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Asteroid May Be a Mini-Planet

The largest known asteroid, 1 Ceres, is 930 kilometers (580 miles) across, and represents about 25 percent of the asteroid belt’s total mass. Recent work using the Hubble Space Telescope’s Advanced Camera for Surveys now shows that the asteroid is nearly round, leading to the belief that it has, like terrestrial planets, a differentiated interior: a dense silicate core surrounded by lighter minerals and covered by a crust of carbon-rich compounds.

But more extrordinary still is the suggestion that water ice is buried beneath the asteroid’s crust. The evidence is tantalizing — Ceres’ density is less than that of Earth’s crust, and its surface gives signs of water-bearing minerals. Wrapped in a solid ice mantle surrounding the asteroid’s core, Ceres could actually possess more water than all the fresh water found on Earth.

Image: NASA’s Hubble Space Telescope took these images of the asteroid 1 Ceres over a 2-hour and 20-minute span, the time it takes the Texas-sized object to complete one quarter of a rotation. One day on Ceres lasts 9 hours. Credit: Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Park).

All these things lead astronomers studying the data to conclude that Ceres is considerably more interesting than once thought. “Ceres is an embryonic planet,” said Lucy A. McFadden of the Department of Astronomy at the University of Maryland, College Park and a member of the team that made the observations. “Gravitational perturbations from Jupiter billions of years ago prevented Ceres from accreting more material to become a full-fledged planet.”

The team’s findings appear in P. C. Thomas, J. Wm. Parker, L. A. McFadden et al., “Differentiation of the asteroid Ceres as revealed by its shape,” Nature 437, 224-226 (08 Sep 2005). See this Cornell University press release for more.