Is there some form of seismic study that could be conducted to determine whether gas giants have cores of heavy elements, or whether they are H/He all the way down? It would certainly open up the debate as to whether they are stars or planets...

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There is a growing tendancy to think of Man as a rational, thinking being, which is absurd.- Marvin the Martian.

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

Actual seismic study would not be necessary; careful observation of the planet's shape and moment of inertia would probably clinch the case. This is in fact what the planned Juno mission will do, provided it gets off the ground.

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

So what's a "gas giant"? Do you mean something like Jupiter? We don't need to know anything about the interior structure of Jupiter to tell whether it's a planet or a star. All we need to know is the total mass, which is ~0.001 solar masses, which automatically makes it far too wimpy to be a "star". Below about 0.08 solar masses there can be no nuclear reactions typical of a star, i.e. PP fusion. Jupiter is well below the limit.

However, it would be nice to know the internal structure in any case, just to learn about planets. In fact the higher order moments of the graviational field can give you lots of information about the interior structure. But since we can only see the atmosphere, I don't think we can rely on seismic methods. For that, you need confidence that you see sound waves that go all the way through the planet, and not just through the gaseous atmosphere.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

I'm not convinced that fusion is a defining characteristic of a star; objects with as little as eight jovian masses have been referred to as brown dwarfs. If an eight-jovian-mass object can be called a brown dwarf, why not a four-jovian-mass object? And if a four-Jovian-mass object, why not two? And if two, why not one?

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There is a growing tendancy to think of Man as a rational, thinking being, which is absurd.- Marvin the Martian.

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

I think the point is, you can always understand the basic physics of what the hydrogen is doing just by looking at the mass. As we want to define stars based on what the hydrogen is doing (when there is lots of hydrogen, including deuterium), Tim's point is that we can characterize stars just by the mass. Now, one may be skeptical that a pure theory model is really that reliable, but that would be a hard position to maintain, as stellar structure calculations have been staggeringly successful so far. So I think your question really comes down to the latter part-- how do we know if Jupiter has a core of heavy elements? Seismology is not the only game in town there, and I think Romanus' approach has greater promise.

Imagine a simple but sturdy probe descending at low speeds into Jupiter. It will remain descending until one of five things happens.

1. It encounters a temperature high enough to melt it.
2. It encounters a pressure high enough to crush it.
3. It encounters a density high enough to make it neutrally buoyant.
4. It encounters a solid surface.
5. It comes to rest at the centre of the planet.

1 and 2 are not independent of one another; by choosing the right materials for the probe (which should be completely solid, with minimal differences in internal density), one might ensure that the high pressures counteract the high temperatures (i.e. the material of the probe will not undergo a phase transition, until the temperature versus pressure curve intersects a phase border).

A probe could probably be designed which will not become neutrally buoyant even in liquefied hydrogen or helium.

The probe should be able to send some simple but powerful signal upwards through the layers above it, which can then be relayed to Earth by satelites orbiting Jupiter (one single satelite in Jovostationary orbit might be enough). The signal should merely convey that the probe is still functioning, whether it is still descending, and whether it is feeling any weight. Temperature and pressure readings might be nice, but would not be essential.

Such a probe might go a long way in determining the internal structure of the planet.

I noted this in a post in one of VenusROVER's threads. We *could* build a probe that has a very large thermal mass so that it could have electronics continuing to operate long after the outer layers start vaporizing.

It is doubtful that we could make something that could survive all the way to the 20,000K core, but we could get well below the melting point of our highest temperature ceramic materials.

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The cut-off between "planet" and "brown dwarf" is 13 Mj, not 8! This is not an arbitrary number (although it is somewhat uncertain - 14 +/-1). Below this mass, the internal temperature is not high enough to initiate DD fusion.

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I have serious dobts here about the feasibility of your project, because option 1 and 2 seem the most likely outcome in any kind of probe.

There was the Galileo Descent probe. It did exactly what you describe and encountered option 1 and 2 after 200 (of 71400) km into the atmosphere. Pressure was 22 bar, temp. was 153 °C. Granted one could have built a sturdier probe (The Soviet Venera probes are ptrobably the record holders), but it is safe to say options one and two will always win, even if you grant 20 times that depth (even that would not penetrate Jupiter to much more than a tenth of its radius).

Then there would be the issue of broadcasting through this kind of atmosphere. I know very little aobut the field, but how would you build a strong enough antenna that could withstand the terrific gravity, and the pressure, and the heat?

