New constraints on the surface of Pluto^⋆

LESIA-Observatoire de Paris, CNRS, UPMC Univ Paris 06, Université Denis Diderot, Sorbonne Paris Cité, 5 place J. Janssen, 92195 Meudon Principal Cedex, France
e-mail: frederic.merlin@obspm.fr

Received: 11 June 2015
Accepted: 31 July 2015

Abstract

Aims. Spectroscopic investigation of the surface of Pluto allows us to constrain the chemical properties of the volatile species of the solar system reservoir. This permits us to obtain the relative abundances of various molecules, their physical properties, as well as their spatial and temporal variation. This also could tell us about the origin of various minor chemical compounds formed during the solar system formation or generated later on by space weathering. This will give us critical information about the evolution processes that may occur in the entire trans-Neptunian objects population, and in particular the biggest objects, which could retain tenuous atmospheres.

Methods. New observations of the surface of Pluto have been carried out along with reanalyses of older observations carried out with the ESO-VLT telescopes and the SINFONI instrument at a mean spectral resolution of 1500. We present three new near-infrared spectra of Pluto observed at different epochs, and covering the H and K spectral bands showing absorption features of methane, nitrogen, and carbon monoxide ices. We ran different spectral models, based on Hapke theory, to constrain the physical and chemical properties of different sides of Pluto.

Results. We have confirmed the spatial and secular variation of the spectral properties of the surface of Pluto. The abundances, sizes, and temperatures of different ices, such as CH₄, CO, and N₂ have been constrained for different parts of the surface of Pluto. The results suggest a temperature probably just above the alpha-beta transition phase of N₂ (close to 36.5 K), and a probable stratification of the dilution state of CO and CH₄. The presence of minor chemical compounds, such as C₂H₆, has been confirmed too, and for data obtained at several sub-Earth east longitudes. Solid C₂H₄ is suggested by the spectral modeling with abundance variation following that of solid C₂H₆ and solid CH₄.