Ceres

In 1801, Giuseppe Piazzi (1746-1826) discovered the first as well as largest Main-Belt asteroid ever found, now commonly designated as 1 Ceres. According to one NASA estimate, Ceres may have over one-third of the total mass of the Main Asteroid Belt. Although relatively round, its diameter actually varies between 579 to 597 miles (or around 932 to 960 kilometers) across. Although rocky and icy protoplanets beyond the Solar System's 2-AU "ice line" formed in the Main Asteroid Belt, the early development of Jupiter prevented protoplanets like Ceres from agglomerating into larger planetary bodies, by sweeping many into pulverizing collisions as well as out into the Oort Cloud or beyond Sol's gravitational reach altogether. On August 24, 2006, the International Astronomical Union voted at the end of its 26th General Assembly to establish a new class of celestial objects in Solar System called a "dwarf planet," which appears to include Ceres.

LESIA, ESO, SwRI,
Keck Observatory



Larger near-infrared image.



In 2006, the texture of 80
percent of Ceres' surface
was mapped with a spatial
resolution of 30 kilometers
or nearly 19 miles (more).

By the IAU's definition, dwarf planets can be found anywhere in the Solar system. Ceres is located with the largest Main-Belt asteroid, between the orbits of Mars and Jupiter, at an average 2.8 AUs from the Sun. Its roughly circular orbit (e= 0.079) takes about 4.6 years to complete but is inclined from the ecliptic at 10.7 degrees). In addition, the next three largest asteroids in the Main Belt (Vesta, Pallas, and Hygeia) may also be "round enough" to be eventually classified as dwarf planets.

Ceres completes a rotation within 9.1 hours. Based on its relatively round shape, density, and rotation rate, astronomers believe that Ceres may have a layered (or "differentiated") interior like those of of the rocky inner planets such as Earth. The dwarf planet appears to have a thin and dusty dark crust that covers a thicker mantle of water ice and a rocky core. Based on its low density (~2.2 grams per cubic centimeters), Ceres may be made of 25 to 30 percent water ice (update from: David Shiga, New Scientist, July 17, 2008; and William B. McKinnon, 2008).

Ann Feild, STScI,
SwRI,
ESA, NASA




Larger illustration.




Computer models
suggest that Ceres
may have a rocky
core with a layer
of water ice under
the crust (more).

Although classified as carbonaceous, Ceres reflects roughly 10 percent of the sunlight that strikes it ("albedo") and so is not as dark as other low-density asteroids called carbonaceous chondrites, which have albedos of around three to five percent. While exposed water ice would not be stable at the surface of Ceres at its relatively close distance to the Sun, water and ammonia do appear to be bound up in crustal minerals on the asteroid's surface. That Ceres has not lost these volatile components from destructive impacts suggests that the asteroid may be a surviving protoplanet from the formation of the Solar System. (More discussion is available from the Planetary Society.)