Ceres (dwarf planet)

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1 Ceres
arcsec / yr
Physical characteristics
Dimensions(966.2 × 962.0 × 891.8)
± 0.2 km[6]
Mean diameter
939.4±0.2 km[6]
2,772,368 km2[7]
Volume434,000,000 km3[8]
Mass
Mean density
2.1616±0.0025 g/cm3[8]
Equatorial surface gravity
0.36±0.15[9][a] (estimate)
Equatorial escape velocity
0.516 km/s[7] 1141 mph
Sidereal rotation period
9.074170±0.000001 h[2]
Equatorial rotation velocity
92.61 m/s[7]
≈4°[11]
North pole right ascension
291.42744°[12]
North pole declination
66.76033°[13]
0.090±0.0033 (V-band)[14]
Surface temp. min mean max
Kelvin ≈110[15] 235±4[16]
C[17]
3.34[2]
0.854″ to 0.339″

Ceres (minor-planet designation: 1 Ceres) is a dwarf planet in the middle main asteroid belt between the orbits of Mars and Jupiter. It was the first known asteroid, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily, and announced as a new planet. Ceres was later classified as an asteroid and then a dwarf planet, the only one always inside Neptune's orbit.

Ceres's small size means that even at its brightest, it is too dim to be seen by the

synodic period. As a result, its surface features are barely visible even with the most powerful telescopes, and little was known about it 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

microbial
life.

In January 2014, emissions of water vapour were detected around Ceres, creating a tenuous, transient atmosphere known as an exosphere.[20]

History

Discovery

In the years between the acceptance of heliocentrism in the 18th century and the discovery of Neptune in 1846, several astronomers argued that mathematical laws predicted the existence of a hidden or missing planet between the orbits of Mars and Jupiter. In 1596, theoretical astronomer Johannes Kepler believed that the ratios between planetary orbits would conform to "God's design" only with the addition of two planets: one between Jupiter and Mars and one between Venus and Mercury.[21] 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?"[21]

In 1772, German astronomer Johann Elert Bode, citing Johann Daniel Titius, published a formula later known as the Titius–Bode law that appeared to predict the orbits of the known planets but for an unexplained gap between Mars and Jupiter.[21][22] This formula predicted that there ought to be another planet with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun.[22] The Titius–Bode law gained more credence with William Herschel's 1781 discovery of Uranus near the predicted distance for a planet beyond Saturn.[21] In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal Monatliche Correspondenz [de] (Monthly Correspondence), sent requests to twenty-four experienced astronomers, whom he dubbed the "celestial police",[22] asking that they combine their efforts and begin a methodical search for the expected planet.[22] Although they did not discover Ceres, they later found the asteroids Pallas, Juno, and Vesta.[22]

One of the astronomers selected for the search was

Mr la Caille",[21] but found that "it was preceded by another".[21] Instead of a star, Piazzi had found a moving starlike object, which he first thought was a comet.[24] Piazzi observed Ceres twenty-four times, the final sighting occurring on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin.[25] 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".[21] 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.[24]

By this time, the apparent position of Ceres had changed (primarily 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. Towards 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

Heinrich W. M. Olbers found Ceres near the predicted position and continued to record its position.[24] At 2.8 AU from the Sun, Ceres appeared to fit the Titius–Bode law almost perfectly; when Neptune was discovered in 1846, eight AU closer than predicted, most astronomers concluded that the law was a coincidence.[26]

The early observers were able to calculate the size of Ceres only to within an order of magnitude. Herschel underestimated its diameter at 260 km (160 mi) in 1802; in 1811, German astronomer Johann Hieronymus Schröter overestimated it as 2,613 km (1,624 mi).[27] In the 1970s, infrared photometry enabled more accurate measurements of its albedo, and Ceres's diameter was determined to within ten percent of its true value of 939 km (583 mi).[27]

