New Horizons Constraints on Charon's Present Day Atmosphere

TL;DR

Using multiple observational techniques during New Horizons' flyby, the study found no evidence of an atmosphere around Charon, setting much stricter upper limits on potential atmospheric pressures than previous constraints.

Contribution

This paper provides the first comprehensive search for Charon's atmosphere using diverse methods, establishing the most stringent upper limits to date on atmospheric presence and composition.

Findings

No detectable atmosphere around Charon within the sensitivity limits.

Upper limits on atmospheric pressures are now 15-110 times lower than previous constraints.

Constraints on atmospheric haze brightness are significantly tighter than prior estimates.

Abstract

We report on a variety of standard techniques used by New Horizons including a solar ultraviolet occultation, ultraviolet airglow observations, and high-phase look-back particulate search imaging to search for an atmosphere around Pluto's large moon Charon during its flyby in July 2015. Analyzing these datasets, no evidence for a present day atmosphere has been found for 14 potential atomic and molecular species, all of which are now constrained to have pressures below 0.3 nanobar, as we describe below, these are much more stringent upper limits than the previously available 15-110 nanobar constraints (e.g., Sicardy et al. 2006); for example, we find a 3$\sigma$ upper limit for an N$_2$ atmosphere on Charon is 4.2 picobars and a 3$\sigma$ upper limit for the brightness of any atmospheric haze on Charon of I/F=2.6x10$^{-5}$. A radio occultation search for an atmosphere around Charon was also conducted by New Horizons but will be published separately by other authors.