Pluto's atmosphere from the 29 June 2015 ground-based stellar occultation at the time of the New Horizons flyby

TL;DR

This study reports ground-based stellar occultation observations of Pluto's atmosphere shortly before the New Horizons flyby, revealing an expanding atmosphere with a stable pressure trend and detailed atmospheric structure insights.

Contribution

It provides the first detailed ground-based atmospheric profile of Pluto during the 2015 occultation, offering a valuable comparison to space-based measurements from New Horizons.

Findings

Pluto's atmosphere is still expanding with increased pressure since 2013.

A central flash indicates a spherical, transparent atmospheric layer about 3 km thick.

Estimated surface pressure ranges from 11.9 to 13.7 microbars.

Abstract

We present results from a multi-chord Pluto stellar occultation observed on 29 June 2015 from New Zealand and Australia. This occurred only two weeks before the NASA New Horizons flyby of the Pluto system and serves as a useful comparison between ground-based and space results. We find that Pluto's atmosphere is still expanding, with a significant pressure increase of 5$\pm$2\% since 2013 and a factor of almost three since 1988. This trend rules out, as of today, an atmospheric collapse associated with Pluto's recession from the Sun. A central flash, a rare occurrence, was observed from several sites in New Zealand. The flash shape and amplitude are compatible with a spherical and transparent atmospheric layer of roughly 3~km in thickness whose base lies at about 4~km above Pluto's surface, and where an average thermal gradient of about 5 K~km$^{-1}$ prevails. We discuss the possibility that small departures between the observed and modeled flash are caused by local topographic features (mountains) along Pluto's limb that block the stellar light. Finally, using two possible temperature profiles, and extrapolating our pressure profile from our deepest accessible level down to the surface, we obtain a possible range of 11.9-13.7~$\mu$bar for the surface pressure.