Issue A&A Volume 458, Number 1, October IV 2006 Page(s) L5 - L8 Section Letters DOI http://dx.doi.org/10.1051/0004-6361:20066028

A&A 458, L5-L8 (2006)
DOI: 10.1051/0004-6361:20066028

Letter

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

J. Licandro^{1, 2}, W. M. Grundy³, N. Pinilla-Alonso⁴ and P. Leisy<sup>1, 2

1</sup>  Isaac Newton Group, PO Box 321, 38700 Santa Cruz de La Palma, Tenerife, Spain
    e-mail: licandro@ing.iac.es
²  Instituto de Astrofísica de Canarias, c/Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
³  Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001-4470, USA
⁴  Fundación Galileo Galilei & Telescopio Nazionale Galileo, PO Box 565, 38700 S/C de La Palma, Tenerife, Spain

(Received 12 July 2006 / Accepted 23 August 2006)

Abstract
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 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 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   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 m), but not at shorter wavelengths (the band around 0.73 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 N₂ is present in the outer surface layers.

Key words: Kuiper Belt

© ESO 2006

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