  A striking feature of the atmosphere of Titan is that no heavy noble gases
other than argon were detected by the Gas Chromatograph Mass Spectrometer
(GCMS) aboard the Huygens probe during its descent to Titan's surface in
January 2005. Here we provide an explanation of the mysterious absence or
rarity of these noble gases in Titan's atmosphere: the thermodynamic conditions
prevailing at the surface-atmosphere interface of the satellite allow the
formation of multiple guest clathrates that preferentially store some species,
including all heavy noble gases, over others. The clean water ice needed for
formation of these clathrates could be delivered by successive episodes of
cryovolcanic lavas that have been hypothesized to regularly cover the surface
of Titan. The formation of clathrates in the porous lavas and their propensity
for trapping Ar, Kr and Xe would progressively remove these species from the
atmosphere of Titan over its history. In some circumstances, a global clathrate
crust with an average thickness not exceeding a few meters could be sufficient
on Titan for a complete removal of the heavy noble gases from the atmosphere.
