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Project Progress

Co-Investigator Robert West and colleagues Joseph Ajello (JPL), Michael Stevens (Naval Research Lab), Darell F. Strobel (Johns Hopkins Univ.), G. Randy Gladstone (Southwest Research Inst.), J. Scott Evans (5Computational Physics Inc.), and E. Todd Bradley (Univ. of Central Florida) discovered airglow emission from Titan’s atmosphere while deep in Saturn’s shadow near equinox in 2009. This is significant because solar XUV photons can provide enough energy to account for the observed nitrogen UV dayglow emissions above 800 km, but a small or sporadic contribution from energetic particles cannot be ruled out. Furthermore, ion production at altitudes deeper than 800 km as inferred from radio occultation cannot be produced by solar XUV stimulation and implies energy deposition from protons and oxygen ions. In this work they examined UV spectra and visible-wavelength images of Titan in Saturn’ s shadow, when XUV stimulation is absent. UV emissions were observed in one of the three sets of spectra, and the intensity of these emissions is about a factor of 10 less than the peak intensity reported on the dayside. They also observed visible-wavelength emissions for the first time. No horizontally-resolved auroral structures were seen in the visible images. At visible wavelengths Titan has a global emission at the haze-top level that is not understood, although cosmic ray ionization and chemiluminescence are candidates needing further investigation.