News Column

Giant Asteroids Struck Earth More Often Than Thought

April 27, 2012

Mike Taugher

The meteor that exploded over California last weekend rained rubble down on the site where gold was discovered in 1848 and where scores of modern-day treasure seekers and clue-seeking researchers are expected to descend this weekend in search of fragments.

The meteor -- just the third known to strike California, according to NASA -- landed the same week researchers published in the journal Nature new studies showing that billions of years ago massive asteroids pummeled the young Earth far more frequently than previously believed.

Those primordial space rocks were as big as the one that formed the features of the man on the moon and the 6-mile-wide one that struck near the Yucatan Peninsula 65 million years ago, killing off the dinosaurs, many mammal species and other life.

Back in those earliest days, however, life on Earth was confined to oceans where oxygen and protection from the sun's rays could be found, unlike on the surface of an atmosphereless planet. Marine life would not have been undisturbed by the collisions, said David Morrison, senior scientist at NASA's Lunar Science Institute in Mountain View.

Using new techniques based not on finding the craters left behind by asteroids but the droplets of molten rock that ascended into space on impact and fell back as tiny sandlike particles, the researchers concluded Earth took a far more savage beating than was thought.

An estimated 70 or so asteroids as big as the one

that killed off dinosaurs struck the planet between 2 billion and 4 billion years ago. (Earth is about 4.5 billion years old, according to scientists.)

Modern astronomers, keen to be on the lookout for a similar cataclysm, have cataloged large asteroids in the sky and say there is no chance one of them will strike the planet in the foreseeable future, Morrison said.

But smaller ones like the one the blew up over the Sierra foothills on Sunday hit the Earth once every few weeks, he said.

It's just that many of them fall in the oceans or other unpopulated areas.

Not so on Sunday, when a meteor the size of a large refrigerator or small car streaked across Northern California and disintegrated in a massive overhead explosion.

"It blew up right over my head Sunday morning," said Ed Allen, an oral historian who portrays James Marshall during re-enactments at Marshall Gold Discovery State Historic Park, where Marshall found gold in the South Fork of the American River at John Sutter's Mill.

Allen said the thunderous explosion lasted 20 to 25 seconds.

Allen said state parks officials will try to coordinate the meteorite rush they expect this weekend by having NASA scientists and high school science club members coordinate people to try to look for pieces together.

Anyone who finds a piece is asked to handle it with tinfoil to prevent contamination and give it to a scientist and be able to show where it was found.

"This has been sitting in the deep freeze of the universe for 4.5 billion years, and it just came to Earth on Sunday," Morrison said. "I think it's pretty cool."



Source: (c)2012 the Contra Costa Times (Walnut Creek, Calif.)


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