Planet Erde - Earth at higher risk of big asteroid strike, satellite data suggest


“It would be in the range of serious crap happening.”


If Zhamanshin crater in Kazakhstan is 30 kilometers wide (red ring) instead of 13 kilometers (black ring), as a new study suggests, the impact that made it would have been far more fierce.J. GARVIN, C. TUCKER, C. ANDERSON, D. SLAYBACK, AND D. MCCLAIN/NASA GODDARD SPACE FLIGHT CENTER; MAXAR WORLDVIEW; EARTHDEM; NASA PLANETARY DEFENSE COORDINATION OFFICE

THE WOODLANDS, TEXAS—At a basic level, humanity’s survival odds come down to one thing: the chances of a giant space rock slamming into the planet and sending us the way of the dinosaurs. One way to calibrate that hazard is to look at the size of Earth’s recent large impact craters. And a provocative new study suggests they are bigger than previously thought—meaning Earth is more at risk of getting hit hard, says James Garvin, chief scientist of NASA’s Goddard Space Flight Center, who presented the work last week at the Lunar and Planetary Science Conference. “It would be in the range of serious crap happening.”

Using a new catalog of high-resolution satellite imagery, Garvin and his colleagues identified large rings around three impact craters and one probable one that are 1 million years old or younger. To Garvin, the rings imply the craters are tens of kilometers wider, and record far more violent events, than researchers had thought.

If Garvin is right—no sure bet—each impact resulted in an explosion some 10 times more violent than the largest nuclear bomb in history, enough to blow part of the planet’s atmosphere into space. Although not as destructive as the impact that killed off the dinosaurs, the strikes would have perturbed the global climate and caused local extinctions.

It’s an extraordinary claim, as Garvin himself admits. “We haven’t proven anything,” he says. Without fieldwork to back up the conclusions, impact researchers are wary of the circles Garvin and his colleagues have drawn on maps—especially because they defy other estimates of impact rates. “I’m skeptical,” says Bill Bottke, a planetary dynamicist at the Southwest Research Institute in Boulder, Colorado. “I want to see a lot more before I believe it.”

Because water and wind quickly erase most impact craters on Earth, researchers estimate impact rates by tallying crater sizes and ages on the Moon. They also study the size of asteroids in orbit near Earth—potential future impactors. Based on those two methods, researchers estimate that an asteroid or comet 1 kilometer wide or larger hits the planet every 600,000 to 700,000 years.

The new study, however, suggests that in the past million years alone, four kilometer-size objects pummeled the continents—and, given that two-thirds of the planet is covered by water, that could mean up to a dozen struck Earth in total, Bottke says. Anna Łosiak, a crater researcher at the Polish Academy of Sciences, doubts the ringlike features identified by Garvin’s team are truly crater rims. If they somehow are, she says, “that would be very scary because it would mean we really don’t understand what’s going on at all—and that there are a lot of space rocks that may come and make a mess.”

The work stems from a database of high-resolution satellite imagery from the company Planet. Garvin and his collaborators used thousands of stereo overlapping images to create 3D maps of the four craters. Adding data from two height-measuring lasers that NASA operates in orbit, including one capable of penetrating tree cover, gave them maps with 4-meter resolution.

They removed features from the maps that were obviously unrelated to the impact. Then they applied an algorithm Garvin had first developed for Mars that searches for circular patterns in the topography. For simple, small craters, it invariably identified the obvious crater rim. But in thousands of runs on the four larger craters, the algorithm frequently identified a rimlike structure much farther out than the accepted rim. For example, Pantasma, an 800,000-year-old crater in Nicaragua, grew from 14.8 kilometers to 35.2 kilometers in diameter.

Experienced crater scientists don’t see the new rims. “Those features are so subtle that I don’t think they say ‘big structural rim,’” says Gordon Osinski, a planetary scientist at Western University. They could instead be rings of debris ejected by the impacts, adds Brandon Johnson, a planetary scientist at Purdue University.

Garvin, however, doesn’t think a mere ridge of debris would still be visible after 1 million years of erosion. He thinks the rings imply large craters on Earth have more variable structures than elsewhere in the Solar System because of high erosion rates. “On Earth, things get messy, particularly when you throw a lot of energy at it,” he says.

For the results to gain credence, Johnson says the team will need to gather more evidence. First, the climate upheaval triggered by impacts as big as Garvin claims should have left its mark in ice cores or ocean or lake sediments. Second, researchers need to visit the sites of the rings to look for the deformed rocks and gravitational variations that would indicate a true crater rim.

Given the stakes, this is one hypothesis that can’t afford to go untested, Johnson says. “We’ve got to go there, check out the geology, and get more detail.”

Quelle: AAAS

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