Ceres (dwarf planet)

Ceres /ˈsɪəriːz/[16] (minor-planet designation: 1 Ceres) is the smallest dwarf planet, the closest dwarf planet to the Sun, and the largest object in the main asteroid belt that lies between the orbits of Mars and Jupiter. With a diameter of 940 km (580 mi), Ceres is both the largest of the asteroids and the only dwarf planet inside Neptune's orbit. It’s the 25th-largest body in the Solar System within the orbit of Neptune.[17]

For the Roman goddess, see Ceres (mythology). For other uses, see Ceres (disambiguation).

Ceres
Ceres in true color in 2015[lower-alpha 1]
Discovery[1]
Discovered byGiuseppe Piazzi
Discovery date1 January 1801
Designations
(1) Ceres
Pronunciation/ˈsɪəriːz/
Named after
Cerēs
  • A899 OF
  • 1943 XB
AdjectivesCererean, -ian /sɪˈrɪəriən/
Orbital characteristics[2]
Epoch 27 April 2019 (JD 2458600.5)
Aphelion2.9796467093 AU
(445,749,000 km)
Perihelion2.5586835997 AU
(382,774,000 km)
2.7691651545 AU
(414,261,000 km)
Eccentricity0.07600902910
4.61 yr
1683.14570801 d
466.6 d
1.278 yr
17.905 km/s
77.37209589°
Inclination10.59406704° to ecliptic
9.20° to invariable plane[3]
80.3055316°
73.5976941°
Proper orbital elements[4]
2.7670962 AU
0.1161977
9.6474122°
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]
Volume434,000,000 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±15[7][lower-alpha 2] (estimate)
Equatorial escape velocity
0.51 km/s[6]
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. min mean max
Kelvin 110 155[12]
C[13]
  • 6.64–9.34 (range)[14]
  • 9.27 (current)[15]
3.34[2]
0.854″ to 0.339″

    Ceres was the first asteroid to be discovered by Giuseppe Piazzi at Palermo Astronomical Observatory on 1 January 1801.[18] It was originally considered a planet, but was reclassified as an asteroid in the 1850s after many other objects in similar orbits were discovered.

    Ceres is the only object in the asteroid belt rounded by its own gravity,[19] although Vesta and perhaps other asteroids were so in the past. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, peaking once at opposition every 15 to 16 months, which is its synodic period.[14] Thus even at its brightest, it is too dim to be seen by the naked eye, except under extremely dark skies. Ceres has been classified as a C-type asteroid[13] and, due to the presence of clay minerals, as a G-type asteroid.[20]

    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.[12] 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.[21] The surface is a mixture of water ice and various hydrated minerals such as carbonates and clay. Cryovolcanoes such as Ahuna Mons form at the rate of about one every fifty million years. In January 2014, emissions of water vapor were detected from several regions of Ceres.[22] This was unexpected because large bodies in the asteroid belt typically do not emit vapor, a hallmark of comets. The atmosphere, however, is transient and of the minimal kind known as an exosphere.[21]

    The robotic NASA spacecraft Dawn entered orbit around Ceres on 6 March 2015.[23][24][25]

    History

    Discovery
    Piazzi's book Della scoperta del nuovo pianeta Cerere Ferdinandea, outlining the discovery of Ceres, dedicated the new planet to Ferdinand I of the Two Sicilies.

    Johann Elert Bode, in 1772, first suggested that an undiscovered planet could exist between the orbits of Mars and Jupiter.[26] Kepler had already noticed the gap between Mars and Jupiter in 1596.[26] 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.[26][27] The pattern predicted that the missing planet ought to have an orbit with a radius near 2.8 astronomical units (AU).[27] William Herschel's discovery of Uranus in 1781[26] 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 twenty-four experienced astronomers (whom he dubbed the "celestial police"), asking that they combine their efforts and begin a methodical search for the expected planet.[26][27] Although they did not discover Ceres, they later found several large asteroids.[27]

    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.[28][29] 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".[26] Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet.[30] 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 Johann Elert Bode of Berlin.[31] 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".[26] In April, Piazzi sent his complete observations to Oriani, Bode, and Jérôme Lalande in Paris. The information was published in the September 1801 issue of the Monatliche Correspondenz.[30]

    By this time, the apparent position of Ceres had changed (mostly due to Earth's orbital motion), 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, Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination.[30] In a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it.[30]

