and IR line centers, high pressures can cause the "wings" (far tails of an absorption line) of strong sub-mm/IR lines to cause more absorption than the lines within the radio band. Although this reference discusses absorption at optical wavelengths, the same principles apply, though with different absorption mechanisms.

Fortunately, on the low-side tails, the farther you get from the line centers, the weaker the absorption is. So the lower you go in frequency, the less absorption you get. Un-fortunately, the lower you go in frequency, the less bandwidth you have for communications, so lower data rates, video resolution, etc. This is the telecom trade to be made: how deep do you need to go, what data rate do you need, what frequencies are practical (low frequencies mean larger, heavier, and clunkier components like antennas, waveguides, etc.), and how much power do you have to pump into the signal to get it out to the receiver? The more power you need, the bigger and heavier is the power supply.

As you go below the 100-bar level other molecules get involved, such as phosphine, with absorption lines all the way down into the MHz range. Even hydrogen shows some collision-induced (molecular-to-molecule collisions, not collisions with the vehicle!) absorptivity. The atmosphere gets pretty opaque down there.

But it gets even worse. The deeper you get, the hotter it gets. At some point (roughly 1000-1500 K) the combination of temperature and pressure is enough to ionize some of the atmospheric constituents, so the atmosphere becomes highly electrically conductive. It's like trying to transmit through metal! Even with @uh-oh's VLF or ELF.

Before I left JPL to pursue life as a consultant, they were still trying to find practical ways to communicate from the 100-bar level at Jupiter to an overhead relay vehicle — an orbiter, a flyby spacecraft, whatever. I proposed an approach that appeared the most promising: use a staged probe, where one part deploys a big parachute and stays high in the atmosphere while another part goes deep and has to communicate only the relatively short distance to the upper part; then the upper part relays to the in-space vehicle. You can even use three or more stages. The main problem was that Jupiter's strong wind shear causes the upper one to be carried laterally far from where the deeper ones are, making the relay link deteriorate quickly. We thought of ways to tackle that, but they all involved fairly complex systems and were looked at as too risky.

The question and comments have mentioned non-electromagnetic means of telecom. Sound isn't very useful, as @AJN says, because it looks like Jupiter's atmosphere is one giant, complex storm. The noise level would be very high, meaning that to get a useful signal-to-noise ratio ('SNR') you'd have to put a lot of power into the sound. @James suggested gravitational waves, but sometime down the line ... way down the line ... way, way down the line! Let me know when I can buy a gravitational-wave generator of useful output that's smaller and lighter than a neutron star! A receiver, too, much smaller, lighter, and more sensitive than LIGO. Most particle-based schemes (like neutrons) also suffer from the dense atmosphere, as @Jon Custer alludes to: the mean free path ('MFP') in the deep atmosphere is short compared to the propagation pathlength needed to get out of the atmosphere. Neutrinos have much longer MFPs but have some of the same problems as gravitational-wave schemes: practical systems need to be much smaller and lighter than Super-Kamiokande.

The suggestion of relay nodes using hot-air balloons is a version of my staged-probe method. The two main problems are the wind shear, which spreads out the stack quickly, and: you don't get much buoyancy from heating air that's mostly hydrogen. The low molecular mass is a big problem, one that requires heating a large mass of air. The more air you try to heat, the larger (and heavier) the balloon envelope gets, and the more heat you have to supply due to convection and radiation losses. Many years ago I did a related calculation: using 238-Pu (the same isotope they use in NASA RTGs as the heat souce), how big does a Pu-heated jovian balloon have to be to support its own weight and the weight of the Pu? Using the highest strength-to-weight-ratio materials available at the time, it finally converged at a balloon radius of some 30 km, and needing mega-tons of Pu. And that's without any payload!

Communicating into and from planetary interiors has been, and continues to be, a problem that has a lot of smart people still scratching their heads.

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    $\begingroup$ I love these Spilker Specials, so chock-full of physics and history. I guess the solution is a cross between Starlink and Loon; an absurdly large number of intelligent balloons saturating a band of altitudes and latitudes with some kind of packet protocol $\endgroup$
    – uhoh
    Sep 14 at 11:13
  • $\begingroup$ Wow! Great answer! X-rays don't penetrate that well in gases, but, given the high bandwidth, could there be a pressure regime where they perform better than radio or optical wavelengths? nasa.gov/feature/goddard/2019/… $\endgroup$ Sep 14 at 15:27
  • $\begingroup$ @WaterMolecule One indicator of where X-rays could be useful is: where in the atmosphere are solar X-rays absorbed out of the solar spectrum? It turns out that is way up in the thermosphere, at pressures of a microbar and less. $\endgroup$ Sep 15 at 23:13
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    $\begingroup$ @uhoh Sorry, the term "Spilker Special" is already taken: my dad, an avid fisherman, created his own lure design that he dubbed the "Spilker Special", some 70 years ago. :-) Balloons, intelligent or not, all have the buoyancy problem I described in my answer. $\endgroup$ Sep 15 at 23:19
  • $\begingroup$ Ya I should have caveated my comment somehow. So as pressure increases going down, by the time the radio repeaters become necessary due to absorption the balloons have already become absurdly large? Was the Spilker Special for fly fishing or trolling (cast, reel-in, repeat) $\endgroup$
    – uhoh
    Sep 15 at 23:28

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