Planetary Sciences News

The Dwarf Planet That Came in from the Cold—Maybe

The presence of ammonia-rich clay on much of the surface of Ceres suggests that this dwarf planet—the largest object in the asteroid belt—may have formed far out in the solar system, then wandered in.

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Astronomers this week have put a new face on the solar system’s largest asteroid Ceres, but it appears to be the face of a stranger to the neighborhood.

The Dawn spacecraft, sent by NASA specifically to explore this little-known dwarf planet in the midst of the asteroid belt, has found evidence that ammonia-rich clays blanket much of the surface of the distant body.

On the basis of this new finding from the Dawn mission, another team of researchers now hypothesizes that Ceres may have more in common with Pluto and other denizens of the outer solar system than with the asteroids that surround it. The ammoniated clay data suggest, the scientists said, that the body either entirely assembled at the solar system’s fringes or formed where it is now but acquired a thick coating of icy material that drifted in from the outskirts.

Either way, they noted, the compositional evidence about Ceres’s surface may lend new credence to models that suggest the asteroid belt includes a plethora of émigrés from the outer solar system’s reservoir of icy objects known as the Kuiper Belt. That migration would have happened soon after the birth of the solar system, shedding light on the origin and initial arrangement of the planets.

Definitive Detection

Carle Pieters of Brown University in Providence, R.I., unveiled the ammoniated clay findings from NASA’s Dawn mission Monday at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in National Harbor, Md. The first hints that the clays, known as ammonia phyllosilicates, cover much of Ceres came from ground-based observations. But because some of the infrared wavelengths required to confirm those hints are absorbed by Earth’s atmosphere, Dawn scientists made additional observations using the spacecraft’s visible and infrared spectrometer to achieve definitive detection, Pieters noted.

At the meeting the same day, Simone Marchi of the Southwest Research Institute in Boulder, Colo., who is also on the Dawn team that identified the clays, presented the new analysis suggesting Ceres might be a visitor from beyond.

Chill Out

The ammoniated clay discovery excited Marchi and his colleagues because laboratory experiments have indicated that the ammonia component of the clays had to originate as particles of pure ammonia ice, which easily vaporizes and can only exist in solid form at chillier locations farther from the Sun than the asteroid belt. The requirement for solid ammonia therefore points to an origin for Ceres, or at least its outer coat, in the vicinity of the outer solar system, Marchi said.

For now, such an origin should only be regarded as a working model, Marchi cautioned, pending higher-resolution spectra and other data Dawn is expected to start collecting in mid-December, after it has settled into to its final, lowest-altitude orbit, just 380 kilometers from the dwarf planet.

Homegrown?

A new chemical analysis developed by planetary scientists Marc Neveu and Steven Desch of Arizona State University in Tempe reveals some of the nuances involved in using the ammonia-rich clays to trace Ceres’s origin.

They calculated that if the infant Ceres had a subsurface ocean and incorporated nitrogen—from any source—into that wet interior, the element could have combined with hydrogen to form the ammonia (NH3) necessary to make the clays. For this reason, “the mere existence of ammoniated clays does not by itself constrain where Ceres formed,” Desch told Eos.

However, Desch and Neveu noted that the farther from the Sun Ceres formed, the more nitrogen the dwarf planet would have accumulated and the more ammonia-rich clays the body would ultimately produce. So if studies with Dawn in its low-altitude orbit find a high abundance of the clays, it could still point to an origin beyond the asteroid belt, they said.

“We can’t say for certain” where Ceres formed, said Desch, “but the [clay] discovery is very significant because it enables quantitative tests in the near future.”

Migration Theories

Theorists have offered differing explanations of what might have kicked outer solar system bodies inward long ago.

According to the so-called Nice model, the solar system’s giant planets—Jupiter, Saturn, Uranus, and Neptune—were originally packed into a much smaller region of space than today, surrounded by an extended disk of rubble. Over several hundred million years, gravitational interactions caused all the giant planets except Jupiter to move outward, plow into the icy rubble, and drive some of it into the asteroid belt and other parts of the inner solar system. In the process, Ceres might have formed in the outer solar system and been hurled inwards, says Hal Levison of the Southwest Research Institute, a coauthor of the model.

A different model, known as the Grand Tack, suggests that objects from the outer solar system migrated much sooner—about 100,000 years after the birth of the giant planets. Gravity maps of Ceres that Dawn is poised to construct will determine the dwarf planet’s internal structure and could help reveal which model, if either, might be correct, Levison said.

Previously Suspected

Years before Dawn visited Ceres, planetary geologist William McKinnon of Washington University in St. Louis, Mo., had already suggested that the dwarf planet had formed beyond the asteroid belt. Among McKinnon’s arguments, he told Eos, Ceres’s bulk composition resembles that of Kuiper Belt objects, and it’s an oddball in the asteroid belt, comprising a third of the belt’s mass.

He had noted that chemical measurements would be critical for identifying Ceres’s provenance. “Which is why I find the definitive detection of ammonia so encouraging,” McKinnon told Eos. “It is a signature of the outer solar system.”

—Ron Cowen, Freelance Science Journalist; email: [email protected]

Citation: Cowen, R. (2015), The dwarf planet that came in from the cold—maybe, Eos, 96, doi:10.1029/2015EO039477. Published on 12 November 2015.

© 2015. The authors. CC BY-NC 3.0