Ceres (dwarf planet)

Last updated

Ceres Ceres symbol.svg
Ceres - RC3 - Haulani Crater (22381131691) (cropped).jpg
Ceres in true color in 2015
Discovery [1]
Discovered by Giuseppe Piazzi
Discovery date1 January 1801
Designations
(1) Ceres
Pronunciation /ˈsɪərz/
Named after
Cerēs
  • A899 OF
  • 1943 XB
Adjectives Cererean, -ian /sɪˈrɪəriən/
Orbital characteristics [2]
Epoch 27 April 2019 (JD  2458600.5)
Aphelion 2.9796467093  AU
(445,749,000 km)
Perihelion 2.5586835997 AU
(382,774,000 km)
2.7691651545 AU
(414,261,000 km)
Eccentricity 0.07600902910
4.61  yr
1683.14570801 d
466.6  d
1.278  yr
Average orbital speed
17.905 km/s
77.37209589 °
Inclination 10.59406704° to ecliptic
9.20° to invariable plane [3]
80.3055316°
73.5976941°
Proper orbital elements [4]
2.7670962  AU
Proper eccentricity
0.1161977
Proper inclination
9.6474122°
Proper mean motion
78.193318  deg  / yr
4.60397 yr
(1681.601 d)
Precession of perihelion
54.070272  arcsec  / yr
Precession of the ascending node
−59.170034  arcsec  / yr
Physical characteristics
Dimensions(964.4 × 964.2 × 891.8) ± 0.2 km [2]
Mean diameter
939.4±0.2 km [2]
Mean radius
469.73 km [5]
2,770,000 km2 [6]
Volume 434,000,000 km3 [6]
Mass (9.3835±0.0001)×1020 kg [2]
0.00016  Earths
0.0128 Moons
Mean density
2.162±0.008 g/cm3 [2]
Equatorial surface gravity
0.28 m/s2 [6]
0.029 g
0.36±0.15 [7] [lower-alpha 1] (estimate)
Equatorial escape velocity
0.51 km/s [6]
Sidereal rotation period
9.074170±0.000001 h [2]
Equatorial rotation velocity
92.61 m/s [6]
≈4° [9]
North pole right ascension
291.42744° [10]
North pole declination
66.76033° [5]
0.090±0.0033(V-band) [11]
Surface temp. minmeanmax
Kelvin ≈110 [12] 235±4 [13]
C [14]
  • 6.64–9.34 (range) [15]
  • 9.27 July, 2021 [16]
3.34 [2]
0.854″ to 0.339″

    Ceres ( /ˈsɪərz/ ; [17] minor-planet designation: 1 Ceres) is the largest object in the main asteroid belt between the orbits of Mars and Jupiter. It comprises a quarter of the mass of the belt and, at 940 km (580 mi) in diameter, is the only asteroid large enough to be rounded by its own gravity. This makes Ceres both the smallest recognized dwarf planet and the only one inside Neptune's orbit.

    Contents

    The first asteroid known, Ceres was discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory. It was originally considered a planet, but was reclassified as an asteroid in the 1850s after over 20 other objects in similar orbits were discovered.

    Despite being closer to Earth than Jupiter, which has been known since antiquity, Ceres's small size means that, from Earth, its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition once during its 15- to 16-month synodic period. Thus even at its brightest, it is too dim to be seen by the naked eye, except under extremely dark skies. Its surface features are barely visible even with the most powerful telescopes, and little was known of them until the robotic NASA spacecraft Dawn entered orbit around Ceres on 6 March 2015.

    Ceres appears to be partially differentiated into a muddy (ice-rock) mantle/core and a less-dense but stronger crust that is at most 30 percent ice. It probably no longer has an internal ocean of liquid water, but there is brine that can flow through the outer mantle and reach the surface. The surface is a mixture of water ice and hydrated minerals such as carbonates and clay. Cryovolcanoes such as Ahuna Mons form at the rate of about one every 50 million years. In January 2014, emissions of water vapor were detected around Ceres, creating a tenuous, transient atmosphere known as an exosphere. This was unexpected because large bodies in the asteroid belt typically do not emit vapor, a hallmark of comets.

    History

    Discovery

    Giuseppe Piazzi, discoverer of Ceres. Giuseppe Piazzi.jpg
    Giuseppe Piazzi, discoverer of Ceres.

    German astronomer Johann Elert Bode, in 1772, first suggested that an undiscovered planet could exist between the orbits of Mars and Jupiter. [18] Theoretical astronomer Johannes Kepler had already noticed the gap between Mars and Jupiter in 1596. [18] Bode based his idea on the Titius–Bode law which is a now-discredited hypothesis that was first proposed in 1766. Bode observed that there was a regular pattern in the size of the orbits of known planets, and that the pattern was marred only by the large gap between Mars and Jupiter. [19] The pattern predicted that the missing planet ought to have an orbit with a radius near 2.8 astronomical units (AU). [19] William Herschel's discovery of Uranus in 1781 [18] near the predicted distance for the next body beyond Saturn increased faith in the law of Titius and Bode, and in 1800, a group headed by Franz Xaver von Zach, editor of the Monatliche Correspondenz, sent requests to 24 experienced astronomers (whom he dubbed the "celestial police"), [19] asking that they combine their efforts and begin a methodical search for the expected planet. [19] Although they did not discover Ceres, they later found the asteroids 2 Pallas, 3 Juno and 4 Vesta. [19]

