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

Abstract

During the New Horizons spacecraft'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 $\alpha$, 0.24$\pm$0.02R at Lyman $\beta$, 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's brightness at Lyman $\alpha$ to be $29.3\pm 1.9$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's atmosphere. We do not detect any emissions from noble gasses and place a 3$\sigma$ 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's airglow. Although completely opaque at Lyman $\alpha$, Pluto'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$\times 10^{15}$ cm${}^{-2}$, corresponding to a column-integrated mixing ratio of $1.6\times 10^{-6}$.

27 pages, 8 figures

References in corpus (6)

• The New Horizons Pluto Kuiper belt Mission: An Overview with Historical Context
• ALICE: The Ultraviolet Imaging Spectrograph aboard the New Horizons Pluto-Kuiper Belt Mission
• Haze in Pluto's Atmosphere
• Alice: The Rosetta Ultraviolet Imaging Spectrograph
• New Horizons: Anticipated Scientific Investigations at the Pluto System
• Production of N2 Vegard-Kaplan and Lyman-Birge-Hopfield emissions on Pluto