Laboratory experiments show that substantial Na-D line emission is produced by 3- to 5-keV proton bombardment of silicate and halite rock powders. Emission intensity and energy efficiency are directly proportional to sodium content and are total dose, dose rate, and proton energy dependent. Highest energy efficiencies were encountered with halite (NaCl) and are of the order of 106 at proton fluxes of ~1013 cm-2 s-1. Na-D photon emission is interpreted as emanating predominantly from sputtered sodium slightly above the target surfaces. Laboratory Na-D intensities from halite fall within the range of Na-D emission intensities recently discovered in the spectrum of the Jovian satellite Io but were achieved with higher flux and power densities than those expected for Io. The Na-D line emission directly excited during surface sputtering by Jovian magnetospheric protons may possibly be an observable component in the Io emission. In any case, sputtering of sodium-rich surface material is a mechanism which can inject sodium (and other material) into the 'atmosphere' and space about Io.