# Pluto's lower atmosphere and pressure evolution from ground-based stellar occultations, 1988-2016

## TL;DR

This study analyzes over a decade of stellar occultations to track Pluto's atmospheric pressure changes and constrains its lower atmosphere structure, revealing a monotonic pressure increase and insights into surface and atmospheric properties.

## Contribution

It provides the first long-term calibrated record of Pluto's surface pressure evolution and constrains the lower atmosphere structure using occultation data and a central flash analysis.

## Key findings

- Pluto's atmospheric pressure increased monotonically from 1988 to 2016.
- The central flash data suggest possible large diurnal temperature variations or hazes at 4-7 km altitude.
- Constraints on N2 ice albedo and emissivity are derived from pressure evolution modeling.

## Abstract

Context. Pluto's tenuous nitrogen (N2) atmosphere undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has been recently (July 2015) observed by the New Horizons spacecraft. Goals are (i) construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) constrain the structure of the lower atmosphere using a central flash observed in 2015. Method: eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between $\sim$5 km and $\sim$380 km altitude levels (i.e. pressures from about 10 microbar to 10 nanobar). Results: (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived; (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia and/or (b) hazes with tangential optical depth of about 0.3 are present at 4-7 km altitude levels and/or (c) the nominal REX density values are overestimated by an implausibly large factor of about 20% and/or (d) higher terrains block part of the flash in the Charon facing hemisphere.