90482 Orcus
Encyclopedia
90482 Orcus is a trans-Neptunian object
in the Kuiper belt
with a large moon. It was discovered on February 17, 2004 by Michael Brown
of Caltech
, Chad Trujillo
of the Gemini Observatory
, and David Rabinowitz
of Yale University
. Precovery
images as early as November 8, 1951 were later identified. It is thought by Michael Brown to be "nearly certainly" a dwarf planet
, although the IAU
has not yet formally designated it as such.
Orcus is a plutino
, locked in a 2:3 resonance with Neptune
, making two revolutions around the Sun, while Neptune makes three. This 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 surface of Orcus is relatively bright with albedo reaching 30%, grey in color and water rich. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane
or ammonia
may also be present. The existence of a satellite allowed astronomers to determine the mass of the system, which is approximately equal to that of the Saturn
ian moon Tethys
. The ratio of masses of Orcus and Vanth is uncertain, possibly anywhere from 1:33 to 1:12. The diameter of Orcus is 800–900 km, while Vanth is thought to be 280 to 380 km, depending on what its albedo turns out to be.
Orcus is a large plutino
(an object in 2:3 orbital resonance
with Neptune
). Orcus's 247-year orbit is shaped similarly to Pluto
's (both have perihelia above the ecliptic
), but is differently oriented. Although at one point its orbit approaches that of Neptune
, the resonance between the two bodies means that Orcus itself is always a great distance away from Neptune (there is always an angular separation of over 60 degrees between them). Over a 14,000 year period Orcus stays more than 18 AU from Neptune. Because their mutual resonance with Neptune constrains Orcus and Pluto to remain in opposite phases of their otherwise very similar motions, Orcus is sometimes described as the "anti-Pluto".
Orcus is currently 47.8 AU from the Sun and will come to aphelion
(farthest distance from the Sun) in 2019. Simulations by the Deep Ecliptic Survey
(DES) show that over the next 10 million years Orcus can acquire a perihelion distance (qmin) as small as 27.8 AU.
The rotation period 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 showed no variability. The value obtained by Ortiz et al., about 10 hours, is often cited in the literature. The rotational poles of Orcus probably coincide with the orbital poles of its moon, Vanth. This means that Orcus is currently viewed pole-on, which could explain the near absence of any rotational modulation of its brightness. The true rotational period may coincide with the 9.5-day orbital period of Vanth.
'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 dead 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. Orcus was approved and published on November 22, 2004.
of Orcus is about 2.3 (comparable with the 2.6 of cubewano 50000 Quaoar
). The detection of Orcus by the Spitzer space telescope
in the far infrared and by Herschel Space Telescope in submillimeter constrains its diameter to . The diameter determined based solely on Spitzer measurements is . Orcus appears to have an albedo of about 22% to 34%, which may be typical of trans-Neptunian objects approaching the 1000 km diameter range.
In the magnitude and size estimates reported above it was assumed that Orcus is a singular object. The presence of a relatively large satellite (Vanth, see below) may change them considerably. The absolute magnitude of Vanth is estimated at 4.88, which means that it is about 11 times fainter than Orcus itself. If the albedos of both bodies are the same then the size of Orcus is about 900 km (assuming single body diameter of 940 km), while the size of Vanth is about 280 km. If, however, the albedo of Vanth is two times lower than that of Orcus, their sizes are 860 and 380 km, respectively.
, or about 3.8% the mass of largest known dwarf planet Eris
. How this mass is partitioned between Orcus and Vanth depends of their relative sizes. If the satellite's size is less than 1/3d of that of the primary, then its mass is only 3% of the total mass. On the other hand, if the size of Vanth is 380 km (see above), then its mass could be as high as 1/13 of the total system mass or about 8% of the mass of Orcus.
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. Thus Orcus appeared to be different from other Trans-Neptunian objects like Ixion
with red visible and often featureless infrared spectra. 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 tholin
s. 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 hydrocarbon
s requires better infrared spectra.
bands of any Kuiper belt object (KBO) that is not associated with the Haumea collisional family
. The large icy satellites of Uranus have infrared spectra quite similar to that of Orcus. Among other trans-Neptunian objects Pluto's moon Charon
appears to be most similar to Orcus. It has a higher albedo but very similar visible and near-infrared spectra. Their densities are also similar and both have water ice rich surfaces. Quaoar—a trans-Neptunian object of similar size—has strong water ice absorption features in its spectra but also has a very red color in the visible implying the presence of the ultrared matter on its surface. Dwarf planet Haumea and objects from its collisional family have much higher albedos and much deeper water absorption bands than Orcus. Finally, (208996) 2003 AZ84—another large object in 3:2 resonance with Neptune—has spectral properties very similar to Orcus.
line 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.
