Editing Orcus (dwarf planet) (section)

== Physical characteristics ==
=== Size and magnitude ===
[[File:Orcus, Earth & Moon size comparison.png|left|thumb|upright|Orcus compared to [[Earth]] and the [[Moon]]]]
[[File:Orcus-apmag19-2009Dec26-11UT.jpg|left|thumb|upright|Long-exposure photograph of Orcus at visual magnitude 19.2]]

The absolute magnitude of Orcus is approximately 2.3.<ref name=TNOsCool8 /> The detection of Orcus by the [[Spitzer Space Telescope]] in the [[far infrared]]<ref name="Stansberry 2007" /> and by [[Herschel Space Telescope]] in [[submillimeter]] estimates its diameter at {{convert|958.4|km|mi|abbr=on}}, with an uncertainty of {{convert|22.9|km|mi|abbr=on}}.<ref name=TNOsCool8 /> Orcus appears to have an [[Albedo#Astronomical albedo|albedo]] of about 21–25%,<ref name=TNOsCool8 /> which may be typical of trans-Neptunian objects approaching the {{convert|1000|km|mi|abbr=on}} diameter range.<ref name="tnoalbedo" /> The magnitude and size estimates were made under the assumption that Orcus is a singular object. The presence of a relatively large satellite, Vanth, may change them considerably. The absolute magnitude of Vanth is estimated at 4.88, which means that it is about 1/11 as bright as Orcus itself.<ref name="Brown2010" /> The [[Atacama Large Millimeter Array|ALMA]] submillimeter measurements taken in 2016 showed that Vanth has a relatively large size of {{convert|475|km|mi|abbr=on}} with an albedo of about 8 percent while Orcus's has a slightly smaller size of {{convert|910|km|mi|abbr=on}}.<ref name="brown2018"/> Using a [[stellar occultation]] by Vanth in 2017, Vanth's diameter has been determined to be {{convert|442.5|km|mi|abbr=on}}, with an uncertainty of {{convert|10.2|km|mi|abbr=on}}.<ref name="Sickafoose2018"/>

=== Mass and density ===
Orcus and Vanth are known to constitute a binary system. The mass of the system has been estimated to be {{val|6.348|0.019|e=20|u=kg}},<ref name="Grundy-orbits"/> approximately equal to that of the [[Saturn]]ian moon [[Tethys (moon)|Tethys]] ({{val|6.175|e=20|u=kg}}).<ref name="Jacobson2006" /> The mass of the Orcus system is about 3.8 percent that of {{dp|Eris}}, the most massive known dwarf planet ({{val|1.66|e=22|u=kg}}).<ref name="Brown2010" /><ref name="Brown Schaller 2007" />

The ratio of the mass of Vanth to that of Orcus was measured astrometrically with the [[Atacama Large Millimeter Array|ALMA]] submillimeter telescope and is {{val|0.16|0.02}} with Vanth containing {{val|13.7|1.3|u=%}} of the total system mass. This also means that the densities of both bodies are about the same at ~{{val|1.5|u=g/cm<sup>3</sup>}}.<ref name="Brown2023"/>

=== 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" />

=== Cryovolcanism ===
Crystalline water ice on the surfaces of trans-Neptunian objects should be completely [[Amorphous solid|amorphized]] by the galactic and Solar radiation in about 10 million years.<ref name="Barucci2008" /> Thus the presence of [[crystal]]line water [[ice]], and possibly [[ammonia]] ice, may indicate that a renewal mechanism was active in the past on the surface of Orcus.<ref name="Barucci2008" /> Ammonia so far has not been detected on any trans-Neptunian object or icy satellite of the outer planets other than [[Miranda (moon)|Miranda]].<ref name="Barucci2008" /> The 1.65&nbsp;[[micrometre|μm]] band on Orcus is broad and deep (12%), as on Charon, {{dp|Quaoar}}, {{dp|Haumea}}, and icy satellites of the [[giant planets]].<ref name="Barucci2008" /> Some calculations indicate that [[cryovolcano|cryovolcanism]], which is considered one of the possible renewal mechanisms, may indeed be possible for trans-Neptunian objects larger than about {{convert|1000|km|mi|abbr=on}}.<ref name="Delsanti2010" /> Orcus may have experienced at least one such episode in the past, which turned the amorphous water ice on its surface into crystalline. The preferred type of volcanism may have been explosive aqueous volcanism driven by an explosive dissolution of methane from water–ammonia melts.<ref name="Delsanti2010" />