Asteroid Ceres may be a freshwater planet

For years astronomers have tried to look closely at Ceres, the largest asteroid in the asteroid belt, a nearly opaque target so featureless that scientists could not even be sure of its shape.

But with the help of the Hubble Space Telescope, a team of researchers obtained enough images in enough detail to determine that Ceres is quite likely a "mini-planet" with enough gravity to make it almost spherical, and suggesting that, like Earth, it may have different layers, including a mantle composed mostly of water ice. The research was reported last week in the journal Nature.

Using Hubble's Advanced Camera for Surveys, the team tracked Ceres through its full nine-hour rotation, and with 700 pixels in each image was able to follow a "bright spot" as it moved around the asteroid, enabling it to determine its polar and equatorial axes.

Team member Lucy McFadden of the University of Maryland said scientists had thought Ceres was homogeneous in composition because of its smooth surface and low-density surface crust.

"But it was too round," McFadden said. If it had been homogeneous, she said, centrifugal forces would have collapsed the light-density material into a more "oblate" shape — flatter on top, swollen in the middle.

"There's something that's keeping it from collapsing on itself," McFadden said, and the likeliest suspect is water ice, which expands when it freezes.

If that is so, Ceres — only 580 miles in diameter — may have more fresh water on it than Earth.