Tiny 'spherules' reveal details about Earth's asteroid impacts
April 25, 2012
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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. (Oberlin College photo/Bruce M. Simonson) |
The spherules were created when asteroids crashed into theEarth, vaporizing rock that expanded into space as a giant vapor plume. Smalldroplets of molten and vaporized rock in the plume condensed and solidified,falling back to Earth as a thin layer. The round or oblong particles werepreserved in layers of rock, and now researchers have analyzed them to recordprecise information about asteroids impacting Earth from 3.5 billion to 35million years ago.
"What we have done is provide the foundation forunderstanding how to interpret the layers in terms of the size and velocity ofthe asteroid that made them," said Jay Melosh, an expert in impactcratering and a distinguished professor of earth and atmospheric sciences,physics and aerospace engineering at Purdue University.
Findings, which support a theory that the Earth endured anespecially heavy period of asteroid bombardment early in its history, aredetailed in a research paper appearing online in the journal Nature on Wednesday (April 25). Thepaper was written by Purdue physics graduate student Brandon Johnson and Melosh.The findings, based on geologic observations, support a theoretical study in acompanion paper in Nature byresearchers at the Southwest Research Institute in Boulder, Colo.
The period of heavy asteroid bombardment - from 4.2 to 3.5billion years ago - is thought to have been influenced by changes in the earlysolar system that altered the trajectory of objects in an asteroid belt locatedbetween Mars and Jupiter, sending them on a collision course with Earth.
"That's the postulate, and this is the first realsolid evidence that it actually happened," Melosh said. "Some of theasteroids that we infer were about 40 kilometers in diameter, much larger thanthe one that killed off the dinosaurs about 65 million years ago that was about12-15 kilometers. But when we looked at the number of impactors as a functionof size, we got a curve that showed a lot more small objects than large ones, apattern that matches exactly the distribution of sizes in the asteroid belt.For the first time we have a direct connection between the crater sizedistribution on the ancient Earth and the sizes of asteroids out inspace."
Because craters are difficult to study directly, impacthistory must be inferred either by observations of asteroids that periodicallypass near the Earth or by studying craters on the moon. Now, the new techniqueusing spherules offers a far more accurate alternative to chronicle asteroidimpacts on Earth, Melosh said.
"We can look at these spherules, see how thick thelayer is, how big the spherules are, and we can infer the size and velocity ofthe asteroid," Melosh said. "We can go back to the earliest era inthe history of the Earth and infer the population of asteroids impacting theplanet."
For asteroids larger than about 10 kilometers in diameter,the spherules are deposited in a global layer.
"Some of these impacts were several times larger thanthe Chicxulub impact that killed off the dinosaurs 65 million years ago,"Johnson said. "The impacts may have played a large role in the evolutionalhistory of life. The large number of impacts may have helped simple life byintroducing organics and other important materials at a time when life on Earthwas just taking hold."
A 40-kilometer asteroid would have wiped out everything onthe Earth's surface, whereas the one that struck 65 million years ago killedonly land animals weighing more than around 20 kilograms.
"Impactcraters are the most obvious indication of asteroid impacts, but craters onEarth are quickly obscured or destroyed by surface weathering and tectonicprocesses," Johnson said. "However, the spherule layers, if preservedin the geologic record, provide information about an impact even when thesource crater cannot be found."
The Purdue researchers studied the spherules usingcomputer models that harness mathematical equations developed originally tocalculate the condensation of vapor.
"There have been some new wrinkles in vaporcondensation modeling that motivated us to do this work, and we were the firstto apply it to asteroid impacts," Melosh said.
The spherules are about a millimeter in diameter.
The researchers also are studying a different type ofartifact similar to spherules but found only near the original impact site.Whereas the globally distributed spherules come from the condensing vaporizedrock, these "melt droplets" are from rock that's been melted and notcompletely vaporized.
"Before this work, it was not possible to distinguishbetween these two types of formations," Melosh said. "Nobody hadestablished criteria for discriminating between them, and we've done thatnow."
One of the authors of the Southwest Research Institutepaper, David Minton, is now an assistant professor of earth and atmosphericsciences at Purdue.
Findings from the research may enable Melosh's team toenhance an asteroid impact effects calculator he developed to estimate whatwould happen if asteroids of various sizes were to hit the Earth. Thecalculator, "Impact: Earth!" allows anyone to calculate potentialcomet or asteroid damage based on the object's mass.
The research has been funded by NASA.
Writer: Emil Venere, 765-494-3470, venere@purdue.edu
Sources: JayMelosh, 765-494-3290, jmelosh@purdue.edu
BrandonJohnson, 765-337-5041, johns477@purdue.edu
Note to Journalists: An electronic copy of the research paper is available by contacting Nature at press@nature.com
ABSTRACT
Impact Spherules: Record of an Ancient Heavy Bombardment of Earth
B.C. Johnsona and H.J. Meloshb,a
aDepartment of Physics, Purdue University
bDepartment of Earth and Atmospheric Sciences, Purdue University
Impact craters are themost obvious indication of asteroid impacts, but craters on Earth are quicklyobscured or destroyed by surface weathering and tectonic processes1. Earth'simpact history is inferred therefore either from estimates of the present-dayimpactor flux as determined by observations of near-Earth asteroids, or fromthe Moon's incomplete impact chronology. Asteroids hitting Earth typicallyvaporize a mass of target rock comparable to the projectile's mass. As this vaporexpands in a large plume or fireball, it cools and condenses into moltendroplets called spherules. For asteroids larger than about 10 kilometers indiameter, these spherules are deposited in a global layer. Spherule layerspreserved in the geologic record accordingly provide information about animpact even when the source crater cannot be found. Here, we report estimatesof the sizes and impact velocities of the asteroids that created globalspherule layers. The impact chronology from these spherule layers reveals thatthe impactor flux was significantly higher 3.5 billion years ago than it isnow. This conclusion is consistent with a gradual decline of the impactor fluxafter the Late Heavy Bombardment.
