Where Does The Surface Of A Gas Planet Begin?

Gas planets have no surfaces, so astronomers have found an alternative method for determining where it begins.

Saturn is one of the four large gas planets in the outer Solar System. It almost only consists of hydrogen and helium and so has no solid surface as we know it from the inner planets such as Mercury, Venus, and Earth. An attempt to land on Saturn would mean that the spacecraft continued to sink, until it was shattered due to the pressure or melted in the heat.

In a gas planet, pressure and temperature increase as you approach the centre. En route, the gases become ever thicker, and at a "depth" of about 1,000 km down, the hydrogen gra-dually becomes liquid. About half way towards the core, the pressure becomes so high that the liquid hydrogen changes state again, becoming electrically conductive (metallic hydrogen).

Astronomers are not so sure what things look like at the centre of Saturn, but they agree that the planet’s core is solid, consisting of rock and different types of ice. In spite of temperatures in the core of up to 10,000 degrees, the ice remains frozen, because the pressure is 10 million times higher than at Earth’s surface, compressing the water molecules into a solid state.

However, to be able to determine the planet’s circumference, etc., astronomers often define Saturn’s surface as the place where the pressure is the same as at sea level on Earth.