Ceres (dwarf planet)

Ceres (minor-planet designation: 1 Ceres) is the largest object in the asteroid belt between the orbits of Mars and Jupiter. Ceres was the first asteroid discovered, on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily. Originally considered a planet, it was reclassified as an asteroid in the 1850s after the discovery of dozens of other objects in similar orbits. In 2006, it was reclassified again as a dwarf planet – the only one always inside Neptune’s orbit – because, at 940 km (580 mi) in diameter, it is the only asteroid large enough for its gravity to make it plastic and to maintain it as a spheroid. In January 2014, emissions of water vapor were detected around Ceres, creating a tenuous, transient atmosphere known as an exosphere. This was unexpected because asteroids typically do not emit vapor, a hallmark of comets.

Ceres’s small size means that even at its brightest it is too dim to be seen by the naked eye, except under extremely dark skies. Its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition (when it is closest to Earth) once every 15- to 16-month synodic period. Its surface features are barely visible even with the most powerful telescopes, and little was known of them until the robotic NASA spacecraft Dawn approached Ceres for its orbital mission in 2015.

Dawn found Ceres’s surface to be a mixture of water ice and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially differentiated into a muddy (ice-rock) mantle/core and a less-dense but stronger crust that is at most 30% ice by volume. Ceres’s small size means that any internal ocean of liquid water it may once have possessed has likely frozen by now. It is not completely frozen, however: brines still flow through the outer mantle and reach the surface, allowing cryovolcanoes such as Ahuna Mons to form at the rate of about one every 50 million years. This makes Ceres the closest known cryovolcanic body to the Sun, and the brines provide a potential habitat for microbial life.

Ceres in true color in 2015

History

Discovery

In the years between the acceptance of heliocentrism and the discovery of Neptune, several astronomers argued that mathematical laws predicted the existence of a hidden or missing planet between the orbits of Mars and Jupiter. In 1596, theoretical astronomer Johannes Kepler believed that the ratios between planetary orbits would only conform to God’s design with the addition of two planets: one between Jupiter and Mars and one between Venus and Mercury. Other theoreticians, such as Immanuel Kant, pondered whether the gap had been created by the gravity of Jupiter; in 1761 astronomer and mathematician Johann Heinrich Lambert asked, “And who knows whether already planets are missing which have departed from the vast space between Mars and Jupiter? Does it then hold of celestial bodies as well as of the Earth, that the stronger chafe the weaker, and are Jupiter and Saturn destined to plunder forever?”

Giuseppe Piazzi, discoverer of Ceres

In 1772, German astronomer Johann Elert Bode, citing Johann Daniel Titius, published a numerical procession known as the Titius–Bode law (now discredited); a formula that appeared to predict the orbits of the known planets but for an unexplained gap between Mars and Jupiter. The pattern predicted that there ought to be another planet with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun. The Titius–Bode law got a boost with William Herschel’s discovery of Uranus near the predicted distance for a planet beyond Saturn. In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal Monatliche Correspondenz (Monthly Correspondence), sent requests to 24 experienced astronomers (whom he dubbed the “celestial police”), asking that they combine their efforts and begin a methodical search for the expected planet. Although they did not discover Ceres, they later found the asteroids 2 Pallas, 3 Juno and 4 Vesta.

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. He was searching for “the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille”, but found that “it was preceded by another”. Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet. Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin. 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”. In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the Monatliche Correspondenz.

By this time, the apparent position of Ceres had changed (mostly due to Earth’s motion around the Sun), and was too close to the Sun’s glare for other astronomers to confirm Piazzi’s observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, mathematician Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination. In a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and fellow celestial policeman Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it. At 2.8 AU from the Sun, Ceres appeared to fit the Titius–Bode law almost perfectly; however, Neptune, once discovered in 1846, was 8 AU closer than predicted, leading most astronomers to conclude that the law was a coincidence.

The early observers were only able to calculate the size of Ceres to within an order of magnitude. Herschel underestimated its diameter at 260 km (160 mi) in 1802, whereas in 1811 German astronomer Johann Hieronymus Schröter overestimated it as 2,613 km (1,624 mi). It was not until the 1970s, when infrared photometry enabled more accurate measurements of its albedo, that Ceres’s diameter was determined to within 10% of its true value of 939 km.

