Monday, November 26, 2012

This 100 km-thick mantle

This 100 km-thick mantle (23%–28% of Ceres by mass; 50% by volume)[62] contains 200 million cubic kilometres of water, which is more than the amount of fresh water on the Earth.[63] This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modelling. Sony VAIO VGN-AW220J/B Battery Also, some characteristics of its surface and history (such as its distance from the Sun, which weakened solar radiation enough to allow some fairly low-freezing-point components to be incorporated during its formation), point to the presence of volatile materials in the interior of Ceres.[9] Alternatively, the shape and dimensions of Ceres may be explained by an interior that is porous and either partially differentiated or completely undifferentiated. Sony VAIO VGN-AW230J/H Battery The presence of a layer of rock on top of ice would be gravitationally unstable. If any of the rock deposits sank into a layer of differentiated ice, salt deposits would be formed. Such deposits have not been detected. Thus it is possible that Ceres does not contain a large ice shell, but was instead formed from low-density asteroids with an aqueous component. Sony VAIO VGN-AW235J/B Battery The decay of radioactive isotopes may not have been sufficient to cause differentiation. The surface composition of Ceres is broadly similar to that of C-type asteroids.[14] Some differences do exist. The ubiquitous features of the Cererian IRspectra are those of hydrated materials, which indicate the presence of significant amounts of water in the interior. Sony VAIO VGN-AW290JFQ Battery Other possible surface constituents include iron-rich clays (cronstedtite) and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites.[14] The spectral features of carbonates and clay are usually absent in the spectra of other C-type asteroids.[14] Sometimes Ceres is classified as a G-type asteroid.[65] Sony VAIO VGN-AW41JF Battery The Cererian surface is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about −38 °C, −36 °F) on 5 May 1991.Only a few Cererian surface features have been unambiguously detected. High-resolution ultraviolet Hubble Space Telescopeimages taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres. Sony VAIO VGN-AW41JF/H Battery This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with the dwarf planet's rotation.Two dark features had circular shapes and are presumably craters; one of them was observed to have a bright central region, Sony VAIO VGN-AW41MF Battery while another was identified as the "Piazzi" feature.[9][66] More recent visible-lightHubble Space Telescope images of a full rotation taken in 2003 and 2004 showed 11 recognizable surface features, the natures of which are currently unknown.[13][67] One of these features corresponds to the "Piazzi" feature observed earlier.[13] Sony VAIO VGN-AW41MF/H Battery These last observations also determined that the north pole of Ceres points in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco. This means that Ceres's axial tilt is very small—about 3°. There are indications that Ceres may have a weak atmosphere and water frost on the surface. Sony VAIO VGN-AW41XH Battery Surface water ice is unstable at distances less than 5 AU from the Sun,[69] so it is expected to sublime if it is exposed directly to solar radiation. Water ice can migrate from the deep layers of Ceres to the surface, but will escape in a very short time. As a result, it is difficult to detect water vaporization. Water escaping from polar regions of Ceres was possibly observed in the early 1990s but this has not been unambiguously demonstrated. Sony VAIO VGN-AW41XH/Q Battery It may be possible to detect escaping water from the surroundings of a fresh impact crater or from cracks in the sub-surface layers of Ceres.[9] Ultraviolet observations by the IUE spacecraft detected statistically significant amounts of hydroxide ion near the Cererean north pole, which is a product of water-vapor dissociation by ultraviolet solar radiation. Sony VAIO VGN-AW41ZF Battery Ceres follows an orbit between Mars and Jupiter, within the asteroid belt, with a period of 4.6 Earth years.[6]The orbit is moderately inclined (i = 10.6° compared to 7° for Mercury and 17° for Pluto) and moderatelyeccentric (e = 0.08 compared to 0.09 for Mars).