Photograph by NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
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These enigmatic bright spots on Ceres’s surface are likely to be water ice and could be associated with a plume of water vapor.

Photograph by NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Mysterious Bright Spots on Dwarf Planet Ceres May Be Water Ice

Strange features are most likely made of ice, and may be venting water vapor into space.

One of several mysterious bright spots on dwarf planet Ceres could be venting a plume of water vapor into space, suggesting that the little world is far from geologically dead.

The possible plume appeared in images taken as NASA’s Dawn spacecraft approached Ceres and was reported at the Lunar and Planetary Science Conference. It’s an exciting find, scientists say, and holds clues to what might lie beneath Ceres’s surface.

The plume’s appearance changes throughout the Cererian day. “During the day, the feature evolves. It brightens,” reported Andreas Nathues of Germany’s Max Planck Institute for Solar System Research. “At dusk time, it gets much fainter. And late dusk, it disappears completely.”

The puzzling bright spots, first spotted by the Hubble Space Telescope, again showed up in recent photos taken by the Dawn spacecraft. Though scientists have speculated the spots contain highly reflective water ice, Dawn hasn't gotten close enough yet to get a good look. But the latest data suggest the bright spots are indeed likely to be icy.

The brightest of these spots—called “Feature 5” for now—is associated with what may be a plume of material, pictures taken from different angles and times of day suggest. Feature 5 is located at the bottom of a 50-mile-wide (80 kilometers) crater and remained visible even when the crater rim blocked the camera’s view of its floor.

“We believe, at the moment, that this could be some kind of outgassing,” Nathues says. “We need higher-resolution data to confirm this.”

Not everyone is quick to accept the preliminary plume explanation. "You would never see a plume itself with that brightness," says Bill McKinnon of Washington University in St. Louis. "Frost and snow fallout on the surface, yes, but we should wait and see."

If it’s there, the plume wouldn’t be the first discovered on Ceres. In early 2014, a team of scientists reported seeing tufts of water vapor hovering over the dwarf planet; whether those tufts came from ice turning into water vapor or from volcanic activity is still unclear.

Mission to an Oddball

Dawn slipped into orbit around Ceres, the largest body in the asteroid belt, on March 6. It’s still moving closer to the dwarf planet, where it will spend the next 16 months studying the icy world’s features, both inside and out.

The mission is finally giving scientists a look at a dwarf planet, a type of small world that hasn’t yet been explored. After years of speculating about Ceres, it's no wonder scientists reported the new results in a scientific session titled “Ceres and Dawn: Your Last Chance to Talk About Ceres Before Dawn Data Wreck Your Theories.”

Large, round, and watery, Ceres is definitely an oddball in the asteroid belt. With a hypothesized subsurface ocean or frozen water layer, it’s much more like a planet or an icy moon than what we typically think of as “asteroids.”

“When we complete our observations, we will show that Ceres is every bit a planet as its terrestrial neighbors Mars, Earth, Venus, and Mercury,” says Christopher Russell of the University of California, Los Angeles, principal investigator of the Dawn mission.

Dawn scientists also gave reporters at the meeting a sneak peek at not-yet-released images, including the first color mosaic of Ceres’s surface and the best look yet at its icy crust, covered in a variety of terrains—rough, smooth, and grooved—and containing many craters and at least one large, flat-bottomed basin.

“There’s a very interesting surface down here,” Russell says. “It is unlike what we have seen before.”

Surface features and regions are being named for deities associated with the harvest (Ceres is the Roman goddess of agriculture, after all), and include such names such as Chahal, Dantal, Ezinu—and Yumyum.

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