Noble gas to be retained. Neon, argon, krypton, and xenon also form clathrate hydrates, where the noble gas is trapped in ice. Noble gases can form endohedral fullerene compounds, in which the noble gas atom is trapped inside a fullerene molecule. In 1993, it was discovered that when , a spherical molecule consisting of 60 carbon atoms, is exposed to noble gases at high pressure, complexes such as can be formed (the "@" notation indicates He is contained inside but not covalently bound to it). As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been obtained. These compounds
Atmosphere of Uranus from the analysis of "Voyager 2" far infrared and radio occultation observations. The currently accepted value is in the upper troposphere, which corresponds to a mass fraction . This value is very close to the protosolar helium mass fraction of , indicating that helium has not settled towards the centre of the planet as it has in the gas giants. The third most abundant constituent of the Uranian atmosphere is methane , the presence of which has been known for some time as a result of the ground-based spectroscopic observations. Methane possesses prominent absorption bands in the visible and near-infrared,