Mercury's internal structure. Computer artwork showing the internal structure of Uranus. The standard model of Uranus's structure is that it consists of three layers: a rocky core at its centre, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The rocky core of Uranus is relatively small, with a mass of 0.55 Earth masses and a radius less than 20 per cent of Uranus's; the mantle is about 13.4 Earth masses, and comprises the bulk of the planet; the upper atmosphere weighs only that of 0.5 Earth masses and extends for the last 20 per cent of Uranus's radius.