Uranus and Neptune are not what I thought: they’re mostly made of fluid ices, not gasses.
A Brief Story
So, last week I was sitting in an invited talk by Dr. David Charbonneau  at the American Astronomical Society’s summer conference. David is looking for exoplanets, and has found many. He’s the kind of person who should really know what he’s talking about when it comes to planets. When he uttered a sentence that went something like this: “Uranus isn’t really a gas giant, it’s an ice giant,” my ears perked up and I started thinking.
Oddly, I had just done a bunch of low-level research into Uranus for my post A Series of Questions II . I ran across a few untrustworthy sites that called Uranus a “slush planet,” but when I went to NASA’s planetary data sheets , which I figured would be solid references, they said Uranus’s atmosphere was 82% Hydrogen, 15% Helium, etc (which I said in my other post). What I completely missed was the word “atmosphere.” So, I joyously wrote that Uranus was mostly Hydrogen and Helium, just like Jupiter and Saturn, and moved on with my life. Until I heard Dr. Charbonneau. Then I got confused. So I talked to both my Astronomy professors from Whitman College (one of whom was there in the audience too!) and did a bunch of web research, since we need to get this right in our Planets Show.
What I Learned
Uranus has three layers: a tiny “rock” core, a thick “ice” “ocean”, and a thin gaseous atmosphere much like most of Jupiter and Saturn’s composition. Let me go on, and explain why there are so many quoted words in that sentence. Note that Uranus is 14.5 times the mass of Earth, and you could fit 63 Earths inside it. Also, only Voyager 2 has flown by it, so our data is limited.
The core of Uranus is between half and four times the mass of the Earth. It is made of something you might call rock – since it’s mostly silicates (that’s what rocks on Earth are) and metals. It is likely that although the rock part is probably molten, the metals could be solid.
The “Icy” “Ocean” Layer
There is a gigantically thick layer between the core and the atmosphere. About 80% of the mass of Uranus (so 11 to 14 times the mass of Earth) is wrapped up in this layer. There’s a bunch of water, some methane, ammonia, nitrogen, and hydrogen sulfide. This layer is termed “ice.” And it is fluid (I didn’t say liquid). And it is hot.
Wait, How Can You Have a Fluid Ice?!
A Note on Phases: It’s ice because of how high the pressure is that deep in a planet. You can change the phase (solid to liquid to gas) of something in two ways: change the temperature, or change the pressure. This is why snow on mountaintops often sublimates (goes straight from solid to gas) rather than melting and then evaporating, and why water boils at a different temperature in Denver than it does in Seattle. (You may have seen “High Altitude Cookbooks” – that’s why). At high enough pressure, no matter what the temperature, water will be ice.
Paraphrasing/Quoting Dr. Andrea Dobson: Why are we using the word “ice”? If you’re talking about rock, you wouldn’t flip out if I said “liquid rock” or “solid rock”, would you? One is specifically called magma, but you can still call it “liquid rock” if you want. Andrea’s “sense of ices is that we’re talking about things that are relatively volatile,” (easy to change the phase of) “but not as volatile as hydrogen or helium.” Some examples are water, methane, N2, ammonia, CO, and sulfur compounds. So the word ice is really being used to say “stuff that’s not rock or gas.”
When it comes to “fluid ice”, Andrea reminded me that, as I learned in my Planetology and Geology classes, you can have plastic/ductile deformation of solids. Whooeee – let me explain a little better there.
Think about a rubber band. Solid, liquid, or gas? Solid, pretty definitely. Can you change the shape of a rubber band? Yes, but it always bounces back. That’s an elastic deformation. You just changed the shape of a solid. Elastic materials always return to their original shape.
Now think about a paperclip. Solid, liquid, or gas? Solid, duh. Can you change the shape of a paperclip? Yes! And it stays that way. That is a plastic deformation of a solid. Plastic materials do not automagically return to their original shape.
Did either the rubber band or the paperclip break? No. So they’re fairly ductile. If you keep changing the shape of them though, which will break first? Probably the paperclip (but try it and let me know your results). The paperclip is less ductile than the rubber band. Ductility is a measure of how much something can be deformed before it breaks.
So, if you take some very basic geology, you learn that below the Earth’s crust is lots of hot rock: magma etc. If you take some medium-level geology classes, you learn that rock is not all liquid, in fact, much or most of it is solid. It still flows around, mixes, etc. because it’s plastic and ductile. It’s still a solid, but it’s acting fluid.
Remember: phase changes are pretty messy. We often think they’re not, because we’re used to the liquid water/solid water phase change, which happens pretty neatly at 0 degrees. Even that’s not as simple as you think though: glaciers flow like very slow rivers, and they’re made of solid ice. Also, the liquid water/gas water transition isn’t all that easy. We think of steam (the white clouds expelled from a boiling tea kettle) as water in gas form, but really, the part you see is tiny droplets of liquid water suspended in the air (a liquid water aerosol). The gas is completely invisible – so right next to the tea kettle spout, where you don’t see anything, is probably pure steam. Try this at home – but don’t burn yourself by looking too closely, and if you’re under a certain age, have a grown-up help you.
The atmosphere, like I said before, is mostly Hydrogen and Helium. But here’s the thing – the atmosphere is only 5-15% of the radius of Uranus. It’s probably also half the mass of the Earth, or maybe a little more – so overall, not much of the composition comes from the atmosphere.
Picture copyright Calvin Hamilton, from: http://explanet.info/Chapter11.htm 
In conclusion, Uranus is mostly made of fluid ices, not gasses.
Neptune’s composition is similar to that of Uranus, with slightly different-sized layers, and slightly different compositions.
Some Good Illustrations:
University of Washington’s Comparative Planetology 
Phase Diagram of Water  (advanced)
Where’d I Get My Info?
Andrea Dobson, Whitman College
Lunine, Jonathan I. The Atmospheres of Uranus and Neptune in the Annual Review of Astronomy and Astrophysics Volume 31, Pages 217-263. 1993.
dePater and Lissaur. Planetary Sciences, Page 2, 245-248. Cambridge. 2001.
Guillot. Interiors of Giant Planets, Annual Review. Earth and planetary Science. 2005.
And, believe it or not, Wikipedia . As of 6/9/2008 the list of references was very good.