An asteroid impact thought to have occurred 3 A cosmic impact thought to have occurred about 3.5 billion years ago was so incredible it covered the entire Earth with ejected material and generated ocean waves that soared more than half a mile high (1 kilometer) when they reached the shore.
That's the account given by Gary Byerly and his colleagues, who say they've uncovered evidence on two continents for the same event, a catastrophe that would be billed as the oldest known meteor impact on Earth.
Byerly, a geologist at Louisiana State University, told SPACE.com that the space rock in question -- it could have been either an asteroid or a comet -- must have been between 12 and 31 miles wide (20-50 kilometers).
It generated "about a billion times more energy than an atomic bomb," Byerly said. The impacting object would have smashed right through 2 miles (about 3 kilometers) of ocean water and hit the rock beneath, generating a cataclysm far more destructive than the impact that led to the demise of dinosaurs in more recent times.
The results of the new study will be published in the Aug. 23 issue of the journal Science.
Tough sleuthing
Earth is not a good record keeper. It tends to fold evidence from ancient events right into its belly, only to spit it out again, in an unrecognizable form, through volcanoes later. There are only two places on Earth suspected of harboring preserved evidence from so long ago. One is in Africa, the other in Australia.
The new study is the first to tie the possible impact to both sites.
Byerly said it is not known, and may never be known, where on Earth the two sites were located at the time.
There were probably no large continental blocks like there are today, though there may have been micro-continents poking through a deep, global ocean. Earth's landmasses were later joined in one super-landmass called Pangaea, and still later broke into the present day continents.
The researchers found zircons, minerals that contain small quantities of uranium and small quantities of lead, which results from radioactive decay and tells geologists the minerals are 3.47 billion years old, give or take 2 million years.
Most geologists agree that the spherical particles studied are "high temperature melt droplets that condensed from a rock vapor," Byerly said. The vapor, he thinks, was the result of an ocean impact that would have carved a crater many hundreds of miles (or kilometers) across. As debris fell back to the ocean, it was "moved around by the impact-generated currents and waves that ripped large pieces of the sea-floor up and created a breccia," or angular fragments of rock also examined in the study.
"In our study, we're looking at the oldest well-preserved sedimentary and volcanic rocks on Earth," said Stanford geologist Donald Lowe, a co-author of the study.
"They are still quite pristine and give us the oldest window that we have on the formative period in Earth's history," Lowe said. "There are older rocks elsewhere, but they've been cooked, heated, twisted and folded, so they don't tell us very much about what the surface of the early Earth was really like."
Why it matters
Little is known about the early evolution of Earth and life on the planet, but space rock impacts are suspected of playing a vital role.
Before the event examined in the new study, a larger object slammed into the planet and
created the Moon. Thereafter, the very ingredients for life, including water, may have arrived in comets and asteroids. In addition, impacts might have seriously challenged early life forms, possibly even resetting the clock of life and forcing it to emerge more than once.Scientists are nearly certain impacts occurred at a frenzied pace between 4.0 and 3.8 billion years ago, a period called the
Late Heavy Bombardment that might have preceded the development of life. The Moon maintains a stark record of strikes from that era, craters that are essentially frozen in time because the Moon does not swallow itself geologically the way Earth does (nor is there much surface erosion on the Moon). What happened to the Moon probably happened to Earth, experts agree.Impacts did not cease at 3.8 billion years ago. Yet as of now there are no known sediment layers that would provide evidence for anything older than what Byerly's team has found.
Byerly and his colleagues have been publishing studies on the ancient sediment layers and possible impacts since the late 1980s. The evidence -- particularly for the timing -- has not convinced all geologists, however.
"I think the evidence is now regarded as 'compelling,'" by his peers, Byerly says. If the evidence is eventually proved out, biologists could become more confident about the role asteroids and comets might have played in the evolution of life.
Life and sex
The oldest solid evidence for life on Earth coincides with the timing of the impact in the new study. However, that evidence is disputed by some biologists. Others believe life might have begun earlier, perhaps more than 4 billion years ago.
Meanwhile, Lowe, the Stanford researcher, said he is not sure what effect an impact would have had on living conditions of the time.
While such an impact today would severely alter the climate, not enough is known of the ancient climate to predict effects. A separate study that Lowe will report soon, for example, indicates the average temperature of the planet back then was very hot, perhaps 185 degrees Fahrenheit (85 Celsius).
Regardless, biologists are pretty certain that only bacteria would have been around to contend with an impact 3.47 billion years ago. Some researchers believe, in fact, that impacts might have provided a necessary stress factor that encouraged
the first sex by causing genetic mutations in otherwise asexual critters that led to the desire to get help in making offspring.More Asteroid & Comet News
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