The asteroid Ceres, once thought to be just a large rock that ran between Mars and Jupiter, has slippery volcanoes and liquid salt water beneath its surface that may have formed an underground ocean in the past, new research suggests.
The study is part of Ceres’ seven studies 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 Ceres’ large Occator crater is caused by salt water that seeps out of fractures in the rocky crust.
This suggests that Ceres has buried reservoirs of salt water and may still have an underground ocean.
“Continued activity in Occator brings additional and independent evidence for a deep salt layer, and upgrades Ceres to the realm of ocean worlds,” Dawn mission scientist Julie Castillo-Rogez of NASA’s Jet Propulsion Laboratory in Pasadena, California, wrote in a review published in Natural Astronomy.
Although the moon of Saturn Enceladus and the European moon of Jupiter are thought to have oceans under the ground, they are both much warmer than the almost frozen reservoirs of salt water on Ceres. But the frozen dwarf planet Pluto may also have oceans under the ground, and Ceres’ new research could have implications for future research into icy worlds, she said.
Ceres, currently about 200 million miles from Earth, is the largest object in the main asteroid belt of our solar system. It is nearly 600 miles across, with gravity strong enough to pull it into a rough atmosphere.
Under the rules of the International Astronomical Union, Ceres makes a dwarf planet – perhaps the only dwarf planet closer than Pluto.
NASA’s Dawn probe was launched in 2007 and has spent years moving to the asteroid belt with its slow but extremely efficient ion engine. From 2011 to 2012 it revolved around 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 it within 25 miles of the asteroid’s surface, before finally running on fuel.
Early Ceres studies with Dawn mission data suggested it had a residual saltwater layer, and recent research has reinforced 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 geological activity,” Raymond said. “Fluids have recently been added to the surface.”
Another study led by planetary scientist Maria Cristina De Sanctis of the National Institute of Astrophysics in Italy in Rome determined the bright spot near the crater of the Octacre contains a form of sodium chloride – table salt – chemically linked to molecules of water.
That chemical form dries out rapidly, leading scientists to conclude that it is “extremely recent” in geological time, De Sanctis said.
“It could have been 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 salt water from reservoirs in the crust to seep to the surface.
Taken together, the studies suggest that Ceres is an active world – although it is very cold, with temperatures of about minus 100 degrees Fahrenheit in 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 muddy mud, but far too much cold for life to evolve.
Yet Ceres is much more interesting than anyone expected, and scientists are pushing for a second probe to track Dawn to the asteroid.
“I was very excited to see Vesta and I was sad to go to Ceres because I thought it would be dark and boring,” De Sanctis said. “But it wasn’t – it was really exciting.”
However, the idea that Ceres once had a buried saltwater layer is controversial, and some scientists think the latest research is 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 salt on the surface suggests water-rock interaction in the history of Ceres, but not necessarily an ocean,” he said in an email. “Even if there was an ocean, it would have to be removed very early in Ceres’ history.”
But others are warmer to the idea.
“I’ve always been convinced that Ceres contains liquids underground,” said astrophysicist Steven Desch, also from Arizona State University.
“These papers … limit the compositions of the liquids and the order in which they were placed on the surface,” he said in an email. “It’s an astonishingly detailed timeline of brine-driven cryovolcanism on Ceres.”
