Astronomy & Astrophysics manuscript no. Delsanti2010 c ESO 2018 June 2, 2018 Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? A portrait of Plutino (90482) Orcus A. Delsanti1,2, F. Merlin3, A. Guilbert–Lepoutre4 , J. Bauer5, B. Yang6, and K.J. Meech6 1 Laboratoire d’Astrophysique de Marseille, Universit´ede Provence, CNRS, 38 rue Fr´ed´eric Joliot-Curie, F-13388 Marseille Cedex 13, FRANCE e-mail: [email protected] 2 Observatoire de Paris, Site de Meudon, 5 place Jules Janssen, 92190 Meudon, FRANCE e-mail: [email protected] 3 University of Maryland, Department of Astronomy, College Park MD 20742, USA e-mail: [email protected] 4 UCLA, Earth and Space Sciences department, 595 Charles E. Young Drive East, Los Angeles CA 90095, USA e-mail: [email protected] 5 Jet Propulsion Laboratory, M/S 183-501, 4800 Oak Grove Drive, Pasadena, CA 91109, USA e-mail: [email protected] 6 NASA Astrobiology Institute at Manoa, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839, USA e-mail: [email protected],[email protected] Received: February 20, 2010; Accepted: May 31, 2010; ABSTRACT Orcus is an intermediate-size 1000km-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 10m–telescope (optical) and the Gemini 8m–telescope (near-infrared). We confirm the unambiguous detection of crystalline water ice as well as absorption in the 2.2µm 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 2.2µm 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 109yrs 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. Key words. Kuiper Belt – Methods: observational, data analysis, numerical — Techniques: photometric, spectroscopic — Radiative transfer arXiv:1006.4962v1 [astro-ph.EP] 25 Jun 2010 1. Introduction methane, as for Pluto. Orcus was discovered in 2004 as one of the largest known In the past decade, several large Kuiper Belt Objects (KBOs) KBOs (Brown et al. 2004) and is a peculiar object from several have been discovered, unveiling the upper part of the size aspects. It revolves around the Sun in a Pluto-like orbit, in 3:2 distribution of these outer Solar System minor bodies. Eris, dis- mean motion resonance with Neptune (with a semi-major axis covered in 2003 (Brown et al. 2005), is the largest object known of 39.16 AU, an inclination of 20.6◦, an eccentricity 0.23, a to date, and its diameter in the 2500–3000km range, exceeding perihelion of 30.26 AU and an aphelion at 48.06 AU). It is that of Pluto, motivated the discussion and the creation of a currently outbound, very close to aphelion. With its 950km di- new class of objects in 2006: the dwarf planets. Several other ameter (Stansberry et al. 2008; Brown et al. 2010), it∼ is now part large objects were discovered since 2003 (Sedna, Makemake, of the intermediate-size objects, and is probably in a transition Haumea, etc) and focused the interest of scientists: the largest regime with respect to volatile surface content. Indeed, smaller of them soon revealed a volatile-rich surface, dominated by KBOs with mostly featureless near-IR reflectance spectra 2 A. Delsanti et al.: A portrait of Plutino (90482) Orcus Table 1. Summarized properties of the Orcus/Vanth binary sys- tem. References are: (1) Noll et al. (2008), (2) Brown et al. (2010), (3): Brown (2008). Parameter Value Reference Angular separation 0.26 (1) System ′′ Total mass 6.3 1020 kg (2) Semi-major axis 8980× 20 km (2) Orbit Period 9.5d± (2) Inclination 90◦ or 306◦ (2) Brightness Frac. 8% (3) Satellite Mass ratio 0.5-0.03a (2) Diameter 280-640kmb (2) a Assuming an albedo ratio of resp. .5 and .1 and equal densities b Assuming an albedo ratio of resp. 1. and .5 and equal densities seem to be totally depleted in volatiles (see Guilbert et al. 2009, and references therein), while the largest KBOs show Fig. 1. A compilation of available visible spectra of Orcus, pre- evidence for the presence of volatile ices such as methane and sented at a resolution of R 600. From bottom to top: this paper, sometimes molecular nitrogen (like Eris, Sedna and Makemake, Fornasier et al. (2004), de∼ Bergh et al. (2005), Fornasier et al. cf. Merlin et al. 2009; Barucci et al. 2005; Brown et al. 2007a, (2009). The three top spectra were shifted by 0.5 in reflectance and references therein). A simple model of atmospheric escape for clarity. of volatiles dedicated to the Kuiper Belt region was computed by Schaller & Brown (2007b) and showed that the objects that are too small and too hot will most probably lose their pristine scenario would then be an initial capture of Vanth, followed by volatile surface inventory (such as CO, CH4 and N2) over the large oscillations of eccentricity an inclination (Kozai cycling), age of the Solar System, while large and cold objects could and when the pericenter drops to a low enough value, tidal have retained these volatiles to the present day. Objects in an evolution can finally lead the orbit to its current circular shape. intermediate regime could have lost some volatiles but retained As noted by Brownet al. (2010), a future discovery of any others, following the different loss rates at work for the various Orcus coplanar satellite would rule out this possibility (as for species. According to this model, Orcus should have lost all its the Pluto-Charon system). Relative color measurements by the volatile content. However, traces of methane (to be confirmed) same team indicate that Vanth is significantly redder than Orcus could explain the near-infrared spectrum of Orcus (Barucci et al. (another unique property among KBO binaries), a characteristic 2008). This object can therefore provide key constraints on that is currently difficult to understand in the framework of a the current surface volatile inventory of intermediate-size KBOs. formation by a giant impact. So far, Orcus’ rotation properties are not uniquely defined: In this work, we review the existing spectroscopic investi- 0.03 to 0.18 magnitude amplitude variations are measured over gations of Orcus and objects with similar spectral and physi- a period ranging from 7 to 21h (Duffard et al. 2009, and ref- cal properties and search for additional constraints on the pres- erences therein), although∼ Sheppard (2007) finds no lightcurve ence of volatiles (and maybe their irradiation products) on an variations within the photometric uncertainties of his measure- object that might have retained some of its original content. We ments. Such a flat lightcurve could be diagnostic of a very first report additional 0.4–2.5µm photometry and spectroscopy slow rotation rate, a small peak-to-peak lightcurve variation or obtained from the Mauna Kea large telescopes. We present a nearly pole-on geometric aspect. Also, given the size of Orcus Hapke radiative transfer modeling of the spectral data, the test- and the relatively slow rotation rate, we can most probably ex- ing for the presence of water ice, methane and ammonia ices pect a circular shape from the relaxation of a fluid body in hy- (and their irradiation products, ethane and ammonium resp.) at drostatic equilibrium. the surface and discuss the results. Finally, we use a new 3D thermal evolution model dedicated to the interiors of KBOs To date, Orcus is known to have a single relatively large (Guilbert-Lepoutre et al. 2010) to describe Orcus’ thermal his- companion, Vanth, in a close circular orbit (see details in Table tory over the age of the Solar System and the observable conse- 1). According to Brown et al. (2010), the satellite is on a nearly quences of this history on the current surface. face-on orbit (with respect to the Sun). Again, Orcus seems to be a unique case in an intermediate regime: larger KBOs have relatively small satellites in circular orbits, while small 2.