Constraints on the composition of Trojan asteroid 624 Hektor

We present a composite spectrum of Trojan asteroid 624 Hektor, 0.3-3.6 [micro]m, and models computed for the full wavelength range with the Hapke scattering theory. The data show that there is no discernible 3-[micro]m absorption band. Such a band would indicate the presence of OH- or [H.sub.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 nitrogen-rich tholins (which show a distinctive absorption band attributed to N-H). The successful models in this study all incorporate magnesium-rich pyroxene (Mg, Fe Si[O.sub.3]), which satisfactorily matches the red 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.sub.2]O ice (30-[micro]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 [micro]m. Thus, the spectrum at 3 [micro]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.sub.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