Ceres and Vesta, the two largest bodies in the main asteroid belt, may be contemporaries and neighbors, but the similarities stop there. Ceres, which is about as wide as Texas, is filled with frozen water and hydrated minerals, making it more like an icy moon of Jupiter or Saturn than Vesta, its dry, rocky sibling, new research shows.
Scientists aren't sure why Ceres and Vesta followed such different evolutionary paths, but they expect more answers as analysis of data collected by NASA's Dawn spacecraft continues. Dawn spent 14 months visiting Vesta before firing up its ion engine and settling into orbit around Ceres in March 2015.
Research published in this week's issues of Science and Nature Astronomy confirms a 30-year-old theory that Ceres is an ice-rich world.
"In Ceres' crust today we think we see about 10 percent (by weight) water ice," Dawn scientist Thomas Prettyman, with the Planetary Science Institute in Tucson, Arizona, told Seeker.
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But water in the crust is just the beginning of the story. Ceres is replete with minerals and clays that can only form in the presence of water. Extrapolating from measurements made by Dawn, Prettyman estimates that the dwarf planet as a whole is 30 percent water.
Vesta had a different history, one that did not involve much water.
"What happened to Vesta is that it completely melted," Prettyman said.
Once its radioactive elements decayed, Vesta, made mostly of silicates, cooled and formed a basaltic crust, a mantle and an iron-rich core.
"This would have had to happen close to the sun, where you couldn't condense volatiles like water because it was too hot," Prettyman added.
Perhaps because Ceres was bigger than Vesta, or positioned farther from the sun, but it was able to accrete water into its body, setting the stage for a cascade of chemical alterations. Some models show that the separation of water and rock caused Ceres to form a frozen shell over a briny liquid layer that may still exist today.
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"The Dawn mission is trying to look back in time to the very earliest stage of the solar system," deputy principal investigator Carol Raymond, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., told reporters at the American Geophysical Union conference in San Francisco.
Of particular interest, she added, is if Ceres' water and rock interacted in such a way that the dwarf planet became chemically suited for life.
In related research, another team of scientists reported that Ceres is filled with cold, dark craters, several of which contain ice. Similar features have been found on the poles of Mercury and the moon.
"Subsurface ice is ubiquitous on Ceres. Exposed ice is rare," said astronomer Norbert Schorghofer, with the University of Hawaii, Manoa.
Image: Representation of Ceres' Occator Crater in false colors shows differences in the surface composition. Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA WATCH VIDEO: What Is The Deep Space Network & How Does It Work?