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Dwarf planet Ceres has salty water and appears geologically active

High-resolution images from NASA’s Dawn Orbiter suggest our solar system’s largest asteroid may have an ancient underground ocean, but some scientists are skeptical.

BySukee BennettNOVA NextNOVA Next
Ceres_Occator_NASA.jpg

The dwarf planet Ceres, photographed by NASA’s Dawn orbiter. False-color renderings highlight differences in its surface materials. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Ceres, the biggest asteroid in our solar system, now has another bragging right. According to new research, Ceres has water seeping onto its surface, suggesting the presence of an ancient underground ocean.

The new findings, documented in seven studies published Monday in the journals Nature Astronomy, Nature Communications, and Nature Geoscience, suggest that, despite being cold and salty (and lacking an atmosphere), Ceres is geologically active.

Also known as a dwarf planet, Ceres dwells in the asteroid belt sandwiched between Mars and Jupiter. At about 588 miles across, it’s about the third of the width of our moon. NASA’s Dawn Orbiter studied Ceres thoroughly from March 2015 to November 2018, getting within 22 miles of the asteroid in its final weeks before running out of fuel. During Dawn’s final year of orbit, it collected high-resolution images of Ceres, which a team of Italian researchers has now analyzed.

The images illustrate the briny liquid seeping out on Ceres’ surface, “as well as mounds and hills that formed when ice melted and refroze after an asteroid impact about 20 million years ago,” Michael Greshko reports for National Geographic. Researchers found that the liquid comes from an underground reservoir of saltwater 25 miles beneath Ceres’ Occator Crater. The reservoir may be hundreds of miles wide.

Occator is a 57-mile-wide impact crater that’s approximately 20 million years old and dappled with bright spots of salt. This week’s findings suggest that these salt deposits formed when cold underground brine oozed out onto the crater’s surface as recently as 1.2 million years ago.

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“Bulging mountains and hills also support the idea that Ceres experiences a kind of ice-cold cryovolcanism, with briny mud or slush acting like molten lava does on Earth,” Greshko writes. “In one region of Occator's crater floor, Dawn spotted hints that brines had dribbled out of ice volcanoes within the past few decades, if not more recently.”

“And the Dawn data shows that the dehydrated salts actually still have a bit of water in them,” Neel V. Patel reports for MIT Technology Review. “That suggests whatever geological activity is encouraging these deposits might still be happening, which would mean Ceres is still an active world.”

Occator_Crater.jpg

A mosaic image of Ceres' Occator Crater using false color (in red) to highlight the recently exposed saltwater pushed up from a deep reservoir under the dwarf planet's crust. Image credit: NASA, JPL-Caltech, UCLA, MPS, DLR and IDA

However, not all scientists are convinced that Ceres is home to a subsurface ocean. Jim Zimbelman, a Smithsonian Institution geologist who was not involved in the latest studies told Scientific American that he remains “pretty skeptical.” Water on the dwarf planet’s surface doesn’t entail the presence of an underground ocean but perhaps a smaller reservoir, some skeptics believe. Mikhail Zolotov of Arizona State University, who was consulted for one of the seven studies but was not directly involved in the work, “dismisses the conclusion that Ceres hosts a brine reservoir as ‘wishful thinking,’” Scott Hershberger reports for Scientific American. In a paper published in January, Zolotov expressed that the presence of water in the form of ice is not required to explain Ceres’ density and that the body’s shape and composition could be explained by organic matter, chondrites (stony, non-metallic meteorites), and a high surface porosity.

But many scientists agree that the possibility of a brine layer on Ceres is compelling and that the Dawn mission, which provided the discovery, was groundbreaking. Dawn was the first probe to orbit two objects beyond Earth’s immediate neighborhood, having visited asteroid Vesta before Ceres. Along with NASA’s New Horizons spacecraft, which flew by Pluto in 2015 and space rock Arrokoth on New Year’s Day of 2019, Dawn has “shown that small, icy bodies are far more active than once thought,” Greshko writes, “stretching how scientists imagine the geology of dozens of alien worlds.”

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