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Evidence of N2-Ice On the Surface of the Icy Dwarf Planet 136472 (2005 FY9)

We present high signal precision optical reflectance spectra of 2005 FY9 taken with the Red Channel Spectrograph and the 6.5-m MMT telescope on 2006 March 4 UT (5000 - 9500 A; 6.33 A pixel-1) and 2007 February 12 UT (6600 - 8500 A; 1.93 A pixel-1). From cross correlation experiments between the 2006 March 4 spectrum and a pure CH4-ice Hapke model, we find the CH4-ice bands in the MMT spectrum are blueshifted by 3 +/- 4 A relative to bands in the pure CH4-ice Hapke spectrum. The higher resolution MMT spectrum of 2007 February 12 UT enabled us to measure shifts of individual CH4-ice bands. We find the 7296 A, 7862 A, and 7993 A CH4-ice bands are blueshifted by 4 +/- 2 A, 4 +/- 4 A, and 6 +/- 5 A. From four measurements we report here and one of our previously published measurements, we find the CH4-ice bands are shifted by 4 +/- 1 A. This small shift is important because it suggest the presence of another ice component on the surface of 2005 FY9. Laboratory experiments show that CH4-ice bands in spectra of CH4 mixed with other ices are blueshifted relative to bands in spectra of pure CH4-ice. A likely candidate for the other component is N2-ice because its weak 2.15 micron band and blueshifted CH4 bands are seen in spectra of Triton and Pluto. Assuming the shift is due to the presence of N2, spectra taken on two consecutive nights show no difference in CH4/N2. In addition, we find no measureable difference in CH4/N2 at different depths into the surface of 2005 FY9.