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 in IAUC 8812 on 22 February 2007. The satellite was given the designation S/2005 (90482) 1 before later being named Vanth. It orbits Orcus in a nearly face-on circular orbit with an eccentricity
less than 0.0036, and an orbital period of 9.53 days. Vanth orbits only 8980 ± 20 km from Orcus and is too close to Orcus for ground-based spectroscopy to determine the surface composition of the satellite. Mike Brown suspects that like the Pluto–Charon system, Orcus and Vanth are tidally locked
. Vanth does not resemble known collisional satellites because its spectrum is very different from that of its primary, and it may be a captured KBO.
On March 23, 2009, Brown asked readers of his weekly column to suggest possible names for the satellite, with the best one to be submitted to the International Astronomical Union
(IAU) on April 5. The name Vanth
, after the Etruscan
goddess who guided the souls of the dead to the underworld, was eventually chosen from among a large pool of submissions. This submission will be assessed by the IAU's Committee for Small Body Nomenclature, which will vote on whether to approve it, in accordance with the normal object naming procedures.
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...
in the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
with a large moon. It was discovered on February 17, 2004 by Michael Brown
Michael E. Brown
Michael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003....
of Caltech
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...
, Chad Trujillo
Chad Trujillo
Chadwick A. "Chad" Trujillo is an astronomer and the co-discoverer of the dwarf planet Eris.Trujillo works with computer software and has examined the orbits of the numerous trans-Neptunian objects , which is the outer area of the solar system that he specialized in. In late August 2005, it was...
of the Gemini Observatory
Gemini Observatory
The Gemini Observatory is an astronomical observatory consisting of two telescopes at sites in Hawai‘i and Chile. Together, the twin Gemini telescopes provide almost complete coverage of both the northern and southern skies...
, and David Rabinowitz
David L. Rabinowitz
David Lincoln Rabinowitz is a researcher at Yale University. He has built CCD cameras and software for the detection of near-Earth asteroids and Kuiper Belt Objects, and his research has helped reduce the assumed number of near-Earth asteroids by half, from 1,000-2,000 to 500-1,000 He has also...
of Yale University
Yale University
Yale University is a private, Ivy League university located in New Haven, Connecticut, United States. Founded in 1701 in the Colony of Connecticut, the university is the third-oldest institution of higher education in the United States...
. Precovery
Precovery
Precovery is a term used in astronomy that describes the process of finding the image of an object in old archived images or photographic plates, for the purpose of calculating a more accurate orbit...
images as early as November 8, 1951 were later identified. It is thought by Michael Brown to be "nearly certainly" a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
, although the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
has not yet formally designated it as such.
Orcus is a plutino
Plutino
In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...
, locked in a 2:3 resonance with Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
, making two revolutions around the Sun, while Neptune makes three. This is much like Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
, 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
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
, Orcus has been seen as the anti-Pluto. This was a major consideration in selecting its name, as the deity Orcus
Orcus
Orcus was a Roman god of the underworld.Orcus can also refer to:* Orcus , a demon prince in the Dungeons & Dragons role-playing game*90482 Orcus, a Trans-Neptunian object* Orcus , a genus of ladybird...
was the Etruscan
Etruscan mythology
The Etruscans were a diachronically continuous population, with a distinct language and culture during the period of earliest European writing, in the Mediterranean Iron Age in the second half of the first millennium BC...
equivalent of the Roman
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...
Pluto
Pluto (mythology)
In ancient Greek religion and myth, Pluto was a name for the ruler of the underworld; the god was also known as Hades, a name for the underworld itself...
, and later became an alternate name for Pluto.
The surface of Orcus is relatively bright with albedo reaching 30%, grey in color and water rich. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
or ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
may also be present. The existence of a satellite allowed astronomers to determine the mass of the system, which is approximately equal to that of the Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
ian moon Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...
. The ratio of masses of Orcus and Vanth is uncertain, possibly anywhere from 1:33 to 1:12. The diameter of Orcus is 800–900 km, while Vanth is thought to be 280 to 380 km, depending on what its albedo turns out to be.
Orbit and rotation
Plutino
In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...
(an object in 2:3 orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
). Orcus's 247-year orbit is shaped similarly to Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
's (both have perihelia above the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
), but is differently oriented. Although at one point its orbit approaches that of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
, the resonance between the two bodies means that Orcus itself is always a great distance away from Neptune (there is always an angular separation of over 60 degrees between them). Over a 14,000 year period Orcus stays more than 18 AU from Neptune. Because their mutual resonance with Neptune constrains Orcus and Pluto to remain in opposite phases of their otherwise very similar motions, Orcus is sometimes described as the "anti-Pluto".
Orcus is currently 47.8 AU from the Sun and will come to aphelion
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...
