Final answer:
Uranus and Neptune have less hydrogen and helium than Saturn and Jupiter because they formed in a region with less of these gases available and were less efficient at attracting them. Their smaller masses and lower internal pressures also prevent the liquefaction of hydrogen, contributing to their unique composition.
Step-by-step explanation:
The reason why Uranus and Neptune have less hydrogen and helium than Saturn and Jupiter is closely related to their formation and the physical conditions in their deep interiors. These two planets are known as ice giants because they consist in large part of compounds of carbon, nitrogen, and oxygen, with each having a core of rock, metal, and ice.
Uranus and Neptune were less efficient at attracting hydrogen and helium gas during their formation due to their smaller masses and the region of the protoplanetary disk they formed in, which had less of these gases available. As a result, they have much smaller atmospheres in proportion to their cores when compared to the gas giants, Jupiter and Saturn.
Moreover, the internal pressures within Uranus and Neptune are too low to liquefy hydrogen, unlike Jupiter and Saturn which have extensive mantles of liquid hydrogen. The densities of Uranus and Neptune are 1.3 g/cm³ and 1.6 g/cm³ respectively, which are much higher than that of Saturn, indicating a different composition that is depleted in hydrogen and helium.