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Solid/liquid Surfaces for Uranus/Neptune, etceteras
algorimancer
post Oct 25 2006, 02:22 AM
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Here's a question for the professionals out there. I'm relatively well-educated with regard to planetary science and basic physics (from an advanced amateur's perspective, anyway), and I've always been bothered by the oft-repeated statement that the gas giant planets lack any definite solid (or liquid) surface. It seems to me that there should at the very least be a zone of phase change, where the gaseous atmosphere changes phase to either solid or liquid (rain/snow equivalent?), and while this may occur at a depth and pressure such that the gas phase has a really high viscosity, I would find it difficult not to interpret this interface as a surface which could be a target by a really robust lander. Am I missing something with regard to the physics here? For example I could envision the phase transition being glass-like, so that the viscosity just smoothly transitions between one phase and the other, without a discontinuity, yet at least in the case of H20, I know that this isn't the case. So what is really going on here? It almost seems like this region is so far beyond the realm of familiar physics that the typical scientist gives up in despair and provides a simplistic non-answer. Clearly Uranus or Neptune would have the most easily accessible surfaces - and according to planetary models similar to those shown on this page:

http://www.astro.washington.edu/larson/Ast...saturanept.html

it looks like those surfaces will be water overlain by H2 gas. Looks like one heck of a planetery ocean, assuming that the water isn't hot ice at these pressures.

Thoughts?
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dvandorn
post Oct 25 2006, 05:15 AM
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I think there is a much better chance of there being a defined "surface" within Uranus and Neptune than within Jupiter and Saturn. According to current theory, the former are "ice giants," with massive amounts of water ice under high pressure surrounding rocky cores. The latter are made primarily of hydrogen all the way down to the cores, which are under such intense pressure that they exist in a semi-solid metallic form.

Phase change is a quirky thing; it's affected by all sorts of things, including temperatures and pressures. And phase changes might look dramatic, but still not consist of anything "solid" enough to support the weight of a lander (especially at the gravity massive Jupiter exerts).

For example, have you ever flown in an airliner? From about 10 km up, it becomes very apparent that the lower cloud layers "float" at the upper boundary of the thickest part of the atmosphere, looking rather like soap suds floating on the surface of water. I'm pretty certain, thinking back to my college meteorology course, that this boundary depends upon phase changes (mostly of water) to define it. It's a rather solid-looking boundary, and while the air below this boundary can sometimes be crystal-clear, it is often (depending on humidity and particulates) as concealing as a muddy stream. It really looks, to the eye, like solid objects ought to be able to sit on this boundary, or at least float at it. And of course we know that this really isn't the case.

The point is that phase changes, especially from gas to liquid states, don't always generate as much structural strength as they "look" like they should. I'm pretty sure that, within Jupiter and Saturn, the boundary between gaseous and liquid hydrogen occurs at such a high pressure (and, therefore, density) that, while it may look significant "to the eye" (so to speak), it really would not present a surface upon which a falling object would be arrested and supported against gravity.

Uranus and Neptune, though -- they're smaller and contain more water and less hydrogen than the two gas giants, and thus might feature far more drastic compositional changes as you descend towards their cores. Pressures and densities are still astounding compared to Earth standards, though, and "hot ice" surfaces below mostly hydrogen atmospheres are distinct possibilities. However, the pressures at such surfaces might be so vast that nothing our current technology can even conceive of would be "robust" enough to reach them intact.

-the other Doug


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nprev
post Oct 25 2006, 05:35 AM
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Many fascinating possibilities here.

