Ceres (dwarf planet)

Ceres

Ceres as seen by Hubble Space Telescope ( ACS). The contrast has been enhanced to reveal surface details.
Discovery
Discovered by	Giuseppe Piazzi
Discovery date	1 January 1801
Designations
MPC designation	1 Ceres
Pronunciation	/ ˈ s ɪər iː z / SEER-eez
Named after	Cerēs
Alternative names	A899 OF; 1943 XB
Minor planet category	dwarf planet main belt
Adjective	Cererian /sɨˈrɪəri.ən/
Orbital characteristics
Epoch 18 June 2009 ( JD 2455000.5)
Aphelion	2.9858 AU (446,669,320 km)
Perihelion	2.5468 AU (380,995,855 km)
Semi-major axis	2.7654 AU (413,690,604 km)
Eccentricity	0.079138
Orbital period	4.60  yr 1679.67 d
Average orbital speed	17.882 km/s
Mean anomaly	113.410°
Inclination	10.587° to Ecliptic 9.20° to Invariable plane
Longitude of ascending node	80.3932°
Argument of perihelion	72.5898°
Proper orbital elements
Proper semi-major axis	2.7670962 AU
Proper eccentricity	0.1161977
Proper inclination	9.6474122°
Proper mean motion	78.193318 deg / yr
Proper orbital period	4.60397 yr (1681.601 d)
Precession of perihelion	54.070272 arcsec / yr
Precession of the ascending node	−59.170034 arcsec / yr
Physical characteristics
Equatorial radius	487.3 ± 1.8 km
Polar radius	454.7 ± 1.6 km
Surface area	2,850,000 sq km
Mass	9.43 ± 0.07×1020 kg 0.00015 Earths 0.0128 Moons
Mean density	2.077 ± 0.036 g/cm3
Equatorial surface gravity	0.27 m/s2 0.028 g
Escape velocity	0.51 km/s
Sidereal rotation period	0.3781 d 9.074170 h
Axial tilt	about 3°
North pole right ascension	19 h 24 min 291°
North pole declination	59°
Albedo	0.090 ± 0.0033 ( V-band geometric)
Surface temp. min mean max Kelvin ? ~168 K 235 K
Spectral type	C
Apparent magnitude	6.64 to 9.34
Absolute magnitude (H)	3.36 ± 0.02
Angular diameter	0.854" to 0.339"

Ceres, minor-planet designation 1 Ceres, is the only dwarf planet in the inner Solar System, and the largest asteroid. It is a rock–ice body 950 km (590 mi) in diameter, and though the smallest identified dwarf planet, it constitutes a third of the mass of the asteroid belt. Discovered on 1 January 1801 by Giuseppe Piazzi, it was the first asteroid to be identified, though it was classified as a planet at the time. It is named after Ceres, the Roman goddess of growing plants, the harvest, and motherly love.

The Cererian surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clays. It appears to be differentiated into a rocky core and icy mantle, and may harbour an ocean of liquid water under its surface. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, and hence even at its brightest it is still too dim to be seen with the naked eye except under extremely dark skies.

The unmanned Dawn spacecraft, launched on 27 September 2007 by NASA, is expected to be the first to explore Ceres after its scheduled arrival there in 2015. The spacecraft left asteroid 4 Vesta about 5 September 2012, which it had been orbiting since July 2011.

Discovery

The idea that an undiscovered planet could exist between the orbits of Mars and Jupiter was suggested by Johann Elert Bode in 1772. Previously, in 1596, Kepler had already noticed the gap between Mars and Jupiter. Bode's considerations were based on the Titius–Bode law, a now discredited hypothesis which had been first proposed by Johann Daniel Titius in 1766, observing that there was a regular pattern in the semi-major axes of the orbits of known planets marred only by the large gap between Mars and Jupiter. The pattern predicted that the missing planet ought to have an orbit with a semi-major axis near 2.8 AU. William Herschel's discovery of Uranus in 1781 near the predicted distance for the next body beyond Saturn increased faith in the law of Titius and Bode, and in 1800, they sent requests to twenty-four experienced astronomers, asking that they combine their efforts and begin a methodical search for the expected planet. The group was headed by Franz Xaver von Zach, editor of the Monatliche Correspondenz. While they did not discover Ceres, they later found several large asteroids.

