In a jungle's depths, a quest for answers

Crater: Maryland researchers travel to the Bolivian wilderness in search of evidence of the impact of an asteroid or comet.

September 14, 2002|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

In an expedition worthy of Indiana Jones, a team of NASA scientists left Maryland this week to search a South American jungle for traces of a fallen "star."

Investigators from NASA's Goddard Space Flight Center in Greenbelt are heading this weekend into a steaming Bolivian jungle, patrolled by jaguars, snakes and piranha, to join Bolivian colleagues at a place called Iturralde Crater.

Barely discernible even from the air, this five-mile-wide circle in the forest may be the bull's-eye where a hurtling asteroid or comet struck with the force of a thousand hydrogen bombs.

Geologists have found more than 145 impact craters around the world, said geophysicist Jim Garvin, NASA's lead scientist for Mars exploration.

But "if this can be proved to be an impact, then it is probably one of the most recent of the bigger impacts in the Earth's history, probably as young as the last 10,000 years," he said. "I dearly hope they can find part of the smoking gun."

Like deer listening for gunshots in the woods, scientists are eager to measure the size and frequency of the Earth's most recent meteorite impacts to get a better notion of how imminent the next such climate-altering impacts might be.

Researchers studying satellite photos in the 1980s noticed a faint but precise circle in the forest canopy at Iturralde, where the type and color of vegetation changed. (A radar image taken from the shuttle Endeavour in 2000, but processed just days ago, clearly shows a circular dent in the forest floor.)

With no volcanoes, sinkholes or river bends to explain the forest circle, the meteorite crater theory emerged. "I can't think of anything else this thing is," Garvin said. "But I haven't been there."

Canadian experts tried in vain to reach the crater more than a decade ago. "They claim they were turned back by too many snakes," said Compton "Jim" Tucker, a NASA biologist with this year's expedition. "But I suspect [the Canadians] were trying to frighten people away. They were hoping to go back."

In 1998, Tucker and team leader Peter Wasilewski, also from Goddard, and Tim Killeen, of Conservation International in Bolivia, launched another expedition, with about a dozen people.

They flew first to the Amazon basin town of Riberalta, to bargain with the native people, the Araona, for permission to pass through their land. Araonas and their attorney demanded $1 million - the price oil companies paid to explore the land.

"We only had $20,000 for the whole expedition," Tucker said. After six days of talks, the team won permission to proceed, at a cost of $5,000, plus 200 D-cell batteries and 500 rounds of .22-caliber ammunition.

The expedition hired Araona bearers for $5 each a day - twice the going rate, Tucker said. From Riberalta, they flew 130 miles to the village of Puerto Araona. There, they boarded dugout canoes and motored down river to Palmasola, a Brazil-nut camp.

Plunging into the forest from there, the team hacked an 11-mile trail with machetes, guided toward Iturralde crater by signals from Global Positioning System satellites. They moved less than a mile a day through the jungle.

The region gets 6 to 12 feet of rain annually. (Baltimore gets 42 inches in a normal year.)

Wildlife is abundant in the absence of human hunters, whose numbers were decimated by disease and mistreatment inflicted by outsiders a century ago during the rubber boom.

The forest today shelters jaguars, monkeys, tapirs, giant anteaters and armadillos. There are hundreds of species of birds, amphibians and snakes. And insects? "You can't begin to count them," Tucker said. Four people on the 1998 team got malaria from mosquito bites. But most troublesome are the sweat bees.

Most tropical forests are salt-poor, Tucker explained. Human sweat draws insects seeking the salt on skin and clothing.

"The first day there are two or three," he said. "The next day there are 200 or 300, and the next there are 2,000 or 3,000. They get caught in your clothes, and so you get stung occasionally." The only relief lies in frequent bathing and rinsing in rivers shared by piranha.

Hiking across the five-mile-wide crater in 1998, Tucker and his team found an outer rim only 6 feet to 7 feet higher than the surrounding forest, and an uplifted central area. In between was a low, boggy ring filled with water-adapted vegetation, explaining the color change seen from space.

It was all consistent with a large impact into deep, soft sediments. Such meteorites don't crash, Tucker said; they "splat." Their craters fill with water and sediment, the outer rims erode, and they soon disappear.

Based on studies of craters on moons and planets where erosion is slight, Garvin said, "you might expect, in the last few million years, five to 10 objects 100 meters in diameter or bigger" to have struck the Earth. (These are big enough to alter the climate, but too small to trigger global extinction.)

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