Name and symbol

Piazzi’s proposed name for his discovery was Ceres FerdinandeaCeres after the Roman goddess of agriculture, whose earthly home, and oldest temple, lay in Sicily; and Ferdinandea in honor of Piazzi’s monarch and patron, King Ferdinand III of Sicily. The latter was not acceptable to other nations and was dropped. Before von Zach’s recovery of Ceres in December 1801, von Zach referred to the planet as Hera and Bode referred to it as Juno. Despite Piazzi’s objections, those names gained currency in Germany before the object’s existence was confirmed. Once it was, astronomers settled on Piazzi’s name.

The adjectival forms of Ceres are Cererian and Cererean, both pronounced /sɪˈrɪəriən/. Cerium, a rare-earth element discovered in 1803, was named after Ceres.

The old astronomical symbol of Ceres is a sickle, ⟨⚳⟩, one of the classical symbols of the goddess Ceres. It was suggested, apparently independently, by von Zach and Bode in 1802. In form it is similar to the symbol ⟨♀⟩ of the planet Venus, but with a break in the circle. It has an occasional variant ⟨⚳⟩, reversed to resemble the initial letter C of the name Ceres. It was replaced with ⟨①⟩, the generic asteroid symbol of a numbered disk, in 1867, but was resurrected for astrological use in 1973.

Classification

The categorization of Ceres has changed more than once and has been the subject of some disagreement. Bode believed Ceres to be the “missing planet” he had proposed to exist between Mars and Jupiter. Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with Pallas, Juno, and Vesta) for over half a century.

As other objects were discovered in the neighborhood of Ceres, astronomers began to suspect that Ceres represented the first of a new class of objects. In 1802, with the discovery of Pallas, Herschel coined the term asteroid (“star-like”) for these bodies, writing that “they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes”. In 1852, astronomer Johann Franz Encke, in the Berliner Astronomisches Jahrbuch, declaring the traditional system of granting planetary symbols too cumbersome for these new objects, instead introduced a new system of placing numbers before their names in order of discovery. Initially, the numbering system began with the fifth asteroid, 5 Astraea, as number 1, but in 1867 Ceres was adopted into the new system under the name 1 Ceres.

By the 1860s, astronomers widely accepted that a fundamental difference existed between the major planets and asteroids such as Ceres, though the word “planet” had yet to be precisely defined. Then, in 2006, the debate surrounding Pluto led to calls for a definition of “planet”, and the possible reclassification of Ceres, perhaps even its reinstatement as a planet. A proposal before the International Astronomical Union (IAU), the global body responsible for astronomical nomenclature and classification, defined a planet as “a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet”. Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun; but on 24 August 2006 the assembly adopted the additional requirement that a planet must have “cleared the 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. Bodies that met the first proposed definition but not the second, such as Ceres, were instead classified as dwarf planets.

Since the IAU declaration in 2006 assuming that Ceres is a dwarf planet, there has been some confusion as to whether it remains an asteroid. A NASA webpage declares that Vesta, the belt’s second-largest object, is the largest asteroid. The IAU has been equivocal on the subject, though its Minor Planet Center, the organization charged with cataloguing such objects, notes that dwarf planets may have dual designations, and the joint IAU/USGS/NASA Gazetteer categorizes Ceres as both asteroid and dwarf planet.

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

Ceres (bottom left), the Moon and Earth, shown to scale

Orbit

Ceres follows an orbit between Mars and Jupiter, near the middle of the asteroid belt, with an orbital period (year) of 4.6 Earth years. Compared to other planets and dwarf planets, Ceres’s orbit is moderately (though not drastically) tilted relative to that of Earth, with an inclination (i) of 10.6° (compared to 7° for Mercury, and 17° for Pluto) and elongated, with an eccentricity (e) = 0.08 (compared to 0.09 for Mars).