One of the astronomers selected for the search was Giuseppe Piazzi at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Giuseppe 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 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. 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 Jérôme Lalande in Paris. 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 the 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. 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.

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

Name

Piazzi originally suggested the name Cerere Ferdinandea for his discovery, after both the mythological figure Ceres (Roman goddess of agriculture, Italian Cerere) and King Ferdinand III of Sicily. "Ferdinandea" was not acceptable to other nations of the world and was thus dropped. Ceres was also called Hera for a short time in Germany. In Greece, it is called Demeter (Δήμητρα), after the Greek equivalent of the Roman goddess Cerēs; in English, that name is used for the asteroid 1108 Demeter. The adjectival form of the name is Cererian, derived from the Latin genitive Cerēris. The old astronomical symbol of Ceres is a sickle, ⚳ (), similar to Venus's symbol ♀, but with a gap in the upper circle (and with a variant under the influence of the initial 'C'); this was later replaced with the numbered disk ①.

The element cerium, discovered in 1803, was named after the asteroid. In the same year, another element was also initially named after Ceres, but its discoverer changed its name to palladium (after the second asteroid, 2 Pallas) when cerium was named.

Status

The classification 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. 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 about half a century.

As other objects were discovered in the area it was realised that Ceres represented the first of a class of many similar bodies. In 1802 Sir William Herschel coined the term asteroid ("star-like") for such bodies, writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes". As the first such body to be discovered, it was given the designation 1 Ceres under the modern system of asteroid numbering.

The 2006 debate surrounding Pluto and what constitutes a 'planet' led to Ceres being considered for reclassification as a planet. 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". Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun. It was not accepted, and in its place an alternate definition came into effect as of 24 August 2006, carrying the additional requirement that a "planet" must have " cleared the neighbourhood around its orbit". By this definition, Ceres is not a planet because it does not dominate its orbit, sharing it with the thousands of other asteroids in the asteroid belt and constituting only about a third of the total mass. It is instead now classified as a dwarf planet.

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", while 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's path and otherwise implies that Ceres is still one of the asteroids. The Minor Planet Centre notes that such bodies may have dual designations. The 2006 IAU decision that classified Ceres as a dwarf planet never addressed whether it is or is not an asteroid, as indeed the IAU has never defined the word 'asteroid' at all, preferring the term ' minor planet' until 2006, and ' 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". NASA continues to refer to Ceres as an asteroid, saying in a 2011 press announcement that "Dawn will orbit two of the largest asteroids in the Main Belt", as do various academic textbooks.

Physical characteristics

Ceres is the largest object in the asteroid belt between Mars and Jupiter. The mass of Ceres has been determined by analysis of the influence it exerts on smaller asteroids. Results differ slightly between researchers. The average of the three most precise values as of 2008 is 9.4×10²⁰ kg. With this mass Ceres comprises about a third of the estimated total 3.0 ± 0.2×10²¹ kg mass of the asteroid belt, which is in turn about 4% of the mass of the Moon. The surface area is approximately equal to the land area of India or Argentina. The mass of Ceres is sufficient to give it a nearly spherical shape in hydrostatic equilibrium. In contrast, other large asteroids such as 2 Pallas, 3 Juno, and in particular 10 Hygiea are known to be somewhat irregular in shape.

Internal structure

Ceres's oblateness is inconsistent with an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle. This 100 km-thick mantle (23%–28% of Ceres by mass; 50% by volume) contains 200 million cubic kilometres of water, which is more than the amount of fresh water on the Earth. This result is supported by the observations made by the Keck telescope in 2002 and by evolutionary modelling. 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.

Alternatively, the shape and dimensions of Ceres may be explained by an interior that is porous and either partially differentiated or completely undifferentiated. The presence of a layer of rock on top of ice would be gravitationally unstable. If any of the rock deposits sank into a layer of differentiated ice, salt deposits would be formed. Such deposits have not been detected. Thus it is possible that Ceres does not contain a large ice shell, but was instead formed from low-density asteroids with an aqueous component. The decay of radioactive isotopes may not have been sufficient to cause differentiation.

Surface

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

The Cererian surface is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about −38 °C, −36 °F) on 5 May 1991.