    One of the astronomers selected for the search was Giuseppe Piazzi, a Catholic priest at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. [20] 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". [18] Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet. [21] Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his observations. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin. [22] 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". [18] In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the Monatliche Correspondenz. [21]

    By this time, the apparent position of Ceres had changed (mostly due to Earth's motion around the Sun), and was too close to the Sun's glare for other astronomers to confirm Piazzi's observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, mathematician Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination. [21] In a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and fellow celestial policeman Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it. [21]

    The early observers were only able to calculate the size of Ceres to within an order of magnitude. Herschel underestimated its diameter as 260 kilometres (160 mi) in 1802, whereas in 1811 German astronomer Johann Hieronymus Schröter overestimated it as 2,613 kilometres (1,624 mi). [23]

    Name

    Piazzi's initial name for his discovery was Ceres Ferdinandea. Ceres after the Roman goddess of agriculture, whose earthly home, and oldest temple, lay in Sicily; "Ferdinandea" in honor of Piazzi's concurrent monarch and patron, King Ferdinand of Sicily. [21] "Ferdinandea" was not acceptable to other nations and was dropped. Prior to Von Zach's confirmation in December 1801, he referred to the planet as Hera , though Bode preferred Juno . Despite Piazzi's objections, these two names gained currency in Germany before the object's existence was confirmed. Once it was, astronomers settled on Piazzi's name of 'Ceres'. [24] Cerium, a rare-earth element discovered in 1803, was named after Ceres. [25] [lower-alpha 2] In July, 1802, William Hyde Wollaston announced the discovery of another new element and his initial intention to name it "Ceresium", after Ceres, though he eventually chose palladium, after the asteroid Pallas. [27]

    The adjectival forms of 'Ceres' are Cererian [28] [29] /sɪˈrɪəriən/ [30] and Cererean [31] (with the same pronunciation), [32] The old astronomical symbol of Ceres is a sickle, , [33] similar to Venus' symbol but with a break in the circle. It has a variant , reversed under the influence of the initial letter 'C' of 'Ceres'. These symbols were later replaced with the generic asteroid symbol of a numbered disk, . [21] [34]

    Classification

    The Four Largest Asteroids.jpg
    Relative sizes of the four largest asteroids. Ceres is furthest left.
    Ceres, Earth & Moon size comparison.jpg
    Ceres (bottom left), the Moon and Earth, shown to scale

    The categorization 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, at a distance of 419 million km (2.8 AU) from the Sun. [18] Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with Pallas, Juno, and Vesta) for half a century. [35]

    As other objects were discovered in the neighborhood of Ceres, it was realized that Ceres represented the first of a new class of objects. [18] In 1802, with the discovery of Pallas, Herschel coined the term asteroid ("star-like") for these bodies, [35] writing that "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes". [36] As the first such body to be discovered, in 1851 Ceres was given the designation 1 Ceres under the modern system of minor-planet designations. [35]

    By the 1860s, the existence of a fundamental difference between asteroids such as Ceres and the major planets was widely accepted, though a precise definition of "planet" was not formulated [35] until 2006, when the debate surrounding Pluto and what constitutes a planet led to Ceres being considered for reclassification as a planet. [37] A proposal before the International Astronomical Union (IAU), the global body responsible for astronomical nomenclature and classification, for the definition of a planet would have 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". [38] Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun. [39] But on 24 August 2006 the assembly adopted the additional requirement that a planet must have "cleared the neighborhood around its orbit". By this definition, 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 25% of the belt's total mass. [40] Bodies that met the first proposed definition but not the second, such as Ceres, were instead classified as dwarf planets.

    Since the IAU declaration in 2006 there has been some confusion as to whether, now that Ceres is officially a dwarf planet, it remains classified as an asteroid. Several sources, such as space.com and even NASA, have declared Vesta, the belt's second-largest object, to be the largest asteroid. [41] [42] The IAU has been equivocal on the subject, often contradicting itself, sometimes in the same source, [43] [44] though its Minor Planet Center, the organization charged with officially cataloguing such objects, notes that they may have dual designations. [45]

    Orbit

    Orbits of Ceres (blue) along with Jupiter and the inner planets (white and gray). The upper diagram shows Ceres's orbit from top down. The bottom diagram is a side view showing Ceres's orbital inclination to the ecliptic. Ceres Orbit c.svg
    Orbits of Ceres (blue) along with Jupiter and the inner planets (white and gray). The upper diagram shows Ceres's orbit from top down. The bottom diagram is a side view showing Ceres's orbital inclination to the ecliptic.