Ceres was once thought to be a member of the Gefion asteroid family, the members of which share similar proper orbital elements, suggesting a common origin through an asteroid collision some time in the past. Ceres was later found have a different composition than the members of the Gefion family and appears to be merely an interloper, coincidentally having similar orbital elements but not a common origin. Ceres’s lack of an asteroid family is believed to be due to the large proportion of ice in its composition, which, if fragmented, would have sublimated to nothing over the age of the Solar System.

Resonances

Due to their small masses and large separations, objects within the asteroid belt rarely fall into gravitational resonances with each other. Nevertheless, Ceres is able to capture other asteroids into temporary 1:1 resonances (making them temporary trojans), for periods from a few hundred thousand to more that two million years. 50 such objects have been identified.

Ceres is close to a 1:1 mean-motion orbital resonance with Pallas (their proper orbital periods differ by 0.2%), but not close enough to be significant over astronomical timescales.

Orbits of Ceres (red, inclined) along with Jupiter and the inner planets (white and gray). The upper diagram shows Ceres’s orbit from top down. The bottom diagram is a side view showing Ceres’s orbital inclination to the ecliptic. Lighter shades indicate above the ecliptic; darker indicate below.

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°. 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 what occurs 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 three times before escaping or being trapped.

Dawn, the first spacecraft to orbit Ceres, determined that the north polar axis points at right ascension 19 h 25 m 40.3 s (291.418°), declination +66° 45′ 50″ (about 1.5 degrees from Delta Draconis), which means an axial tilt of 4°. Over the course of 3 million years, gravitational influence from Jupiter and Saturn has triggered cyclical shifts in Ceres’s axial tilt, ranging from 2 to 20 degrees, meaning that seasonal effects have occurred in the past, with the last period of seasonal activity estimated at 14,000 years ago. Those craters that remain in shadow during periods of maximum axial tilt are the most likely to retain their water over the age of the Solar System.

Permanently shadowed regions capable of accumulating surface ice

Geology

Ceres is the largest asteroid in the main asteroid belt. It has been classified as a C-type or carbonaceous asteroid and, due to the presence of clay minerals, as a G-type asteroid. Its composition is similar, though not identical, to those of carbonaceous chondrite meteorites. In shape it is an oblate spheroid, with an equatorial diameter 8% larger than its polar diameter. Measurements from the Dawn spacecraft found a mean diameter of 939.4 km (583.7 mi) and a mass of 9.39×1020 kg. This gives Ceres a density of 2.16 g/cm3, suggesting that a quarter of its mass is water ice.

Ceres comprises nearly a third of the estimated (3.0±0.2)×1021 kg mass of the asteroid belt, and it has 3½ times the mass of the next asteroid, Vesta, yet it is only 1.3% the mass of the Moon. It is at least close to being in hydrostatic equilibrium, though some deviations from an equilibrium shape have yet to be explained. Assuming it is in equilibrium, Ceres is the only dwarf planet inside the orbit of Neptune. It is approximately the size of the large trans-Neptunian object Orcus (though half again as massive), and it has the surface area of Argentina. Modeling has suggested Ceres’s rocky material is partially differentiated, and that it perhaps possesses a small core, but the data are consistent with a mantle of hydrated silicates and no core.

The mass of 1 Ceres compared to other large asteroids: 4 Vesta2 Pallas10 Hygiea704 Interamnia511 Davida and the remainder of the Main Belt. The unit of mass is ×1018 kg.

Composition

The surface composition of Ceres is homogeneous on a global scale, and is rich in carbonates and ammoniated phyllosilicates that have been altered by water, though water ice in the regolith varies from approximately 10% in polar latitudes to much drier, even ice-free, in the equatorial regions.

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

Tholins, formed from ultraviolet irradiation of simple carbon compounds, were detected on Ceres in Ernutet crater, and most of the planet’s near surface is extremely rich in carbon, at approximately 20% by mass. The carbon content is more than five times higher than in carbonaceous chondrite meteorites analyzed on Earth. The surface carbon shows evidence of being mixed with products of rock-water interactions, such as clays. 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.

The mass of Ceres compared to other small, round astronomical objects: Dione (Saturn IV), Ariel (Uranus I), Charon (Pluto I), Orcus, Quaoar. The unit of mass is ×1021 kg.

