The Dawn spacecraft has found a lot of surprises at Vesta and Ceres. Debra Buczkowski
a geologist and planetary scientist at the Johns Hopkins University Applied Physics Lab, gave a talk recently at the Museum of Flight discussing some of the findings from the mission.
Vesta was Dawn’s first stop, entering orbit around the asteroid on July 15, 2011. Scientists expected to find volcanoes on Vesta. Buczkowski explained that this expectation traces back to meteorites found on Earth that are know to be from Vesta. These are known as HEDs: “howardite–eucrite–diogenite.” These closely resemble igneous rocks found on Earth, and those are made from volcanic activity. But the volcanoes aren’t there.
Before Dawn arrived at Vesta the Hubble Space Telescope showed that Vesta wasn’t spherical, but rather was significantly flattened out at its south pole. Scientists speculated that this was because of an enormous impact, and that proved to be correct. Dawn observed a huge impact crater, now called Rheasilvia Basin, the rim of which is almost as wide as Vesta itself.
“It really should have broken the asteroid apart,” Buczkowski said of the impact that created the basin, which has a huge central peak. Dawn also found a second impact crater, Veneneia Basin, which is almost as large.
Another surprise finding from Dawn is that Vesta is fully differentiated.
“Most of the asteroids are just kind of chunks of rock with one kind of rock all the way through,” Buczkowski explained. “Not Vesta; Vesta actually has a core, it has a mantle, and it has a crust.”
Vesta’s core is about half the diameter of the asteroid itself, about 220 kilometers.
“This is probably why Vesta did not fall apart when the Rheasilvia Basin formed, because it has this huge, massive core,” Buczkowski said.
The surface of Vesta was found to have lots of fractures, features larger that Earth’s Grand Canyon that look like faults. Buczkowski said they did a lot of computer modeling to see if an object the size of Vesta with a core the size of Vesta’s could develop fractures on the crust.
“The stresses that result from that huge impact kind of get redistributed because of the giant core,” she said of the findings. “Instead of being focused around the crater, they move to the equator and fracture at the equator. If we do this same model without the giant core, there’s no fracturing at the equator. So it’s because of the giant core that we have these huge fractures.”
Buczkowski said that was a little disappointing because they were hoping for volcanoes or magma-driven geology. While they didn’t find volcanoes, there is evidence of moving magma that didn’t break through to the surface. Rather, it pushed some of the surface upward, forming mounds.
On to Ceres
Dawn departed Vesta in September 2012 after spending about 14 months in orbit. As Dawn approached the dwarf planet Ceres there was much speculation about extremely bright spots on its surface that were found in Hubble images. Other observations had detected water vapor on Ceres. Since Ceres is relatively large but not dense, scientists were expecting to find ice. But there was more rock and less ice than anticipated. What they did find, Buczkowski said, was evidence of volcanism.
“We’re not expecting magma on Ceres,” she said. “Ceres isn’t dense enough for the kind of magma that we’re used to here on Earth, made out of silicate rocks. This is something called cryomagma; it is basically ice with a little bit of rock.”
The biggest and brightest of the bright spots, named Cerealia Facula, is in the crater Occator. Many of the craters on Ceres are fractured, even on the crater floors, and the many bright spots on Ceres are associated with these fractures.
“What it’s looking like is that we’re having cryomagmatic activity underneath (Occator) crater,” Buczkowski said, “and what’s coming up out of these fractures is a pyroclastic spray, and the water, the volatiles in that, is sublimating away and all it’s leaving is the sodium carbonates.” Those are the bright spots we see all over Ceres.
Dawn also found that Ceres is covered in ammoniated phyllosilicates.
“Ammonia is interesting,” Buczkowski explained. “We don’t expect to find ammonia this close to the Sun, it’s usually something that’s found further out in thhe solar system.” They’re still studying whether Ceres may have formed further from the Sun and migrated in, or if the ammonia somehow made its way to Ceres from the outer solar system.
It turns out that Ceres had quite a few volcanoes, though most of them have now collapsed. There’s one that hasn’t, known as Ahuna Mons, that stands about five kilometers tall. It’s a cryovolcano.
“The volcano that we thought would be on Vesta is on Ceres,” Buczkowski noted. Ahuna Mons may be younger than the others, and also may collapse over time.
Like Vesta, Ceres was found to be differentiated, though only partially so.
“There’s a rocky core, there’s a volatile-rich mantle, and there’s a muddy slurry, a mud ocean” below the crust, Buczkowski said.
Ceres is now considered a dwarf planet, while Vesta still has asteroid status because of its lopsided shape from the giant impact. Buczkowski figures Vesta deserves dwarf-planet status, too. Whatever you call them, she thinks they’re fascinating to study because they’re kind of a bridge between the asteroids and rocky planets.
“These are more involved bodies than just plain, old asteroids,” Buczkowski said. “They’re not just chunks of rock floating in space. They’re actually like little mini-planets. They’ve got a lot of planet-like properties.”
Though they’re pretty small, they can teach us a lot.
“They’re interesting to us because they tell us a lot about how Earth and the other planets formed,” Buczkowski said. “Studying these little protoplanets we actually are looking back to the beginning of the solar system.”
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