Chemical inertness and surface volatility, combined with low abundance, have made the rare (noble) gases a unique trace-elemental and isotopic system for constraining the formation and evolution of the solid Earth and its atmosphere. Here I examine the implications of recent high-pressure measurements of the melting temperatures of heavy rare-gas solids - argon, krypton and xenon - with new diamond-anvil cell methods, together with their pressure-volume relationship, for the total rare-gas inventory of the Earth since its formation. The solid - liquid (melting) transition in these rare - gas solids rises significantly with pressure in the 50 GPa range, such that melting temperatures will exceed the geotherm at pressures of the Earth's transition zone and lower mantle (depths greater than 410-670 km). The densities of condensed rare-gas solids obtained from recent pressure-volume measurements at high compressions also exceed Earth's mantle and core densities. These pressure-induced changes in the physical properties of rare- gas solids, combined with their expected low solubilities and diffusional growth mechanisms, suggest that dense solid or fluid inclusions of rare gases - initially at nanometre scales - would have formed in the Earth's interior and may have resulted in incomplete planetary degassing. Separation of dense solid inclusions into deeper regions during early planet formation could provide a straightforward explanation for the unexpectedly low absolute abundance of xenon observed in the atmospheres of both Earth and Mars.
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