I'm a little confused by this astronomical use of the word "ice". It seems to cover a lot of different materials, from more traditional frozen water to ammonia to nitrogen to methane to carbon dioxide/monoxide to sulphur dioxide.

What is an ice that makes it different from a rock or a metal? Do these compounds' solid states have distinct properties that mark them out as "icy"? The only common link that I can see is that they are all gaseous on Earth.

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Technically ice is the same sort of thing as rock and metal, in that all of those as the solid phase of whatever is being described. For example, the freezing point of copper or iron at standard atmospheric pressure is way above room temperature, but we don't call it "copper ice" or "frozen iron".

In planetary science/astronomy circles, "ice" is basically "frozen volatiles". Usually it's prefaced by what it's the frozen form of - i.e. water ice, methane ice, ammonia ice.

Yes! I too was wondering the same thing. In astronomical terms they are volatile materials that have been frozen into their solid phase. From our point of view those chemicals with low boiling points that are present on Earth as gases are termed as ice. The term is often applied to the substances themselves, whether in solid form or not. For instance, Uranus and Neptune are called "ice giants", because they contain large amounts of methane and ammonia, despite the fact that neither planet contains any actual ice. Wiki has done some work on it.

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I just wrote that wiki section about ten minutes ago, thanks to the info supplied by EDG.

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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.

You lie!

Well someone did. The history indicates that that section was added today.

If you really did

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Don't Uranus and Neptune contain ice in their cores? Ice that is ice because of the high pressure and not because of the temperature.

They're not ice giants. They're gas giants. And if their cores are solid, it's rock, not ice.

Any particular reason why there couldn't be ice in the core area? Perhaps a rocky core surrounded by ice, followed by a vastly greater volume of liquid and finally a gaseous atmosphere.

Actually, you can refer to them with either term. From here:

Physically and chemically, Uranus bears a far closer resemblance to Neptune than it does to the giants Jupiter and Saturn; like Neptune, it is roughly a tenth the mass of Jupiter and has far less elemental hydrogen and helium. Astronomers have therefore begun to refer to both collectively as belonging to a separate category: "ice giants", because they are primarily made of "ices" like water, ammonia, hydrogen sulfide and methane.

Many argue that the differences between the ice giants and the gas giants extend to their formation.

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The name "ice giant" is quite commonly used for Uranus and Neptune; the reason is given in this abstract from a recent Icarus paper.

Grant Hutchison

Edit: And a quote from the current (2007) Encyclopedia of the Solar System article on this topic by Mark S Marley and Jonathan Fortney. "The composition of this region [the deep interior] is thus undoubtedly predominantly icy. However, since the density of rock/ice/gas mixtures can mimic the density of pure ice, the exact composition cannot be known with precision. ... Interestingly, Uranus and Neptune models that do not have have rock cores can be constructed. Other models with cores as large as [one earth mass] are also consistent with the available data."

So is it wrong to say that there is no ice on Neptune or Uranus? If so, that wikipedia entry may need to be altered.

There could be ice or sleet somewhere in their atmospheres, but their cores are way too hot to be made of ice.

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What are their core temps? And what pressures would be required to form ice (of the appropriate composition) at those temps?

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There's definitely water ice in Uranus and Neptune, as well as methane and ammonia. Heck, there's water ice in Jupiter and Saturn too (in the atmospheric clouds at the very least).

U and N are called "icy giants" because they don't have deep layers of liquid and/or metallic and possibly solid hydrogen in them. Instead they have a relatively shallow H2/He atmosphere with ammonia and methane, overlying a high pressure water/ice layer and a rocky core.

So someone needs to go edit that wiki article, becuse U and N certainly do have water ice in them.

It is possible that Wicki is wrong. My guess is we don't know what lies below the cloud tops of Uranus and Neptune. We can make educated guesses. Reasonably, bits of ice of various kinds are found near the cloud tops of Uranus and Neptune.The larger bits fall as hail stones much the same as hail falls from Earth clouds.
Perhaps 100 miles below the cloud tops, it is warm enough to turn the hail to slush then liquid which evaporates to vapors and later gasses. A few thousand miles below the cloud tops the pressure is so high some kinds of matter behave differently than at the rather low pressure of Earth's atmosphere at sea level. Close to the center the temperature is high enough that most compounds decompose and the nuclii of the atoms are stripped of their electrons. For this reason we might call them plasma giant planets. Neil

Erm, no. The pressures and temperatures in the middle of Uranus and Neptune aren't remotely high enough for that. I suppose you're making an educated guess here, but have you actually tried to do some research on this subject yourself? We know more about this than you think we do.

eg. http://sci.esa.int/science-e/www/obj...objectid=12827
http://www.solarviews.com/cap/uranus/uranusint.htm
http://www.sparknotes.com/astronomy/...section4.rhtml

I wrote that too. You could read that as a boast or as a warning.