Name and symbol

Piazzi's proposed name for his discovery was Ceres Ferdinandea: Ceres after the

Roman goddess of agriculture, whose earthly home, and oldest temple, lay in Sicily; and Ferdinandea in honour of Piazzi's monarch and patron, King Ferdinand III of Sicily.[24] 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.[28]

The adjectival forms of Ceres are Cererian[29][30] and Cererean,[31] both pronounced /sɪˈrɪəriən/.[32][33] Cerium, a rare-earth element discovered in 1803, was named after the dwarf planet Ceres.[34][b]

The old

astronomical symbol of Ceres, still used in astrology, is a sickle, ⚳.[24][36] The sickle was one of the classical symbols of the goddess Ceres and was suggested, apparently independently, by von Zach and Bode in 1802.[37] In form, it is similar to the symbol ⟨♀⟩ (a circle with a small cross beneath) of the planet Venus, but with a break in the circle. It had various minor graphic variants, including a reversed form typeset as a 'C' (the initial letter of the name Ceres) with a plus sign. The generic asteroid symbol of a numbered disk, ①, was introduced in 1867 and quickly became the norm.[24][38]

Classification

Ceres (bottom left), the Moon and Earth, shown to scale
Ceres (bottom left), the Moon and Earth, shown to scale
Relative sizes of the four largest asteroids. Ceres is furthest left.
Relative mean diameters of the four largest minor planets in the asteroid belt (dwarf planet Ceres at left)
The mass of 1 Ceres (blue) compared to other large asteroids: 4 Vesta, 2 Pallas, 10 Hygiea, 704 Interamnia, 15 Eunomia and the remainder of the Main Belt. The unit of mass is ×1018 kg.

The categorisation of Ceres has changed more than once and has been the subject of some disagreement. Bode believed Ceres to be the "missing planet" he had proposed to exist between Mars and Jupiter.[21] 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.[39]

As other objects were discovered in the neighbourhood of Ceres, astronomers began to suspect that it represented the first of a new class of objects.

the new system under the name 1 Ceres.[39]

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.[39] In the 1950s, scientists generally stopped considering most asteroids as planets, but Ceres sometimes retained its status after that because of its planet-like geophysical complexity.[41] Then, in 2006, the debate surrounding Pluto led to calls for a definition of "planet", and the possible reclassification of Ceres, perhaps even its general reinstatement as a planet.[42] 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".[43] Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun,[44] but on 24 August 2006 the assembly adopted the additional requirement that a planet must have "cleared the neighbourhood around its orbit". Ceres is not a planet because it does not dominate its orbit, sharing it as it does with the thousands of other asteroids in the asteroid belt and constituting only about forty percent of the belt's total mass.[45] Bodies that met the first proposed definition but not the second, such as Ceres, were instead classified as dwarf planets.[46] Planetary geologists still often ignore this definition and consider Ceres to be a planet anyway.[47]

Ceres is a dwarf planet, but there is some confusion about whether it is also an asteroid. A NASA webpage states that Vesta, the belt's second-largest object, is the largest asteroid.[48] The IAU has been equivocal on the subject,[49][50] though its Minor Planet Center, the organisation charged with cataloguing such objects, notes that dwarf planets may have dual designations,[51] and the joint IAU/

USGS/NASA Gazetteer categorizes Ceres as both asteroid and a dwarf planet.[52]

Orbit

Orbits of Ceres (red, inclined) along with Jupiter and the inner planets (white and grey). 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.

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.[2] Compared to other planets and dwarf planets, Ceres's orbit is moderately tilted relative to that of Earth; its inclination (i) is 10.6°, compared to 7° for Mercury and 17° for Pluto. It is also slightly elongated, with an eccentricity (e) = 0.08, compared to 0.09 for Mars.[2]

Ceres is not part of an

interloper, having similar orbital elements but not a common origin.[55]

Resonances

Due to their small masses and large separations, objects within the asteroid belt rarely fall into gravitational

trojans), for periods from a few hundred thousand to more than two million years. Fifty such objects have been identified.[57] 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.[58]