[6] The diagram illustrates the orbits of Ceres (blue) and several planets (white and grey). Sony VAIO VGN-AW41ZF/B Battery The segments of orbits below the ecliptic are plotted in darker colours, and the orange plus sign is the Sun's location. The top left diagram is a polar view that shows the location of Ceres in the gap between Mars and Jupiter. The top right is a close-up demonstrating the locations of the perihelia (q) and aphelia (Q) of Ceres and Mars. Sony VAIO VGN-AW50DB/H Battery The perihelion of Mars is on the opposite side of the Sun from those of Ceres and several of the large main-belt asteroids, including 2 Pallas and 10 Hygiea. The bottom diagram is a side view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter. In the past, Ceres had been considered to be a member of an asteroid family. Sony VAIO VGN-AW51JGB Battery These groupings of asteroids share similar proper orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres was found to have spectral properties different from other members of the family, and so this grouping is now called the Gefion family, named after the next-lowest-numbered family member, 1272 Gefion. Sony VAIO VGN-AW52JGB Battery Ceres appears to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common origin.[73] The rotational period of Ceres (the Cererian day) is 9 hours and 4 minutes.[11] Ceres is in a near-1:1 mean-motion orbital resonance with Pallas (their orbital periods differ by 0.3%).Sony VAIO VGN-AW53FB Battery However, a true resonance between the two would be unlikely; due to their small masses relative to their large separations, such relationships among asteroids are very rare. Mercury, Venus, Earth, and Mars can all appear to cross the Sun, or transit it, from a vantage point at Ceres. The most common transits are those of Mercury, which usually happen every few years, most recently in 2006 and 2010. Sony VAIO VGN-AW70B/Q Battery The corresponding dates are 1953 and 2051 for Venus, 1814 and 2081 for Earth, and 767 and 2684 for Mars.Ceres is probably a surviving protoplanet (planetary embryo), which formed 4.57 billion years ago in the asteroid belt.[77] While the majority of inner Solar System protoplanets (including all lunar- to Mars-sized bodies) Sony VAIO VGN-AW71JB Battery either merged with other protoplanets to form terrestrial planets or were ejected from the Solar System by Jupiter,[77] Ceres is believed to have survived relatively intact.[26] An alternative theory proposes that Ceres formed in the Kuiper belt and later migrated to the asteroid belt.[78] Another possible protoplanet, Vesta, is less than half the size of Ceres; Sony VAIO VGN-AW72JB Battery it suffered a major impact after solidifying, losing ~1% of its mass.[79] The geological evolution of Ceres was dependent on the heat sources available during and after its formation: friction from planetesimal accretion, and decay of various radionuclides (possibly including short-lived elements like 26Al). Sony VAIO VGN-AW73FB Battery These are thought to have been sufficient to allow Ceres to differentiate into a rockycore and icy mantle soon after its formation.[13][26] This process may have caused resurfacing by water volcanism and tectonics, erasing older geological features.[26] Due to its small size, Ceres would have cooled early in its existence, causing all geological resurfacing processes to cease. Sony VAIO VGN-AW80NS Battery Any ice on the surface would have gradually sublimated, leaving behind various hydrated minerals like clays and carbonates.Today, Ceres appears to be a geologically inactive body, with a surface sculpted only by impacts.[13] The presence of significant amounts of water ice in its composition[8] raises the possibility that Ceres has or had a layer of liquid water in its interior. Sony VAIO VGN-AW80S Battery This hypothetical layer is often called an ocean.[14]If such a layer of liquid water exists, it is believed to be located between the rocky core and ice mantle like that of the theorized ocean on Europa.[26] The existence of an ocean is more likely if solutes (i.e. salts), ammonia, sulfuric acid or other antifreeze compounds are dissolved in the water. Sony VAIO VGN-AW80US Battery When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7.[15] This is generally regarded as too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted person may be able to see this dwarf planet. Ceres will be at its brightest (6.73) on December 18, 2012. Sony VAIO VGN-AW81DS Battery The only other asteroids that can reach a similarly bright magnitude are 4 Vesta, and, during rare oppositions near perihelion, 2 Pallasand 7 Iris.[80] At a conjunction Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars. It can thus be seen with binoculars whenever it is above the horizon of a fully dark sky. Sony VAIO VGN-AW81JS Battery No space probe has visited Ceres. Radio signals from spacecraft in orbit around and on the surface of Mars have been used to estimate the mass of Ceres from its perturbations on the motion of Mars.