    Ceres follows an orbit between Mars and Jupiter, within the asteroid belt and closer to the orbit of Mars, with a period of 4.6 Earth years. [2] Compared to other planets and dwarf planets, Ceres's orbit is moderately though not drastically inclined ( i = 10.6° compared to 7° for Mercury and 17° for Pluto) and eccentric (e = 0.08 compared to 0.09 for Mars). [2]

    Ceres was once thought to be a member of an asteroid family. [46] The asteroids of this family share similar proper orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres was later found to have spectral properties different from other members of the family, which is now called the Gefion family after the next-lowest-numbered family member, 1272 Gefion. [46] Ceres appears to be merely an interloper in the Gefion family, coincidentally having similar orbital elements but not a common origin. [47]

    Resonances

    Ceres is in a near-1:1 mean-motion orbital resonance with Pallas (their proper orbital periods differ by 0.2%). [48] A true resonance between the two would be unlikely; due to their small masses relative to their large separations, such relationships among asteroids are rare. [49] Nevertheless, Ceres is able to capture other asteroids into temporary 1:1 resonant orbital relationships (making them temporary trojans) for periods up to 2 million years or more; 50 such objects have been identified. [50]

    Transits of planets from Ceres

    Mercury, Venus, Earth, and Mars can all transit the Sun from a vantage point on Ceres. The most common transits are those of Mercury, which usually happen every few years. The last transit of Venus from Ceres was in 1953, and the next will be in 2051; the last transit of Earth was in 1814; the next will be in 2081. The last transit of Mars was in 767; the next will be in 2684. [51]

    Rotation and axial tilt

    Permanently shadowed regions capable of accumulating surface ice

    The rotation period of Ceres (the Cererian day) is 9 hours and 4 minutes. It has an axial tilt of 4°. [9] This is small enough for Ceres's polar regions to contain permanently shadowed craters that are expected to act as cold traps and accumulate water ice over time, similar to the situation on the Moon and Mercury. About 0.14% of water molecules released from the surface are expected to end up in the traps, hopping an average of 3 times before escaping or being trapped. [9]

    Hubble Space Telescope observations indicated that the north pole of Ceres pointed in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco, resulting in an axial tilt of approximately 3°. [11] Dawn, the first spacecraft to orbit Ceres, determined that the north polar axis actually 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°. [52]

    Over the course of 3 million years, gravitational influence from Jupiter and Saturn has triggered cyclical shifts in Ceres's axial tilt, ranging from 2 to 20 degrees, meaning that seasonal effects have 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 their water over the age of the Solar System. [53]

    Geology

    Ceres is the largest asteroid in the main asteroid belt. [14] It has been classified both as a C-type asteroid [14] and, due to the presence of clay minerals, as a G-type asteroid. [54] Its composition is similar, though not identical, to those of carbonaceous chondrite meteorites. [55] Ceres has a mean diameter of 939.4 km (583.7 mi) [2] and a mass of 9.39×1020 kg as determined from the Dawn spacecraft. [56] This gives it a density of 2.162±0.008 g/cm3, [2] suggesting up to a quarter of its mass is composed of water. [57] Ceres is an oblate spheroid, with an equatorial diameter eight percent larger than its polar diameter. [2]

    Ceres comprises approximately a quarter of the estimated total 3.0 ± 0.2×1021 kg mass of the asteroid belt, [40] or 1.3% of the mass of the Moon. Ceres is close to being in hydrostatic equilibrium, and thus to being a dwarf planet, though some deviations from an equilibrium shape are not yet to be fully explained. [58] Ceres is the smallest known dwarf planet, and the only dwarf planet inside the trans-Neptunian region. [57] Ceres is intermediate in size between the smaller asteroid Vesta and the larger moon Tethys, and approximately the size of the large trans-Neptunian object Orcus. Its surface area is approximately the same as the land area of India or Argentina. [59] In July 2018, NASA released a comparison of physical features found on Ceres with similar ones present on Earth. [60]

    Ceres is the smallest object likely to be in hydrostatic equilibrium, being 600 km (370 mi) smaller and less than half the mass of Saturn's moon Rhea, the next-smallest likely object. [61] Modeling has suggested Ceres could have a small metallic core from partial differentiation of its rocky fraction, [62] [63] but the data are consistent with a mantle of hydrated silicates and no core. [58]

    Surface

    Hubble images taken over a span of 2 hours and 20 minutes in 2004, showing the "Piazzi" feature Ceres Rotation.jpg
    Hubble images taken over a span of 2 hours and 20 minutes in 2004, showing the "Piazzi" feature

    The surface of Ceres is "remarkably" homogeneous on a global scale, and is rich in carbonates and ammoniated phyllosilicates that have been altered by water, [58] though water ice in the regolith varies from approximately 10% in polar latitudes to much drier, even ice-free, in the equatorial regions. [58] Another large-scale variation is found in three large shallow basins (planitia) with degraded rims; these may be cryptic craters, and two of the three have higher than average ammonium concentrations. [58]

    The water ocean that is thought to have existed early in Ceres's history should have left an icy layer under the surface as it froze. The fact that Dawn found no evidence of such a layer suggests that Ceres's original crust was at least partially destroyed by later impacts, thoroughly mixing the ice with the salts and silicate-rich material of the ancient seafloor and the material beneath. [58]

    Studies by the Hubble Space Telescope reveal that graphite, sulfur, and sulfur dioxide are present on Ceres's surface. The former is evidently the result of space weathering on Ceres's older surfaces; the latter two are volatile under Cererian conditions and would be expected to either escape quickly or settle in cold traps, and are evidently associated with areas with relatively recent geological activity. [64]

