Ceres on ice: Scientists confirm vast stores of frozen water inside dwarf planet

Ceres' frozen water  could yield clues into the history of our own planet, the solar system, and beyond.

Maps of hydrogen draped over shape models of Vesta and Ceres.
Courtesy of T.H. Prettyman and N. Yamashita/Planetary Science Institute | Caption

In 1989, two geophysicists theorized that extensive stores of water-ice exist just beneath the surface of Ceres. Their model suggested that the ice could be 10 to 100 meters (33 to 330 feet) beneath the surface near the equator and closer to the surface – just 1 to 10 meters (3 to 33 feet) deep – from the mid-latitudes to the poles.

That model was created with data obtained from ground-based telescopes, but new data beamed back from NASA's Dawn spacecraft, which arrived at the the dwarf planet (sometimes characterized as an asteroid) in 2015, suggests that their model wasn't far off.

"Lo and behold, they're right," Tom Prettyman, a co-investigator of the NASA Dawn mission and a researcher at the Planetary Science Institute, tells The Christian Science Monitor. "There is ice. And there's very strong evidence for it near the surface at high latitudes on Ceres."

"This builds a lot of confidence in the models that we're using for ice stability," he says, which could help astronomers know where to look for water-ice in the rest of the solar system.

Recent exploration of our solar system has turned up evidence of water on many different planetary bodies. Most are moons with subsurface oceans or icy crusts. But another famous dwarf planet, Pluto, also has water ice, and models hint at a subsurface ocean, too.

"As we're continuing to explore [the solar system], water seems to be very present," Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology who was not part of the research team, tells the Monitor. "'Water, water everywhere' is the key finding that keeps turning up everywhere we explore."

And that could be a boon for scientists. Not only could studying the composition of planetary bodies like Ceres help clarify the early history of the solar system and our own planet, water is a key ingredient for life as we know it. That means that where there is water, there might be life. And any humans planning to venture out into space also need water to survive.

Reinforcing icy models

The model Dr. Prettyman and his colleagues have devised from the new data is remarkably similar to the one proposed in 1989. According to the new model, described in a paper published Thursday in the journal Science, water ice on Ceres becomes abundant starting at latitudes of 40 degrees and could reach 30 percent of the composition of material at the poles. 

"The results of the paper confirm our best hopes about Ceres – that it does contain a significant amount of water," Dr. Binzel says. 

Previous studies of Ceres have been limited to imaging the surface of the dwarf planet, but the Gamma Ray and Neutron Detector (GRaND) on Dawn was able to peer about 1 meter below the surface.

But, Prettyman notes, GRaND isn't sensing ice directly. "There's no gamma ray or neutron signature for ice, per se," he says. Instead, the team looks at measurements of elements like hydrogen and then calculates how much might be in the form of H2O. 

"The water content of Ceres' uppermost surface is from about 16 weight percent at the equator to almost 30 weight percent in the northern hemisphere, at the north pole," Prettyman says. But it may not all be in the form of water ice, he adds. "We think part of it is, and part of it is the form of minerals of hydration," he said, referring to minerals that have water embedded in their chemical structures.

Scientists have debated for years whether any water on Ceres would be in pure water-ice form. One model had suggested Ceres was a water-ice sandwich, with a rocky topping over a layer of mostly pure water-ice atop a rocky core. And although this new research supports the idea of layers, they don't seem to be so clearly separated.

Layered worlds

Understanding Ceres' composition could help scientists understand how the dwarf planet formed, which in turn could yield insight into the early solar system.

When Prettyman and his colleagues examined composition clues from the GRaND data, they spotted less iron in this shallow subsurface than they had expected. 

"The lack of iron in the surface suggests that some of it may have gone deeper, perhaps even to the core of Ceres," Binzel says. That would imply that Ceres underwent a differentiation process, with the heavier elements sinking toward the planetary body's center. 

Water is a key piece of that process, says Josh Emery, a planetary scientist at the University of Tennessee, Knoxville who wasn't part of the research, in an interview with the Monitor.

"As a body is forming, it gets pretty warm inside from radioactive isotopes that can heat the entire interior," he explains. "If there's ice there, it melts the ice into water and drives chemical reactions and can lead to separation by density, by different materials." 

And the low-iron content on the surface of Ceres, along with evidence of clays, suggest water was involved in the early history of Ceres, Binzel says. 

"Layering is something that we see very pervasively in our solar system," says Amy Lovell, an astronomer at Agnes Scott College in Decatur, Ga. who was not involved in the research. So finding evidence of differentiation in the queen of the asteroid belt isn't a surprise, she tells the Monitor.

But one thing doesn't quite fit: Ceres' location in the solar system.

Planetary bodies that got big enough to become differentiated and that are close to the sun, within what is called the snowline, have a metal core and a rocky crust, explains Andy Rivkin, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory who was not involved in the research. Further out in the solar system, bodies have an icy mantle over a rocky core. 

But, so close to the sun, and without a protective atmosphere, ice isn't stable at the surface of Ceres. The heat of the sun would cause any ice on the surface to sublimate away, he tells the Monitor.

That's where Ceres' dusty surface comes in. It serves as a sort of "protective coating," as Dr. Lovell puts it, allowing the ice beneath to stay stable.

So, Dr. Rivkin says, this new research "tells us about the nature of Ceres. It might be what you get if you took something like Enceladus [one of the moons of Saturn], took it out of orbit from Saturn, put it in the middle of the asteroid belt and just let it heat up and see what happened. You might end up with something kind of like Ceres."

And scientists have already proposed that perhaps Ceres migrated from the middle of the solar system, perhaps as a moon of Jupiter or Saturn, to its current location.

Understanding how and where Ceres formed could also hold provide clues into how Earth got its water, Rivkin adds. "There's some evidence that the Earth formed without a lot of water and without a lot of organic materials," he says. It's possible that something like Ceres may have hit early Earth, depositing ice and organics – the building blocks for life as we know it.

On the hunt for water

"Maybe it's not surprising after all" that H2O keeps showing up everywhere, Binzel admits, as hydrogen and oxygen are two of the three most common elements in the universe, "and they like to get together."

But understanding the presence of water and its role in planetary building processes goes beyond our own solar system, Binzel says. "Understanding how the process works here, around our sun, helps us understand how this must be working around all these other stars that we're discovering in planetary systems."

Plus, it has benefits beyond pure science, Dr. Emery adds. If humans are to become "a spacefaring people," he says, "water in any form will be a valuable commodity. To understand where water is and where we can get it in the solar system will be really valuable information."