Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO?

A portrait of Plutino (90482) Orcus

¹ Laboratoire d'Astrophysique de Marseille, Université de Provence, CNRS, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France e-mail: Audrey.Delsanti@oamp.fr
² Observatoire de Paris, Site de Meudon, 5 place Jules Janssen, 92190 Meudon, France e-mail: Audrey.Delsanti@obspm.fr
³ University of Maryland, Department of Astronomy, College Park MD 20742, USA e-mail: merlin@astro.umd.edu
⁴ UCLA, Earth and Space Sciences department, 595 Charles E. Young Drive East, Los Angeles CA 90095, USA e-mail: aguilbert@ucla.edu
⁵ Jet Propulsion Laboratory, M/S 183-501, 4800 Oak Grove Drive, Pasadena, CA 91109, USA e-mail: bauer@scn.jpl.nasa.gov
⁶ NASA Astrobiology Institute at Manoa, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839, USA e-mail: [yangbin;meech]@ifa.hawaii.edu

Received: 20 February 2010
Accepted: 31 May 2010

Abstract

Orcus is an intermediate-size 1000 km-scale Kuiper belt object (KBO) in 3:2 mean-motion resonance with Neptune, in an orbit very similar to that of Pluto. It has a water-ice dominated surface with solar-like visible colors. We present visible and near-infrared photometry and spectroscopy obtained with the Keck 10 m-telescope (optical) and the Gemini 8 m-telescope (near-infrared). We confirm the unambiguous detection of crystalline water ice as well as absorption in the 2.2 μm region. These spectral properties are close to those observed for Pluto's larger satellite Charon, and for Plutino (208996) 2003 AZ₈₄. Both in the visible and near-infrared Orcus' spectral properties appear to be homogeneous over time (and probably rotation) at the resolution available. From Hapke radiative transfer models involving intimate mixtures of various ices we find for the first time that ammonium (NH₄⁺) and traces of ethane (C₂H₆), which are most probably solar irradiation products of ammonia and methane, and a mixture of methane and ammonia (diluted or not) are the best candidates to improve the description of the data with respect to a simple water ice mixture (Haumea type surface). The possible more subtle structure of the 2.2 μm band(s) should be investigated thoroughly in the future for Orcus and other intermediate size Plutinos to better understand the methane and ammonia chemistry at work, if any. We investigated the thermal history of Orcus with a new 3D thermal evolution model. Simulations over 4.5 × 10⁹ yr with an input 10% porosity, bulk composition of 23% amorphous water ice and 77% dust (mass fraction), and cold accretion show that even with the action of long-lived radiogenic elements only, Orcus should have a melted core and most probably suffered a cryovolcanic event in its history which brought large amounts of crystalline ice to the surface. The presence of ammonia in the interior would strengthen the melting process. A surface layer of a few hundred meters to a few tens of kilometers of amorphous water ice survives, while most of the remaining volume underneath contains crystalline ice. The crystalline water ice possibly brought to the surface by a past cryovolcanic event should still be detectable after several billion years despite the irradiation effects, as demonstrated by recent laboratory experiments.