Australia bears the scars of more than 30 major asteroid impacts, of the 176 worldwide.
A sample of Australia's 30-odd confirmed meteorite craters. (Illustration: Michael Payne)
EXPLORING HIS VAST CATTLE station in 1899, Walter Parke came upon a feature in the landscape he could not explain. A 15m-deep, bowl-shaped depression, larger than a football field, had been gouged out of the Central Australian desert. "One of the most curious spots I have ever seen in the country," he wrote in a letter to the anthropologist Frank Gillen. "An immense amphitheatre...To look at it I cannot but think it has been done by human agency, but when or why, goodness knows."
Further investigation revealed 12 craters pitting Parke's property at Henbury station, 115km south-west of Alice Springs. But their origin remained a mystery to Europeans until 1931, when local prospector, J. M. Mitchell, reported finding slugs of iron strewn across the site, "as though they had dropped from a molten mass falling at great speed".
Massive asteroid impact chances "extremely slim"
As one of the oldest and least geologically disturbed continents, Australia has a rich record of meteorite craters. Of 176 confirmed impacts worldwide, our country bears the pockmarks of 30 - and about 20 others await confirmation. Indeed, as you read this, another 1275 potentially hazardous asteroids (meteorite is the name of an asteroid once it has fallen to Earth) are orbiting in space - and that's just the ones we know about.
"An impact in Australia in the future is certain," says Duane Hamacher, an astronomer from Macquarie University in Sydney. "The recent flyby of '2005 YU55', a 400m asteroid - which passed closer to the Earth than the Moon - posed no threat to us, but had it impacted, it would have created a crater more than 6km wide and could have completely eradicated a large city like Sydney or Melbourne."
The 2005 YU55 was the largest object on record to pass this close to the Earth with our foreknowledge, but smaller meteors arrive surprisingly frequently.
"Estimates suggest that an impact or airburst (a meteor that explodes in the atmosphere) with the energy of one million tonnes of TNT will happen over Australian soil about once every few hundred to few thousand years," Duane says. "But the chances of us being affected by an impact event are extremely slim."
Meteorites recorded in Aboriginal Dreamtime
Compared with cataclysmic impacts throughout the Earth's history, the Henbury craters - formed 4200 years ago when a single meteor broke into pieces and spattered the desert - are only tiny dimples. The Arrernte people of Central Australia appear not only to have understood the craters' cosmic origins, but are likely to have witnessed the impact firsthand, Duane says. Eyewitness accounts seem to survive in oral traditions today. "Some of the Dreaming stories talk about stars falling out of the sky and hitting the ground," he says.
An Aboriginal man who accompanied Mitchell to Henbury in the '30s refused to approach within a few hundred metres of any crater. He told Mitchell that tjinka waru (fire spirit) lived in the yabo (rock hole), and that to drink water from the yabo would cause iron to rain down once more.
"So it seems to indicate either that the memory of this event has survived in oral traditions for thousands of years," Duane says, "or that people in the area later deduced something had come down from the sky."
Even as late as the early 1900s, most western scientists still attributed circular craters not to meteorites falling from the sky, but to ancient volcanoes. In this light, he says, the knowledge of the Arrernte people seems profound.
But Dreamtime stories don't explain all of Australia's craters this way. Take the story of Wolfe Creek Meteorite Crater, 880m in diameter and one of the world's best-preserved impact sites, formed in WA's Kimberley some 300,000 years ago, long before the local Djaru people could have seen it happen.
"Some of the Wolfe Creek stories talk about Rainbow Serpents coming up from under the ground and forming the crater (known to the Djaru as Kandimalal)," Duane says. Other stories claim that meteors seeded life on Earth, similar to the 'exogenesis' theory supported by some modern scientists. "Some cultures in the Central Desert associate objects falling out of the sky with the formation of life," he says.
Australian meteorite craters are ancient
In contrast to the relatively young craters at Henbury and Wolfe Creek, some of Australia's craters may date back as far as two billion years. "When you get that far back you start to lose the record," says Dr Alex Bevan, curator of mineralogy and meteoritics at the Western Australian Museum. "Erosion over time can actually remove the record completely. So the remnants of things that are two billion years old are pretty cryptic."
By studying the night sky and analysing well-preserved craters, scientists have built a strong understanding of meteorites, which are the fragments left over after an impact. They have discovered that most meteorites come from the asteroid belt between Mars and Jupiter. In space, rocks up to a few metres in size are termed meteoroids. Larger bodies, like the one that carved a 60km crater at Woodleigh, near Shark Bay, are called asteroids.
Once space rocks enter Earth's atmosphere, they are called meteors. Most meteors burn up and disintegrate harmlessly as 'shooting stars', but others reach our planet's surface - sometimes with catastrophic consequences.
"Strong evidence suggests the impact of an asteroid contributed to the extinction of the dinosaurs," Alex says. "Though it may not have been the only cause."
Evidence of such an impact lies buried in the Yucatán Peninsula, Mexico, in the form of the 65-million-year-old Chicxulub crater. The forces involved in impacts like these are almost unfathomable. The Chicxulub crater is about 300km in diameter. To create it, an asteroid 10km wide would have crashed into Earth with a force of two million hydrogen bombs. It would have lifted 10,000cu.km of dust into the atmosphere and sent giant tsunamis racing around the globe.
Based on the frequency of past events, experts estimate a 1-in-30,000 chance of an impact on this scale happening in the next century. If we see the asteroid approaching early enough, we might somehow be able to nudge it on a different path. "But a situation like the one in the movie Armageddon is simply not feasible in any way," Duane says. "If an asteroid that big were heading straight for Earth, and we only had days or weeks before it hit, all we could do is enjoy what time we have left!"
Rocky and metallic bodies orbiting the Sun, ranging in diameter from several metres up to hundreds of kilometres.
The smallest members of the Solar System - chunks of debris from an asteroid or comet, ranging from a grain of sand to a few metres in diameter.
Space rocks that have entered the Earth's atmosphere, creating a fiery streak of light (a shooting star).
Fragments of space rock left on Earth's surface.
Similar to asteroids, but characterised by massive tails of water vapour, gas and dust that are illuminated by the Sun.
Source: Australian Geographic (Issue 107, March - April, 2007)
Fonte: Australian Geographic