Changes in Pluto’s Atmosphere

Abstract

Pluto’s tenuous atmosphere was probed in 1988 with a stellar occultation observed from the Kuiper Airborne Observatory (KAO, Elliot et al. 1989) and a variety of ground-based sites (Millis et al. 1993). These data, subsequent theoretical modeling (e.g. Strobel et al. 1996), and spectroscopic observations (Owen et al. 1993; Young et al. 1997), gave us the following post-occultation picture of Pluto’s atmosphere (Yelle & Elliot 1997; Elliot, Person, & Qu 2003b): N2 is the dominant constituent of the atmosphere, which also contains small amounts of CH4 and CO. These molecules are in vapor-pressure equilibrium with their surface ices, and this process acts as a thermostat to keep the N2 ice at ~38 K around the body. The temperature of the 1-3 μbar pressure region probed by the occultation was ~100 K. The KAO light curve dropped abruptly, however, just below half-light. This abrupt drop could be due to one of two potential properties of Pluto’s atmosphere: extinction, or a steep thermal gradient. Each of these explanations has strengths and weaknesses (Yelle & Elliot 1997; Elliot, et al. 2003b).