Vesta and the Dawn
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This image of the giant asteroid Vesta was obtained by NASA's Dawn spacecraft in the evening of Nov. 27 Pacific Standard Time, as it was spiraling down from its high altitude mapping orbit to low altitude mapping orbit. Low altitude mapping orbit is the closest orbit Dawn will be making, at an average of 130 miles (210 kilometers) above the giant asteroid's surface. The framing camera obtained this image of an area in the northern mid-latitudes of Vesta from an altitude of about 140 miles (230 kilometers). Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
The Dawn spacecraft has been sending intriguing information back from the dwarf planet Vesta since August. And perhaps no one is more intrigued than Methow Valley physicist Thomas B. McCord, who urged NASA to undertake the Vesta mission and is monitoring it with computers linked to the National Jet Propulsion Laboratory from his Bearfight Institute up the Rendezvous. He spoke about some of the preliminary Vesta findings to the American Geophysical Union annual meeting in San Francisco earlier this month.
McCord has a long history with this piece of heavenly real estate. Forty years ago, he was the first to argue that although it lies in the asteroid belt, Vesta wasn’t an asteroid. That’s because he and other physicists, including McCord’s Methow neighbor John Adams, established that it has a basaltic composition, which means it has melted. That meant it had to have had heat inside, and that ruled out its being an asteroid.
So why does anyone care about Vesta? McCord walked through a theory.
The images show that Vesta’s surface is pockmarked with craters from impacts of asteroids or meteorites, one of them big enough that it likely knocked Vesta into such a tailspin that it upended its poles – flipped it on its head or its backside, as he puts it. The result was a very big hole, captured by Dawn; photos show the crater as a mound, which was created after the impact that blew away surrounding materials.
Why is that hole and its missing material significant? Because, explains McCord, we have meteorites here on earth that are identical in composition to Vesta’s, which suggests they could have come from there. “Finding this huge impact kind of builds the story,” he explains.
And the story is? Ultimately, nothing less than finding the answer to how water formed on Earth - though McCord ascribes no such portentous dimension to this mission. It’s just one in the long line of scientific investigations that may provide an understanding of how water gets around in the cosmos.
Here’s how McCord lays it out: Vesta is completely exposed to space; it has no atmosphere. This means it would be very difficult to keep liquid water there. Yet Vesta shows chemical traces of water. “How did it get there?” he asks. “The sun is seeding it,” he answers.
Vesta, the second largest dwarf plant in the asteroid belt, is bombarded by solar winds that stream electrons and protons, which form hydrogen atoms, onto its battered surface. And in that surface lurks oxygen. “The beating up of the surface from impacts creates grains at the crystal level, and damaged crystals have oxygen atoms in them that don’t have all their (chemical) bonds satisfied. So they are sitting there lusting after something,” he explains. “It’s vaguely pornographic.”
“In comes the hydrogen, and boom!... If you have enough of that happening, you’ll have H2O. The mating gets driven in the direction of saturation,” McCord says. “All they have to do is mate.”
It’s not unreasonable to expect that before Dawn leaves Vesta’s orbit next summer, it may have picked up evidence that indicates some of the objects that hit Vesta were water-bearing, says McCord. Establishing that “wet objects” had hit Vesta would help scientists understand how water has moved through space.
“It’s not fully understood where water comes from,” MCord admits, but there is an understood physical process that creates the building blocks of water. He was part of the team that uncovered evidence of this process on the moon. ”So is it possible that the Earth cooled enough that solar winds could have impregnated the Earth?” he wonders.
The Vesta images also show blocks of dark and bright material, “strange deposits” cropping out of the walls of the crater and on the surface. They’ve never been seen before, McCord says, and no one understands what they are. “My favorite hypothesis at the moment is that they are left-over pieces of the impacting body that hit the surface.”
Vesta was formed 4.5 billion years ago when the solar system was formed, very early in the process of forming planets. “Objects that size could have been building blocks for the planets,” he theorizes. If so, Vesta didn’t get used up in that process but has remained intact enough to entice scientists like McCord to probe its mysteries for possible answers to the eternal question of how Earth became fit for life.
The next stop for Dawn is Ceres in 2015. It’s the biggest dwarf planet in the asteroid belt. It’s been established that Ceres has a lot of water on its surface, he says, and he believes it also has a lot of water in its interior. Ceres has a frozen water mantle and a solid silicate core. “Between the frozen water and the silicate core there should be a layer of liquid water,” McCord predicts. “We don’t know if we’re going to find direct evidence for this liquid layer,” he adds, “But we’re certainly going to try.”
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