Haze in Pluto's atmosphere: Results from SOFIA and ground-based observations of the 2015 June 29 Pluto occultation

Abstract

We report observations of a stellar occultation by Pluto on 2019 July 17. A single-chord high-speed (time resolution = 2 s) photometry dataset was obtained with a CMOS camera mounted on the Tohoku University 60 cm telescope (Haleakala, Hawaii). The occultation light curve is satisfactorily fitted to an existing Pluto's atmospheric model. We find the lowest pressure value at a reference radius of r = 1215 km among those reported after 2012, indicating a possible rapid (approximately 21 +4−5 % of the previous value) pressure drop between 2016 (the latest reported estimate) and 2019. However, this drop is detected at a 2.4σ level only and still requires confirmation from future observations. If real, this trend is opposite to the monotonic increase of Pluto's atmospheric pressure reported by previous studies. The observed decrease trend is possibly caused by ongoing N 2 condensation processes in the Sputnik Planitia glacier associated with an orbitally driven decline of solar insolation, as predicted by previous theoretical models. However, the observed amplitude of the pressure decrease is larger than the model predictions.

A bimodal distribution of haze in Pluto’s atmosphere

“…The existence of haze in Pluto’s atmosphere was confirmed by the New Horizons spacecraft during its flyby in July 2015 11 , 15 , 20 , 21 , as well as a recent occultation 22 . Originating from photolysis of CH 4 and N 2 in Pluto’s upper atmosphere driven by solar ultraviolet radiation, haze particles grow through coagulation as they sediment downwards 12 , 23 , which is similar to processes in Titan’s upper atmosphere and the atmosphere of Triton 13 , 24 .…”

Orbits and Occultation Opportunities of 15 TNOs Observed by New Horizons

“…Lunar scattered light, and therefore lunar elongation, is another concern, as it raises the background level of the sky and reduces the depth of stars that can be observed. Further, to the best of our knowledge, no successful TNO occultation has been captured from an oceangoing vessel, though single-chord occultations have been observed of Pluto and Arrokoth by the NASA/DLR SOFIA flying observatory while over open water (Buie et al 2020;Person et al 2021). A further geographic consideration is the typical climatic conditions of an occultation site, as there is little point to sending a telescope to an area that is almost guaranteed to be cloudy.…”

A bimodal distribution of haze in Pluto's atmosphere

“…The existence of haze in Pluto's atmosphere was confirmed by the New Horizons spacecraft during its flyby in July 2015 (Stern et al 2015, Gladstone et al 2016, Cheng et al 2017), together with a recent occultation (Person et al 2021). Originating from photolysis of CH4, N2, and larger organic molecules in Pluto's upper atmosphere driven by solar ultraviolet radiation, haze particles grow through coagulation of smaller particles as they sediment downwards (Gao et al 2017, Lavvas et al 2021, which is similar to processes in Titan's upper atmosphere (Lavvas et al 2013).…”