2010A&A...521A..35D

We present new visible and near-infrared photometric measurements and near-infrared spectroscopic measurements for three outer solar system small bodies, the Jupiter-coupled object (52872) Okyrhoe and the TNOs (90482) Orcus and (73480) 2002 PN₃₄. We analyzed their surface compositions by modeling their spectra in the visible and near-infrared wavelength ranges. We then compared this new data with previous measurements of Okyrhoe and Orcus to search for heterogeneity on their surfaces. All observations were performed at the European Southern Observatory 8 m Very Large Telescope, UT1 and UT4 at the Paranal Observatory in Chile. We find varying amounts of H₂O ice among these bodies, Okyrhoe shows no trace of it in our spectrum, 73480 has small amounts, and Orcus has large quantities. While we do clearly see for Orcus that a significant fraction of the H₂O ice is in crystalline form from the 1.65-µm feature, we cannot detect the 2.21-µm feature supposedly due to ammonia hydrate, because of the low signal-to-noise of the data. We also do not see any indication of ices more volatile than H₂O, such as CH₄ or CO₂, in the spectrum, so we limit their presence to no more than about 5% based on the data presented here and on high-quality data from Barucci et al. (2008A&A...479L..13B).