Craters

Dawn revealed that Ceres has a heavily cratered surface, though with fewer large craters than expected. Models based on the formation of the current asteroid belt had suggested Ceres should possess 10 to 15 craters larger than 400 km (250 mi) in diameter. The largest confirmed crater on Ceres, Kerwan Basin, is 284 km (176 mi) across. The most likely reason for this is viscous relaxation of the crust slowly flattening out larger impacts.

Ceres’s north polar region shows far more cratering than the equatorial region, with the eastern equatorial region in particular comparatively lightly cratered. The overall size frequency of craters of between 20 and 100 km (10 and 60 mi) is consistent with them having originated in the Late Heavy Bombardment, with craters outside the ancient polar regions likely erased by early cryovolcanism. Three large shallow basins (planitiae) with degraded rims are likely to be eroded craters. The largest, Vendimia Planitia, at 800 km (500 mi) across, is also the largest single geographical feature on Ceres. Two of the three have higher than average ammonium concentrations.

Compositional map of Ceres in false color. Derived from the first mapping cycle at an altitude of 1,470 km (915 mi). Employs a combination of violet (440 nm) near-infrared (750 nm), and infrared (920 nm) filters.

Topographic map of Ceres. The lowest crater floors (indigo), and the highest peaks (white) represent a difference of 15 km (10 mi) elevation. “Ysolo Mons” has been renamed “Yamor Mons.”

Cryovolcanism

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 240 million years. Its relatively high gravitational field suggests it is dense, and thus composed more of rock than ice, and that its placement is likely due to diapirism of a slurry of brine and silicate particles from the top of the mantle. It is roughly antipodal to Kerwan Basin. Seismic energy from the Kerwan-forming impact may have focused on the opposite side of Ceres, fracturing the outer layers of the crust and facilitating the movement of high-viscosity cryomagma (consisting of muddy water ice softened by its content of salts) onto the surface. Kerwan too shows evidence of the effects of liquid water due to impact-melting of subsurface ice.

A 2018 computer simulation suggests that cryovolcanoes on Ceres, once formed, recede due to viscous relaxation over the course of several hundred million years. The team identified 22 features as strong candidates for relaxed cryovolcanoes on Ceres’s surface. Yamor Mons, an ancient, impact-cratered peak, resembles Ahuna Mons despite being much older, due to it lying in Ceres’s northern polar region, where colder temperatures prevent viscous relaxation of the crust. Models suggest that, over the past billion years, one cryovolcano has formed on Ceres on average every 50 million years. The eruptions are not uniformly distributed over Ceres, but may be linked to ancient impact basins. The model suggests that, contrary to findings at Ahuna Mons, Cererian cryovolcanoes must be composed of far less dense material than average for Ceres’s crust, or the observed viscous relaxation could not occur.

An unexpectedly large number of Cererian craters have central pits, perhaps due to cryovolcanic processes, whilst others have central peaks. Hundreds of bright spots (faculae) have been observed by Dawn, the brightest located in the middle of 80 km (50 mi) Occator crater. The bright spot in the center of Occator is named Cerealia Facula, and the group of bright spots to its east, Vinalia Faculae. Occator possesses a 9–10 km-wide pit partially filled by a central dome. The dome post-dates the faculae and is likely due to freezing of a subterranean reservoir, comprable to pingos in Earth’s Arctic region. A haze periodically appears above Cerealia, supporting the hypothesis that some sort of outgassing or sublimating ice formed the bright spots. In March 2016, Dawn found definitive evidence of water molecules on the surface of Ceres at Oxo crater.

On 9 December 2015, NASA scientists reported that the bright spots on Ceres may be due to a type of salt, specifically evaporated brine containing magnesium sulfate hexahydrate (MgSO4·6H2O); the spots were also found to be associated with ammonia-rich clays. Near-infrared spectra of these bright areas were reported in 2017 to be consistent with a large amount of sodium carbonate (Na
2CO
3
) and smaller amounts of ammonium chloride (NH
4Cl
) or ammonium bicarbonate (NH
4HCO
3
). These materials have been suggested to originate from the crystallization of brines that reached the surface. In August 2020, NASA confirmed that Ceres was a water-rich body with a deep reservoir of brine that percolated to the surface in hundreds of locations causing “bright spots”, including those in Occator crater.