    The early observers were only able to calculate the size of Ceres to within an order of magnitude. Herschel underestimated its diameter as 260 km in 1802, whereas in 1811 Johann Hieronymus Schröter overestimated it as 2,613 km.[32][33]

    Name

    Piazzi originally suggested the name Cerere Ferdinandea for his discovery, after the goddess Ceres (Roman goddess of agriculture, Cerere in Italian, who was believed to have originated in Sicily and whose oldest temple was there) and King Ferdinand of Sicily.[26][30] "Ferdinandea", however, was not acceptable to other nations and was dropped. Ceres was called Hera for a short time in Germany.[34] In Modern Greek, it is called Dímitra (Δήμητρα), after Demeter, the Greek equivalent of the Roman Cerēs. (To distinguish the asteroid 1108 Demeter in Greek, the classical form of the name, Dimítir (Δημήτηρ) is used.) All other languages but Chinese use a variant of Ceres/Cerere: e.g. Russian Церера Tseréra, Arabic سيريس Sīrīs, Japanese ケレス Keresu. Even Chinese uses the Latin name for the goddess, as 刻瑞斯 kèruìsī, but it calques the asteroid as 'grain-god(dess) star' (穀神星 gǔshénxīng).

    The regular adjectival forms of the name are Cererian[35][36] /sɪˈrɪəriən/[37] and Cererean[38] (with the same pronunciation),[39] both derived from the Latin oblique stem Cĕrĕr-.[40] The irregular form Ceresian /sɪˈriːziən/ is occasionally seen for the goddess (as in the sickle-shaped Ceresian Lake), as are, by analogy with cereal, the forms Cerean /ˈsɪəriən/[41] and Cerealian /sɛriˈeɪliən/.[42]

    The old astronomical symbol of Ceres is a sickle, ⟨⚳⟩,[43] 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, ⟨①⟩.[30][44]

    Cerium, a rare-earth element discovered in 1803, was named after Ceres.[45][lower-alpha 3] In the same year, another element was also initially named after Ceres, but, when cerium was named, its discoverer changed the latter to palladium, after the second asteroid, 2 Pallas.[47]

    Classification

    The categorization of Ceres has changed more than once and has been the subject of some disagreement. Johann Elert 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.[26] Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno, and 4 Vesta) for half a century.[26][30][48]

    Relative sizes of the four largest asteroids. Ceres is furthest left.

    As other objects were discovered in the neighborhood of Ceres, it was realized that Ceres represented the first of a new class of objects.[26] In 1802, with the discovery of 2 Pallas, William Herschel coined the term asteroid ("star-like") for these bodies,[48] writing that "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes".[49] As the first such body to be discovered, Ceres was given the designation 1 Ceres under the modern system of minor-planet designations. 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 never formulated.[48]

    Ceres (bottom left), the Moon and Earth, shown to scale
    Size comparison of Vesta, Ceres and Eros

    The 2006 debate surrounding Pluto and what constitutes a planet led to Ceres being considered for reclassification as a planet.[50][51] A proposal before the International Astronomical Union 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".[52] Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun.[53] This never happened, however, and on 24 August 2006 a modified definition was adopted, carrying 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.[54] Bodies that met the first proposed definition but not the second, such as Ceres, were instead classified as dwarf planets.

    Ceres is the largest asteroid in the Main Belt.[13] It has sometimes been assumed that Ceres was reclassified as a dwarf planet, and that it is therefore no longer considered an asteroid. For example, a news update at Space.com spoke of "Pallas, the largest asteroid, and Ceres, the dwarf planet formerly classified as an asteroid",[55] whereas an IAU question-and-answer posting states, "Ceres is (or now we can say it was) the largest asteroid", though it then speaks of "other asteroids" crossing Ceres' path and otherwise implies that Ceres is still considered an asteroid.[56] The Gazetteer of Planetary Nomenclature at the IAU lists Ceres under 'Asteroids'.[57] The Minor Planet Center notes that such bodies may have dual designations.[58] The 2006 IAU decision that classified Ceres as a dwarf planet also implied that it is simultaneously an asteroid. It introduces the category of small Solar System body, as objects that are neither planets nor dwarf planets, and states that they 'currently include most of the Solar System asteroids'. The only object among the asteroids that would prevent all asteroids from being SSSBs is Ceres. Lang (2011) comments "the [IAU has] added a new designation to Ceres, classifying it as a dwarf planet. ... By [its] definition, Eris, Haumea, Makemake and Pluto, as well as the largest asteroid, 1 Ceres, are all dwarf planets", and describes it elsewhere as "the dwarf planet–asteroid 1 Ceres".[59] NASA refers to Ceres as a dwarf planet,[60] as do various academic textbooks.[61][62] However, NASA has at least once referred to Vesta as the largest asteroid.[63] Ceres has had the dwarf planet classification since 2006. [64] [65]

