Ever since NASA’s Dawn spacecraft arrived at the dwarf planet Ceres in March 2015, scientists have been arguing over more than 130 curiously bright spots Dawn spied on the surface of the coal-dark world. Most are found in craters—including the biggest and brightest spots, which lie within Occator, a 90-kilometer-wide crater estimated to be relatively youthful at less than 80 million years old.
Are these spots the uncovered surface of a buried layer of ice or perhaps erupting geysers or cryovolcanoes or the shattered remains of space rocks striking Ceres—or something else entirely? Researchers’ best guesses remained in flux for months, changing with each new set of ever-sharper images Dawn delivered as it slowly spiraled down closer to the dwarf planet’s surface.
Late last year Dawn’s scientists used tentative spectroscopic measurements to finally zero in on shiny, ice-suffused deposits of magnesium sulfate salts as the most probable explanation for Occator’s bright spots. Sunlight striking the deposits could boost their brightness by covering them with layers of haze produced from the vaporized ice. But these conclusions remained tentative, contingent on Dawn gathering more definitive data.
And now, in a study published Wednesday, Dawn scientists have again changed their minds about the nature of Occator’s spots and the implications for Ceres. Based on higher-quality spectroscopic observations taken during Dawn’s descent, Maria Cristina De Sanctis of the National Institute of Astrophysics in Rome and several of her colleagues report in Nature that Occator’s spots are made not of magnesium sulfate but of a very different salt—sodium carbonate. “The abundance of [sodium] carbonate requires liquid water in the form of brine,” De Sanctis says. “This is especially intriguing if you think that Ceres is in the so-called ‘habitable zone’ of the solar system,” where liquid water could sustain life as we know it. If Ceres’s subsurface has harbored liquid water in the not-too-distant past, it may still even today.
Whiffs of Salt from the Outer Solar System
According to Mikhail Zolotov of Arizona State University, who wrote an accompanying commentary for the study, the new findings have made Occator’s spots—and the rest of the bright spots on Ceres—much less enigmatic. “To understand Occator is to understand the spots elsewhere. They are a bit like the Old Faithful geyser at Yellowstone.”
The spots, Zolotov says, are confined to impact craters because they likely form from impacts heating the subsurface and releasing briny liquid water or vapor from scant subsurface ice or hydrated minerals. The brines flow up through fractures or porous rock and boil near the vacuum-exposed surface, jetting out as frozen, salty droplets. The water sublimates away, leaving behind shiny, salt-rich residue. In addition to creating new salt deposits by their heat, impacts could also uncover old ones, excavating substantial salt layers built up over geologic time within old craters.
The presence of sodium carbonate on Ceres also adds to an accumulating pile of circumstantial evidence suggesting the dwarf planet may have formed far from the outer edge of the inner solar system where it presently resides. One of the only other places beyond Earth that scientists have found significant signs of sodium carbonate is within plumes venting from the interior of Enceladus, an icy moon of Saturn. Last year De Sanctis and colleagues announced their probable spectroscopic detection on Ceres of ammonia-rich clays—compounds formed from highly volatile raw materials that could not easily endure exposure to the scorching sunlight of the inner solar system, and instead are most often found on bodies in the system’s frigid outer reaches. The presence of such materials on Ceres, De Sanctis says, could also mean the world somehow formed out there before being tossed down into the inner system by a violent gravitational interaction with a passing planet or moon. Alternatively, showers of debris from the outer solar system rich in nitrogen ice and ammonia could have rained on Ceres after it was born in the inner system long, long ago.
What Lies Below?
For years, two competing theories have served as opposing bookends for scientists’ studies of the dwarf planet, which is relatively light for its roughly 900-kilometer diameter. One theory postulates that Ceres’s interior was quite wet and that its low density is due to a thick shell of ice surrounding a rocky core—produced perhaps from the freezing of a buried ocean of liquid water early in the world’s history. The other theory, detailed by Zolotov in 2009, suggests instead that Ceres is relatively dry, and that its lightweight status is simply due to its being an undifferentiated lump of porous rock.
Either scenario is compatible with Ceres’s bright spots; minimal moisture is required to generate brines that well up from the deep to the surface. But there are still major implications to the competing possibilities in this debate. As the most significant possible reservoir of water and other volatile compounds between Mars and Jupiter, Ceres could prove to be a vital oasis for any future human expansion into the solar system. But the two theories hold vastly different prospects for extraterrestrial life—a dryer and more inert Ceres would stretch the definition of “habitability” even closer to a breaking point than a world with a frozen, salt-filled sea.
On a fundamental level, however, the dwarf planet is a remnant from the early history of the solar system, a sizeable scrap of protoplanetary stuff that somehow escaped incorporation into a greater world and instead became the largest known member of the Asteroid Belt, which stretches between the orbits of Mars and Jupiter. Gaining a deeper understanding of its composition, structure and history is perhaps the closest scientists will ever come to knowing the other long-vanished building blocks that went on to form Earth and the solar system’s other planets.
Meanwhile a second, separate study published Wednesday in Nature Geoscience says the “dry Ceres” model is closer to the truth, based on results that suggest a paucity of water ice in the upper several kilometers of the dwarf planet’s subsurface. Using digital terrain maps constructed from Dawn images, a team led by Michael Bland of the U.S. Geological Survey Astrogeology Center at the University of Arizona determined that the morphology of Ceres’s large craters rules out an ice-rich subsurface. On Ceres, ice-dominated craters would change shape over millions and billions of years, their floors flattening and their rims softening as the underlying ice relaxes and flows. The deep, sharp concavities of Ceres’s craters, Bland says, indicate that water ice makes up on average no more than 30 to 40 percent of the subsurface by volume—and perhaps much less.
“It appears to be that my prediction was right, I am sad to say,” Zolotov says. “I think the data now support Ceres being a nondifferentiated body, basically a piece of porous rock—a very large but otherwise typical asteroid without anything exotic like an icy mantle ocean.”
Bland does not entirely agree. Ceres is likely still the most ice-rich object in the Asteroid Belt and still holds many secrets, he says. “Many people latched on to the idea of a water-rich Ceres because it is a little more interesting, but reality is not always as exciting as our expectations,” he says. “This world is probably neither as completely dry as [Zolotov] has suggested nor as wet as its rival model. It is a much more complicated place that lies somewhere in between those two extremes.”
More certainty—or, at least, another change of thinking—may come soon, as the Dawn team publishes additional results from the spacecraft’s ongoing observations of Ceres. Dawn is slated to continue its studies into 2017, until either its funding or its ability to maintain a stable orbit decays, although NASA is also reportedly considering whether the hardy craft could be sent to yet another asteroid (it orbited Vesta previous to its encounter with Ceres) before finally ending its mission.