Reprocessing 35 years of Pluto occultations turns up a 16 percent pressure drop in the lower atmosphere

An occultation does not measure pressure. Pluto crosses in front of a star, the starlight bends as it passes through the atmosphere on its way to a telescope, and the shape of the resulting light curve is inverted into density, temperature and pressure profiles. Stellar occultations give a snapshot of the physical properties of Pluto's atmosphere at a given time, and the path the starlight takes through a thin atmosphere depends on its composition as well as on the temperature and density profiles, Amanda Sickafoose, a planetary scientist at the Planetary Science Institute in Tucson and lead author of the new work, told Eos via email ^[10][11].

Pluto's atmospheric signal is faint enough that one telescope and one analysis method can return different answers from another's, which has made a uniform record hard to assemble ^[8]. Each inversion carries the assumptions of whoever performed it. Sickafoose and her colleagues put the whole 1988 to 2023 archive through consistent processing, 10 new events from 2017 to 2023 included, drawn from more than 15 observing sites ^[9].

Perianne Johnson, a Pluto climate scientist at Purdue University who was not involved in the research, said the uniform treatment is what makes the change believable. "We are hoping to identify slight changes in Pluto's atmosphere, and it can be difficult to ascertain if reported differences are truly changes to the atmosphere or just differences between the way two scientists processed the data and different assumptions they made about unknown atmospheric properties. In this work, we can trust that the reported changes to the atmospheric pressure are real," Johnson said ^[17][18].

"In terms of size and pressure, we find that Pluto's atmosphere remained roughly stable from the New Horizons flyby in 2015 through 2021," Sickafoose said. "Our most recent datasets in 2022 and 2023 suggest that the pressure has dropped, at a level of 16%, in the lower atmosphere" ^[12][13]. Her verb is suggest, and the decline rests on the two most recent observing years of a 35-year record ^[21]. That record covers about 14 percent of one Pluto orbit, which runs 248 years ^[22][1].

The two halves of the atmosphere have diverged under weakening sunlight ^[14]. The upper atmosphere is still haze free while haze in the lower atmosphere has begun to clear, a change astronomers had noted before and attribute to haze particles snowing down as the atmosphere cools ^[15].

The timing is the part still open. A nitrogen-dominated atmosphere that comes and goes with the seasons has been theorised for decades, on the grounds that Pluto's steeply inclined, strongly non-circular orbit swings its temperature through a wide range ^[19][20]. Testing it requires Pluto and a background star to line up, which happens maybe once or twice a year ^[7]; the team's 10 new events across seven seasons work out to about 1.4 a year ^[23]. The paper was published in The Planetary Science Journal in July ^[16].