    Orbit
    Proper (long-term mean) orbital elements compared to osculating (instant) orbital elements for Ceres:
    Element
    type    		a
    (in AU)    		eiPeriod
    (in days)
    Proper[4]2.76710.1161989.6474351,681.60
    Osculating[2]
    (Epoch 23 July 2010 )    		2.76530.07913810.5868211,679.66
    Difference0.00180.037060.9393861.94
    Orbit of Ceres
    Animation of Dawn's trajectory from 27 September 2007 to 5 October 2018
       Dawn  ·   Earth ·   Mars ·   4 Vesta  ·   1 Ceres

    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] The orbit is moderately inclined (i = 10.6° compared to 7° for Mercury and 17° for Pluto) and moderately eccentric (e = 0.08 compared to 0.09 for Mars).[2]

    The diagram illustrates the orbits of Ceres (blue) and several planets (white and gray). The segments of orbits below the ecliptic are plotted in darker colors, 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. In this diagram (but not in general), 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.

    Ceres was once thought to be a member of an asteroid family.[66] 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.[66] Ceres appears to be merely an interloper in the Gefion family, coincidentally having similar orbital elements but not a common origin.[67]

    Resonances

    Ceres is in a near-1:1 mean-motion orbital resonance with Pallas (their proper orbital periods differ by 0.2%).[68] 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.[69] 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; fifty such objects have been identified.[70]

    Transits of planets from Ceres

    Mercury, Venus, Earth, and Mars can all appear to cross the Sun, or transit it, from a vantage point on Ceres. The most common transits are those of Mercury, which usually happen every few years, most recently in 2006 and 2010. The most recent transit of Venus was in 1953, and the next will be in 2051; the corresponding dates are 1814 and 2081 for transits of Earth, and 767 and 2684 for transits of Mars.[71]

    Rotation and axial tilt

    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]

    Geology
    Main article: Geology of Ceres

    Ceres has a mass of 9.39×1020 kg as determined from the Dawn spacecraft.[72] With this mass Ceres composes approximately a third of the estimated total 3.0 ± 0.2×1021 kg mass of the asteroid belt,[73] or 1.3% of the mass of the Moon. Ceres is close to being in hydrostatic equilibrium, and thus to being a dwarf planet. However, there are some deviations from an equilibrium shape that have yet to be fully explained.[19] Among Solar System bodies, 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.[74] In July 2018, NASA released a comparison of physical features found on Ceres with similar ones present on Earth.[75]

    Ceres is the smallest object likely to be in hydrostatic equilibrium, being 600 km smaller and less than half the mass of Saturn's moon Rhea, the next smallest likely (but unproven) object.[76] Modeling has suggested Ceres could have a small metallic core from partial differentiation of its rocky fraction,[77][78] but the data are consistent with a mantle of hydrated silicates and no core.[19]

    Surface
    Main article: List of geological features on Ceres
    Notable geological features on Ceres

    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.[19] However, water ice in the regolith varies from approximately 10% in polar latitudes to much drier, even ice-free, in the equatorial regions.[12][19] 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.[19]

    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.[19]

    Hydrogen concentration in the upper meter of the regolith. Blue denotes a higher concentration and indicates the presence of water ice.

    Ceres' surface is warm enough that surface ice sublimates in the near vacuum. Material left behind by sublimation could explain the dark surface of Ceres compared to the icy moons of the outer Solar System.

