Visible spectroscopy of 2003 UB313:: evidence for N2 ice on the surface of the largest TNO?

Context. 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. Aims. We studied the surface composition of TNO 2003 UB313, the first known TNO larger than Pluto. Methods. We report a visible spectrum covering the 0.35-0.95 mu m spectral range, obtained with the 4.2 m William Herschel Telescope at El Roque de los Muchachos Observatory ( La Palma, Spain). Results. The visible spectrum of this TNO presents very prominent absorptions bands formed in solid CH4. At wavelengths shorter than 0.6 mu m 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 +/- 3 angstrom 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 0.89 mu m), but not at shorter wavelengths (the band around 0.73 mu m 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 N-2 is present in the outer surface layers.