Photo: The topography of Ceres shows the dwarf planet has many large and deep craters, a finding that indicates no more than 40 percent of its subsurface is ice. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA After an asteroid or comet bashed into the dwarf planet Ceres about 80 million years ago and created a massive pit, materials from deep inside Ceres ended up on the surface by some process that likely involved water, a new analysis shows.
Data collected by the Dawn space probe, which is currently orbiting Ceres, shows that the dwarf planet's mysterious bright spots mostly contain sodium carbonate, a kind of salt found in hydrothermal environments on Earth and in icy moons in the outer solar system.
Because an impacting asteroid couldn't have delivered the mineral, scientists conclude that it came from inside Ceres.
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"The heat source may have been triggered by impact heating. Alternatively, internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at depth on Ceres today," Dawn scientist Maria Cristina De Sanctis, with the National Institute of Astrophysics in Rome, and colleagues write in a paper published in this week's Nature.
As Dawn approached Ceres in early 2015, scientists were startled by its early images showing bright spots on the surface of the dwarf planet, most of which were located in a 57-mile wide crater. The basin, named Occator, has a main pit that is about 6 miles across with a central dome that is covered with a highly reflective material.
Ultimately, Dawn revealed that Ceres has more than 130 bright spots, the most prominent of which are located on the floor of Occator crater.
Preliminary findings last year indicated that the bright spots in Occator contained a different type of salt, magnesium sulfate, but the new study shows sodium carbonates are more likely.
The new study also found ammonia salts, such as ammonium chloride and/or ammonium bicarbonate in the crater.
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The discovery of carbonates on Ceres reinforces a link to icy moons in the outer solar system, such as Saturn's Enceladus, which has plumes of icy water shooting into space that contain ammonia, sodium carbonate and sodium bicarbonate, De Sanctis said.
The finding follows a discovery last year that the surface of Ceres contains phyllosilicates, which are ammonia-bearing clays. Because the outer system is rich with ammonia, scientists began to ponder if Ceres, the largest object in the main asteroid belt, actually formed nearer to Neptune's orbit and then migrated inward.
Another idea is that Ceres formed near its present location between Mars and Jupiter, but its building materials came from farther away in the solar system.
"Whether Ceres formed in the outer solar system or formed in its current place but somehow accreted outer solar system material, a contribution of such material would also imply an elevated abundance of bulk organic material," Arizona State University planetary scientist Mikhail Zolotov, who was not involved in the research, wrote in a commentary published in Nature.
In a related study published in this week's Nature Geoscience, another team of scientists analyzed Ceres' craters and determined that most of the largest ones are more than a mile deep. The finding indicates that Ceres' subsurface is dominated by a mix of rock and low-density compounds, with ice accounting for 30 percent to 40 percent of the volume.
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"Only a few percent ice by volume would be sufficient to form aqueous solutions," Zolotov writes. "Perhaps the bigger problem is not the water ice content of the subsurface, but how to melt it: the subsurface of Ceres should normally be too cold."
Zolotov raises the prospect that Ceres' bright deposits may be the remains of Encleadus-like jets. Follow-up studies are needed to determine how and when Ceres might have pulled this off.
Dawn's mission Ceres is due to end this summer, but NASA managers are considering an extension.
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