The largest world in the asteroid belt is sprinkled with bright, enigmatic spots, could have craters that fill with haze each morning, and may even be a transplant from far, far, away, scientists report today in Nature.
These preliminary conclusions are based on data from NASA’s Dawn spacecraft, which has been orbiting the dwarf planet Ceres since March. Large, round and watery, Ceres isn’t quite like the rest of the space rocks that live between Mars and Jupiter. And the more scientists learn about the place, the stranger it gets.
“Ceres has been a mystery,” says UCLA’s Christopher Russell, principal investigator for the Dawn mission, noting that there are no pieces of Ceres that have fallen to Earth. “We had to go out there and see what it was because we didn’t have the clues.”
Dried Up Salty Spots?
Before Dawn even spiraled into orbit around 950-kilometer-wide Ceres, scientists were transfixed by what appeared to be extremely bright splotches on the world’s surface. For months, the Dawn team guessed the splotches might be made of highly reflective water ice –- and even suggested that one of them could be spitting water into space. Now, as often happens in science, it looks like those early guesses weren’t quite right.
As Dawn neared Ceres, the team realized the spots were dimmer than they’d expected – and not nearly bright enough to be water ice. (In fact, Ceres itself is more or less as reflective as freshly laid asphalt – which is to say, not very bright at all.)
“The next brightest thing is salt,” Russell says, referring to the spots. “There’s a number of different salts that could have been made in the interior by the chemistry that goes on between rocks and water.”
The team doesn’t know precisely what the spots are made of, but scientists suggest they could be reflective, sulfate-containing salts – the kind of stuff that might be left behind as ice tucked into salty crystals warms and turns into water vapor, says Andreas Nathues of the Max Planck Institute for Solar System Research.
“The bright spots are remnants of a water ice sublimation process,” he says.
So far, Dawn has taken its closest look at the cluster of spots in Occator crater, which are the biggest and brightest, and also among the youngest. Mostly found inside craters, more than 130 of the enigmatic splotches sprinkle the world’s surface. Nathues notes that the spots’ occurrence within craters is no accident, and that the team suspects impacts trigger their generation by digging into a buried, frozen layer of water. So it’s likely that while water ice still exists in Occator, it has disappeared from the older spots.
“We think these bright spots have the same origin,” Nathues says. “Most of them are today dehydrated.”
But there’s more: Observations suggest that each morning, as the sun rises, a morning haze or mist fills two of Ceres’ craters, Occator and Oxo. The haze isn’t very thick and is likely less than a few hundred meters high – and it disappears by dusk, only to reappear the next morning. Nathues suspects the morning mist could be produced as buried water ice is warmed by the rising sun. If it’s there, that hovering water vapor could explain a 2014 observation made by the Herschel space observatory, which showed that Ceres had thin tufts of water vapor surrounding it.
“The longitudinal position at which Herschel found the strongest water vapor absorption line fits with the longitude of Occator,” Nathues says.
But it’s still too soon to say if the haze is really there, says William McKinnon of the Washington University in St. Louis. He’s unconvinced that what appears to be haze isn’t just an artifact of the angle at which Dawn is peering into these craters, and says he’d like to reserve judgment until the spacecraft can take an even closer look at what’s going on.
“It’s not a slam dunk,” McKinnon says. “Dawn is going into its lowest orbit, so presumably will be getting even more and better pictures of this puppy in the coming months.”
An Outer Solar System Origin?
In 2008, McKinnon suggested that Ceres may have been born in the outer solar system – way far out, near where Pluto and countless other icy worlds live. It has much more water than its fellow asteroids, he argued, and early estimates of its density were similar to those of objects in the Kuiper Belt, an icy debris ring beyond the orbit of Neptune.
At a recent conference, scientists shared some observations that might support this idea, and the same observations were published in Nature today: On Ceres’ surface, Dawn spotted what appear to be clays containing ammonia, which isn’t a compound that’s normally found in the warm inner solar system. Here, ammonia-containing ice would just evaporate. That’s why Maria Cristina De Sanctis, an astronomer at Rome’s National Institute of Astrophysics, says she wasn’t expecting to see this particular type of clay, known as ammoniated phyllosilicates.
“So the question boils down to, where does the ammonia come from?” McKinnon asks. “Maybe stuff from the outer solar system got mixed into the asteroid belt and got widely distributed. Or maybe Ceres as a whole got implanted in the asteroid belt.”
The only solid surfaces where significant amounts of ammonia have been found are in the outer solar system: Charon, Pluto’s largest moon; Orcus, another body in the Kuiper Belt; and Miranda, the innermost moon of Uranus.
But both McKinnon and Andy Rivkin, of the Johns Hopkins University Applied Physics Laboratory, note that recent work has uncovered tiny amounts of ammonia in meteorites that have been chipped off of main belt asteroids. “If it turns out to be ammoniated phyllosilicates, which is a decent if not slam dunk bet, that doesn’t mean it has to be an escaped Kuiper Belt Object or necessarily have these outer solar system pebbles,” Rivkin says.
It’s also possible, they both say, that Ceres was once much more active, and that water percolating through its interior may have helped form and push the the ammonia-containing clays to the surface, concentrating whatever small amounts of ammonia were naturally inside Ceres into a region where Dawn could see it.
Determining which of these scenarios actually took place will be complicated, especially from orbit – but as Dawn snuggles in close to Ceres this week, scientists will give it their best shot.