    Craters

    Prior to the Dawn mission, only a few surface features had been unambiguously detected on Ceres. High-resolution ultraviolet Hubble images taken in 1995 showed a dark spot on its surface, which was nicknamed "Piazzi" in honor of the discoverer of Ceres. [54] 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 bright and dark features moving with Ceres' rotation. [65] Two dark features had circular shapes and were presumed to be craters; one of them was observed to have a bright central region, whereas another was identified as the "Piazzi" feature. [65] Visible-light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed 11 recognizable surface features, the natures of which were then undetermined. [11] [66] One of these features corresponds to the "Piazzi" feature observed earlier. [11] Dawn revealed that Ceres has a heavily cratered surface; nevertheless, Ceres possesses fewer large craters than expected, likely due to past geological processes. [67]

    Cryovolcanism

    PIA20355-Ceres-DwarfPlanet-OccatorCrater-Center-201602-crop.jpg
    A close up of Cerealia Facula
    PIA20348 crop - Ceres' Ahuna Mons top view.jpg
    Ahuna Mons is an estimated 5 km (3 mi) high on its steepest side. [68]

    Ceres has one prominent mountain, Ahuna Mons; this peak appears to be a cryovolcano and has few craters, suggesting a maximum age of no more than a few hundred million years. [69] Its relatively high gravitational field suggests it is dense, and thus composed more of rock than ice, and that its placement is likely due to diapirism of a slurry of brine and silicate particles from the top of the mantle. [70]

    A later computer simulation has suggested that there were originally as many as 22 cryovolcanoes on Ceres that are now unrecognisable due to viscous relaxation. [71] Models suggest that one cryovolcano should form on Ceres every 50 million years. [72]

    An unexpectedly large number of Cererian craters have central pits, perhaps due to cryovolcanic processes, whilst others have central peaks. [73] Hundreds of bright spots (faculae) have been observed by Dawn, the brightest spot ("Spot 5") located in the middle of an 80-kilometer (50 mi) crater called Occator. [74] From images taken of Ceres on 4 May 2015, the secondary bright spot was revealed to actually be a group of scattered bright areas, possibly as many as 10, with an albedo of approximately 40%. [75] The spot in the center of the crater is named Cerealia Facula , [76] and the group of spots to the east - Vinalia Faculae. [77] A haze periodically appears above Spot 5, the best known bright spot, supporting the hypothesis that some sort of outgassing or sublimating ice formed the bright spots. [78] In March 2016, Dawn found definitive evidence of water molecules on the surface of Ceres at Oxo crater. [79]

    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 hexahydrite (MgSO4·6H2O); the spots were also found to be associated with ammonia-rich clays. [80] Near-infrared spectra of these bright areas were reported in 2017 to be consistent with a large amount of sodium carbonate (Na
    2
    CO
    3
    ) and smaller amounts of ammonium chloride (NH
    4
    Cl
    ) or ammonium bicarbonate (NH
    4
    HCO
    3
    ). [81] [82] These materials have been suggested to originate from the crystallization of brines that reached the surface from below. [83] 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 [84] causing "bright spots", including those in Occator crater. [85]

    Carbon

    Tholins were detected on Ceres in Ernutet crater, [86] and most of the planet's surface is extremely rich in carbon, with approximately 20% carbon by mass in its near surface. [87] The carbon content is more than five times higher than in carbonaceous chondrite meteorites analyzed on Earth. [87] The surface carbon shows evidence of being mixed with products of rock-water interactions, such as clays. [87] This chemistry suggests Ceres formed in a cold environment, perhaps outside the orbit of Jupiter, and that it accreted from ultra-carbon-rich materials in the presence of water, which could provide conditions favorable to organic chemistry. [87]

    Boulders

    4423 boulders larger than 105 m (344 ft) have been observed on the surface of Ceres. These boulders are found within or near craters, though not all craters contain boulders. Vast regions of the surface of Ceres lack any large (>100 m (330 ft)) boulders. In addition, the large boulders on Ceres are more numerous at higher latitudes than at lower latitudes. These boulders are brittle and degrade rapidly due to thermal stress (at dawn and dusk, the surface temperature changes rapidly) and meteoritic impacts. Their maximum age is of 150 million years, which is much shorter than the lifetime of boulders on Vesta. [88]

    Internal structure

    Thick outer crust (ice, salts, hydrated minerals)
Salt-rich liquid (brine)
Mantle (hydrated rock) PIA22660-Ceres-DwarfPlanet-Inside-ArtistConcept-20180814.jpg
    • Thick outer crust (ice, salts, hydrated minerals)
    • Salt-rich liquid (brine)
    • Mantle (hydrated rock)

    The active geology of Ceres is driven by ice and brines, with an overall salinity of around 5%. Altogether, Ceres is approximately 40% or 50% water by volume, compared to 0.1% for Earth, and 73% rock by mass. [12]

    The fact that the surface has preserved craters smaller than 300 km (190 mi) in diameter indicate that the outermost layer of Ceres is on the order of 1000 times stronger than water ice. This is consistent with a mixture of silicates, hydrated salts and methane clathrates, with no more than approximately 30% water ice. [58]

    As of 2021, two competing models for Ceres's interior, a 2-layer and a 3-layer model, not counting a possible small metallic core, are proposed:

    Three-layer model

    In the three-layer model, Ceres is thought to consist of an inner muddy mantle of hydrated rock, such as clays, an intermediate layer of brine and rock (mud) down to a depth of at least 100 km (62 mi), and an outer, 40-kilometre (25 mi) thick crust of ice, salts and hydrated minerals. [89] It's unknown if it contains a rocky or metallic core, but the low central density suggests it may retain about 10% porosity. [12] One study estimated the densities of the core and mantle/crust to be 2.46–2.90 and 1.68–1.95 g/cm3, with the mantle and crust being 70–90 km (43–56 mi) thick. Only partial dehydration (expulsion of ice) from the core is expected, though the high density of the mantle relative to water ice reflects its enrichment in silicates and salts. [8] That is, the core, mantle and crust all consist of rock and ice, though in different ratios.