[22] The Dawn spacecraft, launched by NASA in 2007, orbited asteroid 4 Vesta from 15 July 2011 until 5 September 2012[83] and is continuing on to Ceres. Sony VAIO VGN-AW81YS Battery It is scheduled to arrive there in 2015, five months prior to the arrival of New Horizons atPluto.[28] Dawn will thus be the first mission to study a dwarf planet at close range. Dawn's mission profile calls for it to enter orbit around Ceres at an altitude of 5,900 km. The spacecraft will reduce its orbital distance to 1,300 km after five months of study, and then down to 700 km after another five months. Sony VAIO VGN-AW82DS Battery The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutrondetector. These instruments will be used to examine the dwarf planet's shape and elemental composition. Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System (after Eris) and the tenth-most-massive body observed directly orbiting the Sun. Sony VAIO VGN-AW82JS Battery Originally classified as the ninth planet from the Sun, Pluto was recategorized as a dwarf planet and plutoid owing to the discovery that it is only one of several large bodies within the Kuiper belt.[h] Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small, approximately one-sixth the mass of the Earth's Moon and one-third its volume. Sony VAIO VGN-AW82YS Battery It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun thanNeptune. As of 2011, it is 32.1 AU from the Sun.[15] From its discovery in 1930 until 2006, Pluto was classified as a planet. Sony VAIO VGN-AW83FS Battery In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned.[16] In the late 20th and early 21st centuries, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto. Sony VAIO VGN-AW83GS Battery On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres.[18] After the reclassification, Pluto was added to the list of minor planets and given thenumber 134340. Sony VAIO VGN-AW83HS Battery A number of scientists hold that Pluto should continue to be classified as a planet, and that other dwarf planets should be added to the roster of planets along with Pluto.[21][22] Pluto has five known moons, the largest being Charon, discovered in 1978, along with Nix and Hydra, discovered in 2005,[23] and the provisionally named S/2011 (134340) 1, Sony VAIO VGN-AW90NS Battery discovered in 2011,[24] and S/2012 (134340) 1, discovered in 2012.[5] Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits does not lie within either body.[25] However, the IAU has yet to formalise a definition for binary dwarf planets, and as such Charon is officially classified as a moon of Pluto. Sony VAIO VGN-AW90S Battery In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus.[27] Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune. Sony VAIO VGN-AW90US Battery In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X".[28] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.[ Sony VAIO VGN-AW91CDS Battery Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but did not recognise them for what they were. Lowell was not the first to unknowingly photograph Pluto. There are sixteen known pre-discoveries, with the oldest being made by the Yerkes Observatory on August 20, 1909. Sony VAIO VGN-AW91CJS Battery Because of a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,[32] when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, Sony VAIO VGN-AW91CYS Battery a 23-year-old Kansan who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.[32] Tombaugh's task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Sony VAIO VGN-AW91DS Battery Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. Sony VAIO VGN-AW91JS Battery A lesser-quality photograph taken on January 21 helped confirm the movement.[33] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. Sony VAIO VGN-AW92CDS Battery The discovery made headlines across the globe. The Lowell Observatory, which had the right to name the new object, received over 1,000 suggestions from all over the world, ranging from Atlas to Zymal. Sony VAIO VGN-AW91YS Battery Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.[34] Constance Lowell proposed Zeus, then Percival and finally Constance. These suggestions were disregarded.[35] The name Pluto was proposed by Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England. Sony VAIO VGN-AW92CJS Battery Burney was interested in classical mythology as well as astronomy, and considered the name, a name for the god of the underworld, appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library. Sony VAIO VGN-AW92CYS Battery Madan passed the name to Professor Herbert Hall Turner, who then cabled it to colleagues in the United States.