EDG, I edited that line from "any actual ice" to "more than trace amounts of actual ice". Is that good enough?

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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.

In one textbook I read that Neptune and Uranus cores are composed of various exotic forms ice compressed by pressure at immense temperatures - imagine diamond hard ice at 20000 kelvins!
Rocky cores are suspected to be in Jupiter and Saturn, but AFAIK the outer giants contains a lot of ice.

Certainly a lot more that "trace amount" and also I think that icy dust is present in the atmosphere.
Also above the core is a hot liquid soap of various volatiles - water, ammonia, methane...

Use brain.These pressures makes Venus pressure 100 km underground looking like hard vacuum.

I believe there's maybe some confusion caused by that word "ice" in "ice giant". Although ices (= solidified volatiles) went into forming the cores of Uranus and Neptune, these cores are not expected to be solid.

From Marley and Fortney's Interiors of the Giant Planets in the Encyclopedia of the Solar System, 2nd Ed. (2007) :William B Hubbard, Interiors of the Giant Planets in The New Solar System, 4th Ed. (1999):John S Lewis, Physics and Chemistry of the Solar System, Revised Ed. (1997):Lewis goes on to point out that many minerals are quite soluble in hot supercritical water.
He also points out that the measured J₂ gravitational zonal harmonics for the two ice giants are inconsistent with models that use the customary three-layer (atmosphere+ice+rock) depiction, but instead are compatible with only two layers. Either, he suggests, a rocky core with a supercritical fluid above it blending smoothly into the atmosphere, or a discrete atmosphere overlying an ocean consisting of dense liquid that has completely dissolved the rocky component.

Grant Hutchison

Your question regarding 'What is Ice? It looks like you know the answer to that. But there is a conceptual understanding to be defined. Atmospheric pressure and temperature do strange things to a liquid. For example you know that water can not be found on the surface of Mars because the Atmospheric pressure is so low that it becomes gaseous on exposure. The Gas giants have a similar issue with the liquid state of matter. As you descended down into the atmosphere the density increases. While on the surface water as a liquid might not be found. While deep inside that atmosphere the state of matter changes. With the increase of pressure so to the temperature would rise. At the core of the gas giants the pressure is extreme. The state of mater is liquid. ,but not as we would expect to find in your fridge. I hope this helps you.

When you're deep inside a gas giant the pressure is so high the liquid becomes "supercritical", which means that it basically merges with gas. I can't quite get my head around what that means, but basically you'd be dropping through gas that was getting thicker and thicker and wouldn't actually notice that you were in a liquid. .

Thanks, Grant.

Using brain give big headache

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And saying explains nothing.

Grant Hutchison

The alternative, described in the Icarus paper I linked to, is that as you descend into an ice giant you encounter the surface of a liquid ocean, with a definite phase discontinuity.
If I understand the paper correctly (and the thermodynamics is outside my comfort zone) there are two options:
1) You descend through clouds until you reach a depth at which the temperature "burns off" the clouds, and thereafter you descend through a mixed vapour layer until you enter a supercritical regime (as you describe).
2) You descend through clouds until you encounter a critical liquid ocean, which supervenes because the pressure gets high enough before the temperature gets hot enough to edge past the critical point. This is unlikely on Neptune, but might happen on ice giants that are colder than Neptune or which have a higher water content than Neptune.

Grant Hutchison

super-what now? I don't think I've ever heard that word before, and my vocabulary is fairly massive . (is it related to "intervene"? Except somehow getting on top of something instead of between?). I just don't get what it means in the context you've used it in.

It is. Both from Latin venire, "to come": intervene, to come between; supervene, to come immediately after, to be the next thing. Common enough word down my way.
The etymology looks like it's going to involve being on top of something, for sure, but it seems to have ended up with the more general sense I describe, where two things or events come in close proximity or sequence, especially if the one that "supervenes" is in some way unexpected.
It seemed to do the job in referring to the onset of a more or less dramatic phase change at some depth within an ice giant's atmosphere. Sorry if my meaning was opaque.

Grant Hutchison