    The mineral composition can only be determined indirectly for the outer 100 km (62 mi). The 40-kilometre (25 mi) thick solid outer crust 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 100-kilometre (62 mi) limit of detection. Under that is thought to be a mantle dominated by hydrated rocks such as clays. It is not possible to tell if Ceres' deep interior contains liquid or a core of dense material rich in metal. [90]

    Two-layer model

    In one two-layer model, Ceres consists of a core of chondrules and a mantle of mixed ice and micron-sized solid particulates ("mud"). Sublimation of ice at the surface would leave a deposit of hydrated particulates perhaps 20 meters thick. The range of the extent of differentiation is consistent with the data, from a large, 360-kilometre (220 mi) core of 75% chondrules and 25% particulates and a mantle of 75% ice and 25% particulates, to a small, 85-kilometre (53 mi) core consisting nearly entirely of particulates and a mantle of 30% ice and 70% particulates. With a large core, the core–mantle boundary should be warm enough for pockets of brine. With a small core, the mantle should remain liquid below 110 km (68 mi). In the latter case, a 2% freezing of the liquid reservoir would compress the liquid enough to force some to the surface, producing cryovolcanism. [91]

    Another model notes that Dawn data is consistent with 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 70 km (43 mi) thick and have a density of 1.68 g/cm3; with CM-class meteorites (density 2.9 g/cm3), the crust would be approximately 190 km (120 mi) thick and have a density of 1.9 g/cm3. Best-fit from admittance modeling yields a crust approximately 40 km (25 mi) thick with a density of approximately 1.25 g/cm3, and a mantle/core density of approximately 2.4 g/cm3. [58]

    Atmosphere

    In early 2014, the Herschel Space Observatory detected localized (not more than 60 km (37 mi) in diameter) mid-latitude sources of water vapor on Ceres, which each give off approximately 1026 molecules (or 3 kg) of water per second. [92] [93] [lower-alpha 3] Two potential source regions, designated Piazzi (123°E, 21°N) and Region A (231°E, 23°N), have been visualized in the near infrared as dark areas (Region A also has a bright center) by the W. M. Keck Observatory. Possible mechanisms for the vapor release are sublimation from approximately 0.6 km2 (0.23 sq mi) of exposed surface ice, or cryovolcanic eruptions resulting from radiogenic internal heat [92] or from pressurization of a subsurface ocean due to growth of an overlying layer of ice. [96] In 2015, David Jewitt included Ceres in his list of active asteroids. [97] Surface water ice is unstable at distances less than 5 AU from the Sun, [98] 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 escapes in a short time. Surface sublimation would be expected to be lower when Ceres is farther from the Sun in its orbit, whereas internally powered emissions should not be affected by its orbital position. The limited data available was more consistent with cometary-style sublimation, [92] though subsequent evidence from Dawn strongly suggests ongoing geologic activity could be at least partially responsible. [99]

    Studies using Dawn's gamma ray and neutron detector (GRaND) reveal that Ceres is accelerating electrons from the solar wind regularly; the most accepted hypothesis is that these electrons are being accelerated by collisions between the solar wind and a tenuous water vapor exosphere. [100]

    In 2017, Dawn confirmed that Ceres has a transient atmosphere that appears to be linked to solar activity. Ice on Ceres can sublimate when energetic particles from the Sun hit exposed ice within craters. [101]

    Origin and evolution

    Ceres is a surviving protoplanet that formed 4.56 billion years ago, the only one surviving in the inner Solar System, with the rest either merging to form terrestrial planets or being ejected from the Solar System by Jupiter. [102] Despite this, its composition is not consistent with a formation in the asteroid belt. It seems rather that Ceres formed as a centaur, most likely between the orbits of Jupiter and Saturn, and was scattered into the asteroid belt as Jupiter migrated outward. [12] The discovery of ammonia salts in Occator crater supports an origin in the outer Solar System. [103] Other hypotheses support an origin in the asteroid belt; the presence of ammonia ices can be attributed to impacts by comets, and ammonia salts are more likely to be native to the surface. [104]

    The geological evolution of Ceres was dependent on the heat sources available during and after its formation: friction from planetesimal accretion, and decay of radionuclides (possibly including short-lived extinct radionuclides such as aluminium-26). These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation. [63] Ceres possesses a surprisingly small number of large craters, suggesting that viscous relaxation, water volcanism and tectonics may have erased older geological features. [105] Ceres's relatively warm surface temperature implies that any of the resulting ice on its surface would have gradually sublimated, leaving behind hydrated minerals like clay minerals and carbonates. [70]

    Today, Ceres has become considerably less geologically active, 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, in stark contrast to Vesta [106] and of previous expectations that Ceres would have become geologically dead early in its history due to its small size. [107] There are significant amounts of water ice in its crust. [86]

    Potential habitability

    Hydrogen concentration (blue) in the upper meter of the regolith indicating presence of water ice. PIA20353 Ceres Neutron Counts Reflect Hydrogen Abundance.jpg
    Hydrogen concentration (blue) in the upper meter of the regolith indicating presence of water ice.

