Editing Orcus (dwarf planet) (section)

=== Spectra and surface ===

The first spectroscopic observations in 2004 showed that the visible spectrum of Orcus is flat (neutral in color) and featureless, whereas in the [[near-infrared]] there were moderately strong [[water absorption]] bands at 1.5 and 2.0&nbsp;μm.<ref name="Fornasier2004" /> The neutral visible spectrum and strong water absorption bands of Orcus showed that Orcus appeared different from other trans-Neptunian objects, which typically have a red visible spectrum and often featureless [[infrared]] spectra.<ref name="Fornasier2004" /> Further infrared observations in 2004 by the [[European Southern Observatory]] and the [[Gemini telescope]] gave results consistent with mixtures of water [[ice]] and [[carbon]]aceous compounds, such as [[tholin]]s.<ref name="deBergh2005" /> The water and methane ices can cover no more than 50 percent and 30 percent of the surface, respectively.<ref name="Trujillo2005" /> This means the proportion of ice on the surface is less than on [[Charon (moon)|Charon]], but similar to that on [[Triton (moon)|Triton]].<ref name="Trujillo2005" />

Later in 2008–2010 new infrared spectroscopic observations with a higher [[signal-to-noise ratio]] revealed additional spectral features. Among them is a deep water ice absorption band at 1.65&nbsp;μm, which is evidence of the [[crystalline]] water ice on the surface of Orcus, and a new absorption band at 2.22&nbsp;μm. The origin of the latter feature is not completely clear. It can be caused either by [[ammonia]]/[[ammonium]] dissolved in the water ice or by [[methane]]/[[ethane]] ices.<ref name="Barucci2008" /> The [[radiative transfer]] modeling showed that a mixture of water ice, tholins (as a darkening agent), ethane ice, and ammonium ion (NH<sub>4</sub><sup>+</sup>) provides the best match to the spectra, whereas a combination of water ice, tholins, methane ice and ammonia hydrate gives a slightly inferior result. On the other hand, a mixture of only ammonia hydrate, tholins and water ice failed to provide a satisfactory match.<ref name="Delsanti2010" /> As of 2010, the only reliably identified compounds on the surface of Orcus are crystalline water ice and, possibly, dark tholins. A firm identification of ammonia, methane, and other [[hydrocarbon]]s requires a better infrared spectra.<ref name="Delsanti2010" />

Orcus sits at the threshold for trans-Neptunian objects massive enough to retain [[Volatile (astrogeology)|volatiles]] such as methane on the surface.<ref name="Delsanti2010" /> The reflectance spectrum of Orcus shows the deepest water-ice absorption bands of any Kuiper belt object that is not associated with the [[Haumea family|Haumea collisional family]].<ref name="Brown2010" /> The [[Moons of Uranus#Large moons|large icy satellites of Uranus]] have infrared spectra quite similar to that of Orcus.<ref name="Brown2010" /> Among other trans-Neptunian objects, the large plutino {{mpl|2003 AZ|84}} and Pluto's moon Charon both have similar surface spectra to Orcus,<ref name="Barucci2008" /> with flat, featureless visible spectra and moderately strong water ice absorption bands in the near-infrared.<ref name="Delsanti2010" />