Ceres (dwarf planet)

Discovery In the years between the acceptance of heliocentrism in the 18th century and the discovery of Neptune in 1846, several astronomers argued that mathematical laws predicted the existence of a hidden or missing planet between the orbits of Mars and Jupiter. In 1596, theoretical astronomer Johannes Kepler believed that the ratios between planetary orbits would conform to "God's design" only with the addition of two planets: one between Jupiter and Mars and one between Venus and Mercury. This formula predicted that there ought to be another planet with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun. He was searching for "the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another". Instead of a star, Piazzi had found a moving starlike object, which he first thought was a comet. Piazzi observed Ceres twenty-four times, the final sighting occurring on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin. He reported it as a comet, but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet". The early observers were able to calculate the size of Ceres only to within an order of magnitude. Herschel underestimated its diameter at in 1802; in 1811, German astronomer Johann Hieronymus Schröter overestimated it at . In the 1970s, infrared photometry enabled more accurate measurements of its albedo, and Ceres's diameter was determined to within ten percent of its true value of . The adjectival forms of Ceres are Cererian and Cererean, both pronounced . Cerium, a rare-earth element discovered in 1803, was named after Ceres. The old astronomical symbol of Ceres, still used in astrology, is a sickle, ⟨⚳⟩. The sickle was one of the classical symbols of the goddess Ceres and was suggested, apparently independently, by von Zach and Bode in 1802. It is similar in form to the symbol ⟨♀⟩ (a circle with a small cross beneath) of the planet Venus, but with a break in the circle. It had various minor graphic variants, including a reversed form typeset as a 'C' (the initial letter of the name Ceres) with a plus sign. The generic asteroid symbol of a numbered disk, ①, was introduced in 1867 and quickly became the norm. Classification The categorisation of Ceres has changed more than once and has been the subject of some disagreement. Bode believed Ceres to be the "missing planet" he had proposed to exist between Mars and Jupiter. writing that "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes". In 1852 Johann Franz Encke, in the Berliner Astronomisches Jahrbuch, declared the traditional system of granting planetary symbols too cumbersome for these new objects and introduced a new method of placing numbers before their names in order of discovery. The numbering system initially began with the fifth asteroid, 5 Astraea, as number 1, but in 1867, Ceres was adopted into the new system under the name 1 Ceres. Then, in 2006, the debate surrounding Pluto led to calls for a definition of "planet", and the possible reclassification of Ceres, perhaps even its general reinstatement as a planet. A proposal before the International Astronomical Union (IAU), the global body responsible for astronomical nomenclature and classification, defined a planet as "a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet". Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun, but on 24 August 2006 the assembly adopted the additional requirement that a planet must have "cleared the neighbourhood around its orbit". Ceres is not a planet because it does not dominate its orbit, sharing it as it does with the thousands of other asteroids in the asteroid belt and constituting only about forty per cent of the belt's total mass. Bodies that met the first proposed definition but not the second, such as Ceres, were instead classified as dwarf planets. Planetary geologists still often ignore this definition and consider Ceres to be a planet anyway. Ceres is designated as a dwarf planet and an asteroid. A NASA webpage states that Vesta, the belt's second-largest object, is the largest asteroid. The IAU has been equivocal on the subject, though its Minor Planet Center, the organisation charged with cataloguing such objects, notes that dwarf planets may have dual designations, and the joint IAU / USGS / NASA Gazetteer categorises Ceres as both asteroid and a dwarf planet. == Orbit ==

. Lighter shades indicate above the ecliptic; darker indicate below. Ceres follows an orbit between Mars and Jupiter, near the middle of the asteroid belt, with an orbital period of 4.6 Earth years. the members of which share similar proper orbital elements, suggesting a common origin through an asteroid collision in the past. Ceres was later found to have a different composition from the Gefion family Resonances Due to their small masses and large separations, objects within the asteroid belt rarely fall into gravitational resonances with each other. Nevertheless, Ceres is able to capture other asteroids into temporary 1:1 resonances (making them temporary trojans), for periods from a few hundred thousand to more than two million years. Fifty such objects have been identified. Ceres is close to a 1:1 mean-motion orbital resonance with Pallas (their proper orbital periods differ by 0.2%), but not close enough to be significant over astronomical timescales. == Rotation and axial tilt ==

