The Surface of 2003 El61 in the Near-infrared

We report the detection of crystalline water ice on the surface of 2003 EL61. Reflectance spectra were collected from the Gemini North telescope in the 1.0 to 2.4 mwavelength range and from the Keck telescope across the 1.4Y 2.4 mwavelength range. The signature of crystalline water ice is obvious in all data collected. Like the surfaces of many outer solar system bodies, the surface of 2003 EL61 is rich in crystalline water ice, which is energetically less favored than amorphous water ice at low temperatures, suggesting that resurfacing processes may be taking place. The near-infrared color of the object is much bluer than a pure water ice model. Adding a near-infrared blue component such as hydrogen cyanide or phyllosilicate clays improves the fit considerably, with hydrogen cyanide providing the greatest improvement. The addition of hydrated tholins and bitumens also improves the fit, but is inconsistent with the neutralV J reflectance of 2003 EL61. A small decrease in reflectance beyond 2.3 mmay be attributable to cyanide salts. Overall, the reflected light from 2003 EL61 is best fit by a model of 2/3Y4/5 pure crystalline water ice and 1/3Y1/5 near-infrared blue component such as hydrogen cyanide or kaolinite. The surface of 2003EL61 is unlikely to be covered by significant amounts of darkmaterial such as carbon black, as our pure icemodels reproduce published albedo estimates derived from the spin state of 2003 EL61. Subject headings: comets: general — Kuiper Belt — solar system: formation