Because Jupiter is just a huge ball of gas, there is a smooth and continuous increase in the temperature and density as you move towards the core, and there is no distinctive surface.
Jupiter mainly consists of gases like hydrogen and helium. Only a really big planet can hang onto these volatile gases. And they are not suitable for forming anything solid except deep down in the core. So Jupiter does not have a hard surface to stand.
Jupiter’s clouds are thought to be about 50 km thick. Below this there is a 21,000 km thick layer of hydrogen and helium which changes from gas to liquid as the depth and pressure increase. Beneath the liquid hydrogen layer is a 40,000 km deep sea of liquid metallic hydrogen. Beneath this, there might be a solid core which is about one and a half times the size of Earth, but thirty times more massive.
