The Atmosphere of Pluto as Observed by New Horizons

TL;DR

New Horizons observations reveal Pluto's atmosphere is colder, more compact, and dominated by nitrogen, with complex hydrocarbons contributing to haze formation, challenging previous models and raising questions about its long-term variability.

Contribution

This study provides the first detailed in-situ measurements of Pluto's atmosphere, highlighting differences from pre-encounter models and identifying key atmospheric constituents.

Findings

Upper atmosphere is colder and more compact than models predicted.

Nitrogen dominates the atmosphere at altitudes below 1800 km.

Presence of hydrocarbons suggests complex haze production processes.

Abstract

Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. While the lower atmosphere (at altitudes <200 km) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N$_2$) dominates the atmosphere (at altitudes <1800 km or so), while methane (CH$_4$), acetylene (C$_2$H$_2$), ethylene (C$_2$H$_4$), and ethane (C$_2$H$_6$) are abundant minor species, and likely feed the production of an extensive haze which encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state--over seasonal or geologic time scales.