Post by Nemesis on Mar 5, 2003 3:15:41 GMT -5
The three most terrible asteroid impacts in the Earth's history are also the oldest, say geologists working on frozen blobs of melted rock ejected from impacts more than 3.2 billion years ago.
Above is an artist's depiction of the asteroid that killed the dinosaurs 65 million years ago. Researchers have found evidence that there were even more horrible asteroid impacts 3.2 billion years ago.
Any craters from the impacts were erased long ago by Earth's everchanging crust. What does remain, however, are deposits of rock spherules in South Africa's Barberton Greenstone Belt region that were once a fiery rain of molten material blasted from horrific impacts.
"The bottom line is that I think they were bigger than the K/T impact," said geologist Frank Kyte of the University of California at Los Angeles, referring to the impact of a ten-kilometer (seven-mile) wide asteroid that killed the dinosaurs 65 million years ago.
Kyte and a number of other asteroid impact researchers published their report in the March issue of the journal Geology.
Since they were discovered in 1986, the South African spherules have sparked scientific debate over whether they were caused by impacts or some purely earthbound process. The work by Kyte and his colleagues might finally settle the debate because they have found in the spherules something only found in extraterrestrial rocks — an unusual abundance of a rare type of chromium.
For reasons that are buried in the early history of the solar system, meteorites tend to have more chromium-53 and chromium-54 than rocks from the Earth, moon or Mars, which tend to have more chromium-52 (Martian meteorites supplied the information about Mars).
Using a new and painstaking technique to analyze the South African spherules, Kyte and his colleagues were able to determine that the rocks contained amounts of chromium-53 and chromium-54 that pegged them as relatives of a relatively rare kind of meteorite, a carbonaceous chondrite.
Factors they used to estimate the size of the asteroids that spawned the spherules include the thickness of the spherule beds and the likely assumption that the debris rained down over the entire Earth. All that seems to indicate that the Archean asteroids ranged from one to seven times the size of the K/T asteroid.
In other words, the discovery not only confirms the impact origins of the rocks, but it suggests that the asteroids 3.2 billion years ago were a bit different than those that are common today, said planetary scientist Alan Hildebrand of the University of Calgary.
They are rough estimates, based on comparisons to similar beds from the much better preserved K-T impact. Factors they used to estimate the size of the asteroids include the thickness of the spherule beds and the likely assumption that the debris rained down over the entire Earth. All that seems to indicate that the Archean asteroids ranged from one to seven times the size of the KT asteroid.
"In my opinion this is quite a significant result," said Hildebrand. "It should settle the debate on origins."
What isn't settled is just what was going on 3.2 billion years ago to cause three great impacts in just 20 million years. Were asteroid impacts more common throughout the early history of the Earth or was this just a spike of impacts?
Only the discovery of more such spherules of other ages will tell, said Kyte
Source: Discovery News - March 4 2003
Above is an artist's depiction of the asteroid that killed the dinosaurs 65 million years ago. Researchers have found evidence that there were even more horrible asteroid impacts 3.2 billion years ago.
Any craters from the impacts were erased long ago by Earth's everchanging crust. What does remain, however, are deposits of rock spherules in South Africa's Barberton Greenstone Belt region that were once a fiery rain of molten material blasted from horrific impacts.
"The bottom line is that I think they were bigger than the K/T impact," said geologist Frank Kyte of the University of California at Los Angeles, referring to the impact of a ten-kilometer (seven-mile) wide asteroid that killed the dinosaurs 65 million years ago.
Kyte and a number of other asteroid impact researchers published their report in the March issue of the journal Geology.
Since they were discovered in 1986, the South African spherules have sparked scientific debate over whether they were caused by impacts or some purely earthbound process. The work by Kyte and his colleagues might finally settle the debate because they have found in the spherules something only found in extraterrestrial rocks — an unusual abundance of a rare type of chromium.
For reasons that are buried in the early history of the solar system, meteorites tend to have more chromium-53 and chromium-54 than rocks from the Earth, moon or Mars, which tend to have more chromium-52 (Martian meteorites supplied the information about Mars).
Using a new and painstaking technique to analyze the South African spherules, Kyte and his colleagues were able to determine that the rocks contained amounts of chromium-53 and chromium-54 that pegged them as relatives of a relatively rare kind of meteorite, a carbonaceous chondrite.
Factors they used to estimate the size of the asteroids that spawned the spherules include the thickness of the spherule beds and the likely assumption that the debris rained down over the entire Earth. All that seems to indicate that the Archean asteroids ranged from one to seven times the size of the K/T asteroid.
In other words, the discovery not only confirms the impact origins of the rocks, but it suggests that the asteroids 3.2 billion years ago were a bit different than those that are common today, said planetary scientist Alan Hildebrand of the University of Calgary.
They are rough estimates, based on comparisons to similar beds from the much better preserved K-T impact. Factors they used to estimate the size of the asteroids include the thickness of the spherule beds and the likely assumption that the debris rained down over the entire Earth. All that seems to indicate that the Archean asteroids ranged from one to seven times the size of the KT asteroid.
"In my opinion this is quite a significant result," said Hildebrand. "It should settle the debate on origins."
What isn't settled is just what was going on 3.2 billion years ago to cause three great impacts in just 20 million years. Were asteroid impacts more common throughout the early history of the Earth or was this just a spike of impacts?
Only the discovery of more such spherules of other ages will tell, said Kyte
Source: Discovery News - March 4 2003