A gas planet has no solid surface, and therefore an alternative method has been devised to determine where it starts.
Saturn is a gas giant, but that term is misleading, because most of the planet is liquid.
Saturn is one of the major gas planets in the outermost part of the solar system. It consists almost entirely of hydrogen and helium and therefore has no solid surface as we know from the inner planets Mercury, Venus, the Earth and Mars.
Hydrogen changes phase
A spaceship that tries to land on Saturn continues to descend until it is crushed by the pressure or melts by the heat.
The pressure and temperature of the planet rise as you approach the core.
The gases increase in thickness and from a depth of 1000 kilometers the hydrogen gradually becomes liquid.
About halfway to the core, the pressure is now so high that the hydrogen changes state again and becomes electrically conductive - metallic hydrogen.
The core is fixed
It is unclear what the absolute center of Saturn looks like, but astronomers agree that the planet has a solid core consisting of rocks and various types of ice.
Although it is 10,000 ° C in the core, the ice remains frozen because the pressure is 10 million times higher than on the surface of the earth.
The water molecules are thus compressed into a solid form.
In order to determine the circumference of Saturn, astronomers usually define the surface as the place where the pressure equals the air pressure at sea level on Earth: 1 atmosphere.
Lightning forms diamonds on Saturn
In 2013, researchers in the US discovered that possibly centimeters of large diamonds are formed on Saturn.
When lightning strikes methane molecules high in the atmosphere, carbon atoms are formed that together form soot particles.
These fall down, and at a depth of 6,000 km the pressure is so high that the carbon turns into diamond.
Even deeper, where the temperature is more than 8000 ° C, they melt.
Gas giant is largely liquid
Saturn is called a gas giant, but that name is a little misleading. Most of the planet is liquid, because the enormous pressure compresses the gases.