Icy world found inside asteroid.

New observations of Ceres, the largest known asteroid, suggest that frozen water may account for as much as 25 percent of its interior. If this is true, the volume of ice on Ceres would be greater than that of all the fresh water on Earth.

The evidence comes from Hubble Space Telescope images showing that the 930-kilometer-wide asteroid is smooth and almost perfectly round. Simulations show that a body as massive as Ceres can have that shape and texture only if materials inside it have separated into layers of higher- and lower-density compounds. A period of heating and cooling, such as that experienced by the solar system's rocky inner planets, could have caused light material to move toward the asteroid's surface and denser material to sink.

In the Sept. 8 Nature, Peter Thomas of Cornell University and his colleagues suggest that the outer, low-density material is probably ice because Ceres' surface shows signs of water-bearing minerals and because the asteroid's overall density is lower than that of Earth's rocky crust. The proposed ice layer would lie just beneath a thin crust of clay and carbon-rich compounds and above a rocky core, the researchers say.

Ceres is one of several hundred thousand bodies that lie in the main asteroid belt between Mars and Jupiter. The layering provides new evidence that Ceres is a case of arrested development. It's "an embryonic planet" halted by Jupiter's gravity from packing on additional material to become a full-fledged planet, says study coauthor Lucy McFadden of the University of Maryland in College Park.

Next year, NASA plans to launch a mission called Dawn, which will orbit Ceres in 2015 and then move on to Vesta, the second-largest known asteroid.--R.C.