2002 PLUTO OCCULTATIONS J. Elliot et al. Observations were made of an occultation of the star by Pluto. Previous observations in 1988 showed an isothermal atmosphere above a radius of 1215 km. Below this layer there was either a thermal gradient or an extinction layer. The results from the IRTF and the UH 2.2-m telescope show: Evidence for extinction in the atmosphere of Pluto. The surface pressure at the surface of Pluto increased by a factor of 2. The atmosphere has expanded by about 45 km. Reference: Elliot, J.L. et al. 2003, Nature, 424, 165. The expansion of the atmosphere could be due to a decrease in the albedo coupled with a surface temperature rise of 1 K, or to sublimation of the south polar cap due to seasonal variations.

Occultation path The star P131.1 was occulted by Pluto (Rmag = 15.7). The Lick obs. and MKO chords were used for calculating the astrometric solution.

SPECTRA OF PLUTO-CHARON AND P131.1 (SpeX at IRTF; ~0.5 seeing) Wavelength Increasing —> (0.8 to 2.5 µm) Distance along Slit —> <—Pluto-Charon <—Star (P131.1) Wavelength Increasing —> (0.8 to 2.5 µm) Distance along Slit —> (a) PRIOR TO OCCULTATION (separation about 1.5 arc sec) (b) SPECTRA MERGED " This work was made possible by close collaboration between Elliot and Rayner to provide occultation capability with SpeX.

Minimum Stellar flux vs. wavelength Wavelength (  m) Minimum Stellar Flux The trend of less extinction with longer wavelengths is evidence for extinction in the atmosphere of Pluto. These observations are best explained by aggregates of haze particles that are produced photochemically in the atmosphere.

IRTF contributions Planning for stellar occultations with SpeX began at an early stage when SpeX was being designed by John Rayner. The specifications for the electronics and time stamping were set to accommodate occultations. SpeX provided simultaneous wavelength coverage from microns and this was the best data set showing the wavelength dependence of the extinction. To quote the PI: “We had been trying for 14 years to get multi-wavelength occultation observations without success. The high-throughput, high-speed mode on SpeX was optimally designed for this observation and gave us great results, even for such a faint star!”

Occultation light curves The UH 2.2-m light curve was used in the analysis because it had the highest signal-to-noise and time resolution. The KAO data refers to the 1988 occultation. The “kink” observed in 1988 does was not observed in 2002.

Pressure profiles for 1988 and 2002 Inversion of the UH 2.2-m light curve shows a factor of 2 increase in pressure compared to the 1988 KAO observations. Thermal models predict the pressure to decrease starting in 2020 or earlier. Thus future observations are necessary.