[1] The D type asteroids are among the darkest objects known in our solar system. Here, we present infrared spectra of one of the main-belt D type asteroids, 773 Irmintraud. In contrast to previous observations of D type asteroids, we found a gap of reflectance around 3 mm in wavelength. The 3 mm gap is one of the spectral signatures of OH or H 2 O as water ice or in hydrous minerals, which had formed in the processes of aqueous alteration in the early solar system. We suggest that D type asteroids, which are all the while considered as unaltered primitive material, could be aqueously altered. Our data do support the presence of water on a compositionally primitive D type asteroid.

We have carried out infrared high-resolution spectroscopy of the Pluto-Charon system in the L band with the adaptive optics system on the Subaru telescope. The spectrum is dominated by the strong and broad absorption features of methane but includes some additional features. Comparing the spectrum with model calculations, we suggest that absorption features around 3.1, 3.2, and 3.35 mm could be an indication of nonmethane hydrocarbons on Pluto's uppermost surface. Implications of the estimated mass ratio between hydrocarbons for the formation and evolution of Pluto are discussed.

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