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Mud volcanoes and a buried ocean: Scientists say asteroid Ceres is surprisingly active

Data from NASA’s Dawn spacecraft indicates mysterious bright spots on Ceres are evidence of buried liquid — but some scientists aren’t convinced.
Image: Ceres
An animation of the rotation of Ceres, made with data from the Dawn spacecraft. The colorful map at the right shows variations in its gravity field: The red colors are where it is strongest and the blue colors are where it's weakest.NASA / JPL-Caltech

The asteroid Ceres, once thought to be just a large rock orbiting between Mars and Jupiter, has slushy volcanoes and liquid saltwater beneath its surface that may have formed an underground ocean in the past, new research suggests.

The research is part of seven studies of Ceres using data from NASA’s Dawn robot spacecraft, published Monday in the journals Nature Astronomy, Nature Geoscience and Nature Communications. Two of the studies indicate that a bright spot in the large Occator crater of Ceres is caused by saltwater seeping from fractures in the rocky crust.

That suggests Ceres has buried reservoirs of saltwater and may still have an underground ocean.

“Ongoing activity in Occator brings additional and independent evidence for a deep brine layer, and upgrades Ceres to the realm of ocean worlds,” wrote Dawn mission scientist Julie Castillo-Rogez of NASA’s Jet Propulsion Laboratory in Pasadena, California, in an overview published in Nature Astronomy.

Although Saturn's moon Enceladus and Jupiter's moon Europa are thought to have subsurface oceans, they are both much warmer than the nearly-frozen reservoirs of saltwater on Ceres. But the frozen dwarf planet Pluto may also have had subsurface oceans, and the new studies of Ceres could have implications for future investigations of icy worlds, she said.

Ceres, currently about 200 million miles from Earth, is the largest object in our solar system's main asteroid belt. It’s almost 600 miles across, with gravity strong enough to pull it into a rough sphere.

Under the rules of the International Astronomical Union, that makes Ceres a dwarf planet — perhaps the only dwarf planet closer than Pluto.

NASA’s Dawn probe was launched in 2007 and spent years traveling to the asteroid belt with its slow but extremely efficient ion engine. From 2011 until 2012, it orbited the asteroid Vesta — the second-largest object in the asteroid belt at 320 miles across.

Dawn arrived at Ceres in 2015 and studied it until 2018, orbiting to within 25 miles of the asteroid’s surface before it finally ran out of fuel.

Image: Ceres
This image from NASA's Dawn spacecraft of the sunlit northern hemisphere of Ceres was taken in April 2015 from an orbit 14,000 miles above the surface. The dwarf planet's rocky crust is heavily cratered by impacts from asteroids.NASA / JPL-Caltech

Early studies of Ceres using Dawn mission data suggested it had a residual saltwater layer, and the latest studies have strengthened that idea, said planetary scientist Carol Raymond of the Jet Propulsion Laboratory, the lead author of one of the studies.

“The more data we got, it became very clear that there was geologic activity,” Raymond said. “There were fluids arriving at the surface fairly recently.”

Another study led by planetary scientist Maria Cristina De Sanctis of Italy’s National Institute of Astrophysics in Rome determined the bright spot at Occator crater contains a form of sodium chloride — table salt — chemically bound to molecules of water.

That chemical form rapidly dries out, which leads scientists to conclude it is “extremely recent” in geological time, De Sanctis said.

“It could be 100,000 years ago, or a thousand years ago, or even last week," she said.

Another study interprets some other bright spots on Ceres as evidence of “cryovolcanoes” that allow icy saltwater from reservoirs in the crust to seep on to the surface.

Taken together, the studies suggest Ceres is an active world — although it’s very cold, with temperatures of around minus 100 degrees Fahrenheit in the sunlight.

The buried reservoirs, more than eight times saltier than oceans on Earth, are thought to start about 25 miles below the crust and have a temperature of about minus 31 degrees Fahrenheit — warm enough to have the consistency of slushy mud, but much too cold for life to evolve.

Image: Ceres
An artist's rendering of NASA's Dawn spacecraft maneuvering with its ion propulsion system above Ceres. Dawn was launched in 2007 and explored two of the largest objects in the asteroid belt - the protoplanet Vesta in 2011, and Ceres from 2015 until 2018.NASA / JPL-Caltech

Yet Ceres is much more interesting that anyone expected, and scientists are pushing for a second probe to follow Dawn to the asteroid.

“I was very excited to look at Vesta and I was sad to go to Ceres, because I thought it would be dark and boring,” De Sanctis said. “But it was not — it was really exciting.”

The idea that Ceres ever had a buried saltwater layer is controversial, however, and some scientists think the latest studies are not conclusive.

“There is no solid evidence for a past ocean on Ceres based on the papers,” said planetary geochemist Mikhail Zolotov of Arizona State University in Tempe.

“The presence of hydrated silicates and local salts at the surface suggests water-rock interaction in Ceres’ history, but not necessarily an ocean,” he said in an email. “Even if an ocean existed, it should be stripped away very early in the history of Ceres.”

But others are warmer to the idea.

“I have always been convinced that Ceres holds subsurface liquids,” said astrophysicist Steven Desch, also of Arizona State University.

“These papers … constrain the compositions of the liquids and the order in which they were emplaced on the surface,” he said in an email. “It's a stunningly detailed timeline of brine-driven cryovolcanism on Ceres.”