April 25, 2012
New Research Reveals Details About Earth’s Asteroid Impacts
Lee Rannals for RedOrbit.com
Researchers are reporting in the journal Nature that tiny "spherules" embedded in layers of rock have given them details about Earth's asteroid impacts.According to the team, the spherules were created when asteroids crashed into the Earth, vaporizing rock that expanded into space as a giant vapor plume.
Small droplets of molten and vaporized rock in the plume then condensed and solidified, falling back to Earth as a thin layer. These particles were preserved in layers of rock, and the team analyzed them to record precise information about ancient asteroid impacts on Earth, 3.5 billion to 35 million years ago.
"What we have done is provide the foundation for understanding how to interpret the layers in terms of the size and velocity of the asteroid that made them," Jay Melosh, professor Purdue University, said in prepared statement.
The period of heavy asteroid bombardment between 4.2 billion and 3.5 billion years ago is thought to have been influenced by changes in the early solar system that altered the trajectory of objects in an asteroid belt between Mars and Jupiter.
"That's the postulate, and this is the first real solid evidence that it actually happened," Melosh said. "Some of the asteroids that we infer were about 40 kilometers in diameter, much larger than the one that killed off the dinosaurs about 65 million years ago that was about 12-15 kilometers."
When the team looked at the number of impactors as a function of size, they found a time that showed a great deal more small objects than large ones. This pattern, according to Melosh, matches the distribution of sizes in the asteroid belt.
"For the first time we have a direct connection between the crater size distribution on the ancient Earth and the sizes of asteroids out in space," he said.
Impact history is hard to establish because craters are difficult to study directly, so scientists are left with having to study the moon, or asteroids that pass near the Earth.
However, Melosh believes that the new technique using spherules offers a more accurate alternative to chronicle asteroid impacts on Earth.
"We can look at these spherules, see how thick the layer is, how big the spherules are, and we can infer the size and velocity of the asteroid," Melosh wrote. "We can go back to the earliest era in the history of the Earth and infer the population of asteroids impacting the planet."
Purdue physics graduate student Brandon Johnson said some of the impacts were larger than the Chicxulub impact that killed off the dinosaurs 65 million years ago.
"The impacts may have played a large role in the evolutional history of life," Johnson said. "The large number of impacts may have helped simple life by introducing organics and other important materials at a time when life on Earth was just taking hold."
An asteroid about 25-miles wide would have wiped out everything on the Earth's surface, whereas the one that struck 65 million years ago would have killed just land animals that weighed more than 45 pounds.
"Impact craters are the most obvious indication of asteroid impacts, but craters on Earth are quickly obscured or destroyed by surface weathering and tectonic processes," Johnson said. "However, the spherule layers, if preserved in the geologic record, provide information about an impact even when the source crater cannot be found."
The researchers studied the spherules with computer models that used mathematical equations developed to calculate the condensation of vapor.
Melosh said new research in vapor condensation modeling enabled the team to do this work, and they were first able to apply it to asteroid impacts.
The team found that "melt droplets," a different type of artifact similar to spherules, were only near the original impact site. The globally distributed spherules come from the condensing vaporized rock, whereas melt droplets are from rock that has been melted and not completely vaporized.
"Before this work, it was not possible to distinguish between these two types of formations," Melosh said. "Nobody had established criteria for discriminating between them, and we've done that now."
The impact study team also includes Bottke, Dr. David Nesvorny and Dr. Hal Levison, Southwest Research Institute; Dr. David Minton, Purdue University; Prof. Bruce Simonson, Oberlin College and Conservatory; Dr. David Vokrouhlicky, Charles University, Prague, Czech Republic; Dr. Alessandro Morbidelli, Observatorie de la Cote d'Azur, Nice, France; and Dr. Ramon Brasser, Academia Sinica Institute of Astronomy and Astrophysics, Taipei, Taiwan.
Image 1: This image shows an artist´s depiction of a 10-kilometer (6-mile) diameter asteroid striking the Earth. Approximately 70 of these dinosaur killer-sized or larger impacts hit the Earth over a span that lasted between 3.8 and 1.8 billion years ago. They are capable of producing a global millimeter- to centimeter-thick rock layer that contains impact debris: sand-sized droplets, or spherules, of molten rock that rained down from the huge molten plumes made by each mega-impact. Credit: Southwest Research Institute/Art by Don Davis
Image 2: Researchers are learning details about asteroid impacts going back to the Earth's early history by using a new method for extracting precise information from tiny "spherules" embedded in layers of rock. The spherules were created when asteroids crashed into Earth, vaporizing rock that expanded as a giant vapor plume. Small droplets of molten rock in the plume condensed and solidified, falling back to the surface as a thin layer. This sample was found in Western Australia and formed 2.63 billion years ago in the aftermath of a large impact. Credit: Oberlin College photo/Bruce M. Simonson