Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p22b06k&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #P22B-06
Physics
5405 Atmospheres--Composition And Chemistry, 5470 Surface Materials And Properties, 6270 Pluto And Satellites
Scientific paper
Pluto's spectrum in the range of 200 to 250 nm has been extracted from the HST archive observations acquired during 24 orbits. Although the spectral intensity is very weak and smaller than that for Titan by a factor of 100, the spectrum has a signal-to-noise ratio of 10-15 at the resolution of 1 nm. The mean geometric albedo is almost constant at 0.22 in this range. Observations at different rotational phases and at similar phases with a one-year interval reveal two kinds of albedo variations: (1) regular variations with rotational phase and (2) strong UV brightening due to appearance of haze. This brightening is weak at λ >260 nm and very prominent at shorter wavelengths, resulting in an increase of Pluto's geometric albedo at 210-230 nm up to a factor of 2.5. The observed spectrum of the brightening requires haze particles with a strong backscattering component, probably, specifically oriented crystals with specular reflection. The best analog for the Mie spheres is that with r = 0.08 μ m for the refractive index of CH4. Backscattering of these particles at 210 nm exceeds those at 270 and 320 nm by factors of 16 and 5, respectively. This strong UV brightening is the first indication of weather features on Pluto that appears in the global scale. The only known global-scale weather phenomenon in the Solar System is the global dust storms on Mars. Search for gaseous absorptions resulted in upper limits to C4H2, C6H2, HC3N, and C4N2. The first three limits impose important constraints to photochemical models of Pluto's atmosphere. The Cameron bands cannot be used for detection of CO on Pluto because of the very low atmospheric pressure. The first UV spectra of Charon have been extracted from the HST archive observation acquired during 16 orbits. The spectra cover the range of 225-330 nm with a signal-to-noise ratio varying from 70 at 330 to 265 nm to 8 at 240-225 nm. The geometric albedo of Charon is 0.23 and almost constant in the measured range. The North/South ratio is 1.04 +/- 0.02 from 255 to 330 nm. Comparing the extracted spectra with laboratory spectra of H2O, CO2, NH3, and SO2 ices, upper limits to concentrations of these species in the ices of Pluto and Charon have been established. However, these limits are high and nonrestrictive. Some weak features in the spectra of Pluto and Charon are similar to those of chondrites.
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