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Changes in Pluto's atmosphere: 1988-2006


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Elliott, JL and Pasachoff, JM and Pike, RE and Zuluaga, CA and Bosh, AS and Dieters, S and Francis, PJ and Giles, AB and Greenhill, JG and Lade, B and Lucas, R and Person, MJ and Ramm, DJ and Gulbis, AAS and Souza, SP and Adams, ER and Babcock, BA and Gangestad, JW and Jaskot, AE and Kramer, EA (2007) Changes in Pluto's atmosphere: 1988-2006. The Astronomical Journal, 134 (1). pp. 10-13. ISSN 0004-6256

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The 2006 June 12 occultation of the star P384.2 (2UCAC 26039859) by Pluto was observed from five sites in southeastern
Australia with high-speed imaging photometers that produced time-series CCD images. Light curves were
constructed from the image time series and fit by least-squares methods with model light curves. A new modeling
procedure is presented that allows a simultaneous fit of the atmospheric parameters for Pluto and the astrometric
parameters for the occultation to all of the light curves. Under the assumption of a clear atmosphere and using this
modeling procedure to establish the upper atmosphere boundary condition, immersion and emersion temperature
profiles were derived by inversion of the Siding Spring light curve, which had our best signal-to-noise ratio. Above
1230 km radius, atmospheric temperatures are100K and decrease slightly with altitude—the same as observed in
1988 and 2002. Below 1210 km, the temperature abruptly decreases with altitude (gradients 2.2 K km1), which
would reach the expected N2 surface-ice temperature of 40 K in the 1158Y1184 km radius range. This structure is
similar to that observed in 2002, but a much stronger thermal gradient (or stronger extinction) is implied by the 1988
light curve (which shows a ‘‘kink’’ or ‘‘knee’’ at 1210 km). The temperature profiles derived from inversion of the
present data show good agreement with a physical model for Pluto’s atmosphere selected from those presented by
Strobel et al. (1996). Constraints derived from the temperature profiles (and considering the possibility of a deep
troposphere) yield a value of 1152 32 km for Pluto’s surface radius. This value is compared with surface-radius
values derived from the series of mutual occultations and eclipses that occurred in 1985Y1989, and the limitations of
both types of measurements for determining Pluto’s surface radius are discussed. The radius of Pluto’s atmospheric
shadow at the half-intensity point is 1207:9 8:5 km, the same as obtained in 2002 within measurement error.Values
of the shadow radius cast by Pluto’s atmosphere in 1988, 2002, and 2006 favor frost migration models in which Pluto’s
surface has lowthermal inertia. Thosemodels imply a substantial atmosphere when New Horizons flies by Pluto in 2015.
Comparison of the shape of the stellar occultation light curves in 1988, 2002, and 2006 suggests that atmospheric extinction,
which was strong in 1988 (15 months before perihelion), has been dissipating.

Item Type: Article
Keywords: methods: data analysis — occultations — planets and satellites: individual (Pluto)
Journal or Publication Title: The Astronomical Journal
Publisher: University of Chicago Press
Page Range: pp. 10-13
ISSN: 0004-6256
Identification Number - DOI: 10.1086/517998
Additional Information:

© 2007. The American Astronomical Society. All rights reserved.

Date Deposited: 07 Apr 2008 14:28
Last Modified: 18 Nov 2014 03:34
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