Tuesday, July 1, 2008

Asteroid anniversary recalls Earth's rocky history

Summertime — a time for sunny days, beach weekends and of course, leisurely reflections on the end of the world and the monster asteroids that could smack into us. The centennial anniversary of the last big impact, the 1908 Tunguska blast that rocked Siberia, falls Monday, June 30, bringing with it a reminder of the very slight chance that a hunk of space rock out there might have Earth's number.

The Tunguska "event" leveled nearly 800 square miles of swampy woodland in Siberia, traveling from the northwest to deliver a 5-megaton blast seen by hundreds of witnesses, including one who created a postage stamp of the explosion. A space rock about 50 yards long had zoomed into the Earth's atmosphere and exploded in mid-air.

PHOTO GALLERY: This week in science

"People were knocked off their feet hundreds of miles away," writes astronomer Phil Plait in his upcoming book Death from the Skies! These are the Ways the World Will End. Years later, a scientific expedition to the remote region found trees knocked sideways in straight lines radiating 15 miles away from the blast.

Science journals this week brought us more warnings of asteroid hazards, looking even further back in time. Buried under the Chesapeake Bay and its surroundings hides a 35.4 million year-old impact crater about 56 miles across. A team led by Gregory Gohn of the U.S. Geologic Survey reports in the current Science journal that not only was the blast one that dwarfs the Tunguska event, but the crater might still hold some bad news.

Gohn and his colleagues took 3-mile-deep cores from the center of the crater, which is below a farm on Virginia's Eastern Shore. They found, 3,600 feet down in the cores, evidence that the asteroid impact blew a 900-foot thick "megablock" of granite three miles from the crater's center in the first 10 minutes of the explosion. Another 1,800 feet of impact-shifted rock sits atop the granite, with the whole mass hidden away down in the muddy depths of the placid bay.

Inside the granite layer, the team found few microbes — a left-over effect of the blast sterilizing the region, the authors suggest — but lots of trapped seawater. The trapped seawater is a hazard for anyone unlucky enough to drill that deep for drinking water.

In the current Nature, three teams of authors detail a much bigger blast even further back, about 4.4 billion years ago, when the planets had just formed. This one happened to Mars, when an asteroid the size of Pluto scalped the Red Planet's northern hemisphere, delivering a 7,700 billion megaton strike that deformed the planet, leaving the northern lowlands a smooth plain and the southern highlands a mottled badland. Any life on the early planet would have been sterilized instantly, says Jeffrey Andrews-Hanna at the Massachusetts Institute of Technology, who led one of the studies detailing the 6,600 mile-wide impact basin, the biggest one left in the solar system.

The only bigger known strike happened to Earth at about the same time, a 2,390 trillion megaton blast triggered by the collision of a Mars-sized object with Earth's northern hemisphere. The resultant blast left no crater, but is thought to have created the moon from the impact debris.

Asteroids and comets are still out there, of course. SOHO, the international solar astronomy satellite, reported its 1,500th comet discovery on Friday.

And Apophis, a nearly 900-foot-long asteroid, will pass no closer than 18,300 miles of Earth on Friday, April 13, 2029, appearing as a bright spot above the Atlantic Ocean, according to NASA's Jet Propulsion Laboratory. All told, astronomers have spotted more than 5,000 such "Near-Earth Objects" since the 1990s.

In terms of risk to Earth, astronomer David Morrison of NASA's Ames Research Center says a Tunguska-magnitude strike could happen once every two centuries and a bigger impact, a "civilization-threatening" million-megaton strike, could happen once every 2 million years. Even though astronomers have spotted more of these nearby asteroids in the last two decades, the estimated odds of an impact have actually declined, as Morrison notes in a May issue of NEO News, his asteroid newsletter.

Scientists only started to worry about these impacts in the 1960s, when researchers such as Gene Shoemaker realized the moon was covered with impact craters. And in 1980, Science published a study detailing how an asteroid strike, centered in the Gulf of Mexico near the Yucatan peninsula, was implicated in the extinction of dinosaurs about 65 million years ago, raising more concern.

The NASA authorization bill passed last week by the U.S. Congress notes the dinosaur-exterminating blast in calling for the space agency to keep tracking nearby asteroids.

Also in the current Nature, astronomer Alan Harris of the Space Science Institute looks at the history of "Spaceguard," the effort to track nearby asteroids that started in the 1990s. Spaceguard found there is "little risk of a cataclysmic impact in the next century," writes Harris, who adds: "The sky isn't falling, but there are still good reasons for keeping an eye on it."

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