Ceres (dwarf planet)

Ceres

A view of Ceres in natural color, pictured by the Dawn spacecraft in May 2015.[lower-alpha 1]
Discovery[1]
Discovered by	Giuseppe Piazzi
Discovery date	1 January 1801
Designations
MPC designation	1 Ceres
Pronunciation	/ˈsɪəriːz/
Named after	Cerēs
Alternative names	A899 OF; 1943 XB
Minor planet category	Dwarf planet Asteroid belt
Adjectives	Cererian /sᵻˈrɪəriən/, rarely Cererean /sɛrᵻˈriːən/[2]
Orbital characteristics[3]
Epoch 2014-Dec-09 (JD 7006245700050000000♠2,457,000.5)
Aphelion	7011445397740435110♠2.9773 AU (7011445410000000000♠445,410,000 km)
Perihelion	7011382626473889390♠2.5577 AU (7011382620000000000♠382,620,000 km)
Semi-major axis	7011414012107162250♠2.7675 AU (7011414010000000000♠414,010,000 km)
Eccentricity	6998758230000000000♠0.075823
Orbital period	4.60 yr 1,681.63 d
Synodic period	466.6 d 1.278 yr
Average orbital speed	7004179050000000000♠17.905 km/s
Mean anomaly	7000167532584102609♠95.9891°
Inclination	6999184882727663759♠10.593° to ecliptic 9.20° to invariable plane[4]
Longitude of ascending node	7000140201077082228♠80.3293°
Argument of perihelion	7000126574768013133♠72.5220°
Satellites	None
Proper orbital elements[5]
Proper semi-major axis	7000276709620000000♠2.7670962 AU
Proper eccentricity	6999116197700000000♠0.1161977
Proper inclination	6999168379107187070♠9.6474122°
Proper mean motion	78.193318 deg / yr
Proper orbital period	4.60397 yr (1681.601 d)
Precession of perihelion	7001540702720000000♠54.070272 arcsec / yr
Precession of the ascending node	2998408299660000000♠−59.170034 arcsec / yr
Physical characteristics
Dimensions	(7002965200000000000♠965.2 × 7002961200000000000♠961.2 × 7002891200000000000♠891.2) ± 2.0 km[6]
Mean radius	7005473000000000000♠473 km[6]
Surface area	7012277000000000000♠2,770,000 km2[7]
Volume	7017421000000000000♠421,000,000 km3[7]
Mass	7020939300000000000♠(9.393±0.005)×1020 kg[6] 6996150000000000000♠0.00015 Earths 0.0128 Moons
Mean density	7003216100000000000♠2.161±0.009 g/cm3[8]
Surface gravity	0.28 m/s2[7] 0.029 g
Moment of inertia factor	6999370000000000000♠0.37[9][lower-alpha 2] (estimate)
Escape velocity	0.51 km/s[7]
Sidereal rotation period	7004326678400000000♠0.3781 d 7004326670120000000♠9.074170±0.000002 h[10]
Equatorial rotation velocity	7001926100000000000♠92.61 m/s[7]
Axial tilt	4°[11]
North pole right ascension	291.418°[11]
North pole declination	66.764°[11]
Albedo	6998900000000000000♠0.090±0.0033 (V-band geometric)[12]
Surface temp. min mean max Kelvin ? ≈ 168 K[13] 235 K[14]
Spectral type	C[15]
Apparent magnitude	6.64[16] to 9.34[17]
Absolute magnitude (H)	7000336000000000000♠3.36±0.02[12]
Angular diameter	0.854″ to 0.339″

Ceres (/ˈsɪəriːz/;^[18] minor-planet designation: 1 Ceres) is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter. Its diameter is approximately 945 kilometers (587 miles),^[6] making it the largest of the minor planets within the orbit of Neptune. The 33rd-largest known body in the Solar System, it is the only dwarf planet within the orbit of Neptune.^{[lower-alpha 3][19]} Composed of rock and ice, Ceres is estimated to compose approximately one third of the mass of the entire asteroid belt. Ceres is the only object in the asteroid belt known to be rounded by its own gravity (though detailed analysis was required to exclude 4 Vesta). From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, and hence even at its brightest it is too dim to be seen with the naked eye except under extremely dark skies.

