Changes in Pluto's Atmosphere: 1988-2006

The 2006 June 12 occultation of the star P384.2 (2UCAC 26039859) by Pluto was observed from five sites in southeastern\udAustralia with high-speed imaging photometers that produced time-series CCD images. Light curves were\udconstructed from the image time series and fit by least-squares methods with model light curves. A new modeling\udprocedure is presented that allows a simultaneous fit of the atmospheric parameters for Pluto and the astrometric\udparameters for the occultation to all of the light curves. Under the assumption of a clear atmosphere and using this\udmodeling procedure to establish the upper atmosphere boundary condition, immersion and emersion temperature\udprofiles were derived by inversion of the Siding Spring light curve, which had our best signal-to-noise ratio. Above\ud1230 km radius, atmospheric temperatures are100K and decrease slightly with altitude—the same as observed in\ud1988 and 2002. Below 1210 km, the temperature abruptly decreases with altitude (gradients 2.2 K km1), which\udwould reach the expected N2 surface-ice temperature of 40 K in the 1158Y1184 km radius range. This structure is\udsimilar to that observed in 2002, but a much stronger thermal gradient (or stronger extinction) is implied by the 1988\udlight curve (which shows a ‘‘kink’’ or ‘‘knee’’ at 1210 km). The temperature profiles derived from inversion of the\udpresent data show good agreement with a physical model for Pluto’s atmosphere selected from those presented by\udStrobel et al. (1996). Constraints derived from the temperature profiles (and considering the possibility of a deep\udtroposphere) yield a value of 1152 32 km for Pluto’s surface radius. This value is compared with surface-radius\udvalues derived from the series of mutual occultations and eclipses that occurred in 1985Y1989, and the limitations of\udboth types of measurements for determining Pluto’s surface radius are discussed. The radius of Pluto’s atmospheric\udshadow at the half-intensity point is 1207:9 8:5 km, the same as obtained in 2002 within measurement error.Values\udof the shadow radius cast by Pluto’s atmosphere in 1988, 2002, and 2006 favor frost migration models in which Pluto’s\udsurface has lowthermal inertia. Thosemodels imply a substantial atmosphere when New Horizons flies by Pluto in 2015.\udComparison of the shape of the stellar occultation light curves in 1988, 2002, and 2006 suggests that atmospheric extinction,\udwhich was strong in 1988 (15 months before perihelion), has been dissipating