Synoptic CCD Spectrophotometry of Pluto Over the Past 15 Years

Details Synoptic CCD Spectrophotometry of Pluto Over the Past 15 Years Synoptic CCD Spectrophotometry of Pluto Over the Past 15 Years

Nov 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996icar..124..329g&link_type=abstract

Icarus, Volume 124, Issue 1, pp. 329-343.

Physics

27

Scientific paper

Over the 15 years from 1980 to 1994, the same spectrometer has been used to obtain spectrophotometric observations of Pluto+Charon over the wavelength range from 0.5 to 1.0 mum. This time period spanned Pluto's perihelion passage in 1989, as well as the Pluto-Charon mutual event season. The data set is presented and a search made for variations in Pluto's methane absorptions and continuum slope correlating with Pluto's 6.4-day lightcurve as well as possible longer term secular evolution. Four quantities derived from Pluto's spectrum are examined for variation with Pluto's rotational phase. Although the depths of the methane bands are generally deeper away from the lightcurve minimum, they are found not to correlate in a simple way with the lightcurve, in contrast with the behavior of the stronger methane bands at longer wavelengths. This behavior implies that Pluto's CH_4 is not exclusively associated with the brightest or the darkest terrain, and that weak and strong methane bands are sensitive to different methane reservoirs on Pluto's surface. The wavelengths of CH_4 bands also change, being longest at the rotational phases where the CH_4 bands are deepest. The wavelength shifts can be attributed to variable concentration of CH_4 dissolved in N_2 ice. No statistically significant trend of bandwidth with longitude is detected. It is confirmed that the reddest slope of the continuum coincides with the darkest surface terrain. Examining spectral data pairs taken at similar Pluto longitudes, a modest but consistent secular trend is seen, with the depth of the 0.73-μm CH_4 absorption diminishing relative to that of the stronger 0.89-μm band. The observations are interpreted by means of Hapke scattering models. While unique solutions are not possible, it is found that the observed spectra can be matched by geographically diverse models possessing both methane rich and methane poor terrains. Terrain types proposed are a high albedo, CH_4 poor, N_2 dominated surface, a dark, red, tholin-rich surface, and a transitional zone, rich in CH_4, with an intermediate albedo. The slight secular weakening of the 0.73-μm CH_4 band can be interpreted as resulting from differences between Pluto's southern and northern hemispheres or from a temporal evolution in the geographic proportions of the CH_4 rich and CH_4 poor terrains.

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