Final answer:
Jupiter's greater mass compared to Saturn leads to stronger gravitational compression, making it denser despite both planets being primarily composed of hydrogen and helium. The extreme internal pressures in Jupiter's core lead to a higher density than in Saturn's less compressed interior.
Step-by-step explanation:
The reason why Jupiter is denser than Saturn can be attributed largely to its greater mass, leading to a stronger gravitational compression. Both Jupiter and Saturn are primarily composed of hydrogen and helium. However, due to Jupiter being much more massive than Saturn—it is 318 times more massive than Earth, while Saturn is about 25% as massive as Jupiter—the gravitational force is stronger. This additional force compresses the material within Jupiter to a greater extent, resulting in a higher overall density. Additionally, both planets have cores composed of heavier material such as rock and metal, but Jupiter's larger mass means it has a proportionally greater degree of compression in its interior, including both its core and extensive mantles of liquid hydrogen.
The pressures within Jupiter's interior are extreme, with theoretical models predicting a central pressure greater than 100 million bars and a central density of about 31 g/cm³. Saturn, being less massive, experiences lower internal pressures and therefore has a lower density. This greater compression of material in Jupiter contributes to it being denser than Saturn.