Constraints on the Composition of Trojan Asteroid 624 Hektor

Abstract

We present a composite spectrum of Trojan asteroid 624 Hektor, 0.3–3.6 μm, and models computed for the full wavelength range with the Hapke scattering theory. The data show that there is no discernible 3-μm absorption band. Such a band would indicate the presence of OH − or H 2O-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 SiO 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 2O ice (30-μm 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 μm. Thus, the spectrum at 3 μm 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 2O 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 presented here. Our models appear to demonstrate that organic solids are not required to match the red color and low albedos of D-type asteroids.