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 belt's largest object and a "fossil" from the early days of the solar system. In 2007, NASA launched the Dawn spacecraft to the belt to study Ceres up close. After surveying the dwarf planet, tracing its blemishes and examining its sullen features, scientists reasoned it was once home to a global ocean that had frozen over.
On Monday, a suite of seven studies in the journal Nature scrutinize extended mission data from Dawn, peering at Ceres' dull, lifeless shell and finding definitive evidence that it is an ocean world.
"The new results confirm the presence of liquid inside Ceres," says Julie Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion laboratory (JPL) and co-author across six new studies. The discovery of liquids hints that Ceres, the closest dwarf planet to Earth, may have been a habitable world and raises the possibility that these types of worlds may harbor life.
Ceres is a mini-Pluto, around 2.5 times smaller than its more famous cousin. As Dawn whizzed overhead during its prime mission in 2015, the spacecraft photographed mysterious bright spots at the center of Occator crater, a 60-mile-wide scar in Ceres' surface. The stark white light reflecting 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, as its mission was coming to an end, Dawn moved to within 22 miles (35 km) of Ceres' surface. This, according to Castillo-Rogez, improved the resolution of Dawn's imaging by a factor of 10, giving planetary scientists an unprecedented look at the geology and composition of Occator.
In Monday's haul of studies, scientists lay out their case for liquid below the crater and ongoing geological activity within Ceres.
Central to understanding Ceres is Occator's bright spot. Known as Cerealia Facula, previous research showed the reflective sheen at Cerealia was caused by salt residues on the surface, evidence of past water activity. But how did water come to settle in the bowl-shaped Occator? There were two competing theories.
The first hypothesized the salty residues were the result of the impact that created Occator, while the second suggested fluids were still bubbling up from below Ceres' surface. Dawn's close approach was designed to pull apart this problem. One study found, as Castillo-Rogez puts it, a "smoking 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 US researchers found the unmistakable infrared signal of hydrohalite. It's the first time it has been discovered outside Earth. Castillo-Rogez calls it a "major discovery" and explains it must have only recently been exposed, probably "less than around 100 years ago." It provides robust evidence liquid persists on Ceres today, she says, but whether this is a global ocean or pockets of liquid is yet to be determined.
Coupled with gravitational data of Occator and its surrounding region, the Dawn scientists were able to map out the geometry of the underground liquid reservoir. Two other studies allowed researchers to examine the thickness of the bright spot and age it. The composition shows Cerealia is significantly younger than the impact crater itself. It's likely that an impactor smashed into Ceres and formed Occator around 22 million years ago. After impact, a small "melt chamber" of liquid formed and the salty liquid within then moved up to the surface to form the structure of Cerealia about two million years ago.
"We concluded that some low activity at Occator is probably still ongoing," says Andreas Nathues, a planetary scientist at the Max Planck Institute for Solar System Research and first author on one of the new studies. "It was a surprise that the old 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, more diffuse bright spots that also show evidence of salts. The researchers reason the difference between the two regions arises because Vinalia has been fed by a deeper source of liquid welling up deep from within Ceres innards. The liquid travels through fractures to the surface where it freezes out, leaving the salty deposits behind.
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 heavenly bodies, like Jupiter's moon Io, generate internal heat through gravitational interactions with their planets -- a process known as "tidal heating." This process can dominate how those worlds evolve. Castillo-Rogez's work has focused on mid-sized cosmic worlds that don't experience this heating, such as Saturn's moons Iapetus and Phoebe.
Dwarf planets like Ceres have cold hearts. They don't experience tidal heating and, lacking an atmosphere, don't trap any heat either. Without heat, water freezes out. But data from Dawn is now allowing scientists like Castillo-Rogez to show these types of cosmic bodies can maintain liquid water over eons in other ways, thanks to the role of salty liquids and compounds like hydrohalite.
Impact events, which generate a lot of heat, may be a chief driver of icy evolution and the short-lived melt chambers they create could be "transient" habitable chambers for alien life. NASA scientist and co-author Lynnae Quick believes these kinds of chambers may provide a chance for life to arise in ocean worlds and icy moons.
Ocean worlds are becoming increasingly common as we learn more about our solar system. There are Enceladus, Saturn's snowball moon, and both Ganymede and Europa, Jovian moons believed to harbor internal oceans. Even further out, at the edge of the solar system, ocean worlds may persist. Studying Pluto, another cold-hearted dwarf planet (cold-heartedly demoted from official planet status), with NASA's New Horizons spacecraft, researchers found evidence for a global liquid ocean underneath its frozen shell in 2016.
"Maybe there are more objects out there like Ceres," ponders Nathues.
The evolution of these icy bodies has far-reaching implications: Where there is water, there is potential for life. Observing the solar system's ocean worlds will enable scientists to assess how habitable they are, and NASA's Dawn has given us an inside track.
Many astrobiologists have their eyes on Mars. Earth's neighbor is the prime target for a suite of recently launched missions designed to look for signs of ancient life. But ocean worlds might also provide refuge for alien forms.
A raft of ocean worlds are set to be explored in the coming decades by two interplanetary probes. Both NASA and the European Space Agency (ESA) will send spacecraft to Jupiter's moon Europa, a world that NASA scientists believe may be the best spot to check for life. The ESA's spacecraft, known as JUICE, will also flyby Ganymede and Callisto, large Jovian moons believed to maintain underground oceans. Dawn is paving the way to understand what those spacecraft might find there.
"The unprecedented resolution from the Dawn images offer a good reference to support future observations of Europa and Ganymede," says Castillo-Rogez.
While planetary scientists begin to look further ahead and plan for missions to icy moons, Castillo-Rogez says that this is just the beginning of data analysis from NASA's Dawn and that many questions have been opened by the Dawn mission. The results from Occator only cover a fraction of the data beamed back to Earth by Dawn in its final days.
The team's discovery has enabled Ceres to be upgraded from a "candidate" ocean world to a certainty, but there are new mysteries now to solve. To adequately determine whether icy moons and icy bodies like Ceres could harbor life -- or may have in the past -- the nature of the environment below the surface needs to be clearly understood.
"To answer more detailed questions about the ocean, we would need a lander mission," says Nathues.
Castillo-Rogez notes a concept study is being finalized for submission to NASA that would see a sample return mission to Vinalia Faculae, allowing scientists to assess habitability and study the state of organic matter on the surface. Whether such a mission is favorably viewed by NASA's long-term planning committee won't be known until 2022.
Dawn was officially retired on Nov. 1, 2018, bringing an end to its mission. It was not deliberately crashed into the planet for fear it might contaminate the surface. The probe continues a lonely vigil in orbit around Ceres and is likely to do so for the next two decades. If we're to return to nab samples, Dawn will be orbiting overhead, a monument to past success.