Searching for Charon's Atmosphere: Predictions for the New Horizons Approach

Observations of Pluto's companion planet Charon have so far yielded no compelling evidence for a persistent atmosphere. However, with the upcoming encounter of New Horizons at the Pluto-Charon system, sensitivity to detection of a possible Charon atmosphere will increase by orders of magnitude. In particular, it has long been planned for New Horizons to use its Alice ultraviolet spectrograph during solar occultation observations of Charon on July 14, 2015, for this purpose. But in the days before closest encounter, the Alice instrument will also acquire spectra of reflected sunlight from Charon's surface. We examine the effect of absorption by several possible atmospheres around Charon composed of N$_2$, CH$_4$, and CO, with plausible surface pressures of 0.1, 1, and 10 nanobar (nbar). We show that this reflectance technique is sensitive to surface pressures on Charon down to the 0.1 nbar level, and should provide significant evidence of a possible atmosphere on Charon before closest encounter. It may also be the only way to detect a local atmosphere domed over the subsolar region on Charon.