Final answer:
Uranus has a small, rocky core and a deep mantle of slushy water ice and rock due to its inefficient capture of hydrogen and helium compared to Jupiter and Saturn, resulting in a smaller atmosphere relative to the core.
Step-by-step explanation:
Uranus has a small, rocky core and a deep slushy mantle of ice water and rock. The core of Uranus, as well as Neptune's, is primarily composed of rock, metal, and ice and represents a significant portion of the planet's mass. Unlike the larger gas giants, Jupiter and Saturn, Uranus and its sister planet Neptune did not efficiently attract massive quantities of hydrogen and helium during their formation.
Consequently, they have much smaller atmospheres relative to their cores. These cores are believed to be the original rock-and-ice bodies that formed before the capture of gas from the surrounding nebula, and exist under immense pressures and temperatures that would render familiar forms of rock and ice unrecognizable.