    Although Ceres is not as actively discussed as a potential home for microbial extraterrestrial life as Mars, Europa, Enceladus, or Titan, it is the most water-rich body in the inner Solar System after Earth, [70] and there is evidence that its icy mantle was once a watery subterranean ocean. [87] Although it does not experience tidal heating, like Europa or Enceladus, it is close enough to the Sun, and contains enough long-lived radioactive isotopes, to preserve liquid water in its subsurface for extended periods. [70] The remote detection of organic compounds and the presence of water with 20% carbon by mass in its near surface could provide conditions favorable to organic chemistry. [87] Ceres is rich in carbon, hydrogen, oxygen and nitrogen, but the two other crucial biogenic elements, sulfur and phosphorus, have proven elusive. [70] [108] The relaxation of Ceres' topography across its surface is evidence for a liquid layer some 60 km (37 mi) below the surface, or at least pockets of brine, that may persist to the present. [70]

    Observation and exploration

    Observation

    When in opposition near its perihelion, Ceres can reach an apparent magnitude of +6.7. [109] This is too dim to be visible to the average naked eye, but under ideal viewing conditions, keen eyes with 20/20 vision may be able to see it. The only other asteroids that can reach a similarly bright magnitude are Vesta and, when in rare oppositions near their perihelions, Pallas and 7 Iris. [110] When in conjunction, Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars; [15] thus it can be seen with such binoculars in a naturally dark and clear night sky around new moon.

    Before Dawn

    On 13 November, 1984, an occultation of a star by Ceres observed in Mexico, Florida and across the Caribbean. [111] On 25 June 1985, Hubble attained ultraviolet images of Ceres with 50-kilometre (31 mi) resolution. [54] In 2002, the Keck telescope attained infrared images with 30-kilometre (19 mi) resolution using adaptive optics. [112] In 2003 and 2004, Hubble attained 30-kilometre (19 mi) resolution in visible light; the best prior to the Dawn mission. [11] On 22 December, 2012, Ceres occulted the star TYC 1865-00446-1 over parts of Japan, Russia, and China. [113] Ceres' brightness was magnitude 6.9 and the star, 12.2. [113] In 2014, the Herschel Space Observatory found Ceres to possess a tenuous atmosphere of water vapor. [114]

    Proposed exploration

    In 1981, a proposal for an asteroid mission was submitted to the European Space Agency (ESA). Named the Asteroidal Gravity Optical and Radar Analysis (AGORA), this spacecraft was to launch some time in 1990–1994 and perform two flybys of large asteroids. The preferred target for this mission was Vesta. AGORA would reach the asteroid belt either by a gravitational slingshot trajectory past Mars or by means of a small ion engine. That proposal was refused by ESA. A joint NASA–ESA asteroid mission was then drawn up for a Multiple Asteroid Orbiter with Solar Electric Propulsion (MAOSEP), with one of the mission profiles including an orbit of Vesta. NASA indicated they were not interested in an asteroid mission. Instead, ESA set up a technological study of a spacecraft with an ion drive. Other missions to the asteroid belt were proposed in the 1980s by France, Germany, Italy, and the United States, but none were approved. [115]

    Dawn mission

    Artist's conception of Dawn spacecraft, travelling from Vesta to Ceres Dawn Flight Configuration 2.jpg
    Artist's conception of Dawn spacecraft, travelling from Vesta to Ceres

    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 recommended as a high priority a mission to explore the asteroid belt using a spacecraft with an ion engine. Funding for this program remained problematic for nearly a decade, but by 2004 the Dawn vehicle had passed its critical design review. [116]

    It was launched on 27 September 2007, as the space mission to make the first visits to both Vesta and Ceres. On 3 May 2011, Dawn acquired its first targeting image 1.2 million kilometers from Vesta. [117] After orbiting Vesta for 13 months, Dawn used its ion engine to depart for Ceres, with gravitational capture occurring on 6 March 2015 [118] at a separation of 61,000 km (38,000 mi), [119] four months prior to the New Horizons flyby of Pluto.

