Largest Asteroid May Be 'Mini Planet' With Water Ice

The observations by NASA's Hubble Space Telescope also show thatCeres shares characteristics of the rocky, terrestrial planets likeEarth. Ceres' shape is almost round like Earth's, suggesting that theasteroid may have a "differentiated interior," with a rocky inner coreand a thin, dusty outer crust.

"Ceres is an embryonic planet," said Lucy A. McFadden of theDepartment of Astronomy at the University of Maryland, College Park anda member of the team that made the observations. "Gravitationalperturbations from Jupiter billions of years ago prevented Ceres fromaccreting more material to become a full-fledged planet."

The finding will appear Sept. 8 in a letter to the journal Nature.The paper is led by Peter C. Thomas of the Center for Radiophysics andSpace Research at Cornell University in Ithaca, N.Y., and also includesproject leader Joel William Parker of the Department of Space Studiesat Southwest Research Institute in Boulder, Colo.

Ceres is approximately 580 miles (930 kilometers) across, about thesize of Texas. It resides with tens of thousands of other asteroids inthe main asteroid belt. Located between Mars and Jupiter, the asteroidbelt probably represents primitive pieces of the solar system thatnever managed to accumulate into a genuine planet. Ceres comprises 25percent of the asteroid belt's total mass. However, Pluto, our solarsystem's smallest planet, is 14 times more massive than Ceres.

The astronomers used Hubble's Advanced Camera for Surveys to studyCeres for nine hours, the time it takes the asteroid to complete arotation. Hubble snapped 267 images of Ceres. From those snapshots, theastronomers determined that the asteroid has a nearly round body. Thediameter at its equator is wider than at its poles. Computer modelsshow that a nearly round object like Ceres has a differentiatedinterior, with denser material at the core and lighter minerals nearthe surface. All terrestrial planets have differentiated interiors.Asteroids much smaller than Ceres have not been found to have suchinteriors.

The astronomers suspect that water ice may be buried under theasteroid's crust because the density of Ceres is less than that of theEarth's crust, and because the surface bears spectral evidence ofwater-bearing minerals. They estimate that if Ceres were composed of 25percent water, it may have more water than all the fresh water onEarth. Ceres' water, unlike Earth's, would be in the form of water iceand located in the mantle, which wraps around the asteroid's solid core.

Besides being the largest asteroid, Ceres also was the firstasteroid to be discovered. Sicilian astronomer Father Giuseppe Piazzispotted the object in 1801. Piazzi was looking for suspected planets ina large gap between the orbits of Mars and Jupiter. As more suchobjects were found in the same region, they became known as "asteroids"or "minor planets."