Reflectance of amorphous-cubic NH3 frosts and amorphous-hexagonal H2O frosts at 77K from 1400 to 3000Å

The ultraviolet reflectance of Enceladus: Implications for surface composition

“…Both the Hapke and the Goguen spectra are higher than expected at the shorter wavelengths, possibly due to a red leak (suspected for the Goguen measurement) or to non-ice contaminants in the sample; Hapke et al (1981) suggest that the reflectance maximum centered near 130 nm is due to surface scattering related to an absorption maximum in water vapor at that wavelength. Pipes et al (1974) report that the reflectance in the 140-170 nm range is <2%.…”

“…(Dressler and Schnepp (1960) also measured the 194 nm absorption peak in their crystalline sample at 77 K, but not at 125 K.) Samples deposited at temperatures >85 K were found to be crystalline (cubic) or largegrain polycrystalline, with much weaker 194 nm exciton absorption peaks. Pipes et al (1974) measured the directional-hemispherical reflectance of both hexagonal and amorphous ammonia frosts $5 mm thick at 77 K. Hapke et al (1981) measured the bidirec- Pipes et al (1974) reflectance data (for both hexagonal and amorphous ice samples) are also shown, scaled to approximately agree with the model in the near-UV. Pipes et al (1974) report that the reflectance in the 140-170 nm range is <2%.…”

“…Pipes et al (1974) measured the directional-hemispherical reflectance of both hexagonal and amorphous ammonia frosts $5 mm thick at 77 K. Hapke et al (1981) measured the bidirec- Pipes et al (1974) reflectance data (for both hexagonal and amorphous ice samples) are also shown, scaled to approximately agree with the model in the near-UV. Pipes et al (1974) report that the reflectance in the 140-170 nm range is <2%. tional reflectance of ammonia frost which was likely amorphous; the temperature of the ice was not reported.…”

“…Enceladus requires a species that is absorbing over a very narrow wavelength range ($170 to $280 nm). If Martonchik et al (1984) and Dawes et al (2007; samples deposited at 75 K) and reflectance measurements from Pipes et al (1974) and Hapke et al (1981). The difference in wavelength of the absorption edge between the spectral models and the laboratory-measured reflectance spectra could be due to differences in crystallinity and/or temperature in the samples.…”

“…Pipes et al (1974) measured the directional-hemispherical reflectance of $5 mm thick samples of both hexagonal and amorphous water frosts at 77 K. Hapke et al (1981) measured the bidirectional reflectance of water ice which was likely amorphous; the temperature of the ice was not reported. Jay Goguen and JPL colleagues (personal communication) measured the reflectance of a sample ($cm thick) of pure water ice at 100 K at phase and emission angles of 45°under vacuum.…”