Photograph by NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
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This image shows the northern terrain on the sunlit side of dwarf planet Ceres as seen by NASA's Dawn spacecraft on April 14 and 15, 2015.


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

Largest Object in Asteroid Belt May Have Come From Elsewhere

Ammonia detected on Ceres’ surface suggests a cold birth beyond Neptune’s orbit.

New and controversial observations from NASA’s Dawn spacecraft, which has orbited the dwarf planet Ceres since March, suggest the 4.6-billion-year-old body may have been knocked into the main asteroid belt from the solar system’s chillier outskirts.

Dawn’s discovery of ammoniated clays on the world’s surface—reported November 9 at the Division for Planetary Sciences annual meeting—points to this intriguing scenario. Their presence indicates that Ceres was born somewhere beyond the orbit of Neptune, where the sun’s weaker glare would not vaporize and scatter the ammonia as the minerals formed. Then, sometime in the following 500 million years, gravitational heave-hos may have thrown the dwarf planet inward, delivering it to the main asteroid belt between Mars and Jupiter.

There is, of course, another possibility.

“Either Ceres formed farther out altogether and then was implanted in the main belt,” says Simone Marchi of the Southwest Research Institute, “or it grew at its current position with the contribution of outer solar system materials.”

The idea of Ceres as an inner solar system transplant isn’t a complete surprise. For starters, Ceres doesn’t look like any of its neighboring space rocks: It’s round, it’s the biggest thing in the belt by far, and it is much more watery than anything else nearby. It’s more like a warmer version of the icy moons orbiting Jupiter and Saturn.

“Ceres is basically unique, in terms of objects that we’ve been to,” says Andy Rivkin of the Johns Hopkins Applied Physics Laboratory. “The Moon is rocky; Pluto is icy. Ceres is the only one you look at with aspects of both, and that makes it a great way to learn about both.”

The dwarf planet’s soft, flat-bottomed craters, enigmatic bright spots, a possible ice volcano, and tufts of water vapor hint at a world with something exciting going on beneath the surface. Even more telling, perhaps, is that its density is similar to that of Pluto, which lives in the region beyond Neptune’s orbit.

“It sure smells like something that came from the outer solar system,” Caltech astronomer Mike Brown told National Geographic back in January.

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A composite image from NASA's Dawn spacecraft shows Ceres' Occator crater, home to a collection of intriguing bright spots of unknown composition.

Researchers have been studying Ceres for a long time, but ground-based telescopes haven’t been able to convincingly identify ammonia, since they have had to contend with Earth’s obstructive atmosphere. The orbiting Dawn spacecraft has a more ideal perch to observe how molecules on the dwarf planet’s surface reflect various wavelengths of light. It was in those wavelengths that Dawn investigator Carle Pieters and her colleagues spied the signature of ammoniated phyllosilicates, which are minerals similar to clays on Earth, mixed in with other materials.

Ceres today is much too warm for a volatile molecule like ammonia to survive on its own—it would just drift away as vapor. That means the ammonia must have become bound to those minerals when the mix was somewhere much colder, meaning Ceres either flew in from afar, or was bombarded by ammonia-bearing materials from farther out.

Of the two scenarios, Bill McKinnon of Washington University in St. Louis finds Ceres’ frigid origin more plausible. “The idea that pebbles of ammonia would drift in to coat Ceres seems like a kludge,” he said, noting that everything in the mid- and outer-asteroid belt would also be covered in ammonia, which is not observed.

But determining which scenario actually happened will be a tough task for the team. Each history should be associated with a unique distribution of crater sizes and numbers on Ceres, kind of like a fingerprint. In principle, the team only needs to read Ceres’ craters to figure out which prediction fits. However, “the global distribution of craters—and in particular the lack of large craters—is at odds with both scenarios,” says Marchi.

The issues arise because Ceres’ outer layer, a viscous mixture that’s “more like icy dirt than dirty ice,” according to Michael Bland of the U.S. Geological Survey’s Astrogeology Science Center, is smoothing over craters and erasing them over time. The problem is particularly pronounced with the bigger, older craters, and that will make it a bit tricky to reconstruct Ceres’ history.

Of course, it’s also possible that the detection of ammonia itself is flawed. Some researchers, wary of the result, maintain that Ceres’ surface spectra are better explained by brucite, a magnesium-based mineral that readily forms in the asteroid belt. “I do not understand how they concluded so firmly that it’s not brucite,” says Rivkin, who has challenged earlier claims of ammoniated compounds on Ceres.

But it’s too soon to say the results are wrong: The complete analysis will be published soon in the journal Nature, and it’s possible the results will withstand closer scrutiny. “It may be that ammoniated phyllosilicates have taken Game Five to go ahead three games to two in a best-of-seven series,” says Rivkin.

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