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    Categories: Solar System

Asteroid Facts: Ceres

Image Credit: Dawn / NASA

The image above shows a near true color picture of the C-type (carbonaceous) asteroid 1 Ceres, the first asteroid ever to be discovered, and the biggest object in the asteroid belt. With a diameter of 945 km (587 miles), it is also the largest of the minor planets within the orbit of Neptune, and the 33rd biggest object in the solar system overall, accounting for about 33% of all the mass in the asteroid belt.

For a considerable time after its discovery by Giuseppe Piazzi in 1801 it was considered to a be proper planet, until many other objects were found in similar orbits, leading to its reclassification as an asteroid in the mid-1800”s. Although Ceres is reasonably easy to spot from Earth with binoculars and small to medium sized telescopes, its brightness varies from 6.7 to 9.3 over a period of 15-16 months.

Quick Facts

• Aphelion: 2.9773 AU
• Perihelion: 2.5577 AU
• Eccentricity: 0.075823
• Orbital period: 4.60 years (1,681.63 days)
• Equatorial rotation velocity: 92.61 m/sec
• Average orbital speed: 17.905 km/sec
• Mean proper motion: 78.193318 degrees/year
• Dimensions: 945 km (Diameter)
• Volume: 421,000,000 cubic kilometres
• Mass: 9.393 ± 0.005 × 1020 kilograms
• Surface area: 2,770 000 square kilometres
• Mean density: 2.161±0.009 gram/cubic centimetre
• Escape velocity: 0.51 km/sec
• Apparent magnitude: Variable from 6.64 to 9.34

Origin

Due to its mass and size, it is likely that Ceres is a surviving protoplanet remnant. Current models of how the solar system formed suggest that while all lunar to Mars-sized bodies had either been incorporated into the rocky planets, or been ejected from the solar system by Jupiter and/or the other gas giants, Ceres had survived the formation of the solar system relatively unscathed. However, a competing theory holds that Ceres had formed in the Kuiper belt and migrated inwards as the large planets moved outwards into the Kuiper belt, since the presence of ammonia salts in the Occator crater on Ceres suggests that it (Ceres) had not formed in the inner solar system.

Physical Properties

Ceres is the only object in the asteroid belt that is known to be in hydrostatic equilibrium, meaning that it is the only asteroid belt body whose own gravity has caused it to be roughly spherical in shape. This was confirmed after detailed analysis revealed that this was not the case with 4 Vesta. Ceres is also the smallest known body that is confirmed to be in hydrostatic equilibrium after Saturn’s satellite Rhea, which is 600 km bigger and more than twice as massive as Ceres.

Composition

Ceres is composed mainly of ice and rock, and its slightly oblate shape is the result of its internal structure, which is partially differentiated. In terms of its structure, Ceres consists of a large rocky core that is overlaid by a 100-km-thick icy mantle that is estimated to hold about 200 million cubic kilometres of water.

If this estimate turns out to be accurate, it would mean that Ceres contains more water than all the fresh water on Earth, which is supported by several observations made with the Keck telescope in 2002, as well as by complex evolutionary models. Moreover, studies have shown that a liquid water ocean under a thick layer of water ice may have persisted to present times. However, recent studies have suggested that Ceres could possibly have a small inner iron-rich core due to partial differentiation of the rocky fraction of its core.

Surface

Although the surface of Ceres is largely similar to those of most other C-type asteroids in terms of chemical composition, there are some significant differences. For instance, some persistent and ubiquitous features in Ceres’ infrared spectrum indicate the presence of hydrated materials that include iron-rich clay minerals such as cronstedtite, and carbonate minerals such as dolomite and siderite, all of which are common in carbonaceous chondrite meteorites. The presence of these materials requires that relatively large amounts of water be present in the asteroid’s interior, which is absent in the spectra of most other C-type asteroids. For this reason, Ceres is sometimes classified as a G-type asteroid.

Studies based on Hubble Space Telescope-derived data have also revealed the presence of graphite, sulphur dioxide, and sulphur on Ceres’ surface. While the exact origins of these materials remain somewhat unclear, the presence of graphite is almost certainly the result of space weathering on some of Ceres’ older surfaces, while the latter two are volatile under Cerean conditions. As such, sulphur, and sulphurous compounds released by recent geological activity on the surface either would have escaped rapidly, or would have become trapped in cold spots on the asteroid.

In terms of surface features, the Dawn spacecraft found that, the asteroid’s surface is heavily cratered, although the total number of large impact craters was lower than was expected by most investigators. However, a relatively large number of large craters have central pits, likely as the result of cryovolcanic or other geological process, while many other craters have prominent central peaks. Ceres also has one prominent mountain, a cryovolcanic structure named Ahuna Mons, which is estimated to be no more than a few hundred million years old, based on the relative absence of impact craters on the mountain.

A recent computer simulation has also suggested that other ice volcanoes may have existed on Ceres at various times, but that these have become indistinguishable from the surrounding terrain through the process of viscous relaxation. In simple terms, this means that the surface of Ceres is partially viscous (fluid), and that fluid motions continually mould and shape the surface so that some features are destroyed while others are created.

Temperaure

Surface temperatures on Ceres are relatively high. With the Sun at the zenith, the maximum surface temperature (based on Ceres’ position on 5 May 1991) was estimated be in the region of 235K (-38 °C, -36 °F). At this temperature, ice is unstable, which could explain the dark surface of the asteroid since minerals mixed into the ice would remain on the surface when the ice fraction sublimates.

Atmosphere

Based on data obtained from the Herschel Space Observatory in 2014, it appears that Ceres has a tenuous atmosphere consisting primarily of water vapor. The origin of the water vapor has been shown to derive from several localized sources of water vapor in mid-latitudes, that each releases about 3 kilograms of water second.

Possible mechanisms include the sublimation of exposed water ice over a surface area of about 0.6 square km, although cryovolcanic eruptions resulting from radiogenic internal heat is likely to produce more vapor than sublimation of surface ice. In fact, observations made by the Dawn spacecraft provide strong evidence that ongoing geological activity is a major contributor to the water vapour in Ceres’ atmosphere, since the asteroids’ atmosphere appears to be reasonably stable. If sublimation of surface ice were the main source of water vapor, the density of the atmosphere would vary according to the asteroid’s position relative to the Sun.

Peter Christoforou :