Ahuna Mons is an estimated 5 km (3 mi) high on its steepest side.

Cerealia and Vinalia Faculae

Tectonic features

Although Ceres lacks plate tectonics, with the vast majority of its surface features linked either to impacts or to cryovolcanic activity, several potentially tectonic features have been tentatively identified on its surface, particularly in its eastern hemisphere. The Samhain Catenae, kilometer-scale linear fractures on Ceres’s surface, lack any apparent link to impacts and bear a stronger resemblance to pit crater chains, which are indicative of buried normal faults. Also, several craters on Ceres have shallow, fractured floors consistent with cryomagmatic intrusion.

Boulders

Dawn observed 4,423 boulders larger than 105 m (344 ft) in diameter on the surface of Ceres. These boulders are likely formed through impacts, and thus are found within or near craters, though not all craters contain boulders. Vast regions of the surface of Ceres lack any craters larger than 100 m (300 ft) in diameter. Large boulders are more numerous at higher latitudes. Boulders on Ceres are brittle and degrade rapidly due to thermal stress (at dawn and dusk, the surface temperature changes rapidly) and meteoritic impacts. Their maximum age is calculated to be 150 million years, much shorter than the lifetime of boulders on Vesta.

Internal structure

The active geology of Ceres is driven by ice and brines. Water leached from rock is estimated to possess salinity of around 5%. Altogether, Ceres is approximately 50% water by volume, compared to 0.1% for Earth, and 73% rock by mass.

Ceres’s largest craters are several kilometers deep, inconsistent with an ice-rich shallow subsurface. The fact that the surface has preserved craters almost 300 km (200 mi) in diameter indicates that the outermost layer of Ceres is roughly 1000 times stronger than water ice. This is consistent with a mixture of silicates, hydrated salts and methane clathrates, with no more than approximately 30% water ice by volume.

Gravity measurements from Dawn have generated three competing models for Ceres’s interior. In the three-layer model, Ceres is thought to consist of an inner muddy mantle of hydrated rock, such as clays, an intermediate layer comprising a muddy mixture of brine and rock down to a depth of at least 100 km (60 mi), and an outer, 40 km (25 mi) thick crust of ice, salts and hydrated minerals. It is not possible to tell if Ceres’s deep interior contains liquid or a core of dense material rich in metal, but the low central density suggests it may retain about 10% porosity. One study estimated the densities of the core and mantle/crust to be 2.46–2.90 and 1.68–1.95 g/cm3 respectively, with the mantle and crust together being 70–190 km (40–120 mi) thick. Only partial dehydration (expulsion of ice) from the core is expected, though the high density of the mantle relative to water ice reflects its enrichment in silicates and salts. That is, the core (if it exists), the mantle and crust all consist of rock and ice, though in different ratios.

Three-layer model of Ceres’s internal structure:

  • Thick outer crust (ice, salts, hydrated minerals)
  • Salt-rich liquid (brine) and rock
  • Mantle (hydrated rock)

The mineral composition can only be determined (indirectly) for the outer 100 km (60 mi). The 40 km (25 mi)–thick solid outer crust is a mixture of ice, salts, and hydrated minerals. Under that is a layer that may contain a small amount of brine. This extends to a depth of at least the 100 km (60 mi) limit of detection. Under that is thought to be a mantle dominated by hydrated rocks such as clays.

In one two-layer model, Ceres consists of a core of chondrules and a mantle of mixed ice and micron-sized solid particulates (“mud”). Sublimation of ice at the surface would leave a deposit of hydrated particulates perhaps 20 meters thick. The range of the extent of differentiation is consistent with the data, from a large, 360 km (220 mi) core of 75% chondrules and 25% particulates and a mantle of 75% ice and 25% particulates, to a small, 85 km (55 mi) core consisting nearly entirely of particulates and a mantle of 30% ice and 70% particulates. With a large core, the core–mantle boundary should be warm enough for pockets of brine. With a small core, the mantle should remain liquid below 110 km (68 mi). In the latter case, a 2% freezing of the liquid reservoir would compress the liquid enough to force some to the surface, producing cryovolcanism.

