An ocean world in the asteroid belt

Remnants of an ancient water ocean lie buried beneath the icy crust of the dwarf planet Ceres – or at least the parts of it left behind. That’s the tantalizing find presented on August 10 by scientists working on NASA’s Dawn mission. Their research was documented in a series of articles published in Nature.

Ceres is by far the largest object in the asteroid belt, which surrounds the inner planets between Mars and Jupiter. But unlike its rocky neighbors, Ceres is a giant ball of ice. It contains more water than any world in the Sun’s interior except Earth. That knowledge had long led some astronomers to suspect that Ceres may have once had a subsurface ocean, which is part of the reason why NASA sent the Dawn spacecraft there.

However, some models predicted that Ceres’ ocean would have frozen long ago, forming the world’s thick, icy crust.

Now, after five years of studying a series of strange surface features around recently formed craters, astronomers think they are seeing signs of a large, underground body of salty fluid. Variations in Ceres’ gravity field support this, implying that the underground reservoir of salt water could extend horizontally for hundreds of kilometers beneath the ice and reach a depth of about 40 kilometers.

“Previous research showed that Ceres had a global ocean, an ocean that (still) would have no reason to exist and should have been frozen by now,” study co-author and Dawn team member Maria Cristina De Sanctis of the National Institute of Astrophysics in Rome explains Astronomy. “These latest discoveries have shown that part of this ocean could have survived and be present beneath the surface.”

If future missions can confirm the results, it will mean that somewhere around the size of Utah’s Great Salt Lake there is a very salty, very muddy mass of liquid on a dwarf planet just 950 km across (about the size of Texas). . .

Astronomers believe that the water’s extreme saltiness, which lowers its freezing point, allowed it to remain liquid for so long. Also, a class of compounds called hydrates, which are cages of water that trap gas or salt compounds, could change the way heat moves through the dwarf planet’s crust.

Researchers used similar reasoning, applying it to data from NASA’s New Horizons mission, to also argue that Pluto hides a global ocean of liquid water beneath its icy crust.

“Oceans should be common features of dwarf planets, based on what New Horizons learned at Pluto and Dawn at Ceres,” said Dawn project scientist Julie Castillo-Rogez of NASA’s Jet Propulsion Laboratory, who co-authored one of the studies . Astronomy.

The new find raises interesting questions about whether Ceres could be habitable by extraterrestrial life. And it could place Ceres in a fast-growing group of potential icy ocean worlds revealed in recent years.

Ceres is the only dwarf planet in the inner solar system and traps a third of the entire mass in the asteroid belt. Astronomers think Ceres is a protoplanet, the fossilized remains of a world that never fully formed. But its growth was halted before it could become a full-fledged planet. Having such a history means that Ceres likely has an early record of our solar system’s primordial past – hence the name Dawn.

The strange white spots of Ceres

The Dawn mission launched in 2007 with an unconventional ion thruster that first orbited Vesta, the asteroid belt’s second largest object, for 14 months before venturing to Ceres in 2012. No mission had ever orbited two alien worlds before.

“Vesta is a dry body that looks almost like the moon,” said Dawn principal investigator Carol Raymond of JPL Astronomy. ‘We knew that Ceres was a very water-rich object that had retained volatiles from the moment it formed. The two sat there like plums. The low-hanging fruit.”

Ceres began revealing its secrets to astronomers when Dawn first glimpsed the dwarf planet in early 2015. A few strange white spots stood out from afar, shining like cat’s eyes in the dark. More of these bright features became apparent as we approached, and they became the focus of scientists’ efforts to understand Ceres.

Much of Ceres’ story was clear within days of Dawn’s arrival, but scientists still believed they had more to learn, so NASA extended Dawn’s mission for a second time. This allowed the spacecraft to continue collecting data until 2018, when it finally ran out of fuel. This final set of surveys was collected during that extended phase.

And as Dawn collected higher-resolution images, it began to unravel intimate details of the Earth’s surface and its ancient history. Among other things, the spacecraft spotted a lone mountain rising some 21,000 feet above the surface, higher than Denali, North America’s tallest mountain.

Ceres’ white spots are located in Occator Crater, which stretches 56 miles (92 kilometers) from the world’s northern hemisphere. Another place with a prominent bright spot is the smaller Haulani Crater, named after the Hawaiian goddess of plants. It is one of the youngest features of the dwarf planet.

According to the research, it appears that when the impacts hit this region, they penetrated a reservoir of muddy, salty water that lay beneath the plain.

In one of the articles published on August 10, a team of scientists unravels the history of the Occator crater in detail. They think a space rock struck this location about 20 million years ago, piercing the ice crust to the salty reservoir below. However, within hours the crater quickly froze over.

But when it did, it closed into a large chamber of meltwater beneath the center of the crater, allowing fluids and chemicals to continue mixing with the larger reservoir below. This structure allowed salty, chemically rich water to erupt from the center of the crater as recently as 2 million years ago, creating the fascinating white spots.

However, Ceres could have erupted even more recently. Before Dawn reached the dwarf planet, the European Space Agency’s Herschel telescope detected water vapor coming from the same region. And if fluids are still not seeping from the cracks in the Occator crater, then the minerals in the area should have already evaporated.

“It’s really like a smoking gun, because you would expect it to have disappeared if it had been there even close to the surface for millions of years,” Raymond says.

Ceres as a place of residence?

Scientists are still not entirely sure what Ceres has in common with our solar system’s other icy ocean worlds, such as Jupiter’s moon Europa and Saturn’s Enceladus. However, some of the minerals found on Ceres were also found in the water plumes erupting from Enceladus, creating a link between the two bodies.

All these discoveries together are changing astronomers’ ideas about our solar system. Half a century ago, they thought Earth’s oceans made it a unique home for life in our solar system. But it now appears that there could be dozens of potential ocean worlds in the inner and outer Solar System. That finding is “one of the most profound discoveries in planetary science in the space age,” says S. Alan Stern of the Southwest Research Institute and head of NASA’s New Horizons mission. Astronomy.

In the coming decades, astronomers are aiming for a large number of missions to explore these ocean worlds in more detail. And Ceres’ relatively short distance from Earth could help them make the case for a visit in the not-so-distant future.

On Monday, as the team’s new research was published, Castillo-Rogez formally submitted a study outlining a $1 billion mission that would actually land on Ceres. If astronomers show interest in the idea as part of their ten-year research, and NASA decides to fund it, the spacecraft would fly as a New Frontier mission sometime before 2032. Meanwhile, the European Space Agency is also studying a possible return mission.

“Ceres is a lot closer and a lot easier to get to than these moons in the outer solar system,” Raymond says. “So it is a very attractive target.”