Finally, there is "Jovostationary". Stationary wrt to what, on a gas giant that has a differential rotation?

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Getting signals out would be a huge problem. We usually use electromagnetic radiation, but there is no frequency range that could escape from near the center of Jupiter.

On the presumption that we were sending such an insanely large probe that it had thermal mass to make it a significant way down, we could build an accelerator in the probe which emitted enough neutrinos that we could see a pulsed signal. This might require building something like IceCube, only larger on Callisto.

edit: Obviously I'm not talking about doing this any time in the next couple hundred years.

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Forming opinions as we speak

Doesn't stop people from referring to lone planetary mass objects as brown dwarfs. From no less an authority than the IAU:

also, see this article:

http://www.nasa.gov/vision/universe/...-20051129.html

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There is a growing tendancy to think of Man as a rational, thinking being, which is absurd.- Marvin the Martian.

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

I feel that it should be possible to build a probe able to withstand temperatures up to 3500 Kelvin, which is nearly the melting point of tungsten under Earthly pressure. I assume that even higher melting points could be achieved under Jovian pressures, but we would need a materials specialist to look into this.

I don't feel that carrying excess mass to cool the probe by evaporation would be a good idea, because this would enormously increase expenses (it has all to be lifted from Earth in the first place and lifted from the central Solar System in the second place). For the same reason I would not be in favour of carrying refrigerators. Lets keep it simple and robust. The less there is actually going on inside the probe, the less there is which can fail.

I also feel that it should be possible to build a probe able to withstand the pressures encountered inside a diamond anvil, which I think is in the ten thousands of bars. This should get it a nice part of the way down -- perhaps enough to encounter a solid surface.

The signal, I feel, would be the biggest problem. One should use electromagnetic radiation, preferably beamed straight up in the form of a laser, and one should choose a wavelength which doesn't get absorbed by the main constituents of the upper layers, especially hydrogen. The receiver (which won't have to withstand Jovian circumstances) might be a lot more sophisticated than the transmitter.

Jovostationary orbit is an orbit whose period is equal to Jupiter's rotation period of some ten hours. As Amaltheia has an orbital period of some twelve hours, this orbit will be below the natural satelites, but of course it will be above the atmosphere, which would otherwise get into orbit.

Umm, except that very same IAU tid-bit lists them as "sub-brown dwarfs?"

I must admit I don't "feel" so. Is this anything but daydreaming? Has any funtioning electronical device ever withstood 3500 K or diamond anvil for any length of time? Even if this were true, how is this miracle ever going to reach Jupiter, as a functioning (and that means delicate) spacecraft?

Just saying "I feel we can do this" is not going to advance this discussion.

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Non sunt multiplicanda entia praeter necessitatem.

I did give some reasons for my "feelings".

And I don't understand at all why a spacecraft must be fragile. Indeed, it had better not be fragile, if it is to survive being launched from Earth.

How about a buoyant "escape probe" within the primary probe?

Inside the escape probe are the electronics to record the conditions encountered and once a certain condition is met then the escape probe launches out of the primary probe up to an altitude point where it can transmit the data - then it plunges to its fate.

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My invisible elf ??? Why he is made of dark matter and lives off of dark energy !!!
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That's a wonderful idea.

Another idea which occurred to me is using a number of relay probes, sinking at certain distances above each other.

Along somewhat similar lines, it should be possible to "x-ray" a planet with very high energy neutrinos. Extremely high energy neutrinos are much more likely to interact with matter. So, you build a really powerful accelerator to generate a neutrino beam that would be sent through whatever planet you wanted to image. Then have a detector on the other side. You could get a nice density map, based on the number of neutrinos that got through (actually, most would get through, but you could map the percentages). CAT scan a planet!

This is another thing we won't do tomorrow, and earth would probably be the first target.

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Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

Actually, Van Rijn, we already neutrino scan the Earth. The Sudbury Neutrino Observatory and Kamiokande2 both see a day-night oscillation in the solar neutrino flux. Even the old IMB saw some of this. Fermilab generates a beam picked up after a crustal excursion. I drive through a few of the Bates' Linac's neutrinos in Middleton every day on my way to work....they're causing pretty little Cherenkov cones in my vitreous humor, but not my sense of humor... Pete.

