Ceres in cutaway

Ceres' shape is almost round like Earth's, suggesting that the asteroid may have a "differentiated interior," with a rocky inner core, a thick water-ice mantle, and a thin, dusty outer crust.

NASA, ESA, and A. Feild (STScI)

Last summer's vote by members of the International Astronomical Union elevated Ceres from being merely the largest member of the asteroid belt to a prime candidate for "dwarf planet" status. And from what astronomers have learned about it recently, Ceres is making a good case for that promotion.

The most recent insights come from a trio of astronomers led by Andrew S. Rivkin (Applied Physics Laboratory), who examined the big asteroid's near-infrared spectral signature last year. Previous work had already established the presence of clay-like minerals that include water as part of their molecular structure. Rivkin's team has used the body's infrared fingerprint to refine the kinds of materials that might lie on its surface. The best candidates, he reported at a recent meeting of planetary scientists, are iron-rich clays that contain roughly 5% carbonates — just the kind of minerals that would form on what was once a wet surface.

Curiously, the surface exhibits no hint of water ice, which would be readily detected by infrared spectroscopy.

The new results follow on the heels of extensive observations with the Hubble Space Telescope, conducted last year, that imply Ceres is more than just a rocky jumble. Peter C. Thomas (Cornell University) and others determined that its slightly squashed shape and spin rate match what would be expected for a body that had differentiated (segregated) into a rocky core and a water-ice exterior. A thin rind of rock and dust may be all that's hiding an icy layer 60 to 120 kilometers (40 to 80 miles) thick.

Ceres from Hubble

The surface of Ceres, as revealed by the Hubble Space Telescope's Advanced Camera for Surveys, includes several brighter and darker regions that could be caused by impacts. Ceres itself is slightly wider at its equator than at its poles.

NASA, ESA, J. Parker (Southwest Research Institute), and others

Hubble's images are no longer the last word on Ceres' appearance. Last month a team of European researchers led by Benoit Carry (LESIA, France) released views of the asteroid taken four years ago with the Keck Observatory's powerful adaptive-optics camera. They find that Ceres has a smooth shape overall, slightly fatter across its midsection (481 km) than through its poles (447 km).

According to Carry, the surface of Ceres displays a wealth of bright and dark markings, some of which might be due to regional differences in composition.

All of this is whetting the appetite of scientists involved in Dawn, a NASA spacecraft that narrowly avoided outright cancellation earlier this year. Now scheduled for launch next summer, Dawn will spend time orbiting both Ceres and Vesta, an equally intriguing asteroid.


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