When NASA’s Dawn spacecraft arrived at Ceres in 2015, scientists were expecting to find a relic from the early days of the Solar System – one that could help unravel the secrets of how other worlds, including the Earth, formed.
Instead, they discovered that the asteroid – which at 946 kilometres in diameter is large enough to be classified as a dwarf planet – is a complex world its own right, with numerous signs of geological activity in the not-too-distant past, plus signs of a subsurface ocean.
Now, a suite of seven papers in the journals Nature Astronomy, Nature Geoscience and Nature Communications has upped the ante. [See below for links to abstracts.]
Ceres isn’t just recently geologically active: it’s very recently so. For all we know, says Carol Raymond, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, and the mission’s principal investigator, it could still be active today.
The focus of the excitement is a 90-kilometre crater named Occator, where an impact punched a 4000-metre deep hole in Ceres’ northern highlands, only 20 million years ago.
“It’s basically the Tycho of Ceres,” says Paul Schenk, a planetary scientist at the Lunar and Planetary Institute, Houston, Texas, and author of one of the studies.
Tycho is a bright crater on the Moon that is easily visible with binoculars or a small telescope (and sometimes with the naked eye).
From the beginning, Occator drew interest because its floor had mysterious bright spots that later turned out to be a type of sodium-enriched carbonate that can only form in hydrothermal environments akin to earthly hot-springs and geyser basins.
But what was their source?
To learn more, Dawn spent the last phase of its mission, until it ran out of manoeuvering fuel in 2018, in an orbit that swept only 35 kilometres above Occator – so close it was able to photograph the crater floor to a resolution of about 3.5 metres per pixel. “That’s pretty damn good,” Schenk says.
Better yet, the photos were taken from multiple angles, creating a stereoscopic effect. “I like to work in that,” Schenk says. “They give a very good insight into the 3D structure of the surface.”
From this, he says, it’s clear that heat from the impact melted the ice beneath the crater, causing muddy brine to rise from the depths through thousands of little vents, even after the crater’s surface had re-solidified, peppering it with pits and domes.
But that’s only part of the story. Residual heat from the impact would gradually have faded, Raymond says, making it difficult to explain why some of the carbonate deposits are so fresh.
In fact, she says, based on the tiny number of small impact craters they’ve collected since they formed, some of these deposits can’t be more than two million years old – if that. “Everything points to [them] being really young,” she says.
It appears that in addition to melting the ice close to the surface, the impact punched deep enough into Ceres’ crust to create cracks through which brine could continue to well up from a subsurface ocean about 35 kilometres below the surface.
Evidence for this, Raymond says, comes from high-resolution gravity measurements that the spacecraft was able to take each time its orbit dived close above the Occator region – data that suggests that a liquid layer still lies beneath the crater.
Meanwhile, another research team, led by Maria Cristina De Sanctis of the Istituto Nazionale di Astrofisica, Rome, Italy, found that some of the geologically young deposits contained hydrohalite.
“That’s basically hydrated table salt,” says Raymond. It’s also evidence that the hydrothermal activity creating the deposits on Occator’s surface aren’t just recent, but very, very recent.
Hydrohalite, De Sanctis and Raymond say, dehydrates very rapidly on Ceres – so quickly that these deposits aren’t just geologically young; they are newborns.
“It would dehydrate in a matter of years, 100 years maximum,” Raymond says. “It could be oozing up through cracks right now.”
Though, she adds, it’s possible that the hydrohalites seen by De Sanctis’s team were actually below the surface until something recently uncovered them. “But it certainly is a very recent event,” she says.
Meanwhile, scientists hope to return to Ceres. A proposal to do so is being prepared for NASA’s 2023-2032 Decadal Survey of priority missions for the next decade. The target? What else: a lander and possible sample-return mission in Occator Crater.