THE EARTHS MISSING XENON - A COMBINATION OF EARLY DEGASSING AND OF RARE-GAS LOSS FROM THE ATMOSPHERE

The under-abundance of Xe in the Earth's atmosphere relative to solar system abundances has been long recognised and is usually referred to as the ''missing xenon problem''. It is suggested here that this featu re of the Earth's composition is the result of processes operative dur ing its accretion and earliest degassing history. In essence, it arise s from several competing processes. One of these is sorption of rare g ases onto the accreting material, which favours an excess of Xe relati ve to lighter rare gases. Another is the subsequent fractional degassi ng of melts formed during or shortly after accretion, which favours pr eferential degassing of Xe over lighter rare gases from the Earth's in terior. These two processes are combined with a major loss of rare gas from the Earth's early atmosphere, possibly accompanying hydrogen hyd rodynamic escape.