[37] The object was officially named on March 24, 1930.Each member of the Lowell Observatory was allowed to vote on a short-list of three: Minerva (which was already the name for an asteroid), Sony VAIO VGN-AW92DS Battery Cronus (which had lost reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See), and Pluto. Pluto received every vote.[40] The name was announced on May 1, 1930.[36] Upon the announcement, Madan gave Venetia five pounds (£5) (£234 as of 2012),[41]as a reward.[36] Sony VAIO VGN-AW92JS Battery The choice of name was partly inspired by the fact that the first two letters of Pluto are the initials of Percival Lowell, and Pluto's astronomical symbol ( ) is a monogram constructed from the letters 'PL'.[42] Pluto's astrological symbol resembles that of Neptune ( ), but has a circle in place of the middle prong of the trident ( ).Sony VAIO VGN-AW92YS Battery The name was soon embraced by wider culture. In 1930, Walt Disney introduced for Mickey Mouse a canine companion, named Pluto apparently in the object's honour, although Disney animator Ben Sharpsteen could not confirm why the name was given.[43] In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto, Sony VAIO VGN-AW93FS Battery in keeping with the tradition of naming elements after newly discovered planets, following uranium, which was named after Uranus, and neptunium, which was named after Neptune.[44] In Chinese, Japanese and Korean the name was translated as underworld king star (冥王星), as suggested by Houei Nojiri in 1930.[47] Sony VAIO VGN-AW93GS Battery Many other non-European languages use a transliteration of "Pluto" as their name for the object; some Indian languages use a form of Yama, the Guardian of Hell in Hindumythology, such as the Gujarati Yamdev. Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's Planet X. Sony VAIO VGN-AW93HS Battery Estimates of Pluto's mass were revised downward throughout the 20th century. Astronomers initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1931 Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1948 bringing the mass down to roughly that of Mars. Sony VAIO VGN-AW93ZFS Battery In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto's albedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of the Earth.[52] (Pluto's albedo is 1.3–2.0 times greater than that of Earth.[2]) Sony VAIO VGN-AW93ZGS Battery In 1978, the discovery of Pluto's moon Charon allowed the measurement of Pluto's mass for the first time. Its mass, roughly 0.2% that of the Earth, was far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternative Planet X, notably by Robert Sutton Harrington,failed. Sony VAIO VGN-AW93ZHS Battery In 1992, Myles Standish used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5%, to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.[55] Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.[56] Sony VAIO VGN-BZ11EN Battery Lowell had made a prediction of Planet X's position in 1915 that was fairly close to Pluto's position at that time;Ernest W. Brown concluded almost immediately that this was a coincidence,[57] a view still held today.Pluto's orbital period is 248 Earth years. Its orbital characteristics are substantially different from those of the planets, Sony VAIO VGN-BZ11MN Battery which follow nearly circular orbits around the Sun close to a flat reference plane called the ecliptic. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity means a small region of Pluto's orbit lies nearer the Sun than Neptune's. The Pluto–Charon barycentre came to perihelion on September 5, 1989, Sony VAIO VGN-BZ11XN Battery and was last closer to the Sun than Neptune between February 7, 1979 and February 11, 1999.[58] In the long term Pluto's orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forward and backward in time), after intervals longer than the Lyapunov time of 10–20 million years, calculations become speculative: Sony VAIO VGN-BZ12VN Battery Pluto's tiny size makes it sensitive to unmeasurably small details of the Solar System, hard-to-predict factors that will gradually disrupt its orbit.[59][60] Millions of years from now, Pluto may well be ataphelion, at perihelion or anywhere in between, with no way for us to predict which. This does not mean Pluto's orbit itself is unstable, Sony VAIO VGN-BZ12XN Battery but its position on that orbit is impossible to determine so far ahead. Several resonances and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering. Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits are aligned so that they can never collide or even approach closely. Sony VAIO VGN-BZ31VT Battery There are several reasons why. At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 AUabove that of Neptune, preventing a collision.[ Sony VAIO VGN-BZ31XT Battery

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