2006A&A...458L...5L

The recent discovery of two large trans-Neptunian objects (TNOs) 2003 UB₃₁₃ and 2005 FY₉, with surface properties similar to those of Pluto, provides an exciting new laboratory for the study of processes considered for Pluto and Triton: volatile mixing and transport; atmospheric freeze-out and escape, ice chemistry, and nitrogen phase transitions. We studied the surface composition of TNO 2003 UB₃₁₃, the first known TNO larger than Pluto. We report a visible spectrum covering the 0.35-0.95µm spectral range, obtained with the 4.2m William Herschel Telescope at ``El Roque de los Muchachos'' Observatory (La Palma, Spain). The visible spectrum of this TNO presents very prominent absorptions bands formed in solid CH₄. At wavelengths shorter than 0.6 µm the spectrum is almost featureless and slightly red (S'=4%). The icy-CH₄ bands are significantly stronger than those of Pluto and slightly weaker than those observed in the spectrum of another giant TNO, 2005 FY₉, implying that methane is more abundant on its surface than in Pluto's and close to that of the surface of 2005 FY₉. A shift of 15 ±3Å relative to the position of the bands of the spectrum of laboratory CH₄ ice is observed in the bands at larger wavelengths (e.g. around 0.89µm), but not at shorter wavelengths (the band around 0.73 µm is not shifted) this may be evidence for a vertical compositional gradient. Purer methane could have condensed first while 2003 UB₃₁₃ moved towards aphelion during the last 200 years, and as the atmosphere gradually collapsed, the composition became more nitrogen-rich as the last, most volatile components condensed, and CH₄ diluted in N₂ is present in the outer surface layers.