New Horizons Alice sky Lyman-α at Pluto encounter: Importance for photochemistry

Abstract:

The third zone of our solar system, including the Pluto system, has a unique illumination environment at UV wavelengths. While direct solar Lyman-α emissions dominate the signal at 121.6 nm at classical solar system distances, the contribution of illumination by Interplanetary Medium (IPM) Lyman-α sky-glow is roughly on par at Pluto (Gladstone et al. 2015). The Pluto-Alice UV imaging spectrograph on New Horizons conducted several dedicated sky scans to measure the IPM Lyman-α both en route to and while at Pluto. These scans provide 6° by 360° great-circle swaths while spinning the spacecraft. Three sets of scans conducted en route are reported in Gladstone et al. (2012). During the Pluto encounter, sets of scans with six such swaths evenly spaced ~30° apart for all-sky coverage were obtained just before closest approach and again just after. These measurements agree well with brightness variations expected for IPM brightnesses peaking in the sunward direction and interspersed with detections of UV bright stars and other sky features. Previous studies estimated contributions of ~2/3rds direct solar Lyα and 1/3rd IPM Lyα. Our early results suggest that these model predictions need revision. These findings have important implications for determining the rates of photochemical reactions within Pluto's atmosphere that are driven by UV photons at 121.6 nm. Similarly, new constraints are provided to the rates of photolysis on Charon's polar winter nightside. These constraints are useful for understanding the volatile transport and long-term stability of the dark red region near Charon's pole discovered by New Horizons.