(farthest distance from the Sun) in 2019. Simulations by the Deep Ecliptic Survey
Deep Ecliptic Survey
The Deep Ecliptic Survey is a project to find Kuiper belt objects , using the facilities of the National Optical Astronomy Observatory .The principal investigator is Bob Millis....
(DES) show that over the next 10 million years Orcus can acquire a perihelion distance (qmin) as small as 27.8 AU.
The rotation period 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 showed no variability. The value obtained by Ortiz et al., about 10 hours, is often cited in the literature. The rotational poles of Orcus probably coincide with the orbital poles of its moon, Vanth. This means that Orcus is currently viewed pole-on, which could explain the near absence of any rotational modulation of its brightness. The true rotational period may coincide with the 9.5-day orbital period of Vanth.
Name
Under the guidelines of the International Astronomical UnionInternational Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
's naming conventions
Astronomical naming conventions
In ancient times, only the Sun and Moon, a few hundred stars and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year...
, objects with a similar size and orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
to that of Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
are named after underworld
Underworld
The Underworld is a region which is thought to be under the surface of the earth in some religions and in mythologies. It could be a place where the souls of the recently departed go, and in some traditions it is identified with Hell or the realm of death...
deities. Accordingly, the discoverers suggested naming the object after Orcus
Orcus
Orcus was a Roman god of the underworld.Orcus can also refer to:* Orcus , a demon prince in the Dungeons & Dragons role-playing game*90482 Orcus, a Trans-Neptunian object* Orcus , a genus of ladybird...
, a god of the dead in Etruscan and Roman mythology
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...
. The name was also a private reference to the homonymous Orcas Island
Orcas Island
Orcas Island is the largest of the San Juan Islands, which are located in the northwestern corner of Washington state in San Juan County, Washington.-History:...
, where Brown's wife Diane had lived as a child and which they visit frequently. Orcus was approved and published on November 22, 2004.
Size and magnitude
The absolute magnitudeAbsolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
of Orcus is about 2.3 (comparable with the 2.6 of cubewano 50000 Quaoar
50000 Quaoar
50000 Quaoar is a rocky trans-Neptunian object in the Kuiper belt with one known moon. Discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology from images acquired at the Samuel Oschin Telescope at Palomar Observatory, it is thought by...
). The detection of Orcus by the Spitzer space telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
in the far infrared and by Herschel Space Telescope in submillimeter constrains its diameter to . The diameter determined based solely on Spitzer measurements is . Orcus appears to have an albedo of about 22% to 34%, which may be typical of trans-Neptunian objects approaching the 1000 km diameter range.
In the magnitude and size estimates reported above it was assumed that Orcus is a singular object. The presence of a relatively large satellite (Vanth, see below) may change them considerably. The absolute magnitude of Vanth is estimated at 4.88, which means that it is about 11 times fainter than Orcus itself. If the albedos of both bodies are the same then the size of Orcus is about 900 km (assuming single body diameter of 940 km), while the size of Vanth is about 280 km. If, however, the albedo of Vanth is two times lower than that of Orcus, their sizes are 860 and 380 km, respectively.
Mass
Since Orcus is known to be a binary system, the mass of the system has been estimated to be 6.32 ± 0.05 kgKilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
, or about 3.8% the mass of largest known dwarf planet Eris
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...
. How this mass is partitioned between Orcus and Vanth depends of their relative sizes. If the satellite's size is less than 1/3d of that of the primary, then its mass is only 3% of the total mass. On the other hand, if the size of Vanth is 380 km (see above), then its mass could be as high as 1/13 of the total system mass or about 8% of the mass of Orcus.
Spectra and surface
28978 Ixion
28978 Ixion is a Kuiper belt object discovered on May 22, 2001. Ixion is a plutino and a very likely dwarf planet; its diameter of 650 km estimated by Spitzer makes it about the fifth largest plutino. It is named after Ixion, a figure from Greek mythology...
with red visible and often featureless infrared spectra. Further Infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
observations in 2004 by the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
and the Gemini telescope give results consistent with mixtures of water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
and carbonaceous
Carbonaceous
Carbonaceous is the defining attribute of a substance rich in carbon. Particularly, carbonaceous hydrocarbons are very unsaturated, high-molecular-weight hydrocarbons, having an elevated carbon:hydrogen ratio....
compounds, such as tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...
s. 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
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
, but similar to that on Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
.
Later in 2008–2010 new infrared spectroscopic observations with a higher signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
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
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
/ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
dissolved in the water ice or by methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
/ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
ices. The radiative transfer
Radiative transfer
Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission and scattering processes. The equation of radiative transfer describes these interactions mathematically...
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 hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s requires better infrared spectra.
