Solid/liquid Surfaces for Uranus/Neptune, etceteras Options

[algorimancer]

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?

[dvandorn]

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

[Rob Pinnegar]

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.)