Constraints on the composition of Trojan asteroid 624 Hektor

We present a composite spectrum of Trojan asteroid 624 Hektor, 0.3-3.6 mum, and models computed for the full wavelength range with the Hapke scatterin g theory. The data show that there is no discernible 3-mum absorption band. Such a band would indicate the presence of OH- or H2O-bearing silicate min erals, or macromolecular carbon-rich organic material of the kind seen on t he low-albedo hemisphere of Saturn's satellite Iapetus. The absence of spec tral structure is itself indicative of the absence of the nitrogen-rich tho lins (which show a distinctive absorption band attributed to N-H). The succ essful models in this study all incorporate magnesium-rich pyroxene (Mg, Fe SiO3), which satisfactorily matches the red color of Hektor. Pyroxene is a mafic mineral common in terrestrial and lunar lavas, and is also identifie d in Main Belt asteroid spectra. An upper limit to the amount of crystallin e H2O ice (30-mum grains) in the surface layer of Hektor accessible to near -infrared remote sensing observations is 3 wt%. The upper limit for serpent ine, as a representative of hydrous silicates, is much less stringent, at 4 0%, based on the shape of the spectral region around 3 mum. Thus, the spect rum at 3 mum does not preclude the presence of a few weight percent of vola tile material in the uppermost surface layer of Hektor. Below this "optical " surface that our observations probe, any amount of H2O ice and other vola tile-rich materials might exist. All of the models we calculated require a very low-albedo, neutral color material to achieve the low geometric albedo that matches Hektor; we use elemental carbon. If elemental carbon is prese nt on Hektor, it could be of organic or inorganic origin. By analogy, other D-type asteroids could achieve their red color, low albedo, and apparent a bsence of phyllosilicates from compositions similar to the models presented here. Our models appear to demonstrate that organic solids are not require d to match the red color and low albedos of D-type asteroids. (C) 2001 Acad emic Press.