    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 recent geological activity.[79]

    Observations prior to Dawn
    HST images taken over a span of 2 hours and 20 minutes in 2004

    Prior to the Dawn mission, only a few surface features had been unambiguously detected on Ceres. High-resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface, which was nicknamed "Piazzi" in honor of the discoverer of Ceres.[20] 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 Ceres' rotation.[80][81] 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.[80][81] Visible-light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed eleven recognizable surface features, the natures of which were then undetermined.[11][82] One of these features corresponds to the "Piazzi" feature observed earlier.[11]

    These last 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 later 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°.,[83]

    Observations by Dawn

    Dawn revealed that Ceres has a heavily cratered surface; nevertheless, Ceres does not have as many large craters as expected, likely due to past geological processes.[84][85] An unexpectedly large number of Cererian craters have central pits, perhaps due to cryovolcanic processes, and many have central peaks.[86] 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.[87][88] A later computer simulation has suggested that there were originally other cryovolcanoes on Ceres that are now unrecognisable due to viscous relaxation.[89] Several bright spots have been observed by Dawn, the brightest spot ("Spot 5") located in the middle of an 80-kilometer (50 mi) crater called Occator.[90] 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 ten. These bright features have an albedo of approximately 40%[91] that are caused by a substance on the surface, possibly ice or salts, reflecting sunlight.[92][93] 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.[93][94] In March 2016, Dawn found definitive evidence of water molecules on the surface of Ceres at Oxo crater.[95][96]

    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.[97] 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    ).[98][99] These materials have been suggested to originate from the recent crystallization of brines that reached the surface from below.[100][101][102][103] In August 2020, NASA confirmed that Ceres was a water-rich body with a deep reservoir of brine that percolated to the surface in various locations causing "bright spots", including those in Occator crater.[104][105]

    Carbon

    Organic compounds (tholins) were detected on Ceres in Ernutet crater,[106][107] and most of the planet's surface is extremely rich in carbon,[108] with approximately 20% carbon by mass in its near surface.[109][110] The carbon content is more than five times higher than in carbonaceous chondrite meteorites analyzed on Earth.[110] The surface carbon shows evidence of being mixed with products of rock-water interactions, such as clays.[109][110] 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.[109][110] Its presence on Ceres is evidence that the basic ingredients for life can be found throughout the universe.[108]

    Ceres – Maps of bright areas
     
    December 2017
     
    December 2015. The map shows places with ice water
    Bright spots on Ceres in visible and infrared:
    "Spot 1" (top row) ("cooler" than surroundings);
    "Spot 5" (bottom) ("similar in temperature" as surroundings) (April 2015)
    "Bright Spot 5" in the crater Occator. Imaged by Dawn from 385 km (239 mi) (LAMO)
    Ahuna Mons is an estimated 5 km (3 mi) high on its steepest side.[111] Imaged by Dawn from 385 km (239 mi) in December 2015.

    Internal structure
    Internal structure of Ceres, as informed by the Dawn spacecarft (August 2018). The layers (from the surface inwards) are: a thick outer crust composed of a mixture of ice, salts, and hydrated minerals; an intermediate layer containing some salt rich liquid, or brine; and, at the centre, the mantle, which is dominated by hydrated rocks.
    Map of Cererian gravity fields: red is high; blue, low.

    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 weight.[12]

    The fact that the surface has preserved craters smaller than 300 km 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.[19]

    The thickness and density of the crust is not well constrained. There are competing 2-layer and 3-layer models of the Cererian interior, not counting a possible small metallic core.

    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, and an outer, 40-km thick crust of ice, salts and hydrated minerals.[112] 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 70–190 km thick. Only partial dehydration (expulsion of ice) from the core is expected, while 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. The 40-km 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-km 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.[113]

    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. There are range to the extent of differentiation that is consistent with the data, from a large, 360-km core of 75% chondrules and 25% particulates and a mantle of 75% ice and 25% particulates, to a small, 85-km 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. In the latter case, a 2% freezing of the liquid reservoir would compress the liquid enough to force some to the surface, producing cryovolcanism.[114]

    The model may be compared to estimates that Ceres has averaged one cryovolcano every 50 million years.[115]

    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 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 thick and have a density of 1.9 g/cm3. Best-fit from admittance modeling yields a crust approximately 40 km thick with a density of approximately 1.25 g/cm3, and a mantle/core density of approximately 2.4 g/cm3.[19]

    Atmosphere

    There are indications that Ceres has a tenuous water vapor atmosphere outgassing from water ice on the surface, making it an active asteroid.[116][117][118][119]

    Surface water ice is unstable at distances less than 5 AU from the Sun,[120] 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 very short time.

