Ceres, a dwarf planet in the asteroid belt, hides an underground ocean

In the asteroid belt, an immense region of space between Mars and Jupiter, millions of rocky bodies serenely move around the sun in a timeless cosmic dance. Queen among the dancers is Ceres, the largest object of the belt and a “fossil” from the early days of the solar system. In 2007, NASA launched the Dawn spacecraft to the belt to study Ceres closely. After examining the dwarf planet, its traces and its cunning features, scientists reasoned that it was once home to a global ocean that had frozen.

On Monday, a suite of seven studies in the journal Nature reviewed Dawn’s extended mission data, peering into Ceres’ dull, lifeless shell and finding conclusive evidence that it is an ocean world.

“The new results confirm the presence of liquid in Ceres,” said Julie Castillo-Rogez, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL) and co-author of six new studies. The discovery of liquids suggests that Ceres, the closest dwarf planet to Earth, was an ordinary world and raises the possibility that these types of worlds are harboring life.

Ceres is a mini-Pluto, about 2.5 times smaller than its more famous cousin. When Dawn Overheard during its prime mission in 2015, the spacecraft photographed mysterious bright spots in the center of the Occator crater, a 60-kilometer-wide scar in the surface of Ceres. The bright white light that reflected from within turned out to be a crater-within-a-crater. NASA scientists decided they needed an even closer look.

Between June and October 2018, when his mission came to an end, Dawn moved within 22 miles (35 km) of Ceres’ surface. This improved, according to Castillo-Rogez, the resolution of Dawn’s image by a factor of 10, giving planetary scientists an unusual look at the geology and composition of Ocator.

In Monday’s study, scientists left their case for fluid beneath the crater and ongoing geological activity within Ceres.

Bright spots

Central to Ceres’ understanding is the bright spot of Occator. Known as Cerealia Facula, previous research showed that the reflective luster in Cerealia was caused by salt residues on the surface, evidence of activity of past water. But how did water get into the shaped Okkator? There were two competing theories.

The first hypothesis that the saline remains were the result of the impact that Okkator made, while the second suggested liquids were still bubbling from beneath the surface of Ceres. Dawn’s narrow approach was designed to take this issue apart. One study found, as Castillo-Rogez puts it, a “smoke gun”: hydrohalite.

Hydrohalite is composed of sodium chloride – sea salt – surrounded by water molecules. Using images obtained by Dawn’s infrared mapping device, a team of Italian and American researchers found the unmistakable infrared signal of hydrohalite. It is the first time it has been discovered outside Earth. Castillo-Rogez calls it a “great discovery” and explains that it should only have been exposed recently, probably “less than about 100 years ago.” It provides robust evidence that liquidity persists on Ceres today, she says, but whether this is a global ocean as pockets of liquid remains to be determined.

Combined with gravity data from Okkator and its surrounding region, Dawn scientists were able to map the geometry of the underground fluid reservoir. Two other studies allowed researchers to examine the thickness of the bright spot and age it. The composition shows that Cerealia is significantly younger than the impact crater itself. It is likely that an impactor in Ceres throttled and formed Okkator about 22 million years ago. After impact, a small “melting chamber” of fluid formed and the saline fluid in it then moved to the surface to form the structure of Cerealia about two million years ago.

“We conclude that some low activity at Occator probably still persists,” says Andreas Nathues, a planetary scientist at the Max Planck Institute for Solar System Research and first author of one of the new studies. “It was a surprise that the ancient ocean is not completely frozen.”

Dawn’s imagery also helps explain another surface feature on Ceres. To the east of Cerealia lies the Vinalia Faculae, a set of thinner, diffused clear spots that also provide evidence of salt. The researchers reason that the difference between the two regions arises because Vinalia is fed by a deeper source of fluid welling deep inside the Ceres interiors. The fluid travels through fractures to the surface where it freezes, leaving the saline deposits behind.

Cold hearts

Castillo-Rogez has spent the last two decades studying the evolution of icy worlds in our solar system, trying to understand how they came to be.

Some celestial bodies, such as Jupiter’s moon Io, generate internal heat through gravitational interactions with their planets – a process known as ‘tidal warming’. This process can dominate how these worlds evolve. Castillo-Rogez’s work has focused on medium-sized cosmic worlds that do not experience this warming, like the months of Saturn Iapetus and Phoebe.

Dwarf planets like Ceres have cold hearts. They do not experience tidal warming and, lacking an atmosphere, also do not capture heat. Without heat, water freezes out. But data from Dawn now show scientists like Castillo-Rogez that these types of cosmic bodies can retain liquid water over eons in other ways, thanks to the role of saline fluids and compounds such as hydrohalite.

Impact events, which generate a lot of heat, can be a major driver of icy evolution and the short-lived melting rooms they create could be “transient” habitable rooms for early life. NASA scientist and co-author Lynnae Quick believes that these types of rooms could provide an opportunity to originate in ocean worlds and icy moons.

Ocean worlds are becoming more and more frequent as we learn more about our solar system. There are Enceladus, the snowball moon of Saturn, and both Ganymede and Europe, Jovian moons believed to hide internal oceans. Even further out, at the edge of the solar system, ocean worlds can persist. Studies of Pluto, another cold-hearted dwarf planet (cold-hearted off official planetary status), with NASA’s New Horizons spacecraft, researchers have found evidence for a global fluid ocean beneath its frozen shell in 2016.

“Maybe there are more objects than Ceres,” Nathues thinks.

The evolution of these icy bodies has far-reaching implications: Where there is water, there is potential for life. Observing the ocean worlds of the solar system will enable scientists to assess how habitable they are, and NASA’s Dawn has given us an insight.

Ocean worlds

Folle astrobiologists have their eyes on Mars. The neighbor of earth is the main target for a suite of recently launched missions designed to search for signs of ancient life. But ocean worlds might also give way to alien forms.

A fleet of ocean worlds will be explored in the coming decades by two interplanetary probes. Both NASA and the European Space Agency (ESA) will send spacecraft to the moon Europe of Jupiter, a world that NASA scientists believe may be the best place to check on life. ESA’s spacecraft, known as JUICE, will also fly Gbymede and Callisto, large Jovian moons, believed to be subterranean oceans. Dawn paves the way to understand what those spaceships might find there.

“The unusual resolution of the Dawn images provides a good reference to support future observations of Europe and Ganymede,” says Castillo-Rogez.

While planetary scientists are beginning to look ahead and plan for missions to icy moons, Castillo-Rogez says this is just the beginning of NASA’s Dawn data analysis and that many questions have been raised by the Dawn mission. Occator’s results show only a fraction of the data that Dawn radiated to Earth in recent days.

The team’s discovery has enabled Ceres to be upgraded from a “candidate” ocean world to a certainty, but there are now new mysteries to solve. To adequately determine whether icy moons and icy bodies like Ceres could have lived – or perhaps have in the past – the nature of the environment beneath the surface must be clearly understood.

“To answer more detailed questions about the ocean, we would need a lander mission,” Nathues says.

Castillo-Rogez notes that a draft study is being finalized for submission to NASA that would show a sample mission to Vinalia Faculae, allowing scientists to observe and study the state of organic matter on the surface. Whether such a mission will be viewed favorably by NASA’s long-term planning committee will not be known until 2022.

Dawn was officially retired on November 1, 2018, ending his mission. It was not deliberately crashed into the planet for fear that it might contaminate the surface. The probe is pursuing a lone guard in orbit around Ceres and will likely do so for the next two decades. As we return to nab samples, Dawn will roam the neighborhood, a monument to past success.