Tucked into the asteroid belt between Mars and Jupiter, the dwarf planet Ceres is a small world that holds big surprises. A slew of new research from NASA’s Dawn spacecraft advances the case that—in its own cold, salty way—Ceres is a geologically active body, with ice volcanoes and surviving pockets of an ancient ocean.
About a year’s worth of data collected by Dawn from late 2017 through late 2018—during its final orbits before running out of fuel—show that the dwarf planet probably has briny liquid seeping out on its surface, as well as mounds and hills that formed when ice melted and refroze after an asteroid impact about 20 million years ago.
The idea that liquid water could persist on Ceres—a world that’s less than a third of the moon’s width—would have once seemed outlandish. But now that humankind has seen it up close, we know that frigid, tiny Ceres is geologically alive.
The findings help address Ceres’s central mystery: a 57-mile-wide impact crater known as Occator that’s covered with perplexing bright spots of salt. As recently as 1.2 million years ago, cold underground brine oozed out onto Occator’s floor to form these salty deposits, the new research suggests.
Bulging mountains and hills also support the idea that Ceres experiences a kind of ice-cold cryovolcanism, with briny mud or slush acting like molten lava does on Earth. In one region of Occator's crater floor, Dawn spotted hints that brines had dribbled out of ice volcanoes within the past few decades, if not more recently.
“We’ve provided strong evidence that Ceres is geologically active in the present, [or] at least in the very recent past,” says Dawn’s principal investigator Carol Raymond, the manager of the NASA Jet Propulsion Laboratory’s Small Bodies Program in Pasadena, California. “And there’s some tantalizing evidence that it could be ongoing.”
Beyond exotic volcanoes, the new findings add Ceres to the growing list of worlds that had all the required ingredients for life at one point or another: liquid water, energy, and carbon-bearing organic molecules. Thanks to the heat of asteroid impacts, scientists say that Ceres may have been habitable—though not necessarily inhabited—for short periods of time.
“We’ve got this recent, warm, wet geologic system that has all the ingredients that we think you need for life,” says Kirby Runyon, a planetary geologist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, who wasn’t involved with the studies.
A dwarf planet up close
Seven studies published today in three journals—Nature Astronomy, Nature Communications, and Nature Geoscience—relay data from the final leg of the Dawn mission, which orbited Ceres from 2015 to 2018. For the mission's grand finale, Dawn swooped to within 22 miles of Ceres's surface, snapping pictures with a stunning resolution of 10 feet per pixel, equivalent to seeing a golf ball from more than a quarter mile away.
Ever since Dawn detected Occator’s bright spots in 2015, scientists have puzzled over how they formed. Researchers quickly learned the features were made of salts, likely deposited in the crater from brines seeping up onto Ceres’s surface. The question was where the brines came from.
Researchers think Occator Crater is roughly 20 million years old. The impact that created it would have generated immense amounts of heat, turning the normally frigid landscape into a frothy bath of churning saltwater. But using computer simulations, the Dawn team found that the collision’s heat largely dissipated within five million years or so.
Some of the salty bright spots were laid down within the past four million years, so the impact couldn’t have created them. Instead, the fluids must have been coming from an ancient, deep reservoir of liquid brine.
Ceres’s gravity revealed the brine’s probable sources, thanks to the fact that a planet’s gravitational tug can vary slightly from area to area based on the local landscape and crust density. Researchers could track this variation at Ceres by measuring small-scale changes in Dawn’s velocity as the spacecraft orbited the dwarf planet.
When researchers combined this data with Ceres’s topography, they found that the ground beneath Occator was less dense than the surrounding crust. Two reservoirs of brine, ellipsoids shaped like giant M&M’s, appear to sit below the crater. The bigger one, some 260 miles wide, lies 30 miles directly beneath the crater at the base of Ceres’s crust. A smaller briny reservoir about 120 miles wide sits to the crater’s southeast, 12 miles beneath the surface.
“If you were to drill, you might be able to reach an aquifer, and then you’d get very cold brine to come out,” says Bill McKinnon, a planetary scientist at Washington University in St. Louis who wasn’t involved in the new studies.
Oozing leftovers of an ancient ocean
These brine pockets are the relics of a larger, possibly global ocean that once existed on Ceres, the team concluded. As anyone who’s driven on a salted road in winter has seen, dissolved salts can keep water in a liquid state at temperatures colder than its usual freezing point. In the case of Ceres, the brines are estimated to be about minus 22°F, requiring a lot of salt and possibly a mixture of muddy, fine-grained minerals to stay in liquid form.
The brines are “definitely not for scuba diving—it’s like a big swamp,” says study co-author Julie Castillo-Rogez, a JPL planetary scientist and Dawn team member.
Whatever slammed into Ceres and created Occator Crater likely kickstarted the icy volcanism that brought briny material to the surface. Unlike volcanoes on Earth, cryovolcanoes on Ceres develop as ice in the dwarf planet’s crust freezes and expands, compressing and pressurizing pockets of underground brine.
The Occator impact cracked Ceres’s crust, leaving fractures that deep brines could use to wend their way up to the surface. Once they spilled out, the water evaporated, leaving the bright, salty deposits that we see today.
Some observations even suggest that the activity on Ceres is ongoing. In one of the seven studies, a team led by Maria Cristina De Sanctis, a planetary scientist at Italy’s National Institute of Astrophysics, found evidence that Occator’s bright spots include hydrated sodium chloride. The water component of this salt should boil off into space within a hundred years of coming to the surface, the researchers say. Since the material is still hydrated, though, Ceres’s icy volcanoes could still be chugging along.
“It’s very likely that this volcano is still active, in the sense that water, in a lower amount, is still rising,” says Dawn team member Andreas Nathues, a planetary scientist at Germany’s Max Planck Institute for Solar System Research and co-author of several of the new studies.
The solar system’s many icy worlds
Dawn and NASA’s New Horizons spacecraft, which flew by Pluto in 2015, have shown that small, icy bodies are far more active than once thought, stretching how scientists imagine the geology of dozens of alien worlds.
Like Ceres and its bright spots, “every planet seems to have its something special,” says McKinnon, a New Horizons co-investigator. “Geology is going to rhyme—but it’s not going to repeat.”
A team led by Castillo-Rogez submitted a proposal to NASA today for a Ceres sample-return mission, which would launch no earlier than 2031, since it can take years to approve, design, and build a spacecraft. The mission would collect a hundred grams of material from Occator’s floor and send it back to Earth.
That may not sound like much material to work with, but the samples would represent much more pristine and primitive material than anything we’ve yet been able to study, Raymond says. “By getting into the details of what is in these bodies, we’re going to learn a tremendous amount.”