Ceres was the first asteroid to be discovered (by Giuseppe Piazzi at Palermo on 1 January 1801). 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 appears to be differentiated into a rocky core and an icy mantle, and may have a remnant internal ocean of liquid water under the layer of ice.^[20][21] The surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clay. 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 robotic NASA spacecraft Dawn entered orbit around Ceres on 6 March 2015.^[23][24][25] 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^[26]) were seen in a 19 February 2015 image, leading to speculation about a possible cryovolcanic origin^[27][28][29] or outgassing.^[30] 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.^[31] However, on 2 September 2016, NASA scientists released a paper in Science that claimed that a massive ice volcano called Ahuna Mons is the strongest evidence yet for the existence of these mysterious ice volcanoes.^[32][33] On 11 May 2015, NASA released a higher-resolution image showing that, instead of one or two spots, there are actually several.^[34] 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.^[35] In June 2016, near-infrared spectra of these bright areas were found to be consistent with a large amount of sodium carbonate (Na
2CO
3), implying that recent geologic activity was probably involved in the creation of the bright spots.^[36][37][38]

In October 2015, NASA released a true color portrait of Ceres made by Dawn.^[39] In February 2017, organics were reported to have been detected on Ceres in Ernutet crater (see image).^[40]

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.^[41] Kepler had already noticed the gap between Mars and Jupiter in 1596.^[41] 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 semi-major axes of the orbits of known planets, and that the pattern was marred only by the large gap between Mars and Jupiter.^[41][42] The pattern predicted that the missing planet ought to have an orbit with a semi-major axis near 2.8 astronomical units (AU).^[42] William Herschel's discovery of Uranus in 1781^[41] 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.^[41][42] Although they did not discover Ceres, they later found several large asteroids.^[42]

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.^[43][44] 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".^[41] Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet.^[45] 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 of Berlin.^[46] 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".^[41] 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.^[45]

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.^[45] In only 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.^[45]

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.^[47][48]

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.^[41][45] "Ferdinandea", however, was not acceptable to other nations and was dropped. Ceres was called Hera for a short time in Germany.^[49] In Greece, it is called Demeter (Δήμητρα), after the Greek equivalent of the Roman Cerēs;^{[lower-alpha 4]} in English, that name is used for the asteroid 1108 Demeter.

The regular adjectival forms of the name are Cererian and Cererean,^[50] derived from the Latin genitive Cereris,^[2] but Ceresian is occasionally seen for the goddess (as in the sickle-shaped Ceresian Lake), as is the shorter form Cerean.

The old astronomical symbol of Ceres is a sickle, ⟨⚳⟩ (),^[51] 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 were later replaced with the generic asteroid symbol of a numbered disk, ⟨①⟩.^[45][52]

Cerium, a rare-earth element discovered in 1803, was named after Ceres.^{[53][lower-alpha 5]} In the same year another element was also initially named after Ceres, but when cerium was named, its discoverer changed the name to palladium, after the second asteroid, 2 Pallas.^[55]

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.^[41] 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.^[41][45][56]

Sizes of the first ten main-belt objects discovered profiled against the Moon. Ceres is far left (1).

As other objects were discovered in the neighborhood of Ceres, it was realized that Ceres represented the first of a new class of objects.^[41] In 1802, with the discovery of 2 Pallas, William Herschel coined the term asteroid ("star-like") for these bodies,^[56] writing that "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes".^[57] 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.^[56]

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.^[58][59] 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".^[60] Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun.^[61] 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 a third of the mass of the belt. Bodies that met the first proposed definition but not the second, such as Ceres, were instead classified as dwarf planets.

