THE recent occultation of a 12th magnitude star by Pluto provided a unique opportunity for studying its atmosphere. Analyses of measurements made at the Hobart observatory in Australia and the Kuiper Airborne Observatory (KAO) have been published1,2. It is generally agreed that Pluto possesses a substantial atmosphere, with a surface pressure of ∼l0bar. Both occultation measurements are sensitive primarily to the atmospheric conditions at the 1-μbar level. Analysis of the Hobart data reveals an atmospheric scale height of 46-57 km at a radial distance of 1,240-1,290 km, whereas the scale height derived from the KAO data is 59.7 ± 1.5 km at 1,214 ± 20 km. The existence of an optically thick dust layer along the line of sight at the limb has been inferred from the KAO measurements, raising doubts about the true surface radius of Pluto2. The measured scale heights are consistent with a purely methane atmosphere at a temperature of 50-61 K for the Hobart data1 and 67±6K for the KAO data2. These values are close to the surface temperature3. Here we examine the energy balance in the atmosphere and conclude that the temperature near 1 μ bar is ∼100K rather than the surface temperature; consequently, the mean molecular weight of the atmosphere is close to 25a.m.u., and a molecule heavier than (and in addition to) methane must be present in the atmosphere.
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