A second two-layer 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 (40 mi) thick and have a density of 1.68 g/cm3; with CM-class meteorites (density 2.9 g/cm3), the crust would be approximately 190 km (120 mi) thick and have a density of 1.9 g/cm3. Best-fit modeling yields a crust approximately 40 km (25 mi) thick with a density of approximately 1.25 g/cm3, and a mantle/core density of approximately 2.4 g/cm3.

Atmosphere

In 2017, Dawn confirmed that Ceres has a transient atmosphere of water vapor derived from exposed surface ice evaporated by the Sun. Hints of an atmosphere had appeared in early 2014, when the Herschel Space Observatory detected localized mid-latitude sources of water vapor on Ceres, no more than 60 km (40 mi) in diameter, which each give off approximately 1026 molecules (or 3 kg) of water per second. Two potential source regions, designated Piazzi (123°E, 21°N) and Region A (231°E, 23°N), were visualized in the near infrared as dark areas (Region A also has a bright center) by the Keck Observatory. Possible mechanisms for the vapor release are sublimation from approximately 0.6 km2 (0.2 sq mi) of exposed surface ice, cryovolcanic eruptions resulting from radiogenic internal heat, or pressurization of a subsurface ocean due to thickening of an overlying layer of ice. In 2015, David Jewitt included Ceres in his list of active asteroids. Surface water ice is unstable at distances less than 5 AU from the Sun, so it is expected to sublime if it is exposed directly to solar radiation. Water ice can migrate from the deep layers of Ceres to the surface, but escapes in a short time. Surface sublimation would be expected to be lower when Ceres is farther from the Sun in its orbit, whereas internally powered emissions should not be affected by its orbital position. The limited data previously available was more consistent with cometary-style sublimation, though subsequent evidence from Dawn strongly suggests ongoing geologic activity could be at least partially responsible.

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

Origin and evolution

Ceres is a surviving protoplanet that formed 4.56 billion years ago; alongside Pallas and Vesta, the only remaining in the inner Solar System, with the rest either merging to form terrestrial planets, being shattered in collisions or being ejected by Jupiter. Despite this, its composition is not consistent with a formation within the asteroid belt. It seems rather that Ceres formed between the orbits of Jupiter and Saturn, and was scattered into the asteroid belt as Jupiter migrated outward. The discovery of ammonia salts in Occator crater supports an origin in the outer Solar System, as ammonia is far more abundant in that region.

The early geological evolution of Ceres was dependent on the heat sources available during and after its formation: impact energy from planetesimal accretion and decay of radionuclides (possibly including short-lived extinct radionuclides such as aluminium-26). These may have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation, and even a liquid water ocean. This water ocean 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.

Ceres possesses a surprisingly small number of large craters, suggesting that viscous relaxation and cryovolcanism has erased older geological features. The presence of clays and carbonates requires chemical reactions at temperatures above 50 °C, consistent with hydrothermal activity.

Ceres has become considerably less geologically active over time, with a surface dominated by impact craters; nevertheless, evidence from Dawn reveals that internal processes have continued to sculpt Ceres’s surface to a significant extent, in stark contrast to Vesta and to previous expectations that Ceres would have become geologically dead early in its history due to its small size.

Potential habitability

Although Ceres is not as actively discussed as a potential home for microbial extraterrestrial life as Mars, Europa, Enceladus, or Titan are, it has the most water of any body in the inner Solar System after Earth, and the likely brine pockets under its surface could provide habitats for life. Although it does not experience tidal heating, like Europa or Enceladus, it is close enough to the Sun, and contains enough long-lived radioactive isotopes, to preserve liquid water in its subsurface for extended periods. The remote detection of organic compounds and the presence of water mixed with 20% carbon by mass in its near surface could provide conditions favorable to organic chemistry. Of the biochemical elements, Ceres is rich in carbon, hydrogen, oxygen and nitrogen, but phosphorus has yet to be detected, and sulfur, despite being suggested by Hubble UV observations, was not detected by Dawn.

