ar X iv : a st ro - p h / 06 01 61 8 v 1 2 6 Ja n 20 06 The Surface of 2003 EL 61 in the Near Infrared

We report the detection of crystalline water ice on the surface of 2003 EL61. Reflectance spectra were collected from Gemini North telescope from 1.0 to 2.4 μm wavelength range, and from the Keck telescope across the 1.4 to 2.4 μm wavelength range. The signature of crystalline water ice is clear and obvious in all data collected. Like the surface 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 cold temperatures, suggesting 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 neutral V − J reflectance of 2003 EL61. A small decrease in reflectance beyond 2.3 μm may be attributable to cyanide salts. Overall, the reflected light from 2003 EL61 is best fit by a model of 2/3 to 4/5 pure crystalline water ice and 1/3 to 1/5 near infrared blue component such as hydrogen cyanide or kaolinite. The surface of 2003 EL61 is unlikely to be covered by significant amounts of dark material such as carbon black, as our pure ice models reproduce published albedo estimates derived from the spin state of 2003 EL61. Subject headings: comets: general — Kuiper Belt — solar system: formation