New Horizons Upper Limits on O2 in Pluto’s Present Day Atmosphere

Abstract

Abstract The surprising discovery by the Rosetta spacecraft of molecular oxygen (O2) in the coma of comet 67P/Churyumov–Gerasimenko challenged our understanding of the inventory of this volatile species on and inside bodies from the Kuiper Belt. That discovery motivated our search for oxygen in the atmosphere of Kuiper Belt planet Pluto, because O2 is volatile even at Pluto’s surface temperatures. During the New Horizons flyby of Pluto in 2015 July, the spacecraft probed the composition of Pluto’s atmosphere using a variety of observations, including an ultraviolet solar occultation observed by the Alice UV spectrograph. As described in these reports, absorption by molecular species in Pluto’s atmosphere yielded detections of N2, as well as hydrocarbon species such as CH4, C2H2, C2H4, and C2H6. Our work here further examines this data to search for UV absorption from molecular oxygen (O2), which has a significant cross-section in the Alice spectrograph bandpass. We find no evidence for O2 absorption and place an upper limit on the total amount of O2 in Pluto’s atmosphere as a function of tangent height up to 700 km. In most of the atmosphere, this upper limit in line-of-sight abundance units is ∼3 × 1015 cm−2, which, depending on tangent height, corresponds to a mixing ratio of 10−6 to 10−4, far lower than in comet 67P/CG.