90482 Orcus is a trans-Neptunian object in the Kuiper belt with a large moon. It is considered to be a dwarf planet by some astronomers, but the IAU has not formally designated it as such.
Discovery
Orcus was discovered on February 17, 2004 by Michael E. Brown of Caltech, Chadwick A. Trujillo of the Gemini Observatory, and David Lincoln Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified.
Naming
Under the guidelines of the International Astronomical Union's naming conventions, objects with a similar size and orbit to that of Pluto are named after underworld deities.
Accordingly, the discoverers suggested naming the object after Orcus, a god of the underworld, punisher of broken oaths in Etruscan and Roman mythology.
The name was also a private reference to the homonymous Orcas Island, where Brown's wife Diane had lived as a child and which they visit frequently.
Stats
Estimated Diameter: 950 km (850 - 950 km)
Aphelion: 48.07 AU
Perihelion: 30.27 AU
Semi-major axis: 39.17 AU
Orbital Period: 245.18 years
Rotation period: 13.19 hrs
Date discovered: 2004.2.17
Satellite: 1
Classification: TNO, KBO - Plutino
Orbit
Orcus is a plutino, locked in a 2:3 resonance with Neptune, making two revolutions around the Sun, while Neptune makes three.
Orcus is much like Pluto, except that it is constrained to always be in the opposite phase of its orbit from Pluto: Orcus is at aphelion when Pluto is at perihelion and vice versa. Because of this, along with its large moon Vanth that recalls Pluto's large moon Charon, Orcus has been seen as the anti-Pluto.
This was a major consideration in selecting its name, as the deity Orcus was the Etruscan equivalent of the Roman Pluto, and later became an alternate name for Pluto.
The rotation period of the primary is not known. Different photometric surveys have produced different results. Some show low amplitude variations with periods ranging from 7 to 21 hours, while others show no variability. However the value obtained by Ortiz et al., about 10.5 hours seems to be the most likely.
Spectra and surface
The first spectroscopic observations in 2004 showed that the visible spectrum of Orcus is flat (neutral in color) and featureless, while in the near-infrared there were moderately strong water absorption bands at 1.5 and 2.0 μm.
Further Infrared observations in 2004 by the European Southern Observatory and the Gemini telescope give results consistent with mixtures of water ice and carbonaceous compounds, such as tholins.
The water and methane ices can cover no more than 50% and 30% of the surface, respectively. This means the proportion of ice on the surface is less than on Charon, but similar to that on Triton.
Later in 2008–2010 new infrared spectroscopic observations with a higher signal-to-noise ratio revealed additional spectral features. Among them are a deep water ice absorption band at 1.65 μm, which is an evidence of the crystalline water ice on the surface of Orcus, and a new absorption band at 2.22 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.
The radiative transfer modeling showed that a mixture of water ice, tholins (as a darkening agent), ethane ice and ammonium ion (NH4+) provides the best match to the spectra, while 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. So, 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 hydrocarbons requires better infrared spectra.
Cryovolcanism
The presence of crystalline water ice, and possibly ammonia ice may indicate that a renewal mechanism was active in the past on the surface of Orcus. Ammonia so far has not been detected on any TNO or icy satellite of the outer planets other than Miranda. The 1.65 μm band on Orcus is broad and deep (12%), as on Charon, Quaoar, Haumea, and icy satellites of giant planets.
On the other hand the crystalline water ice on the surfaces of TNOs should be completely amorphized by the galactic and Solar radiation in about 10 million years. Some calculations indicate that cryovolcanism, which is considered one of the possible renewal mechanisms, may indeed be possible for TNOs larger than about 1000 km.
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.
Models of internal heating via radioactive decay suggest that Orcus may be capable of sustaining an internal ocean of liquid water.
Moon
Using observations with the Hubble Space Telescope from November 13, 2005, Mike Brown and T.A. Suer detected a satellite. The discovery of a satellite of Orcus was reported on 22 February 2007.
The satellite was given the designation S/2005 (90482) 1 before later being named Vanth, after the Etruscan goddess who guided the souls of the dead to the underworld.
How big is it?
Orcus is so far away in the outer solar system that we don't know for sure how large it is. Because all we see is a dot of light, which is sunlight reflected off the surface of the TNO. But we don't know if the object is bright because it is large or if it is bright because it is highly reflective or both.
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