Ceres is the largest object in the asteroid belt.^[15] It is sometimes assumed that Ceres has been 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",^[62] 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.^[63] The Minor Planet Center notes that such bodies may have dual designations.^[64] The 2006 IAU decision that classified Ceres as a dwarf planet never addressed whether it is or is not an asteroid. Indeed, the IAU has never defined the word 'asteroid' at all, having preferred the term 'minor planet' until 2006, and preferring the terms 'small Solar System body' and 'dwarf planet' after 2006. 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".^[65] NASA continues to refer to Ceres as an asteroid,^[66] as do various academic textbooks.^[67][68]

Orbit

Orbit of Ceres

Ceres follows an orbit between Mars and Jupiter, within the asteroid belt, with a period of 4.6 Earth years.^[3] 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).^[3]

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.^[69] 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.^[69] Ceres appears to be merely an interloper in the Gefion family, coincidentally having similar orbital elements but not a common origin.^[70]

Resonances

Ceres is in a near-1:1 mean-motion orbital resonance with Pallas (their proper orbital periods differ by 0.2%).^[71] 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.^[72] 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.^[73]

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.^[74]

Rotation and axial tilt

The rotation period of Ceres (the Cererian day) is 9 hours and 4 minutes.^[75] It has an axial tilt of 4°. 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.^[76]

Geology

Ceres has a mass of 7020939000000000000♠9.39×10²⁰ kg as determined from the Dawn spacecraft.^[77] With this mass Ceres composes approximately a third of the estimated total 3.0 ± 0.2×10²¹ kg mass of the asteroid belt,^[78] which is in turn approximately 4% of the mass of the Moon. Ceres is massive enough to give it a nearly spherical, equilibrium shape.^[79] Among Solar System bodies, Ceres is intermediate in size between the smaller Vesta and the larger Tethys. Its surface area is approximately the same as the land area of India or Argentina.^[80]

Surface

The surface composition of Ceres is broadly similar to that of C-type asteroids.^[15] Some differences do exist. The ubiquitous features in Ceres' IR spectrum are those of hydrated materials, which indicate the presence of significant amounts of water in its interior. Other possible surface constituents include iron-rich clay minerals (cronstedtite) and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites.^[15] The spectral features of carbonates and clay minerals are usually absent in the spectra of other C-type asteroids.^[15] Sometimes Ceres is classified as a G-type asteroid.^[81]

Ceres' surface is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (approximately −38 °C, −36 °F) on 5 May 1991.^[14] Ice is unstable at this temperature. Material left behind by the sublimation of surface ice 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 Cerean conditions and would be expected to either escape quickly or settle in cold traps, and are evidently associated with areas with recent geological activity.^[82]

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.^[81] 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.^[83][84] 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.^[83][84] 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.^[12][85] One of these features corresponds to the "Piazzi" feature observed earlier.^[12]

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°.^[12][79] 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°.^[6]

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.^[86][87] An unexpectedly large number of Cererian craters have central pits, perhaps due to cryovolcanic processes, and many have central peaks.^[88] 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.^[89][90] A later computer simulation has suggested that there were originally other cryovolcanoes on Ceres that are now unrecognisable due to viscous relaxation.^[91] 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.^[92] 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%^[93] that are caused by a substance on the surface, possibly ice or salts, reflecting sunlight.^[94][95] 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.^[95][96] In March 2016, Dawn found definitive evidence of water molecules on the surface of Ceres at Oxo crater.^[97][98] JPL states: "This water could be bound up in minerals or, alternatively, it could take the form of ice."

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.^[35] Another team thinks the salts are sodium carbonate.^[37][38]

Map of bright spots on Ceres (released 10 December 2015).

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.^[99] Imaged by Dawn from 385 km (239 mi) in December 2015.