The rotation period of Ceres (the Cererian day) is 9 hours and 4 minutes; Ceres has an axial tilt of 4°, Dawn, the first spacecraft to orbit Ceres, determined that the north polar axis points at right ascension 19 h 25 m 40.3 s (291.418°), declination +66° 45' 50" (about 1.5 degrees from Delta Draconis), which means an axial tilt of 4°. This means that Ceres currently sees little to no seasonal variation in sunlight by latitude. Gravitational influence from Jupiter and Saturn over the course of the last three million years has triggered cyclical shifts in Ceres's axial tilt, ranging from two to twenty degrees, meaning that seasonal variation in sun exposure has occurred in the past, with the last period of seasonal activity estimated at 14,000 years ago. Those craters that remain in shadow during periods of maximum axial tilt are the most likely to retain water ice from eruptions or cometary impacts over the age of the Solar System. == Geology ==

planetary-mass objects, arranged by the order of their orbits outward from the Sun (from left: Mercury, Venus, Earth, the Moon, Mars and Ceres) Ceres is the largest asteroid in the main asteroid belt. It is an oblate spheroid, with an equatorial diameter 8% larger than its polar diameter. This gives Ceres a density of , Ceres makes up 40% of the estimated mass of the asteroid belt, and it has times the mass of the next asteroid, Vesta, but it has only the mass of the Moon, and its surface gravity is that of Earth ( of the Moon's). It is close to being in hydrostatic equilibrium, but some deviations from an equilibrium shape have yet to be explained. Ceres is the only widely accepted dwarf planet with an orbital period less than that of Neptune. but the data is also consistent with a mantle of hydrated silicates and no core. Ceres's internal differentiation may be related to its lack of a natural satellite, as satellites of main belt asteroids are mostly believed to form from collisional disruption, creating an undifferentiated, rubble pile structure. Surface Composition The surface composition of Ceres is homogeneous on a global scale, and it is rich in carbonates and ammoniated phyllosilicates that have been altered by water, Organic compounds were detected in the Ernutet crater, and at least another eleven regions are candidates for their presence. Most of the planet's near surface is rich in carbon, at approximately 20% by mass. The carbon content is more than five times higher than in carbonaceous chondrite meteorites analysed on Earth. Craters , is across. Ceres's north polar region shows far more cratering than the equatorial region, with the eastern equatorial region in particular comparatively lightly cratered. Three large shallow basins (planitiae) with degraded rims are likely to be eroded craters. Two of the three have higher than average ammonium concentrations. Tectonic features Although Ceres lacks plate tectonics, with the vast majority of its surface features linked either to impacts or to cryovolcanic activity, Cryovolcanism Ceres has one prominent mountain, Ahuna Mons; this appears to be a cryovolcano and has few craters, suggesting a maximum age of 240 million years. Kerwan too shows evidence of the effects of liquid water due to impact-melting of subsurface ice. A 2018 computer simulation suggests that cryovolcanoes on Ceres, once formed, recede due to viscous relaxation over several hundred million years. The team identified 22 features as strong candidates for relaxed cryovolcanoes on Ceres's surface. Yamor Mons, an ancient, impact-cratered peak, resembles Ahuna Mons despite being much older, due to it lying in Ceres's northern polar region, where lower temperatures prevent viscous relaxation of the crust. The eruptions may be linked to ancient impact basins but are not uniformly distributed over Ceres. Hundreds of bright spots (faculae) have been observed by Dawn, the brightest in the middle of Occator Crater. The bright spot in the centre of Occator is named Cerealia Facula, and the group of bright spots to its east, Vinalia Faculae. Occator possesses a pit 9–10 km wide, partially filled by a central dome. The dome post-dates the faculae and is likely due to freezing of a subterranean reservoir, comparable to pingos in Earth's Arctic region. A haze periodically appears above Cerealia, supporting the hypothesis that some sort of outgassing or sublimating ice formed the bright spots. In March 2016 Dawn found definitive evidence of water ice on the surface of Ceres at Oxo crater. On 9 December 2015, NASA scientists reported that the bright spots on Ceres may be due to a type of salt from evaporated brine containing magnesium sulfate hexahydrate (MgSO4·6H2O); the spots were also found to be associated with ammonia-rich clays. These materials have been suggested to originate from the crystallisation of brines that reached the surface. In August 2020 NASA confirmed that Ceres was a water-rich body with a deep reservoir of brine that percolated to the surface in hundreds of locations causing "bright spots", including those in Occator Crater. Internal structure The active geology of Ceres is driven by ice and brines. Water leached from rock is estimated to possess a salinity of around 5%. Altogether, Ceres is approximately 50% water by volume (compared to 0.1% for Earth) and 73% rock by mass. Ceres's largest craters are several kilometres deep, inconsistent with an ice-rich shallow subsurface. The fact that the surface has preserved craters almost in diameter indicates that the outermost layer of Ceres is roughly 1000 times stronger than water ice. This is consistent with a mixture of silicates, hydrated salts and methane clathrates, with no more than 30% water ice by volume. Gravity measurements from Dawn have generated three competing models for Ceres's interior. It is not possible to tell if Ceres's deep interior contains liquid or a core of dense material rich in metal, That is, the core (if it exists), the mantle and crust all consist of rock and ice, though in different ratios. Ceres's mineral composition can be determined (indirectly) only for its outer . The solid outer crust, thick, is a mixture of ice, salts, and hydrated minerals. Under that is a layer that may contain a small amount of brine. This extends to a depth of at least the limit of detection. Under that is thought to be a mantle dominated by hydrated rocks such as clays. A second two-layer model suggests a partial differentiation of Ceres into a volatile-rich crust and a denser mantle of hydrated silicates. A range of densities for the crust and mantle can be calculated from the types of meteorite thought to have impacted Ceres. With CI-class meteorites (density 2.46 g/cm3), the crust would be approximately thick and have a density of 1.68 g/cm3; with CM-class meteorites (density 2.9 g/cm3), the crust would be approximately thick and have a density of 1.9 g/cm3. Best-fit modelling yields a crust approximately thick with a density of approximately 1.25 g/cm3, and a mantle/core density of approximately 2.4 g/cm3. == Exosphere ==

