Final answer:
Uranus and Neptune's higher densities compared to Saturn are attributed to their higher proportion of hydrogen compounds and rock, leading to denser cores and less hydrogen and helium gas in their atmospheres.
Step-by-step explanation:
The reason why the densities of Uranus and Neptune are higher than that of Saturn, despite Saturn having a much greater mass, is that they have a higher proportion of hydrogen compounds and rock in their composition. Both Uranus and Neptune have denser cores made of 'ice' and 'rock,' and are depleted in hydrogen and helium relative to Saturn. This compositional difference implies Uranus and Neptune were less efficient at accumulating hydrogen and helium gas from the protoplanetary nebula during their formation, leading to significantly smaller gas envelopes in proportion to their cores.
Saturn's lower density can be partly explained by its larger volume of liquid hydrogen compared to its relatively small amount of metallic hydrogen. In addition, the stronger gravity at Saturn's size does not compress its interior as much as in the case of the smaller, but compositionally denser, Uranus and Neptune. Thus, the internal structures of these planets are critical to understanding their overall densities.