Rotation and axial tilt

Permanently shadowed regions capable of accumulating surface ice

The rotation period of Ceres (the Cererian day) is 9 hours and 4 minutes;[11] the small equatorial crater of Kait is selected as its prime meridian.[59] Ceres has an axial tilt of 4°,[11] small enough for its 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.[11]

Dawn, the first spacecraft to orbit Ceres, determined that the north polar axis points at right ascension 19 h 25 m 40.3 s (291.418°), declination +66° 45' 50" (about 1.5 degrees from Delta Draconis), which means an axial tilt of 4°. This means that Ceres currently sees little to no seasonal variation in sunlight by latitude.[60] Over the course of three million years, gravitational influence from Jupiter and Saturn has triggered cyclical shifts in Ceres's axial tilt, ranging from two to twenty degrees, meaning that seasonal variation in sun exposure has occurred in the past, with the last period of seasonal activity estimated at 14,000 years ago. Those craters that remain in shadow during periods of maximum axial tilt are the most likely to retain water ice from eruptions or cometary impacts over the age of the Solar System.[61]

Geology

Ceres is the largest asteroid in the main asteroid belt.[17] It has been classified as a C‑type or carbonaceous asteroid[17] and, due to the presence of clay minerals, as a G-type asteroid.[62] It has a similar, but not identical, composition to that of carbonaceous chondrite meteorites.[63] It is an oblate spheroid, with an equatorial diameter 8% larger than its polar diameter.[2] Measurements from the Dawn spacecraft found a mean diameter of 939.4 km (583.7 mi)[2] and a mass of 9.38×1020 kg.[64] This gives Ceres a density of 2.16 g/cm3,[2] suggesting that a quarter of its mass is water ice.[65]

Ceres comprises 40% of the estimated (2394±5)×1018 kg mass of the asteroid belt, and it has 3+12 times the mass of the next asteroid, Vesta, but it is only 1.3% the mass of the Moon. It is close to being in hydrostatic equilibrium, but some deviations from an equilibrium shape have yet to be explained.[66] Assuming it is in equilibrium, Ceres is the only dwarf planet that is always within the orbit of Neptune.[65] Modelling has suggested Ceres's rocky material is partially differentiated, and that it may possess a small core,[67][68] but the data is also consistent with a mantle of hydrated silicates and no core.[66] Because Dawn lacked a magnetometer, it is not known if Ceres has a magnetic field; it is believed not to.[69][70] Ceres's internal differentiation may be related to its lack of a natural satellite, as satellites of main belt asteroids are mostly believed to form from collisional disruption, creating an undifferentiated, rubble pile structure.[71]

Surface

Composition

The surface composition of Ceres is homogeneous on a global scale, and is rich in

phyllosilicates that have been altered by water,[66] though water ice in the regolith varies from approximately 10% in polar latitudes to much drier, even ice-free, in the equatorial regions.[66]

Studies using the Hubble Space Telescope show graphite, sulfur, and sulfur dioxide 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 so are evidently associated with areas with relatively recent geological activity.[72]

Organic compounds were detected in Ernutet Crater,[73] and most of the planet's near surface is rich in carbon, at approximately 20% by mass.[74] The carbon content is more than five times higher than in carbonaceous chondrite meteorites analysed on Earth.[74] The surface carbon shows evidence of being mixed with products of rock-water interactions, such as clays.[74] 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 favourable to organic chemistry.[74]

  • Black-and-white photographic map of Ceres, centred on 180° longitude, with official nomenclature (September 2017)
    Black-and-white photographic map of Ceres, centred on 180° longitude, with official nomenclature (September 2017)
  • Ceres, polar regions (November 2015): North (left); south (right). The south pole is in shadow. "Ysolo Mons" has since been renamed "Yamor Mons."[75]
    Ceres, polar regions (November 2015): North (left); south (right). The south pole is in shadow. "Ysolo Mons" has since been renamed "Yamor Mons."[75]