    Dawn's 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 13,500 km (8,400 mi) on 23 April 2015, staying for only approximately one orbit (15 days). [120] [121] The spacecraft subsequently reduced its orbital distance to 4,400 km (2,700 mi) for its second observational orbit ("survey") for three weeks, [122] then down to 1,470 km (910 mi) ("HAMO;" high altitude mapping orbit) for two months [123] and then down to its final orbit at 375 km (233 mi) ("LAMO;" low altitude mapping orbit) for at least three months. [124]

    The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These instruments examined Ceres' shape and elemental composition. [125] On 13 January 2015, Dawn took the first images of Ceres at near-Hubble resolution, revealing impact craters and a small high-albedo spot on the surface, near the same location as that observed previously. Additional imaging sessions, at increasingly better resolution took place on 25 January 4, 12, 19 and 25 February 1 March, and 10 and 15 April. [126] In October 2015, NASA released a true-color portrait of Ceres made by Dawn. [127] In February 2017, tholins were detected on Ceres in Ernutet crater. [86]

    Animation of Dawn's trajectory around Ceres from 1 February 2015 to 6 October 2018

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Dawn *
Ceres Animation of Dawn trajectory around Ceres.gif
    Animation of Dawn 's trajectory around Ceres from 1 February 2015 to 6 October 2018
       Dawn  ·  Ceres

    Pictures with a resolution previously unattained were taken during imaging sessions starting in January 2015 as Dawn approached Ceres, showing a cratered surface. Two distinct bright spots (or high-albedo features) inside a crater (different from the bright spots observed in earlier Hubble images) [128] were seen in a 19 February 2015 image, leading to speculation about a possible cryovolcanic origin [129] or outgassing. [130] On 2 September 2016, scientists from the Dawn team argued in a Science paper that a massive cryovolcano called Ahuna Mons is the strongest evidence yet for the existence of these mysterious formations. [131] On 11 May 2015, NASA released a higher-resolution image showing that the spots were actually composed of multiple smaller spots. [132] 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 hexahydrite (MgSO4·6H2O); the spots were also found to be associated with ammonia-rich clays. [80] In June 2016, near-infrared spectra of these bright areas were found to be consistent with a large amount of sodium carbonate (Na
    2
    CO
    3
    ), implying that recent geologic activity was probably involved in the creation of the bright spots. [133] In July 2018, NASA released a comparison of physical features found on Ceres with similar ones present on Earth. [60] From June to October 2018, Dawn orbited Ceres from as close as 35 km (22 mi) and as far away as 4,000 km (2,500 mi). [134] The Dawn mission ended on 1 November 2018 after the spacecraft ran out of fuel. [135]

    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. [136] The Chinese Space Agency is designing a sample-return mission from Ceres that would take place during the 2020s. [137]

    Maps

    Dawn's operational lifetime around Ceres lasted 3 years, allowing for its entire surface to be mapped.

    PIA20351-Ceres-DwarfPlanet-EllipticalMap-HAMO-20160322.jpg

    Map of Ceres (Elliptical; HAMO; color; March 2016)

    PIA21755-CeresMap-CraterNames-20170901.jpg

    Black-and-white photographic map of Ceres, centered on 180° longitude, with official nomenclature (September 2017)

    PIA20918-Ceres-Dawn-GlobalMap-Annotated-20160926.jpg

    Topographic map of Ceres (September 2016).
    15 km (10 mi) of elevation separate the lowest crater floors (indigo) from the highest peaks (white). [138]

    PIA20126-Ceres-PolarRegions-Dawn-20151023.jpg

    Ceres, polar regions (November 2015): North (left); south (right).

    See also

    Notes

    1. The value given for Ceres is the mean moment of inertia, which is thought to better represent its interior structure than the polar moment of inertia, due to its high polar flattening. [8]
    2. In 1807 Klaproth tried to change the name to "cererium", to avoid confusion with the root cēra 'wax' (as in cereous 'waxy'), but it did not catch on. [26]
    3. This emission rate is modest compared to those calculated for the tidally driven plumes of Enceladus (a smaller body) and Europa (a larger body), 200 kg/s [94] and 7000 kg/s, [95] respectively.

    Related Research Articles

    Asteroid Minor planet that is not a comet

    An asteroid is a minor planet of the inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt. The term "asteroid" refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter. Larger asteroids are often called planetoids.

    4 Vesta Second largest asteroid of the main asteroid belt

    Vesta is one of the largest objects in the asteroid belt, with a mean diameter of 525 kilometres (326 mi). It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the virgin goddess of home and hearth from Roman mythology.

    2 Pallas Large asteroid of the main asteroid belt

    Pallas is the second asteroid to have been discovered, after 1 Ceres. Like Ceres, it is believed to have a mineral composition similar to carbonaceous chondrite meteorites, though significantly less hydrated than Ceres. It is the third-largest asteroid in the Solar System by both volume and mass, and is a likely remnant protoplanet. It is 79% the mass of 4 Vesta and 22% the mass of Ceres, constituting an estimated 7% of the mass of the asteroid belt. Its estimated volume is equivalent to a sphere 505 to 520 kilometers in diameter, 90–96% the volume of Vesta.

    Asteroid belt Circumstellar disk (accumulation of matter) in an orbit between those of Mars and Jupiter

    The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes but much smaller than planets, called asteroids or minor planets. This asteroid belt is also called the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids.

    <i>Dawn</i> (spacecraft) Mission to main-belt asteroids

    Dawn is a retired space probe that was launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt: Vesta and Ceres. In the fulfillment of that mission—the ninth in NASA's Discovery Program—Dawn entered orbit around Vesta on July 16, 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012. It entered orbit around Ceres on March 6, 2015. In 2017, NASA announced that the planned nine-year mission would be extended until the probe's hydrazine fuel supply was depleted. On November 1, 2018, NASA announced that Dawn had depleted its hydrazine, and the mission was ended. The spacecraft is currently in a derelict, but stable, orbit around Ceres.

