Here's a breakdown:

* Rock: The crust of Earth, Mars, Venus, and Mercury is primarily made of solid rock.

* Liquid: Some planets and moons have a liquid outer layer, like the oceans of water on Earth, or the liquid methane oceans on Titan (a moon of Saturn).

* Ice: Some planets and moons have an icy outer layer, like the icy surface of Europa (a moon of Jupiter) or the ice giants Uranus and Neptune.

So, the answer depends on the specific planet!