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A second rate theory explains after the fact.
A first rate theory predicts.
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I described something like this in a science fiction story. In that story, scientists built an array of subsurface neutrino detectors on a planet of Alpha Centauri A. They took more than a century to build the array, and it would function only once -- when Alpha Centauri B passed behind Alpha Centauri A as seen from the planet. Which would not happen more than once in 50,000 years. In effect, they used one star as a neutrino source to scan the other star.

But the Sun passes behind Jupiter every few days as seen from Ganymede, so scanning Jupiter with neutrinos would actually be possible, if hideously expensive.

A few examples. The PDF preprints or reprints are downloadable in PDF.

• Neutrino tomography - Learning about the Earth's interior using the propagation of neutrinos; Walter Winter, Proceedings of "Neutrino sciences 2005: Neutrino geophysics'', December 14-16, 2005, Honolulu, USA. Submitted to Earth, Moon, and Planets; February 2006
• On neutrino absorption tomography of the Earth Reynoso & Sampayo, Astroparticle Physics 21(3): 315-324, June 2004
• Tomography of the Earth's core using supernova neutrinos; Lindner, et al., Astroparticle Physics 19(6): 755-770, September 2003

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

That is not the same thing as a neutrino scan. You are promoting your argument that matter affects neutrino oscillation (as opposed to it being simply a difference in time from where the neutrino started), but that is speculation at this point.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

Now that's interesting, thanks. It seems there is a bit of work to be done, but perhaps it isn't quite as far out as I suspected. Okay, a revision to my prediction: It will probably be some time before we can do high resolution planetary neutrino absorption tomography.

In principle, it should be possible to image faults or anything else that shows up as a density difference.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

How big is the region in the Sun where the neutrinos are being generated?

For a translucency picture taken without a lens the source must be more or less pointlike (that is, small in relation to its distance). The Sun as seen from Earth is not pointlike, but perhaps the neutrino-producing region is. If it is not, any picture wil be blurred, the finest details being more or less the apparent size of the source.

Which is one of the reasons why the idea might actually work better for Jupiter than for Earth. (The other reason is that Jupiter will intercept a larger fraction of the neutrinos than Earth does, giving better contrast.) But perhaps the first sharp neutrino pictures of a planet will not feature Jupiter but Neptune.

Distant supernovae are of course pointlike, but probably not bright enough.

Once the density of free electrons and ions is high enough, a gas giant's 'atmosphere' will cease to be transparent enough for any descending probe to get its message out, using long wavelength EM.

At what depth does the Jovian atmosphere have enough such charge carriers to limit any radio/microwave/THz signals to <100 km (say)?

A few hundred km of Jovian gas (H, He, various compounds containing H, O, N, ...) plus clouds (H₂O, NH₃, ...) has an optical depth far greater than 1 - but how transparent is that to various bands in the IR? How many hundred (thousand?) km of Jovian atmosphere could be probed in the radio part of the EM spectrum?

Yes, you are right. The atmosphere will be essentially opaque to electromagnetic ratiation at all depths where the temperature is high enough to ionize its main constituent. This would be H₂, which would be ionized into H₂⁺ + e^-.

However, the temperature needed to do this would depend on the density, because the ionization process doubles the number of particles. With high densities a higher temperature would be needed. Density will not increase with depth as sharply as pressure does, however, so there is probably a level beyond which the probe will no longer be able to communicate.

If anyone here knows how to calculate this depth, please do.

Good question - they are generated where nuclear fusion takes place, in the core. For the Sun, the core has a radius of ~0.2 (wrt its photosphere), or ~5' as seen from Earth (though I expect that if we could image the Sun in neutrinos, it would should 'limb darkening').They're also unpredictable, and last just a short time. However, intensity could be addressed by making the detector large enough - 1000km on each side say (think of Callisto fitted with AMANDA-type detectors).

I feel that would be too expensive, especially in view of the fact that there would be no guarantees for a supernova to occur in Callisto's orbital plane.

I noticed that none of the nearby giants which might plausibly go supernova in the next few millennia are exacly on the ecliptic, so none of these could be used to picture the Sun or other Solar System objects.

By the way, how fast does the neutrino flux from a supernova decline? The light decay takes weeks, but if the neutrino decay is much faster, the supernova would not only have to be in the orbital plane, but also on a more or less 180 degree angle to the receiver.

The Sun rings 'like a bell', and helioseismology can be used to tease out some details of its internal structure and composition.

If there were a number of probes, floating in the Jovian atmosphere below the clouds, to what extent could seismology be used to trace Jupiter's internal structure and composition?