Only a few Cererian surface features have been unambiguously detected. High-resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres. 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 the dwarf planet's rotation. Two dark features had circular shapes and are presumably craters; one of them was observed to have a bright central region, while another was identified as the "Piazzi" feature. More recent visible-light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed 11 recognizable surface features, the natures of which are currently unknown. One of these features corresponds to the "Piazzi" feature observed earlier.

These last observations also determined that the north pole of Ceres points in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco. This means that Ceres's axial tilt is very small—about 3°.

Atmosphere

There are indications that Ceres may have a weak atmosphere and water frost on the surface. 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 will escape 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 sub-surface layers of Ceres. Ultraviolet observations by the IUE spacecraft detected statistically significant amounts of hydroxide ion near the Cererean north pole, which is a product of water-vapor dissociation by ultraviolet solar radiation.

Potential for extraterrestrial life

While not as actively discussed as a potential home for extraterrestrial life as Mars or Europa, the potential presence of water ice has led to speculation that life may exist there, and that evidence for this could be found in hypothesized ejecta that could have come from Ceres to Earth.

Orbit

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

The diagram illustrates the orbits of Ceres (blue) and several planets (white and grey). The segments of orbits below the ecliptic are plotted in darker colours, 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. 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.

In the past, Ceres had been considered to be a member of an asteroid family. These groupings of asteroids share similar proper orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres was found to have spectral properties different from other members of the family, and so this grouping is now called the Gefion family, named after the next-lowest-numbered family member, 1272 Gefion. Ceres appears to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common origin.

The rotational period of Ceres (the Cererian day) is 9 hours and 4 minutes.

Ceres is in a near-1:1 mean-motion orbital resonance with Pallas (their orbital periods differ by 0.3%). 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.

Transits of planets from Ceres

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

Origin and evolution

Ceres is probably a surviving protoplanet (planetary embryo), which formed 4.57 billion years ago in the asteroid belt. While 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, Ceres is believed to have survived relatively intact. An alternative theory proposes that Ceres formed in the Kuiper belt and later migrated to the asteroid belt. Another possible protoplanet, Vesta, is less than half the size of Ceres; it suffered a major impact after solidifying, losing ~1% of its mass.

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 elements like ²⁶Al). These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation. This process may have caused resurfacing by water volcanism and tectonics, erasing older geological features. Due to its small size, Ceres would have cooled early in its existence, causing all geological resurfacing processes to cease. Any ice on the surface would have gradually sublimated, leaving behind various hydrated minerals like clays and carbonates.

Today, Ceres appears to be a geologically inactive body, with a surface sculpted only by impacts. The presence of significant amounts of water ice in its composition raises the possibility that Ceres has or had a layer of liquid water in its interior. This hypothetical layer is often called an ocean. If such a layer of liquid water exists, it is believed to be located between the rocky core and ice mantle like that of the theorized ocean on Europa. 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.

Observations

When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7. 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 this dwarf planet. Ceres will be at its brightest (6.73) on 18 December 2012. 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. 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 observational milestones for Ceres include:

• An occultation of a star by Ceres observed in Mexico, Florida and across the Caribbean on 13 November 1984.
• Ultraviolet Hubble Space Telescope images with 50 km resolution taken on 25 June 1995.
• Infrared images with 30 km resolution taken with the Keck telescope in 2002 using adaptive optics.
• Visible light images with 30 km resolution (the best to date) taken using Hubble in 2003 and 2004.

Occultations

On 22 December 2012, 1 Ceres occulted the star TYC 1865-00446-1 over parts of Japan, Russia, and China. Ceres' level of brightness will be magnitude 6.9 and the star, 12.2.

Exploration

No space probe has visited Ceres. Radio signals from spacecraft in orbit around and on the surface of Mars have been used to estimate the mass of Ceres from its perturbations on the motion of Mars.

The Dawn spacecraft, launched by NASA in 2007, orbited asteroid 4 Vesta from 15 July 2011 until 5 September 2012 and is continuing on to Ceres. It is scheduled to arrive there in 2015, five months prior to the arrival of New Horizons at Pluto. Dawn will thus be the first mission to study a dwarf planet at close range.

Dawn's mission profile calls for it to enter orbit around Ceres at an altitude of 5,900 km. The spacecraft will reduce its orbital distance to 1,300 km after five months of study, and then down to 700 km after another five months. The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These instruments will be used to examine the dwarf planet's shape and elemental composition.