Internal structure

Diagram showing a possible internal structure of Ceres

Ceres' oblateness is consistent with a differentiated body, a rocky core overlain with an icy mantle.^[79] This 100-kilometer-thick mantle (23%–28% of Ceres by mass; 50% by volume)^[100] contains up to 200 million cubic kilometers of water, which would be more than the amount of fresh water on Earth.^[101] This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modeling.^[20][83] Also, some characteristics of its surface and history (such as its distance from the Sun, which weakened solar radiation enough to allow some fairly low-freezing-point components to be incorporated during its formation), point to the presence of volatile materials in the interior of Ceres.^[83] It has been suggested that a remnant layer of liquid water may have survived to the present under a layer of ice.^[20][21]

Shape and gravity field measurements by Dawn confirm Ceres is a body in hydrostatic equilibrium with partial differentiation^[8][102] and isostatic compensation, with a mean moment of inertia of 0.37 (which is similar to that of Callisto at ~0.36).^[9] The densities of the core and outer layer are estimated to be 2.46–2.90 and 1.68–1.95 g/cm³, with the latter being about 70–190 km thick. Only partial dehydration of the core is expected. The high density of the outer layer (relative to water ice) reflects its enrichment in silicates and salts.^[9] Ceres is the smallest object confirmed to be in hydrostatic equilibrium, being 600 km smaller and less than half the mass of Saturn's moon Rhea, the next smallest such object.^[103] Modeling has suggested Ceres could have a small metallic core from partial differentiation of its rocky fraction.^[104]

Atmosphere

There are indications that Ceres has a tenuous water vapor atmosphere outgassing from water ice on the surface.^{[105][106][107]}

Surface water ice is unstable at distances less than 5 AU from the Sun,^[108] 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. As a result, it is difficult to detect water vaporization. Water escaping from polar regions of Ceres was possibly observed in the early 1990s but this has not been unambiguously demonstrated. It may be possible to detect escaping water from the surroundings of a fresh impact crater or from cracks in the subsurface layers of Ceres.^[83] Ultraviolet observations by the IUE spacecraft detected statistically significant amounts of hydroxide ions near Ceres' north pole, which is a product of water vapor dissociation by ultraviolet solar radiation.^[105]

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 7026100000000000000♠10²⁶ molecules (or 3 kg) of water per second.^{[109][110][lower-alpha 6]} 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^[109] or from pressurization of a subsurface ocean due to growth of an overlying layer of ice.^[21] 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;^[109] however, subsequent evidence from Dawn strongly suggests ongoing geologic activity could be at least partially responsible.^[113][114]

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.^[115]

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.^[116]

Origin and evolution

Ceres is possibly a surviving protoplanet (planetary embryo), which formed 4.57 billion years ago in the asteroid belt.^[20] Although the majority of inner Solar System protoplanets (including all lunar- to Mars-sized bodies) either merged with other protoplanets to form terrestrial planets or were ejected from the Solar System by Jupiter,^[117] Ceres is thought to have survived relatively intact.^[20] An alternative theory proposes that Ceres formed in the Kuiper belt and later migrated to the asteroid belt.^[118] The discovery of ammonia salts in Occator crater supports an origin in the outer Solar System.^[119] Another possible protoplanet, Vesta, is less than half the size of Ceres; it suffered a major impact after solidifying, losing ~1% of its mass.^[120]

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.^[12][20] This process may have caused resurfacing by water volcanism and tectonics, erasing older geological features.^[20] 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.^[15]

Today, Ceres has become considerably less geologically active, with a surface sculpted chiefly by impacts;^[12] nevertheless, evidence from Dawn reveals that internal processes have continued to sculpt Ceres's surface to a significant extent, in stark contrast to Vesta^[121] and of previous expectations that Ceres would have become geologically dead early in its history due to its small size.^[20][122] The presence of significant amounts of water ice in its composition^[79] and evidence of recent geological resurfacing, raises the possibility that Ceres has a layer of liquid water in its interior.^[20][122] This hypothetical layer is often called an ocean.^[15] If such a layer of liquid water exists, it is hypothesized to be located between the rocky core and ice mantle like that of the theorized ocean on Europa.^[20] The existence of an ocean is more likely if solutes (i.e. salts), ammonia, sulfuric acid or other antifreeze compounds are dissolved in the water.^[20]