    In early 2014, using data from the Herschel Space Observatory, it was discovered that there are several localized (not more than 60 km in diameter) mid-latitude sources of water vapor on Ceres, which each give off approximately 1026 molecules (or 3 kg) of water per second.[121][122][lower-alpha 4] 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 of exposed surface ice, or cryovolcanic eruptions resulting from radiogenic internal heat[121] or from pressurization of a subsurface ocean due to growth of an overlying layer of ice.[125] 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;[121] however, subsequent evidence from Dawn strongly suggests ongoing geologic activity could be at least partially responsible.[126][127]

    Studies using Dawn's gamma ray and neutron detector (GRaND) reveal that Ceres is accelerating electrons from the solar wind regularly; although there are several possibilities as to what is causing this, the most accepted is that these electrons are being accelerated by collisions between the solar wind and a tenuous water vapor exosphere.[128]

    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.[129]

    Origin and evolution

    Ceres is a surviving protoplanet (planetary embryo) 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.[130] However, 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.[131] However, the presence of ammonia ices can be attributed to impacts by comets, and ammonia salts are more likely to be native to the surface.[132]

    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 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.[78] This process may have caused resurfacing by water volcanism and tectonics, erasing older geological features. Ceres's relatively warm surface temperature implies that any of the resulting ice on its surface would have gradually sublimated, leaving behind various hydrated minerals like clay minerals and carbonates.

    Today, Ceres has become considerably less geologically active, with a surface sculpted chiefly by impacts; nevertheless, evidence from Dawn reveals that internal processes have continued to sculpt Ceres's surface to a significant extent, in stark contrast to Vesta[133] and of previous expectations that Ceres would have become geologically dead early in its history due to its small size.[134] There are significant amounts of water ice in its crust.[107]

    Potential habitability

    Although Ceres is not as actively discussed as a potential home for microbial extraterrestrial life as Mars, Europa, Enceladus, or Titan, there is evidence that its icy mantle was once a watery subterranean ocean. 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.[109][110]

    Observation and exploration

    Observation
    Polarimetric map of Ceres[135]

    When in opposition near its perihelion, Ceres can reach an apparent magnitude of +6.7.[136] This is generally regarded as too dim to be visible to the 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 4 Vesta and, when in rare oppositions near their perihelions, 2 Pallas and 7 Iris.[137] 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.

    Some notable observations and milestones for Ceres include the following:

    Proposed exploration
    First asteroid image (Ceres and Vesta) from Mars – viewed by Curiosity (20 April 2014)

    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. However, the 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.[142] Exploration of Ceres by fly-by and impacting penetrator was the second main target of the second plan of the multiaimed Soviet Vesta mission, developed in cooperation with European countries for realisation in 1991–1994 but canceled due to the Soviet Union disbanding.

    The Chinese Space Agency is designing a sample-return mission from Ceres that would take place during the 2020s.[143]

    The Calathus Mission is a concept to Occator Crater at Ceres, to return a sample of the bright carbonate faculae and dark organics to Earth.[144][145]

    Dawn mission
    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 several years, but by 2004 the Dawn vehicle had passed its critical design review.[146]

    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.[147] After orbiting Vesta for 13 months, Dawn used its ion engine to depart for Ceres, with gravitational capture occurring on 6 March 2015[148] at a separation of 61,000 km,[149] 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 on 23 April 2015, staying for only approximately one orbit (fifteen days).[25][150] The spacecraft subsequently reduced its orbital distance to 4,400 km for its second observational orbit ("survey") for three weeks,[151] then down to 1,470 km ("HAMO;" high altitude mapping orbit) for two months[152] and then down to its final orbit at 375 km ("LAMO;" low altitude mapping orbit) for at least three months.[153]

    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.[154] 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.[155]

    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[156]) were seen in a 19 February 2015 image, leading to speculation about a possible cryovolcanic origin[157][158][159] or outgassing.[160] On 3 March 2015, a NASA spokesperson said the spots are consistent with highly reflective materials containing ice or salts, but that cryovolcanism is unlikely.[161] However, on 2 September 2016, scientists from the Dawn team claimed in a Science paper that a massive cryovolcano called Ahuna Mons is the strongest evidence yet for the existence of these mysterious formations.[162][163] On 11 May 2015, NASA released a higher-resolution image showing that, instead of one or two spots, there are actually several.[164] 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.[97] 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.[100][101][103] In July 2018, NASA released a comparison of physical features found on Ceres with similar ones present on Earth.[75] 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).[165][166] The Dawn mission ended on 1 November 2018 after the spacecraft ran out of fuel.