Hydrogen concentration (blue) in the upper meter of the regolith indicating presence of water ice

Observation

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

On 13 November 1984, an occultation of the star BD+8°471 by Ceres was observed in Mexico, Florida and across the Caribbean, allowing better measurements of its size, shape and albedo. On 25 June 1985, Hubble attained ultraviolet images of Ceres with 50 km (30 mi) resolution.In 2002, the Keck Observatory attained infrared images with 30 km (20 mi) resolution using adaptive optics.

Before the Dawn mission, only a few surface features had been unambiguously detected on Ceres. High-resolution ultraviolet images taken by the Hubble Space Telescope in 1995 showed a dark spot on its surface, which was nicknamed “Piazzi” in honor of the discoverer of Ceres. It was thought to be a crater. Visible-light images of a full rotation taken by Hubble in 2003 and 2004 showed 11 recognizable surface features, the natures of which were undetermined. One of these corresponded to the Piazzi feature. Near-infrared images over a whole rotation, taken with adaptive optics by the Keck Observatory in 2012, showed bright and dark features moving with Ceres’s rotation. 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. Dawn would eventually reveal Piazzi to be a dark region in the middle of Vendimia Planitia, close to the crater Dantu, and the other dark feature to be within Hanami Planitia and close to Occator crater.

An enhanced Hubble image of Ceres, the best acquired from Earth, taken in 2004

Proposed exploration

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

Dawn mission

In the early 1990s, NASA initiated the Discovery Program, which was intended to be a series of low-cost scientific missions. In 1996, the program’s study team recommended as a high priority a mission to explore the asteroid belt using a spacecraft with an ion engine. Funding for this program remained problematic for nearly a decade, but by 2004 the Dawn vehicle had passed its critical design review.

Dawn was launched on 27 September 2007, as the first space mission to visit either Vesta or Ceres. On 3 May 2011, Dawn acquired its first targeting image 1,200,000 km (750,000 mi) from Vesta. After orbiting Vesta for 13 months, Dawn used its ion engine to depart for Ceres, with gravitational capture occurring on 6 March 2015 at a separation of 61,000 km (38,000 mi), four months prior to the New Horizons flyby of Pluto.

The spacecraft instrumentation included a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These instruments examined Ceres’s shape and elemental composition. On 13 January 2015, as Dawn approached Ceres, the spacecraft took its first images at near-Hubble resolution, revealing impact craters and a small high-albedo spot on the surface, 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.

Artist’s conception of Dawn spacecraft, travelling from Vesta to Ceres

Dawn’s mission profile called for it to study Ceres from a series of circular polar orbits at successively lower altitudes. It entered its first observational orbit (“RC3”) around Ceres at an altitude of 13,500 km (8,400 mi) on 23 April 2015, staying for only approximately one orbit (15 days). The spacecraft subsequently reduced its orbital distance to 4,400 km (2,700 mi) for its second observational orbit (“survey”) for three weeks, then down to 1,470 km (910 mi) (“HAMO;” high altitude mapping orbit) for two months and then down to its final orbit at 375 km (233 mi) (“LAMO;” low altitude mapping orbit) for at least three months. In October 2015, NASA released a true-color portrait of Ceres made by Dawn. In 2017, Dawn’s mission was extended to perform a series of closer and tighter orbits around Ceres until the hydrazine used to maintain its orbit ran out.

Dawn soon discovered evidence of cryovolcanism. Two distinct bright spots (or high-albedo features) inside a crater (different from the bright spots observed in earlier Hubble images) were seen in a 19 February 2015 image, leading to speculation about a possible cryovolcanic origin or outgassing. On 2 September 2016, scientists from the Dawn team argued in a Science paper that Ahuna Mons was the strongest evidence yet for cryovolcanic features. On 11 May 2015, NASA released a higher-resolution image showing that the spots were actually composed of multiple smaller spots. 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. 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.

Animation of Dawn‘s trajectory around Ceres from 1 February 2015 to 1 February 2025

   Dawn ·   Ceres

Future missions

In 2020, an ESA team proposed the Calathus Mission concept, a followup mission to Occator Crater, to return a sample of the bright carbonate faculae and dark organics to Earth. The Chinese Space Agency is designing a sample-return mission from Ceres that would take place during the 2020s.