Potential habitability

Although not as actively discussed as a potential home for microbial extraterrestrial life as Mars, Titan, Europa or Enceladus, there is evidence that Ceres' icy mantle was once a watery subterranean ocean, and that has led to speculations that life could have existed there,^{[123][124][125][126]} and that hypothesized ejecta bearing microorganisms could have come from Ceres to Earth.^[127][128]

Observation and exploration

Observation

Polarimetric map of Ceres^[129]

When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7.^[16] This is generally regarded as too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted person may be able to see it. The only other asteroids that can reach a similarly bright magnitude are 4 Vesta, and, during rare oppositions near perihelion, 2 Pallas and 7 Iris.^[130] At a conjunction Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars. It can thus be seen with binoculars whenever it is above the horizon of a fully dark sky.

Some notable observations and milestones for Ceres include:

• 1984 November 13: An occultation of a star by Ceres observed in Mexico, Florida and across the Caribbean.^[131]
• 1995 June 25: Ultraviolet Hubble Space Telescope images with 50-kilometer resolution.^[81][132]
• 2002: Infrared images with 30-km resolution taken with the Keck telescope using adaptive optics.^[84]
• 2003 and 2004: Visible light images with 30-km resolution (the best prior to the Dawn mission) taken using Hubble.^[12][85]
• 2012 December 22: Ceres occulted the star TYC 1865-00446-1 over parts of Japan, Russia, and China.^[133] Ceres' brightness was magnitude 6.9 and the star, 12.2.^[133]
• 2014: Ceres was found to have an atmosphere with water vapor, confirmed by the Herschel space telescope.^[134]
• 2015: The NASA Dawn spacecraft approached and orbited Ceres, sending detailed images and scientific data back to Earth.

Exploration

Artist's conception of Dawn, travelling from Vesta to Ceres

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.^[135] 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.

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

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.^[136]

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.^[137] After orbiting Vesta for 13 months, Dawn used its ion engine to depart for Ceres, with gravitational capture occurring on 6 March 2015^[138] at a separation of 61,000 km,^[139] four months prior to the New Horizons flyby of Pluto.

Dawn's mission profile calls 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][140] The spacecraft will subsequently reduce its orbital distance to 4,400 km for its second observational orbit ("survey") for three weeks,^[141] then down to 1,470 km ("HAMO;" high altitude mapping orbit) for two months^[142] and then down to its final orbit at 375 km ("LAMO;" low altitude mapping orbit) for at least three months.^[143] The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These instruments will examine Ceres' shape and elemental composition.^[144] 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.^[145]

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.^[146]

The Chinese Space Agency is designing a sample retrieval mission from Ceres that would take place during the 2020s.^[147]

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 0° longitude, with official nomenclature (August 2015)

Topographic map of Ceres (September 2016). 15 km (10 mi) of elevation separate the lowest crater floors (indigo) from the highest peaks (white).[148]

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 the dwarf planet Ceres is divided into 15 quadrangles. They are named after the first craters whose names the IAU approved in July 2015.^[149] The map image(s) were taken by the Dawn space probe.

90°N 180°E / 90°N 180°E / 90; 180

0°N 0°E / 0°N 0°E / 0; 0

0°N 180°E / 0°N 180°E / 0; 180

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.

[]

Gallery

Ceres in half shadow from 40,000 km (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.^[150]

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,^[151] 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)

• 
  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 Dec 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 Dec 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 Dec 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)

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

See also

• Ceres (dwarf planet) in fiction
• Mining of Ceres
• Former classification of planets
• List of exceptional asteroids
• List of geological features on Ceres
• List of Solar System objects by size

Notes

References

External links

Wikimedia Commons has media related to Ceres (dwarf planet).