Craters

Topographic map of Ceres. The lowest crater floors (indigo) and the highest peaks (white) represent a difference of 15 km (10 mi) elevation.[76] "Ysolo Mons" has been renamed "Yamor Mons."[75]

Dawn revealed that Ceres has a heavily cratered surface, though with fewer large craters than expected.[77] Models based on the formation of the current asteroid belt had predicted Ceres should have ten to fifteen craters larger than 400 km (250 mi) in diameter.[77] The largest confirmed crater on Ceres, Kerwan Basin, is 284 km (176 mi) across.[78] The most likely reason for this is viscous relaxation of the crust slowly flattening out larger impacts.[77][79]

Ceres's north polar region shows far more cratering than the equatorial region, with the eastern equatorial region in particular comparatively lightly cratered.

Vendimia Planitia, at 800 km (500 mi) across,[77] is also the largest single geographical feature on Ceres.[81] Two of the three have higher than average ammonium concentrations.[66]

Dawn observed 4,423 boulders larger than 105 m (344 ft) in diameter on the surface of Ceres. These boulders likely formed through impacts, and are found within or near craters, though not all craters contain boulders. 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 estimated to be 150 million years, much shorter than the lifetime of boulders on Vesta.[82]

Tectonic features

Although Ceres lacks

normal faults. Also, several craters on Ceres have shallow, fractured floors consistent with cryomagmatic intrusion.[85]

Cryovolcanism

A smooth-sided mountain rising from a grey surface
Ahuna Mons is an estimated 5 km (3 mi) high on its steepest side.[86]
Icy patches against a grey, flat background
Cerealia and Vinalia Faculae

Ceres has one prominent mountain,

diapirism of a slurry of brine and silicate particles from the top of the mantle.[53] 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 triggering the movement of high-viscosity cryomagma (muddy water ice softened by its content of salts) onto the surface.[88] Kerwan too shows evidence of the effects of liquid water due to impact-melting of subsurface ice.[78]

A 2018 computer simulation suggests that cryovolcanoes on Ceres, once formed, recede due to viscous relaxation over several hundred million years. The team identified 22 features as strong candidates for relaxed cryovolcanoes on Ceres's surface.[87][89] Yamor Mons, an ancient, impact-cratered peak, resembles Ahuna Mons despite being much older, due to it lying in Ceres's northern polar region, where lower temperatures prevent viscous relaxation of the crust.[84] Models suggest that, over the past billion years, one cryovolcano has formed on Ceres on average every fifty million years.[84] The eruptions are not uniformly distributed over Ceres, but may be linked to ancient impact basins.[84] 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.[87]

An unexpectedly large number of Cererian craters have central pits, perhaps due to cryovolcanic processes; others have central peaks.

Oxo crater.[97]

On 9 December 2015, NASA scientists reported that the bright spots on Ceres may be due to a type of salt from evaporated brine containing

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
).[99][100] These materials have been suggested to originate from the crystallisation of brines that reached the surface.[101] 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[102] causing "bright spots", including those in Occator Crater.[103]

Internal structure

a cutaway image of the interior of Ceres
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 active geology of Ceres is driven by ice and brines. Water leached from rock is estimated to possess a salinity of around 5%. Altogether, Ceres is approximately 50% water by volume (compared to 0.1% for Earth) and 73% rock by mass.[15]

Ceres's largest craters are several kilometres 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 30% water ice by volume.[66][104]

Gravity measurements from Dawn have generated three competing models for Ceres's interior.[15] In the three-layer model Ceres is thought to consist of an outer, 40 km (25 mi) thick crust of ice, salts and hydrated minerals and an inner muddy "mantle" of hydrated rock, such as clays, separated by a 60 km (37 mi) layer of a muddy mixture of brine and rock.[105] It is not possible to tell if Ceres's deep interior contains liquid or a core of dense material rich in metal,[106] but the low central density suggests it may retain about 10% porosity.[15] 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.[10] That is, the core (if it exists), the mantle and crust all consist of rock and ice, though in different ratios.

