Final answer:
Gas giants have cores composed of heavy elements in solid state, referred to as 'rock and ice' bodies, which formed before capturing gas. The cores are primarily made up of heavy elements like iron, silicon, oxygen, carbon, nitrogen, and hydrogen.
Step-by-step explanation:
The solid cores of gas giants like Jupiter and Saturn are made of heavier elements. These cores, which formed prior to the gas from the surrounding nebula being captured, are sometimes referred to as "rock and ice" bodies. Even though these cores are referred to as "rock" and "ice," the high temperatures and pressures present in gas giants cause these materials to change into unrecognizable forms. Heavy elements like iron, silicon, oxygen, carbon, nitrogen, and hydrogen make up the majority of the cores.