Dwarf planet Ceres has a patchwork past and a hazy present

The dwarf planet Ceres may have a mottled past and a strangely active present, according to data sent back by NASA’s Dawn spacecraft.

The findings, published in a pair of papers in the journal Nature, reveal (respectively) that the early solar system may have undergone a surprising amount of mixing and that Ceres today may be active enough to power the release of hazes into its atmosphere.

“Before about, say, 10 or 15 years ago, we thought that material more or less stayed where it formed -- and so the idea [was] that Ceres would always have been where it was,” said Andrew Rivkin, a planetary scientist at Johns Hopkins University Applied Physics Laboratory who was not involved in the papers.


But as astronomers have discovered planets around other stars and studied the physics of their orbits, he added, “there’s been this recognition that planets can potentially move quite a lot.”

Ceres stands out among its peers in the asteroid belt that stretches between Mars and Jupiter, and not just because it’s the largest of the bunch. It’s also a dwarf planet — a body that has enough mass that its gravity pulls it into a spherical, planet-esque shape. Ceres also has attracted attention because its surface had seemed from a distance to be quite smooth, and because there were hints that there could be water coming off the surface.

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Ceres is thought to be the seed of a fully fledged world that never quite made it to the big league. The Dawn spacecraft has visited both Ceres and protoplanet Vesta, a fellow giant asteroid, because they are fossils from the solar system’s early adolescent years. Because Ceres is round and relatively wet, while Vesta is more lumpy and dry, they offer differing — and perhaps complementary — narratives for what that tumultuous, developing time really looked like.

Using Dawn’s Visible-Infrared Mapping Spectrometer VIR, the scientists were able to take images of the dwarf planet’s surface in the spectral range of 0.4 to 5 micrometers — a range that included some telltale wavelengths that had been obscured from ground telescopes’ view by ground cover on Earth. The “light fingerprint” in the Dawn data revealed signs of ammoniated phyllosilicates — a type of ammonia-rich clay-like mineral — all over the dwarf planet’s surface. That was something of a surprise to the researchers, said study co-author Carol Raymond, deputy principal investigator for the Dawn mission.

“Ammonia would not be stable in the vicinity of Ceres, so the fact that you see these all over Ceres and not some regional variation, the fact that you’re seeing it pervasively on the surface of Ceres means that it has to be incorporated in the body,” Raymond said.

The scientists think the ammonia didn’t originate in the main belt, which is still relatively close to the sun. Instead, they must have come from a far more distant region called the Kuiper belt, a giant ring of icy and rocky debris that begins beyond the orbit of the farthest planet, Neptune. Here, ammonia could remain stable for long periods of time and be incorporated into large fragments of rock. (Demoted dwarf planet Pluto is perhaps the most famous of these Kuiper Belt Objects.) Smaller, ammonia-rich chunks would have been more easily dragged into the inner solar system, eventually joining with the other chunks that would form Ceres and becoming incorporated into the body.

If this is indeed what happened, it means that there was a lot more mixing going on in the early solar system than previously thought.

“It would have some significant implications for how much material was being exchanged between the icier outer reaches and the inner solar system,” Raymond said. It could mean that some of the models for its early development might soon have to be revisited. “Dynamical models are only as good as the observations that they’re trying to explain.”

That mixing could have implications for the development of Earth and the origins of life, Rivkin added.

After all, all this material was dragged into the inner solar system to form Ceres, then “who knows how many of those smaller pieces hit the Earth and maybe brought Earth a lot of its water and carbon and the like,” he said.

A second paper picks away at the 130-plus bright spots that have become the dwarf planet’s mysterious hallmark. Using the spacecraft’s Framing Camera, the researchers identified the bright material as a kind of magnesium sulfate known as hexahydrite. These salty spots were likely left behind after water-ice sublimated off the surface, transforming directly from a solid into a gas. This could mean that there is a lot of very salty, or briny, ice lying beneath the surface.

In the 60-mile-wide Occator crater — home to the most prominent of the bright spots — there appear to be dark streaks that could be fractures crossing the pit, along with the remains of a central peak. And some shots of the crater seem to reveal a haze hovering near the surface and covering the crater floor. This might be linked to the hints of water vapor that were picked up by the Herschel space observatory earlier — especially since it’s visible at noon and gone at dusk and dawn.

“The haze observations are really surprising,” Rivkin said. “I don’t think anyone expected to see anything like that on Ceres or on any kind of asteroid.”

It’s possible that the haze activity is akin to what happens on a comet, when water vapor lifts dust and ice particles off the surface, creating the comet’s lovely tails.

“We have evidence that Ceres is exhaling water vapor,” Raymond said, who added that they haven’t detected water ice yet. “We’re by no means in the position to say that … we know what process is causing this.”

Follow @aminawrite on Twitter for more interplanetary science news.


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