    In October 2015, NASA released a true-color portrait of Ceres made by Dawn.[167] In February 2017, organics (tholins) were detected on Ceres in Ernutet crater (see image).[106][107]

    Dawn's arrival in a stable orbit around Ceres was delayed after, close to reaching Ceres, it was hit by a cosmic ray, making it take another, longer route around Ceres in back, instead of a direct spiral towards it.[168]

    Maps

    Map of Ceres (red=IR-bright;green=high albedo areas;blue=UV-bright) (September 2015)

    Map of Ceres (centered on 180° longitude; color; March 2015)

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

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

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

    Topographic map of Ceres (September 2016).
    15 km (10 mi) of elevation separate the lowest crater floors (indigo) from the highest peaks (white).[169]

    Hemispheric topographic maps of Ceres, centered on 60° and 240° east longitude (July 2015).

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

    Ceres – Survey Maps (June 2015)
    Overall
    Kerwan section
    (PDF version)
    Asari-Zadeni section
    (PDF version)
    Occator section
    (PDF version)

    Map of quadrangles

    The following imagemap of Ceres is divided into 15 quadrangles. They are named after the first craters whose names the IAU approved in July 2015.[170] The map image(s) were taken by the Dawn space probe.

    90°N 180°E
    North Polar Area
    Asari
    Ac-H-1
    Coniraya
    Ac-H-2
    Dantu
    Ac-H-3
    Ezinu
    Ac-H-4
    Fejokoo
    Ac-H-5
    Haulani
    Ac-H-6
    Kerwan
    Ac-H-7
    Nawish
    Ac-H-8
    Occator
    Ac-H-9
    Rongo
    Ac-H-10
    Sintana
    Ac-H-11
    Toharu
    Ac-H-12
    Urvara
    Ac-H-13
    Yalode
    Ac-H-14
    Zadeni
    Ac-H-15
    South Polar Area
    Topographic map of Ceres as of February 2015. Darker areas represent lower elevations, and brighter areas represent higher elevations.
    []
    Ceres − high-resolution view (20 September 2017)
    Ceres in half shadow from 40,000 km, or 24850 miles (25 February 2015)
    Dawn Ceres mosaic – 19 February 2015
    Ceres from Dawn, 47,000 kilometers (29,000 mi) away. At this distance, Ceres is approximately the apparent size of the full moon (19 February 2015). The large impact basin in the lower portion of the left image appears relatively young.[171]
    Ceres at 84,000 kilometers (52,000 mi) away (12 February 2015), at half the apparent size of the full moon. Relative to these images, those at left were taken at similar longitudes but a more northerly latitude,[172] and are rotated approximately 45° clockwise.
    • 2004
      Hubble Space Telescope
    • 14 April 2015; Dawn
      22,000 km (14,000 mi)
    • 6 May 2015; Dawn
      13,600 km (8,500 mi)
    • 16 May 2015; Dawn
      7,500 km (4,700 mi)
      (1;2;3)
    • 22 May 2015; Dawn
      5,100 km (3,200 mi)
      (1;2;3;4;5) (Ezinu crater: context;closeup)
    Animation of Dawn's trajectory around Ceres from 1 February 2015 to 6 October 2018
       Dawn ·   Ceres
    Mapping orbits and resolution[173] – Photos of Ceres by Dawn on Commons
    Orbit phaseNo.Dates[174]Altitude
    (km; mi)    		Orbital periodResolution
    (km/px)    		Improvement
    over Hubble    		Notes
    RC31st23 April 2015 – 9 May 201513,500 km (8,400 mi)15 days1.324×
    Survey2nd6 June 2015 – 30 June 20154,400 km (2,700 mi)3.1 days0.4173×
    HAMO3rd17 August 2015 – 23 October 20151,450 km (900 mi)19 hours0.14 (140 m)217×
    LAMO/XMO14th16 December 2015 – 2 September 2016375 km (233 mi)5.5 hours0.035 (35 m)850×
    XMO25th5 October 2016 – 4 November 20161,480 km (920 mi)19 hours0.14 (140 m)217×[175][176][177]
