New Horizons Upper Limits on O 2 in Pluto’s Present Day Atmosphere

The surprising discovery by the Rosetta spacecraft of molecular oxygen (O 2 ) 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 O 2 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 N 2 , as well as hydrocarbon species such as CH 4 , C 2 H 2 , C 2 H 4 , and C 2 H 6 . Our work here further examines this data to search for UV absorption from molecular oxygen (O 2 ), which has a significant cross-section in the Alice spectrograph bandpass. We find no evidence for O 2 absorption and place an upper limit on the total amount of O 2 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 × 10 15 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.