In 2017, Dawn confirmed that Ceres has a transient atmosphere of water vapour. Hints of an atmosphere had appeared in early 2014, when the Herschel Space Observatory detected localised mid-latitude sources of water vapour on Ceres, no more than in diameter, which each give off approximately molecules (3 kg) of water per second. Two potential source regions, designated Piazzi (123°E, 21°N) and Region A (231°E, 23°N), were visualised in the near infrared as dark areas (Region A also has a bright centre) by the W. M. Keck Observatory. Possible mechanisms for the vapour release are sublimation from approximately of exposed surface ice, cryovolcanic eruptions resulting from radiogenic internal heat, In 2015, David C. Jewitt included Ceres in his list of active asteroids. Surface water ice is unstable at distances less than 5 AU from the Sun, so it is expected to sublime if exposed directly to solar radiation. Proton emission from solar flares and CMEs can sputter exposed ice patches on the surface, leading to a positive correlation between detections of water vapour and solar activity. Water ice can migrate from the deep layers of Ceres to the surface, but it escapes in a short time. Surface sublimation would be expected to be lower when Ceres is farther from the Sun in its orbit, and internally powered emissions should not be affected by its orbital position. The limited data previously available suggested cometary-style sublimation, Studies using ''Dawn's'' gamma ray and neutron detector (GRaND) reveal that Ceres accelerates electrons from the solar wind; the most accepted hypothesis is that these electrons are being accelerated by collisions between the solar wind and a tenuous water vapour exosphere. Bow shocks like these could also be explained by a transient magnetic field, but this is considered less likely, as the interior of Ceres is not thought to be sufficiently electrically conductive. The rate of this vapour diffusion scales with grain size and is heavily affected by a global dust mantle consisting of an aggregate of approximately 1 micron particles. Exospheric replenishment through sublimation alone is very small, with the current outgassing rate being only 0.003 kg/s. Various models of an extant exosphere have been attempted including ballistic trajectory, DSMC, and polar cap numerical models. Results showed a water exosphere half-life of 7 hours from the ballistic trajectory model, an outgassing rate of 6 kg/s with an optically thin atmosphere sustained for tens of days using a DSMC model, and seasonal polar caps formed from exosphere water delivery using the polar cap model. The mobility of water molecules within the exosphere is dominated by ballistic hops coupled with interaction of the surface, however less is known about direct interactions with planetary regoliths. == Origin and evolution ==

Ceres is a surviving protoplanet that formed 4.56 billion years ago; alongside Pallas and Vesta, one of only three remaining in the inner Solar System, with the rest either merging to form terrestrial planets, being shattered in collisions or being ejected by Jupiter. Despite Ceres's current location, its composition is not consistent with having formed within the asteroid belt. It seems rather that it formed between the orbits of Jupiter and Saturn, and was deflected into the asteroid belt as Jupiter migrated outward. The early geological evolution of Ceres was dependent on the heat sources available during and after its formation: impact energy from planetesimal accretion and decay of radionuclides (possibly including short-lived extinct radionuclides such as aluminium-26). These may have been sufficient to allow Ceres to differentiate into a rocky planetary core and icy mantle, or even a liquid water ocean, The presence of clays and carbonates requires chemical reactions at temperatures above 50 °C, consistent with hydrothermal activity. It has become considerably less geologically active over time, with a surface dominated by impact craters; nevertheless, evidence from Dawn reveals that internal processes have continued to sculpt Ceres's surface to a significant extent contrary to predictions that Ceres's small size would have ceased internal geological activity early in its history. == Habitability ==

