Pluto's atmosphere

Abstract The stellar occultation by Pluto on June 9, 1988, was observed with a high-speed CCD photometer attached to the 0.9-m telescope aboard NASA's Kuiper Airborne Observatory (KAO). The occultation lightcurve, which probed two regions on the sunrise limb separated by about 200 km, reveals a clear upper atmosphere that overlies an extinction layer with an abrupt upper boundary. The observations demonstrate that the extinction layer extends along the portion of the sunrise limb bounded by the immersion and emersion regions, as well as long the corresponding portion of the sunset limb on the opposite side of the planet. In all, the total limb probed by the KAO data for extinction represents nearly half of Pluto's circumference. Hence, the extinction layer may surround the entire planet. A model atmosphere is presented, from which is derived an occultation lightcurve that closely matches the data. In addition to the standard parameters describing the occultation curve by an isothermal atmosphere, our model includes the radius of the upper boundary of the extinction and the radius of unit observed optical depth as free parameters. Fits of this model to the immersion and emersion lightcurves show no significant differences in the derived atmospheric structure. A preliminary geometrical solution, based on three occultation chords, yields a half-light radius of 1214 ± 20 km. At this level, the mean scale height derived from the model fits to the KAO data is 59.7 ± 1.5 km. The corresponding ratio of temperature to mean molecular weight is 4.2 ± 0.4°K/amu, with the principal source of error arising from the uncertainty in the mass of Pluto. The extinction layer, whose upper boundary lies 25 km below the half-light level, has a minimum thickness of 46 km, a minimum vertical optical depth of 0.19, and a scale height of 33.4 ± 6.9 km. For a pure methane atmosphere, our results imply (for the clear atmosphere at the half-light level) a temperature of 67 ± 6°K, a number density of 8.3 × 10 13 cm −3 , and a pressure of 0.78 ωbar. Our occultation data are also consistent with a predominantly nitrogen atmosphere (such as that of Titan), in which case the temperature would be 117 ± 11°K. The substantially smaller scale height of the extinction layer may arise from properties of the “particles” causing the extinction or may indicate a lower temperature in this region. Since our analysis indicates that the extinction layer is optically thick at the limb of Pluto, determinations of Pluto's radius by methods that use reflected light, such as speckle interferometry and observations of the mutual events, give results that refer to the “visible disk” of Pluto and not on the planet's solid surface. Unit optical depth of the extinction layer (observed along the line of sight) lies at 1174 ± 20 km, a level consistent with the radius of Pluto derived from the mutual events (1142 ± 21 km). The mutual event radius is also consistent with the deepest level probed by the occultation: it lies at a radius of 1143 ± 20 km, which represents an upper limit on the surface radius. For a pure methane atmosphere, a surface pressure as low as 3 ωbar (the vapor pressure of methane at 50°K) would be consistent with the occultation data.