  We use a statistical-thermodynamic model to investigate the formation and
composition of noble-gas-rich clathrates on Pluto's surface. By considering an
atmospheric composition close to that of today's Pluto and a broad range of
surface pressures, we find that Ar, Kr and Xe can be efficiently trapped in
clathrates if they formed at the surface, in a way similar to what has been
proposed for Titan. The formation on Pluto of clathrates rich in noble gases
could then induce a strong decrease in their atmospheric abundances relative to
their initial values. A clathrate thickness of order of a few centimeters
globally averaged on the planet is enough to trap all Ar, Kr and Xe if these
noble gases were in protosolar proportions in Pluto's early atmosphere. Because
atmospheric escape over an extended period of time (millions of years) should
lead to a noble gas abundance that either remains constant or increases with
time, we find that a potential depletion of Ar, Kr and Xe in the atmosphere
would best be explained by their trapping in clathrates. A key observational
test is the measurement of Ar since the Alice UV spectrometer aboard the New
Horizons spacecraft will be sensitive enough to detect its abundance $\sim$10
times smaller than in the case considered here.
