Methane and Ethane on the Bright Kuiper Belt Object 2005 FY9

The spectrum of the bright Kuiper Belt object 2005 FY9 from 0.34 to 2.5 m is dominated by the red coloring of manyoutersolarsystemobjectsintheopticalwavelengthregimeandbyabsorptionduetomethaneinthenear-infrared. The solid methane absorption lines are significantly broader on 2005 FY9 than on any other solar system body, in- dicatinglongopticalpathlengthsthroughthemethane.Theselongpathlengthscanbeparameterizedasamethanegrain size of approximately 1 cm in a Hapke reflectance model. In addition to large-grained methane, the infrared spectrum also indicates the clear presence of ethane, an expected product of UV photolysis of methane. No evidence for N2 or CO, both known to be present on Pluto, is found. We suggest that the large differences between the spectrum of 2005 FY9 and that of Pluto and 2003 UB313 is due to a depletion of nitrogen on the surface of 2005 FY9 that leads to large methane grains, abundant sites for ethane formation through UV photolysis, and highly irradiated tholin-like material.