The evolution of the Jovian planets and the terrestrial planets has been very different, and as a result their structure and composition are also quite different.
The Jovian planets have no surface. Rather, they only have gradual transitions in the atmosphere. Jupiter's main molecular constituents are hydrogen (90%) and helium (10%). The outermost part of the planet is in the form of clouds. (As seen in the schematic). As one gets closer to the core, the pressure within the planet increases and the hydrogen and helium change to liquid form.
Most of Jupiter is liquid, however the
transitions are very gradual and thus, the layers are not strict
like on terrestrial planets.
This illustration shows the main layers of Jupiter's internal structure. The atmospheric clouds (shown in the above schematic) are followed by a liquid hydrogen layer. This layer begins at aproximately 1000 km below the outermost clouds. This layer functions like the earth's ocean producing currents and convection patterns that carry heat from the interior to the exterior.
After the liquid hydrogen layer there is a liquid metallic hydrogen layer. This layer also produces currents and convection patterns. However, because of its metallic nature, it can conduct electricity. This property couple with Jupiter's fast spin, help produce its very powerful magnetosphere.
After the liquid metallic hydrogen layer is the core. The core is
probably about the size of the Earth.