Letter to the Editor

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

¹ 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 μm 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 μm the spectrum is almost featureless and slightly red (%). 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 μm), 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.