Although Ceres is not as actively discussed as a potential home for microbial extraterrestrial life as Mars, Europa, Enceladus, or Titan are, it has the most water of any body in the inner Solar System after Earth, but phosphorus has yet to be detected, and sulfur, despite being suggested by Hubble UV observations, was not detected by Dawn. == Observation and exploration ==

Observation When in opposition near its perihelion, Ceres can reach an apparent magnitude of +6.7. This is too dim to be visible to the average naked eye, but under ideal viewing conditions, keen eyes may be able to see it. Vesta is the only other asteroid that can regularly reach a similarly bright magnitude, while Pallas and 7 Iris do so only when both in opposition and near perihelion. When in conjunction, Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars; thus, it can be seen with such binoculars in a naturally dark and clear night sky around new moon. On 25 June 1995, Hubble obtained ultraviolet images of Ceres with resolution. In 2002, the W. M. Keck Observatory obtained infrared images with resolution using adaptive optics. Before the Dawn mission, only a few surface features had been unambiguously detected on Ceres. High-resolution ultraviolet Hubble images in 1995 showed a dark spot on its surface, which was nicknamed "Piazzi" in honour of the discoverer of Ceres. One of them corresponded to the Piazzi feature. Dawn eventually revealed Piazzi to be a dark region in the middle of Vendimia Planitia, close to the crater Dantu, and the other dark feature to be within Hanami Planitia and close to Occator Crater. Dawn mission '' trajectory around Ceres from 1 February 2015 to 1 February 2025 In the early 1990s, NASA initiated the Discovery Program, which was intended to be a series of low-cost scientific missions. In 1996, the program's study team proposed a high-priority mission to explore the asteroid belt using a spacecraft with an ion thruster. Funding remained problematic for nearly a decade, but by 2004, the Dawn vehicle passed its critical design review. Dawn, the first space mission to visit either Vesta or Ceres, was launched on 27 September 2007. On 3 May 2011, Dawn acquired its first targeting image from Vesta. After orbiting Vesta for thirteen months, Dawn used its ion thruster to depart for Ceres, with gravitational capture occurring on 6 March 2015 at a separation of , four months before the New Horizons flyby of Pluto. On 13 January 2015, as Dawn approached Ceres, the spacecraft took its first images at near-Hubble resolution, revealing impact craters and a small high-albedo spot on the surface. Additional imaging sessions, at increasingly better resolution, took place from February to April. Dawns mission profile called for it to study Ceres from a series of circular polar orbits at successively lower altitudes. It entered its first observational orbit ("RC3") around Ceres at an altitude of on 23 April 2015, staying for only one orbit (15 days). The spacecraft then reduced its orbital distance to for its second observational orbit ("survey") for three weeks, then down to ("HAMO;" high altitude mapping orbit) for two months and then down to its final orbit at ("LAMO;" low altitude mapping orbit) for at least three months. In October 2015, NASA released a true-colour portrait of Ceres made by Dawn. In 2017, Dawns mission was extended to perform a series of closer orbits around Ceres until the hydrazine used to maintain its orbit ran out. Dawn soon discovered evidence of cryovolcanism. Two distinct bright spots (or high-albedo features) inside a crater (different from the bright spots observed in earlier Hubble images) were seen in a 19 February 2015 image, leading to speculation about a possible cryovolcanic origin or outgassing. On 2 September 2016, scientists from the Dawn team argued in a Science paper that Ahuna Mons was the strongest evidence yet for cryovolcanic features on Ceres. On 9 December 2015, NASA scientists reported that the bright spots on Ceres may be related to a type of salt, particularly a form of brine containing magnesium sulfate hexahydrate (MgSO4·6H2O); the spots were also found to be associated with ammonia-rich clays. In June 2016, near-infrared spectra of these bright areas were found to be consistent with a large amount of sodium carbonate (), implying that recent geologic activity was probably involved in the creation of the bright spots. From June to October 2018, Dawn orbited Ceres from as close as to as far away as . The Dawn mission ended on 1 November 2018 after the spacecraft ran out of fuel. Future missions In 2020, an ESA team proposed the Calathus Mission concept, a followup mission to Occator Crater, to return a sample of the bright carbonate faculae and dark organics to Earth. The China National Space Administration is designing a sample-return mission from Ceres that would take place during the 2020s. == See also ==