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C. Max, B. Macintosh, S. Gibbard (Lawrence Livermore Nat'l Lab), H. Roe, I. de Pater (Univ. of Calif. at Berkeley), A. Ghez (UCLA), S. Acton, P. Wizinowich (W. M. Keck Observatory), O. Lai (Canada France Hawaii Telescope Facility)
We present near-IR images of Neptune and Titan with a resolution of 0.05 and 0.04 arc seconds respectively, obtained with the new adaptive optics (AO) system at the 10-m Keck II telescope. The resolution and contrast of these images allows us to fit radiative transfer atmospheric models including latitudinal and longitudinal variations. In the case of Neptune the images show both circumferential bands and bright cloud or storm features. We first model Neptune's clear atmosphere, un- contaminated by scattered light from the bright storms and bands. We can measure for the first time the precise infrared spatial extent of storm features and calculate their reflectance in broad-band and methane-absorption near-IR band-passes. These data allow us to model the changed haze distribution within the storm, relative to the clear atmosphere. In the case of Titan we use a radiative transfer model incorporating fractal haze properties, and are able to separate the contributions of Titan's atmosphere and surface to determine the optical depth of Titan's haze layer and to construct surface albedo maps. We find that Titan's trailing hemisphere is overall very dark, with a surface reflectance <0.05 consistent with the presence of solid or liquid hydrocarbons or perhaps very large-grained ice. The leading hemisphere is brighter, but with dark patches of very low albedo similar to the trailing hemisphere. These observations demonstrate that AO on an 8-10 meter telescope is a powerful new tool for the quantitative study of objects in the outer solar system, surpassing the resolution of the NICMOS instrument on the Hubble Space Telescope by a factor of 2-4.
This research was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-ENG-48.
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