We present a composite spectrum of Trojan asteroid 624 Hektor, 0.3-3.6 Β΅m, and models computed for the full wavelength range with the Hapke scattering theory. The data show that there is no discernible 3-Β΅m absorption band. Such a band would indicate the presence of OH -or H 2 O-bearing silicate minerals, or macromolecular carbon-rich organic material of the kind seen on the low-albedo hemisphere of Saturn's satellite Iapetus. The absence of spectral structure is itself indicative of the absence of the nitrogenrich tholins (which show a distinctive absorption band attributed to N-H).
The successful models in this study all incorporate magnesiumrich pyroxene (Mg, Fe SiO 3 ), which satisfactorily matches the red 1 Guest observer, United Kingdom Infrared Telescope facility (UKIRT). 2 Guest observer, NASA Infrared Telescope Facility (IRTF). color of Hektor. Pyroxene is a mafic mineral common in terrestrial and lunar lavas, and is also identified in Main Belt asteroid spectra. An upper limit to the amount of crystalline H 2 O ice (30-Β΅m grains) in the surface layer of Hektor accessible to near-infrared remote sensing observations is 3 wt%. The upper limit for serpentine, as a representative of hydrous silicates, is much less stringent, at 40%, based on the shape of the spectral region around 3 Β΅m. Thus, the spectrum at 3 Β΅m does not preclude the presence of a few weight percent of volatile material in the uppermost surface layer of Hektor. Below this "optical" surface that our observations probe, any amount of H 2 O ice and other volatile-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 present 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 absence of phyllosilicates from compositions similar to the models 348