Do you think that radar imaging (or sounding, as applicable) of Uranus' and Neptune's "surfaces" would be a worthwhile experiment someday, oDoug? At the very least, measuring the deepest-feasible-layer dielectric properties plus depth of return sounds like an experiment that would provide useful data... huh.gif


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deglr6328
post Oct 25 2006, 08:34 AM
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Here, watch this video of ethane in a pressure vessel transitioning in and out of the supercritical state to get some idea of why there may really be no defined liquid surface transition. The equation of state of materials at such incredibly high pressures like inside Jupiter is difficult to understand and model.
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remcook
post Oct 25 2006, 08:56 AM
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At DPS Sushil Atreya had an interesting talk about a potential ocean on Neptune and Uranus

QUOTE
An Ocean Of Water-ammonia On Neptune And Uranus: Clues From Tropospheric Cloud Structure
Presentation Time: Monday, 10:30 a.m. - 12:00 noon
Sushil K. Atreya1, K. H. Baines2, P. A. Egeler1
1Univ. of Michigan, 2JPL.
Presentation Number: 05.08
We discuss how the existence of a putative ionic ocean may manifest itself in the tropospheric cloud structure of Uranus and Neptune. An optically infinite cloud, presumably of H2S-ice, has been postulated at 3-5 bar to explain the microwave absorption [1] and the methane [2] observations. Our equilibrium cloud condensation model shows that no clouds forms in this region when similar enrichment factors are employed for all heavy elements [3,4]. However, an H2S-ice cloud can form in the 3-5 bar region, if the S/N ratio is greatly supersolar. This can happen if ammonia was depleted substantially in the deep troposphere. The loss of ammonia in the conventional aqueous-ammonia solution or cloud at kilobar level or in the NH4SH cloud is insignificant [3]. On the other hand, existence of a water ocean in the deep atmosphere would allow ammonia to dissolve readily in water, resulting in a water-ammonia ionic ocean [H3O+ (NH4+) OH-] and depleting NH3 in turn above this level. Interior models suggest the existence of such an ocean [5,6], and Molecular Dynamics models and lab experiments also show that at P>50 GPa and T>1000 K, water is dynamically ionized, consisting of H3O+, OH- and H2O [7]. The intrinsic magnetic field of Uranus and Neptune detected by Voyager [8] could also be due to an active dynamo in such a water-ammonia ionic ocean. [1]de Pater I, et al., 1991. Icarus, 91, 220; [2]Baines K, Hammel H, 1994. Icarus, 109, 20; [3]Atreya, SK, Wong, AS, 2005. Space Sci. Rev., 116, 121; [4]Atreya SK, et al. 2005AGUFM.P11A0088A; [5] Hubbard W, in Uranus, Neptune, NASA CP 2330, 291, 1984; [6]Podolak M, et al., 1991, in Uranus, 1991. 48; [7]Goncharov et al., Phys. Rev. Ltrs., 94, 125508, 2005; [8]Ness N, et al., 1986 Science 233, 85, and 1989 Science 246, 1473
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Rob Pinnegar
post Oct 25 2006, 02:59 PM
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QUOTE (dvandorn @ Oct 24 2006, 11:15 PM) *
However, the pressures at such surfaces might be so vast that nothing our current technology can even conceive of would be "robust" enough to reach them intact.

I guess it would all hinge upon building something that could operate while immersed in whatever fluid makes up the liquid part of Uranus and Neptune. There's no way we could build anything that would stay "water-tight" at those pressures. It would have to be *designed* to leak, and all the individual parts would have to be able to function under huge hydrostatic pressure.

I don't even want to speculate on the engineering challenges that would be involved in this type of thing, except to say that they most likely lie within the realm of science fiction. (For now.)
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Guest_Myran_*
post Oct 25 2006, 03:16 PM
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So far I have been able to understand these things I think you are correct deglr6328.
Its is possible that all the gigant planets have a layer of liquid of some kind, in the case of Juptier it would likely be liquid metallic hydrogen.
But neither of those worlds would have anything resembling the surface of Earths oceans, and some years ago I remember trying to explain that to a person, why there wouldnt be a clean transition where the liqud layer would start. Now if I only could remember who the person was I would like to show that film of yours. smile.gif
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algorimancer
post Oct 25 2006, 05:50 PM
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Wow, that stirred up lot's of responses. The ethane phase change video is interesting, however I notice that a distinct layer of liquid existed outside of the gas phase, so I'm not clear on the relevance, other than to reinforce that interesting things are possible in materials near their critical point, under laboratory conditions. I liked the Atreya abstract, and the references led me to some neat papers discussing the possibility of a water/ammonia ocean on these bodies, whereas others posited a transition to hot ice (presumably there could be multiple layers as you go downwards, especially considering the wacky H20 phase diagram, not to mention blends with ammonia). It seems pretty clear that there is a thick layer of gaseous H2 overlaying a REALLY deep layer of predominantly H20. It sounds like the question of the interface as a distinct surface is essentially "we don't really know yet". Considering the depth of H2 I would expect the H20 "surface" to be completely dark, presumably at an atmospheric pressure rather higher than that on Venus, and I'm not sure whether we know enough to say how hot it would likely be. Further, considering that the "surface" is not merely a phase change but also a material change, I find it difficult to envision it as other than a genuine surface.

