At their centers, all the jovian planets have cores made of:
new elements produced by the high pressure; elements which we do not have on Earth
hydrogen and helium in the form of gas
a solid mixture of rocky and icy materials under great pressure
methane, ammonia, and sulfur compounds
hydrogen and helium in the form of liquids

The core of Jupiter and Saturn is made up of rock,metal and hydrogen compounds, while the core of Uranus and Neptune is made up of rock,metals,water, methane, and ammonia.

Explanation:

The Jovian planets include, Jupiter, Saturn, Uranus, and Neptune. These planets when compared to terrestrial planets are small, with dense cores and surrounded by layers of gas.