Comparison with moons and other TNOs
Orcus straddles the edge for trans-Neptunian objects massive enough to retain volatiles such as methane on the surface. The reflectance spectrum of Orcus shows the deepest water-ice absorptionWater absorption
During the transmission of electromagnetic radiation through a medium containing water molecules, portions of the electromagnetic spectrum are absorbed by water molecules...
bands of any Kuiper belt object (KBO) that is not associated with the Haumea collisional family
Haumea family
The Haumea family is the only identified trans-Neptunian collisional family; that is, the only group of trans-Neptunian objects with similar orbital parameters and spectra that suggest they originated in the disruptive impact of a progenitor body...
. The large icy satellites of Uranus have infrared spectra quite similar to that of Orcus. Among other trans-Neptunian objects Pluto's moon Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
appears to be most similar to Orcus. It has a higher albedo but very similar visible and near-infrared spectra. Their densities are also similar and both have water ice rich surfaces. Quaoar—a trans-Neptunian object of similar size—has strong water ice absorption features in its spectra but also has a very red color in the visible implying the presence of the ultrared matter on its surface. Dwarf planet Haumea and objects from its collisional family have much higher albedos and much deeper water absorption bands than Orcus. Finally, (208996) 2003 AZ84—another large object in 3:2 resonance with Neptune—has spectral properties very similar to Orcus.
Cryovolcanism
The presence of crystalCrystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
, and possibly ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
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
Miranda (moon)
-External links:* at * at The Nine8 Planets* at Views of the Solar System* * from the...
. The 1.65 μm band on Orcus is broad and deep (12%), as on Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
, Quaoar
50000 Quaoar
50000 Quaoar is a rocky trans-Neptunian object in the Kuiper belt with one known moon. Discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology from images acquired at the Samuel Oschin Telescope at Palomar Observatory, it is thought by...
, Haumea, and icy satellites of giant planets. On the other hand the crystalline water ice on the surfaces of TNOs should be completely amorphized
Amorphous solid
In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....
by the galactic and Solar radiation in about 10 million years. Some calculations indicate that cryovolcanism
Cryovolcano
A cryovolcano is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form...
, 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
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
suggest that Orcus may be capable of sustaining an internal ocean of liquid water.
Satellite
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
less than 0.0036, and an orbital period of 9.53 days. Vanth orbits only 8980 ± 20 km from Orcus and is too close to Orcus for ground-based spectroscopy to determine the surface composition of the satellite. Mike Brown suspects that like the Pluto–Charon system, Orcus and Vanth are tidally locked
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
. Vanth does not resemble known collisional satellites because its spectrum is very different from that of its primary, and it may be a captured KBO.
On March 23, 2009, Brown asked readers of his weekly column to suggest possible names for the satellite, with the best one to be submitted to the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
(IAU) on April 5. The name Vanth
Vanth
Vanth is a chthonic figure in Etruscan mythology shown in a variety of forms of funerary art, such as in tomb paintings and on sarcophagi. Vanth is a female demon in the Etruscan underworld that is often accompanied either by additional Vanth figures or by another demon, Charun . Both Vanth and...
, after the Etruscan
Etruscan mythology
The Etruscans were a diachronically continuous population, with a distinct language and culture during the period of earliest European writing, in the Mediterranean Iron Age in the second half of the first millennium BC...
goddess who guided the souls of the dead to the underworld, was eventually chosen from among a large pool of submissions. This submission will be assessed by the IAU's Committee for Small Body Nomenclature, which will vote on whether to approve it, in accordance with the normal object naming procedures.
External links and sources
- MPEC 2004-D09 announcing the discovery but attributing it to Raymond J. Bambery, Steven H. Pravdo, Michael D. Hicks, Kenneth J. LawrenceKenneth J. LawrenceKenneth J. Lawrence is an American astronomer.He has discovered numerous asteroids, and also co-discovered periodic comet 152P/Helin-Lawrence. The asteroid 4969 Lawrence is named in his honor.-References:...
, Daniel MacDonald, Eleanor F. HelinEleanor F. HelinEleanor Francis "Glo" Helin was an American astronomer. She was principal investigator of the Near Earth Asteroid Tracking program of NASA's Jet Propulsion Laboratory. Some sources give her name as Eleanor Kay Helin...
and Robert Thicksten / NEATNear Earth Asteroid TrackingNear-Earth Asteroid Tracking is a program run by NASA and Jet Propulsion Laboratory to discover near-Earth objects. The NEAT project began in December 1995 and ran until April 2007.-History:... - MPEC 2004-D13 correcting MPEC 2004-D09
- Chad Trujillo's page on 2004 DW
- First BBC article
- AstDys orbital elements
- Orbital simulation from JPL (Java) / Ephemeris