Spectral reflectance change and luminescence of selected salts during 2–10 KeV proton bombardment: Implications for Io

Abstract

Radiation damage and luminescence, caused by magnetospheric charged particles, have been suggested by several authors as mechanisms for explaining some of the peculiar spectral/albedo features of Io. We have pursued this possibility by measuring the uv-visual spectral reflectance and luminescent efficiency of several proposed Io surface constituents during 2 to 10-keV proton irradiation at room temperature and at low temperature (120 < T < 140°K). The spectral reflectance of NaCl and KCl during proton irradiation exhibits the well-known F-center absorption bands at 4580 and 5560 Å. Na 2SO 4 shows a generalized darkening which increases toward longer wavelengths. NaNO 3 shows a spectral reflectance change indicative of the partial alteration of NaNo 3 to NaNo 2. NaNO 2 shows no change. The luminescent efficiencies of NaCl and KCl are ∼10 −4 at 300°K and increase by one-half order of magnitude at ∼130°K. The efficiencies of K 2CO 3, Na 2CO 3, Na 2SO 4, and NaNO 3 are 10 −4, 10 −4, 10 −5 and 10 −6, respectively, at 300°K and they all decrease by one-half order of magnitude at ∼130°K. These results indicate that magnetospheric proton irradiation of Io could cause spectral features in its observed ultraviolet and visible reflection spectrum if salts such as those studied here are present on its surface. However, because the magnitude of these spectral effects is dependent on competing factors such as surface temperature, incident particle energy flux, solar bleaching effects, and trace element abundance, we are unable at this time to make a quantitative estimate of the strength of these spectral effects on Io. The luminescent efficiencies of pure samples that we have studied in the laboratory suggest that charged-particle induced luminescence from Io's surface might be observable by a spacecraft such as Voyager when viewing Io's dark side.