Question

Scientists think several moons of outer planets may have liquid water. How could it be warm enough for liquid water to exist that far from the Sun?

Answer 1

The same reason that there can be volcanoes on some of those moons.

Explanation:

While the surface temperatures of all the moons in the solar system are much much too low to have liquid water, when you start going deeper than the surface the temperature increases in the same manner that it increases on Earth once you go down past the crust. This heat is primarily residual from the formation of the planet but also radioactive decay of materials in the planet. At any rate once you go a few miles below the surface of these moons the temperature rises enough that liquid water can exist.