  Given current uncertainties in the cratering rates and geological histories
of icy objects in the outer solar system, it is worth considering how the ages
of icy surfaces could be constrained with measurements from future landed
missions. A promising approach would be to determine cosmic-ray exposure ages
of surface deposits by measuring the amounts of cosmogenic Lithium, Beryllium
and Boron at various depths within a few meters of the surface. Preliminary
calculations show that ice that has been exposed to cosmic radiation for one
billion years should contain these cosmogenic nuclei at concentrations of a few
parts per trillion, so any future experiment that might attempt to perform this
sort of measurement will need to meet stringent sensitivity requirements.
