  We present evidence that excesses in Be in polluted white dwarfs (WDs) are
the result of accretion of icy exomoons that formed in the radiation belts of
giant exoplanets. Here we use excess Be in the white dwarf GALEX J2339-0424 as
an example. We constrain the parent body abundances of rock-forming elements in
GALEX J2339-0424 and show that the overabundance of beryllium in this WD cannot
be accounted for by differences in diffusive fluxes through the WD outer
envelope nor by chemical fractionations during typical rock-forming processes.
We argue instead that the Be was produced by energetic proton irradiation of
ice mixed with rock. We demonstrate that the MeV proton fluence required to
form the high Be/O ratio in the accreted parent body is consistent with
irradiation of ice in the rings of a giant planet within its radiation belt,
followed by accretion of the ices to form a moon that is later accreted by the
WD. The icy moons of Saturn serve as useful analogs. Our results provide an
estimate of spallogenic nuclide excesses in icy moons formed by rings around
giant planets in general, including those in the solar system. While excesses
in Be have been detected in two polluted WDs to date, including the WD
described here, we predict that excesses in the other spallogenic elements Li
and B, although more difficult to detect, should also be observed, and that
such detections would also indicate pollution by icy exomoons formed in the
ring systems of giant planets.