Ceres's mineral composition can be determined (indirectly) only for its outer 100 km (60 mi). The solid outer crust, 40 km (25 mi) thick, is a mixture of ice, salts, and hydrated minerals. Under that is a layer that may contain a small amount of brine. This extends to a depth of at least the 100 km (60 mi) limit of detection. Under that is thought to be a mantle dominated by hydrated rocks such as clays.[106]

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 twenty metres 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.[107]

A second two-layer model suggests a partial differentiation of Ceres into a volatile-rich crust and a denser mantle of hydrated silicates. A range of densities for the crust and mantle can be calculated from the types of meteorite thought to have impacted Ceres. With CI-class meteorites (density 2.46 g/cm3), the crust would be approximately 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 modelling 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.[66]

Atmosphere

In 2017, Dawn confirmed that Ceres has a transient atmosphere of water vapour.

David Jewitt included Ceres in his list of active asteroids.[114] Surface water ice is unstable at distances less than 5 AU from the Sun,[115] so it is expected to sublime if 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, and internally powered emissions should not be affected by its orbital position. The limited data previously available suggested cometary-style sublimation,[109] but evidence from Dawn suggests geologic activity could be at least partially responsible.[116]

Studies using Dawn's gamma ray and neutron detector (GRaND) reveal that Ceres accelerates electrons from the solar wind; the most accepted hypothesis is that these electrons are being accelerated by collisions between the solar wind and a tenuous water vapour exosphere.[117][118] Bow shocks like these could also be explained by a transient magnetic field, but this is considered less likely, as the interior of Ceres is not thought to be sufficiently electrically conductive.[118]

Origin and evolution

Ceres is a surviving protoplanet that formed 4.56 billion years ago; alongside Pallas and Vesta, one of only three remaining in the inner Solar System,[119] with the rest either merging to form terrestrial planets, being shattered in collisions[120] or being ejected by Jupiter.[121] Despite Ceres's current location, its composition is not consistent with having formed within the asteroid belt. It seems rather that it formed between the orbits of Jupiter and Saturn, and was deflected into the asteroid belt as Jupiter migrated outward.[15] The discovery of ammonium salts in Occator Crater supports an origin in the outer Solar System, as ammonia is far more abundant in that region.[122]

The early geological evolution of Ceres was dependent on the heat sources available during and after its formation: impact energy from

core and icy mantle, or even a liquid water ocean,[66] soon after its formation.[68] This 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.[66]

Ceres possesses surprisingly few large craters, suggesting that viscous relaxation and cryovolcanism have erased older geological features.[123] The presence of clays and carbonates requires chemical reactions at temperatures above 50 °C, consistent with hydrothermal activity.[53]

It has become considerably less geologically active over time, with a surface dominated by impact craters; nevertheless, evidence from Dawn reveals that internal processes have continued to sculpt Ceres's surface to a significant extent[124] contrary to predictions that Ceres's small size would have ceased internal geological activity early in its history.[125]

Habitability

Europa, Enceladus, or Titan are, it has the most water of any body in the inner Solar System after Earth,[53] and the likely brine pockets under its surface could provide habitats for life.[53] It does not experience tidal heating, like Europa or Enceladus, but it is close enough to the Sun, and contains enough long-lived radioactive isotopes, to preserve liquid water in its subsurface for extended periods.[53] The remote detection of organic compounds and the presence of water mixed with 20% carbon by mass in its near surface could provide conditions favourable to organic chemistry.[74] Of the biochemical elements, Ceres is rich in carbon, hydrogen, oxygen and nitrogen,[126] but phosphorus has yet to be detected,[127] and sulfur, despite being suggested by Hubble UV observations, was not detected by Dawn.[53]

Observation and exploration

Observation

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.[18]

On 13 November 1984, an

Keck Observatory obtained infrared images with 30 km (20 mi) resolution using adaptive optics.[131]

Before the Dawn mission only a few surface features had been unambiguously detected on Ceres. High-resolution

Dawn mission