Everything we know about Uranus comes from looking through a telescope. Only one spacecraft, Voyager 2, has ever made a close flyby of the planet. Astronomers suspect there is lots of water on Uranus. Since they’ve never actually sampled the surface of the planet, how could they know?
It all comes down to density.The density of Uranus is the second least in the Solar System, after Saturn. In fact, it has a density that’s only a little higher than water. Since water is very common in the outer Solar System, astronomers suspect that the whole planet is made of mostly water. But it’s not like any water you’ve ever seen.
The temperature at the cloud tops of Uranus is 57 K (-357 F), and that temperature increases as you go down at a very predictable rate. It’s believed that the temperature at the center of Uranus is about 5,000 K. Liquid water can’t survive those kinds of temperatures without boiling away, unless you hold it under huge pressure. The water should be a vapor, but the heat and pressure turns it into a superheated liquid.
Did you know that there might be oceans on Neptune? Here’s an article about it.
And here’s some more information about water on Uranus from the Internet. NASA has an article that talks about superheated water on Uranus.
We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.