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

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 AZ 84 . 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 (NH + 4 ) and traces of ethane (C 2 H 6 ), 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 4.5×10 9 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.

New 3D thermal evolution model for icy bodies application to trans-Neptunian objects

“…If the thermal conductivity of the surface layers is additionally very low, the presence of crystalline water ice could simply be due to the erosion of the surface by impact cratering, revealing the underlying material. These conclusions highlight the importance of objects like Quaoar (ρ bulk > 2800 kg m Desch et al 2009;Delsanti et al 2010, for example).…”

TNOs are Cool: A survey of the trans-Neptunian region

“…Orcus is a 1000 km-sized object (Stansberry et al 2008), known to have a satellite, Vanth, and a surface rich in water ice in both amorphous and crystalline forms (Fornasier et al 2004b;de Bergh et al 2005;Trujillo et al 2005;Barucci et al 2008;Guilbert et al 2009;Delsanti et al 2010;DeMeo et al 2010). It is one of the few binary TNOs in an orbit resonant with that of Neptune (it is a plutino populating the 3:2 resonance with Neptune).…”

Prerequisites for explosive cryovolcanism on dwarf planet-class Kuiper belt objects

“…First, short-lived ammonia hydrates have been detected on the surfaces of Charon and, likely, Orcus (Brown and Calvin, 2000;Cook et al, 2007;Merlin et al, 2010;Delsanti et al, 2010). These compounds should be destroyed by solar ultraviolet radiation and galactic cosmic rays on timescales shorter than 1 to 50 Myr (Jewitt and Luu, 2004;Cook et al, 2007), and no mechanism other than recent localized resurfacing by cryovolcanism has conclusively explained their presence on Charon's surface (Jewitt and Luu, 2004;Cook et al, 2007;McKinnon et al, 2008;Desch et al, 2009;Brown, 2012).…”