• Destination Ceres:Breakfast at Dawn – NASA
• Dawn mission home page at JPL
• A simulation of the orbit of Ceres
• JPL Ephemeris
• How Gauss determined the orbit of Ceres from keplersdiscovery.com
• Hilton, James L. (1999). "U.S. Naval Observatory Ephemerides of the Largest Asteroids". The Astronomical Journal. 117 (2): 1077–1086. Bibcode:1999AJ....117.1077H. doi:10.1086/300728. 
• Animated reprojected colorized map of Ceres (22 February 2015)
• Rotating relief model of Ceres by Seán Doran (about 60% of a full rotation; starts with Occator midway above center)

Ceres
Geography	General Geology of Ceres Craters Abellio Achita Annona Anura Aristaeus Asari Attis Azacca Begbalel Belun Besua Bonsu Braciaca Centeotl Centeotl Chaminuka Coniraya Consus Cozobi Dada Dantu Darzamat Datan Doliku Duginavi Ernutet Ezinu Fejokoo Fluusa Gaue Geshtin Ghanan Hakumyi Hamori Hatipowa Haulani Heneb Homshuk Ialonus Ikapati Inamahari Insitor Jaja Jarimba Jarovit Juling Kaikara Kait Kerwan Kiriamma Kirnis Kondos Kumitoga Kupalo Laukumate Liber Lociyo Lono Meanderi Megwomets Messor Mondamin Nawish Nepen Ninsar Occator Oltagon Omonga Oxo Piuku Rao Razeka Rongo Roskva Sekhet Shakaema Sintana Tafakula Tahu Takel Tawals Tibong Toharu Tupo Urvara Victa Vinotonus Xochipilli Yalode Zadeni Montes Ahuna Mons Liberalia Mons Yamor Mons Catenae Baltay Catena Gerber Catena Junina Catenae Pongal Catena Samhain Catenae Uhola Catenae Rupēs Niman Rupes Plana Hanami Planum Tholi Aymuray Tholi Cosecha Tholus Dalien Tholus Hosil Tholus Kwanzaa Tholus Mikeli Tholus Wangala Tholus Planitiae Vendimia Planitia Fossae Nabanna Fossa Sulci Nar Sulcus Other features Bright spots
Astronomy	Classification Asteroid Definition of planet Dwarf planet Planet 2006 definition of planet Discovery Barnaba Oriani Carl Friedrich Gauss Francis Wollaston Franz Xaver von Zach Giuseppe Piazzi Heinrich Olbers Jérôme Lalande Johann Elert Bode Tobias Mayer Possible trojans (185105) 2006 SV23 1372 Haremari
Exploration	Dawn mission Ceres Polar Lander (proposed)
Related	Ceres in fiction Colonization In astrology Solar System bodies formerly considered to be planets

