The steep red spectrum of 1992 AD: An asteroid covered with organic material?

Abstract

A spectrum of the newly discovered asteroid 1992 AD (now numbered 5145) was obtained by us with our CCD camera and spectrometer 1992 February 01.23. The reflection spectrum of 1992 AD displays a very steep and constant red slope between 0.5 and 1.0 μm and exhibits no absorption nor emission features. The red slope is steeper than that of any presently known Solar System object. The reflectivity ratio between 1.0 and 0.55 μm is a factor of 3.5 (1.36 magnitudes), or using a slight extrapolation, a factor of 4.90 (1.72 magnitudes) for the wavelength octave 1.0 to 0.5 μm. The steep red slope is difficult to match with conventional silicate or meteoritic materials. While allotropes of sulfur may give a partial match, the best match is provided by the steep red spectra of mixtures of tholins, the residues left after subjecting organic molecules to an energetic radiation environment.