The charging of ice grains in planetary plasmas is studied, including
the effects of secondary electron emission and backscattering of the i
ncident electrons. It is shown that existing charging models can not b
e simply extrapolated to the low-energy electron regime (below 30 eV)
common in planetary magnetospheric plasmas. We derive expressions for
the electrical potential of a grain immersed in a low-energy plasma wh
ich more carefully account for electron reflection and the threshold f
or secondary electron emission. Using plasma parameters from Voyager P
LS experiment, we calculate the potential of Saturn's E ring grains to
vary from -5.5 V at 4 R(s) to 5 V at 10 R(s).