Dwarf planets
Asteroid belt	IAU: Ceres No longer rounded and therefore not dwarf planets: Vesta
Centaurs	Possibly: Chariklo Chiron
Centaurs (extended)	Possibly: 1995 SN55 (lost) Ceto 2010 TY53
Plutinos	IAU: Pluto Additional objects proposed by Brown and Tancredi: Orcus Ixion 2003 AZ84 2004 UX10 Huya Possibly: 2003 UZ413 2003 VS2 2002 VR128 2004 PF115 2002 XV93 2007 JH43 2001 QF298
Twotinos	Possibly: 2002 WC19
Other resonances/ unknown resonances:	IAU: Haumea Possibly: 2002 XW93 1999 DE9 1999 KR16 2001 YH140 2011 GM27 2010 VK201 2013 FZ27 2014 UM33 1999 CD158
Cubewanos:	IAU: Makemake Additional objects proposed by Brown and Tancredi: 2002 MS4 Salacia Quaoar Varuna Varda 2004 GV9 2002 AW197 2002 TX300 2005 RR43 2003 OP32 2005 RN43 Possibly: Chaos Altjira Sila-Nunam 2002 UX25 2007 JJ43 2005 UQ513 2004 TY364 2003 QW90 1998 SN165 2002 KX14 Praamzius 2004 NT33 2010 FX86 2002 CY248 2010 VR11 Rhadamanthus
Scattered disc	IAU: Eris Additional objects proposed by Brown and Tancredi: 2007 OR10 1996 TL66 2001 UR163 2005 RM43 Possibly: 2002 TC302 2004 XA192 2005 QU182 2006 QH181 2007 UK126 2010 EK139 2010 KZ39 2010 RE64 2010 RF43 2010 TJ 2013 FY27 2008 OG19 1996 GQ21 2012 HH2 1999 CC158 2010 VZ98
Area uncertain	Captured satellites that were once dwarf planets: Triton (captured by Neptune) Phoebe (captured by Saturn, and no longer rounded) Possibly: 2011 FW62 (lost) 2000 YW134 V774104
Detached objects	Possibly: 2004 XR190 2010 GB174 2008 ST291 1995 TL8 2005 TB190 2003 QX113 2003 FY128 2004 VN112 2000 CR105
Sednoids	Objects proposed by Brown and Tancredi: Sedna Possibly: 2012 VP113
Former candidates	Pallas Vesta Hygiea
See also: Charon Vanth Dysnomia Hiʻiaka Mesoplanet Planemo List of trans-Neptunian objects List of possible dwarf planets List of Solar System objects List of Solar System objects by size

The Solar System
The Sun Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Haumea Makemake Eris
Planets	Terrestrial planets Mercury Venus Earth Mars Giant planets Jupiter Saturn Uranus Neptune Dwarf planets Ceres Pluto Haumea Makemake Eris
Rings	Jovian Saturnian (Rhean) Charikloan Chironean Uranian Neptunian
Moons	1 Terrestrial (Moon) Other near-Earth objects 2 Martian Phobos Deimos 69 Jovian Ganymede Callisto Io Europa all 62 Saturnian Titan Rhea Iapetus Dione Tethys Enceladus Mimas Hyperion Phoebe all 27 Uranian Titania Oberon Umbriel Ariel Miranda all 14 Neptunian Triton Proteus Nereid all 5 Plutonian Charon Nix Hydra Kerberos Styx 2 Haumean Hiʻiaka Namaka 1 Makemakean (S/2015 (136472) 1) 1 Eridian (Dysnomia)
Lists	Solar System objects By size By discovery date Minor planets Gravitationally rounded objects Possible dwarf planets Natural satellites Comets
Small Solar System bodies	Meteoroids Minor planets moons Comets Damocloids Mercury-crossers Venus-crossers Venus trojans Near-Earth objects Earth-crossers Earth trojans Mars-crossers Mars trojans Asteroid belt Asteroids first discovered: Ceres Pallas Juno Vesta Families Notable asteroids Kirkwood gap Main-belt comets Jupiter trojans Jupiter-crossers Centaurs Saturn-crossers Uranus trojans Uranus-crossers Neptune trojans Cis-Neptunian objects Trans-Neptunian objects Neptune-crossers Plutoids Kuiper belt Plutinos Cubewanos Scattered disc Detached objects Sednoids Hills cloud Oort cloud
Hypothetical objects	Vulcan Vulcanoids Phaeton Planet V Theia Fifth giant Planets beyond Neptune Tyche Nemesis Planet Nine
Exploration (outline)	Discovery Astronomy Timeline Spaceflight Robotic spacecraft Human spaceflight Colonization List of probes Timeline Mercury Venus Moon Mars Ceres Asteroids Mining Comets Jupiter Saturn Uranus Neptune Pluto Deep space
Outline of the Solar System Portals Solar System Astronomy Earth sciences Mars Jupiter Uranus Cosmology Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Virgo Supercluster → Laniakea Supercluster → Observable universe → Universe Each arrow (→) may be read as "within" or "part of".