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We present evidence that excesses in Be in polluted white dwarfs (WDs) arethe result of accretion of icy exomoons that formed in the radiation belts ofgiant exoplanets. Here we use excess Be in the white dwarf GALEX J2339-0424 asan example. We constrain the parent body abundances of rock-forming elements inGALEX J2339-0424 and show that the overabundance of beryllium in this WD cannotbe accounted for by differences in diffusive fluxes through the WD outerenvelope nor by chemical fractionations during typical rock-forming processes.We argue instead that the Be was produced by energetic proton irradiation ofice mixed with rock. We demonstrate that the MeV proton fluence required toform the high Be/O ratio in the accreted parent body is consistent withirradiation 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 theWD. The icy moons of Saturn serve as useful analogs. Our results provide anestimate of spallogenic nuclide excesses in icy moons formed by rings aroundgiant planets in general, including those in the solar system. While excessesin Be have been detected in two polluted WDs to date, including the WDdescribed here, we predict that excesses in the other spallogenic elements Liand B, although more difficult to detect, should also be observed, and thatsuch detections would also indicate pollution by icy exomoons formed in thering systems of giant planets.

Icy Exomoons Evidenced by Spallogenic Nuclides in Polluted White Dwarfs