I find it surprising that the interface conditions appear not to have been simulated in the laboratory. Seems like a nice (series of) easy to publish paper there.

The radar sounding question is a good one. Conceivably, if it is possible at all, something like the Arecibo radar could ping it successfully (at the right wavelength). That would be neat.

It concerns me a bit that the "there is no surface on the gas giants" thing may have been repeated so religiously that noone bothers to investigate whether there actually is a surface and, if so, what sort of conditions exist there.
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ngunn
post Oct 25 2006, 07:40 PM
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This is something I have wondered about too, and I recall reading an article or two about real oceans on U and N, probably spinoffs from the papers you mention. I have been trying to picture a sort of 'artists impression' of the scene. Problems? Zero illumination unless you have either lightning or luminescent organisms. I'm not sure which is less likely. As for sending a probe, what kind of signal could you possibly use for getting data out? It's a bit like the deep interior of our own world - a lot less accessible than Alpha Centauri. Exceptionally interesting nonetheless.
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dvandorn
post Oct 25 2006, 10:58 PM
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QUOTE (nprev @ Oct 25 2006, 12:35 AM) *
Do you think that radar imaging (or sounding, as applicable) of Uranus' and Neptune's "surfaces" would be a worthwhile experiment someday, oDoug? At the very least, measuring the deepest-feasible-layer dielectric properties plus depth of return sounds like an experiment that would provide useful data... huh.gif

Yes, I think that some form of imaging might someday be possible. And it would be a very worthwhile experiment. I think sounding is a lot more likely to yield data than radar -- something like an active seismic experiment. Of course, you'd need to ram something the size of Triton into Neptune to get the kind of seismic energy you'd need... ohmy.gif

-the other Doug


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algorimancer
post Oct 26 2006, 01:07 PM
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QUOTE (dvandorn @ Oct 25 2006, 05:58 PM) *
Of course, you'd need to ram something the size of Triton into Neptune to get the kind of seismic energy you'd need... ohmy.gif

Hmmm... take some stray KBO in a chaotic orbit and give it just the right tweak at the right time, and that's not entirely out of the question. Perhaps something Nereid-sized as opposed to Triton-sized smile.gif
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ustrax
post Sep 21 2007, 04:12 PM
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I wasn't sure where to post this...
"An international team of astronomers has found that Neptune's south pole is warmer than other parts of the planet.
Temperatures at its south pole are about 18 degrees warmer than elsewhere on the planet."

From CNN.

I like Neptune! Really! For me it is the most beautiful planet in the Solar System... smile.gif

Here's ESO release.


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jrdahlman
post Sep 22 2007, 05:44 AM
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(Never noticed this thread before. I thought it was recent, not last year's.)

Radar-scanning Uranus or Neptune, as mentioned in the earlier posts, is a facinating idea. It's too bad it'll be many years before radar or any other science instrument goes into orbit around these worlds. But it did remind me of a radar that does just happen to be in the neighborhood of a gas giant....

Just curious, but what would Cassini's radar see if it was pointed at Saturn itself?

I know, I know. Every turn, camera-click, radar-ping, and LED-blink for Cassini is planned years in advance, and they would never waste time on a crazy target just on a whim. But still... would it really see anything?
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JRehling
post Sep 22 2007, 06:06 AM
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[...]
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JRehling
post Sep 19 2008, 10:35 PM
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[...]
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