The atmosphere of Uranus, like that of Neptune, is markely different from those of the larger gas giants, Jupiter and Saturn. While still composed primarily of hydrogen and helium, it possesses a higher proportion of volatiles (dubbed "ices") such as water, ammonia and methane. Unlike Jupiter and Saturn, Uranus is not believed to possess a metallic hydrogen mantle or envelope below its upper atmosphere. Instead, its inner regions are believed to consist of an "ocean" composed of ammonia, water and methane, which then makes a gradual transition without a clear boundary into a gaseous atmosphere dominated by hydrogen and helium. Due to these differences, many astronomers group Uranus and Neptune into their own separate category, the ice giants, to distinguish them from Jupiter and Saturn.