    10 Hygiea Major asteroid

    Hygiea is a major asteroid located in the main asteroid belt. With a diameter of 434 kilometres (270 mi) and a mass estimated to be 3% of the total mass of the belt, it is the fourth-largest asteroid in the Solar System by both volume and mass. In some spectral classifications it is the largest of the dark C-type asteroids with a carbonaceous surface, in others it is second after 1 Ceres.

    Facula

    A facula, Latin for "little torch", is literally a "bright spot". The term has several common technical uses. It is used in planetary nomenclature for naming certain surface features of planets and moons, and is also a type of surface phenomenon on the Sun. In addition, a bright region in the projected field of a light source is sometimes referred to as a facula, and photographers often use the term to describe bright, typically circular features in photographs that correspond to light sources or bright reflections in a defocused image.

    1000 Piazzia

    1000 Piazzia, provisional designation 1923 NZ, is a carbonaceous background asteroid from the outer region of the asteroid belt, approximately 48 kilometers in diameter. It was discovered on 12 August 1923, by German astronomer Karl Reinmuth at Heidelberg Observatory in southern Germany. The C-type asteroid has a rotation period of 9.5 hours. It was named after Italian Giuseppe Piazzi, who discovered 1 Ceres.

    Dwarf planet Planetary-mass object

    A dwarf planet is a planetary-mass object that does not dominate its region of space and is not a satellite. That is, it is in direct orbit of the Sun and is massive enough to be plastic – for its gravity to maintain it in a hydrostatically equilibrious shape – but has not cleared the neighborhood of its orbit of similar objects. The prototype dwarf planet is Pluto. The interest of dwarf planets to planetary geologists is that, being possibly differentiated and geologically active bodies, they may display planetary geology, an expectation borne out by the 2015 New Horizons mission to Pluto.

    Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and thus surmised as essential for extraterrestrial life.

    Geology of solar terrestrial planets Geology of Mercury, Venus, Earth, Mars and Ceres

    The geology of solar terrestrial planets mainly deals with the geological aspects of the four terrestrial planets of the Solar System – Mercury, Venus, Earth, and Mars – and one terrestrial dwarf planet: Ceres. Earth is the only terrestrial planet known to have an active hydrosphere.

    10001 Palermo, provisional designation 1969 TM1, is a Vestian asteroid and a slow rotator from the inner regions of the asteroid belt, approximately 4 kilometers (2.5 miles) in diameter. It was discovered on 8 October 1969, by Soviet–Russian astronomer Lyudmila Chernykh using a 0.4-meter double astrograph at the Crimean Astrophysical Observatory in Nauchnij on the Crimean peninsula. The asteroid is likely elongated in shape and has a long rotation period of 213 hours. It was named for the Italian city of Palermo to commemorate the discovery of Ceres two hundred years earlier.

    Rheasilvia Impact crater on the surface of the asteroid 4 Vesta

    Rheasilvia is the most prominent surface feature on the asteroid Vesta and is thought to be an impact crater. It is 505 km (314 mi) in diameter, which is 90% the diameter of Vesta itself, and is 95% the mean diameter of Vesta, 529 km (329 mi). However, the mean is affected by the crater itself. It is 89% the mean equatorial diameter of 569 km (354 mi), making it one of the largest craters in the Solar System, and at 75°S latitude, covers most of the southern hemisphere. The peak in the center of the crater rises 22.5 km (14.0 mi) from its base, or 73,819 ft (22,500 m) making it the tallest mountain known in the Solar System.

    Planetary surface Where the solid (or liquid) material of the outer crust on certain types of astronomical objects contacts the atmosphere or outer space

    A planetary surface is where the solid material of the outer crust on certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets, dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus of a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.

    Permanently shadowed crater Permanently shadowed region of a body in the Solar System

    A permanently shadowed crater is a depression on a body in the Solar System within which lies a point that is always in darkness.

    Bright spots on Ceres

    Several bright surface features were discovered on the dwarf planet Ceres by the Dawn spacecraft in 2015.

    Ahuna Mons

    Ahuna Mons is the largest mountain on the dwarf planet and asteroid Ceres. It protrudes above the cratered terrain, is not an impact feature, and is the only mountain of its kind on Ceres. Bright streaks run top to bottom on its slopes; these streaks are thought to be salt, similar to the better known Cererian bright spots, and likely resulted from cryovolcanic activity from Ceres's interior. It is named after the traditional post-harvest festival Ahuna of the Sumi Naga people of India. In July 2018, NASA released a comparison of physical features, including Ahuna Mons, found on Ceres with similar ones present on Earth.

    Occator (crater)

    Occator is an impact crater located on Ceres, the largest object in the main asteroid belt that lies between the orbits of Mars and Jupiter, that contains "Spot 5", the brightest of the bright spots observed by the Dawn spacecraft. It was known as "Region A" in ground-based images taken by the W. M. Keck Observatory on Mauna Kea.

    Geology of Ceres

    The geology of Ceres consists of the characteristics of the surface, the crust and the interior of the dwarf planet Ceres. The surface of Ceres is comparable to the surfaces of Saturn's moon Rhea and Tethys, and Uranus's moon Umbriel and Oberon.

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