The largest known asteroid could contain more fresh water than Earth and looks like our planet in other ways, according to a new study that further blurs the line between planets and large space rocks.

Astronomers took 267 images of asteroid Ceres using the Hubble Space Telescope. From these images and subsequent computer simulations, they suggest Ceres may have a rocky inner core and a thin, dusty outer crust.

A team led by Peter Thomas of Cornell University said today that Ceres is nearly spherical, which suggests that gravity controls its shape. Also, the asteroid's non-uniform shape indicates that material is not evenly distributed throughout the inside.

These and other new clues, including Ceres' low density, point to an interior loaded with frozen water, the astronomers said.

The results are detailed in the Sept. 8 issue of the journal Nature.

Big and round

Ceres has long been considered one of the tens of thousands of asteroids that make up the asteroid belt between Mars and Jupiter. At 580 miles (930 km) in diameter – about the size of Texas – it's the largest asteroid in the belt, accounting for about 25 percent of the belt's total mass.

Astronomers had thought Ceres might never have been heated enough to create layers of material.

But computer models now suggest Ceres has a differentiated interior – dense material in the core and lighter stuff near the surface. Possible configurations include a mantle rich in water ice around a rocky core.

If this mantle is composed of at least 25 percent water, Ceres would have more fresh water than Earth, according to a statement released by the Space Telescope Science Institute, which operates Hubble for NASA and the European Space Agency.

"The most likely scenario from the knowledge we have on how other objects form, it probably has a rocky core and a mantle. That mantle is probably some watery, icy mix, with other dirt and constituents. That mantle could be as much as ¼ of the whole object," study coauthor Joel Parker of the Southwest Research Institute told SPACE.com. "Even though it's a small object compared to Earth, there could be a lot of water."

On Earth, fresh water makes up only a thin layer just a few miles deep in some places, less in others. The water layer proposed for Ceres, while smaller in circumference, is many miles thicker.

The total volume of water on Earth is about 1.4 billion cubic kilometers, around 41 million of which is fresh water. If Ceres' mantle accounts for 25 percent of the asteroid's mass, that would translate to an upper limit of 200 million cubic kilometers of water, Parker said.

Since all the nine "regular" planets have differentiated interiors, this new view of Ceres has some astronomers calling Ceres a "mini-planet," adding fuel to an ongoing debate over exactly what qualifies as a planet.

Embroyonic world

Other researchers recently announced the discovery of 2003 UB313, a round object in our solar system 1-1/2 times larger than Pluto and about three times further away from the Sun. But even an object of this size – at 2,100 miles in diameter roughly four times the size of Ceres – doesn't receive universal endorsement as being a planet.

One astronomer, Brian Marsden, who runs the Minor Planet Center where data on small bodies is collected, says that if Pluto is considered a planet, then any other round worlds should also be considered planets. Under this definition, which some other astronomers subscribe to, Ceres 2003 UB313 and a handful of other large objects would be named planets. The alternative, Marsden and others say, is to stop calling Pluto a planet. 

Another explanation is that Ceres is a sort of 'baby' planet – an underdeveloped version of Earth and other rocky planets. Looked at this way, Ceres appears as other fledgling planets might have looked more than 4 billion years ago.

The leading theory for planet formation holds that small rocks collided, stuck and gradually grew. Depending on location and orbit, a developing world may or may not have encountered enough raw material to become as large as the four traditional rocky planets.

"Ceres is an embryonic planet," said observation team member Lucy McFadden of the Department of Astronomy at the University of Maryland. "Gravitational perturbations from Jupiter billions of years ago prevented Ceres from accreting more material to become a full-fledged planet."

In 2015 scientists will get a close up look at Ceres when the NASA Dawn mission orbits the asteroid. A closer look should provide more clues about the asteroid's composition.

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