    XMO36th5 December 2016 – 22 February 20187,520–9,350 km
    (4,670–5,810 mi)    		≈8 days0.9 (est)34× (est)[176]
    XMO47th22 April 2017 – 22 June 201713,830–52,800 km
    (8,590–32,810 mi)    		≈29 days[178]
    XMO58th30 June 2017 – 16 April 20184,400–39,100 km
    (2,700–24,300 mi)    		30 days[178][179][180]
    XMO69th14 May 2018 – 31 May 2018440–4,700 km
    (270–2,920 mi)    		37 hours[181]
    XMO7 (FINAL)10th6 June 2018 – present35–4,000 km
    (22–2,485 mi)    		27.2 hours[165][166][182][183]
    • 23 April 2015; Dawn
      1st Map Orbit - RC3
      13,600 km (8,500 mi)
      (1;2;3;4;5;6;7;8;9;10;11;12;13;14;15;16;17;18;19;20;21;22;23;24) (3D: 1;5;6;7) (animation)
    • 6 June 2015: Dawn
      2nd Map Orbit - SRVY
      4,400 km (2,700 mi)
      (1;2;3;4;5;6;7;8;9;10;11;12;13;14;15;16;17;18;19;20;21;22;23;24;25;26;27;28;29;30;31;32;33;34;35;36;37;38;39;40;41;42;43;44;45;46;47;48;49;50;51;52;53;54)
    • 17 August 2015: Dawn
      3rd Map Orbit - HAMO
      1,470 km (910 mi)
      (1;2;3;4;5;6;7;8;9;10;11;12;13;14;15;16;17;18;19;20;21;22;23;24;25;26;27;28;29;30;31;32;33;34;35;36;37;38;39;40;41;42;43;44;45;46;47;48;49;50;51;52;53;54;55;56;57;58;59;60;61;62;63;64;65;66;67;68;69;70;71;72;73;74;75;76;77;78;79;80;81;82;83;84;85;86;87;88;89)
    • 10 December 2015: Dawn
      4th Map Orbit - LAMOa
      385 km (239 mi)
      (a;b;c;d;e;1;2;3;4;5;6;7;8;9;10;11;12;13;14;15;16;17;18;19;20;21;22;23;24;25;26;27;28;29;30;31;32;33;34;35;36;37;38;39;40;m1;m2;m3;41;42;43;44;45;46;47;48;49;50;51;52;53;54;55;56;57;58;59;60;61;62;63;64;65;66;67;68;69;70;71;72;73;74;75;76;77;78;79;80;81;82;83)
    • 10 December 2015: Dawn
      4th Map Orbit - LAMOb
      385 km (239 mi)
      (84;85;86;87;88;89;90;91;92;93;94;95;96;97;98;99;100;101;102;103;104;105;106;107;108;109;110;111;112;113;114;115;116;117;118;119;120;121;122;123;124;125;126;127;128;129;130;131;132;133;134;135;136;137;138;139;140;141;142;143;144;145;146;147;148;149)
    • 10 December 2015: Dawn
      4th Map Orbit - LAMOc
      385 km (239 mi)
      (150;151;152;153;154;155;156;157;158;159;160;161;162;163;164;165;166;167;168;169;170;171;172;173;174;175;176;177;178;179;180;181;182;183;184;185;186;187;188;189;190;191;192;193;194;195;196;197;198;199;200;201;202;203;204;205;206;207 – end)
    • 6 June 2018: Dawn
      Final Map Orbit
      35 km (22 mi)
      OccatorCrater:
      (Boulders;Landslides;EasternRim-1;EasternRim-2)
      (view on commons)

    True-color images

    Animations
    • 4 February 2015; Dawn
      90,000 km (56,000 mi)
    • 29 April 2017: Dawn
      20,000 km (12,000 mi)
    • 4 May 2015; Dawn
      13,600 km (8,500 mi)
    • 4 May 2015; Dawn
      13,600 km (8,500 mi)
      bright spots
    Ceres flyover animations
    Surface features exaggerated
    (simulated; 01:15; 8 June 2015)[184]
    Focus on Occator Crater
    (false colors; 01:12; 9 December 2015)
    Flight over Ceres
    (color; 03:43; 29 January 2016)

    See also

    Notes
    1. Photograph by the Framing Camera (FC) instrument aboard the Dawn spacecraft on 2 May 2015, during a "rotation characterization" orbit, 13,642 kilometres (8,477 mi) above the surface of Ceres. Visible at center and center right are two bright spots, a phenomenon common on Ceres, in Oxo and Haulani craters respectively. Ahuna Mons is also visible in the image as a noticeable, bluff hill, seen just right of bottom.
    2. 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]
    3. 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.[46]
    4. 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[123] and 7000 kg/s,[124] respectively.

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