Paper detail

Pluto's Ultraviolet Spectrum, Surface Reflectance, and Airglow Emissions

During the New Horizons spacecraft&#39;s encounter with Pluto, the Alice ultraviolet spectrograph conducted a series of observations that detected emissions from both the interplanetary medium (IPM) and Pluto. In the direction of Pluto, the IPM was found to be 133.4$\pm$0.6R at Lyman $α$, 0.24$\pm$0.02R at Lyman $β$, and <0.10R at He I 584Å. We analyzed 3,900s of data obtained shortly before closest approach to Pluto and detect airglow emissions from H I, N I, N II, N$_2$, and CO above the disk of Pluto. We find Pluto&#39;s brightness at Lyman $α$ to be $29.3\pm1.9$R, in good agreement with pre-encounter estimates. The detection of the N II multiplet at 1085Å marks the first direct detection of ions in Pluto&#39;s atmosphere. We do not detect any emissions from noble gasses and place a 3$σ$ upper limit of 0.14 R on the brightness of the Ar I 1048Å line. We compare pre-encounter model predictions and predictions from our own airglow model, based on atmospheric profiles derived from the solar occultation observed by New Horizons, to the observed brightness of Pluto&#39;s airglow. Although completely opaque at Lyman $α$, Pluto&#39;s atmosphere is optically thin at wavelengths longer than 1425Å. Consequently, a significant amount of solar FUV light reaches the surface, where it can participate in space weathering processes. From the brightness of sunlight reflected from Pluto, we find the surface has a reflectance factor (I/F) of 17% between 1400-1850Å. We also report the first detection of an C$_3$ hydrocarbon molecule, methylacetylene, in absorption, at a column density of ~5$\times10^{15}$ cm$^{-2}$, corresponding to a column-integrated mixing ratio of $1.6\times10^{-6}$.