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 ± 0.6 R at Ly α , 0.24 ± 0.02 R at Ly β , and <0.10 R at He i  584 Å. We analyzed 3900 s 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 Ly α to be 29.3 ± 1.9 R, in good agreement with preencounter estimates. The detection of the N ii multiplet at 1085 Å marks the first direct detection of ions in Pluto’s atmosphere. We do not detect any emissions from noble gases and place a 3 σ upper limit of 0.14 R on the brightness of the Ar i  1048 Å line. We compare preencounter 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 Ly α , Pluto’s atmosphere is optically thin at wavelengths longer than 1425 Å. Consequently, a significant amount of solar far-UV 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 and 1850 Å. We also report the first detection of a C 3 hydrocarbon molecule, methylacetylene, in absorption, at a column density of ∼5 × 10 15 cm −2 , corresponding to a column-integrated mixing ratio of 1.6 × 10 −6 .