IT WAS nearly noon, and the tropical sun was beating down on a rugged, remote expanse of Peruvian plains. Villagers from the settlements of Carancas and Desaguadero were going about their business when they suddenly saw a blazing light, as bright as the sun, streak across the sky from the Andes in the north-east towards the coastal peaks. Seconds later, there was a loud boom and the ground shook.
The villagers rushed to the scene and were astonished to find a yawning pit, some 2 metres deep and 13 metres wide, in the reddish, rocky soil. Water had pooled inside it, bubbling and steaming, and a foul smell wafted out. Within days, dozens of these onlookers reported feeling sick, and word of the event quickly travelled around the world.
For astronomers, geologists and others who specialise in impacts of extraterrestrial bodies, this apparent crater-forming meteorite strike on 15 September 2007 was a once-in-a-lifetime event – but the story was perplexing from the start. Initial reports from the scene were contradictory, and some things didn’t add up: the boiling water and mystery illness, for instance, seemed better suited to The X-Files than an actual impact. For most researchers, the first reaction was scepticism.
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In the intervening months, that scepticism has turned to acceptance as the evidence in favour of a meteorite impact has mounted. This is where the mystery deepens, however. In December, Peter Schultz, a leading specialist in extraterrestrial impacts from Brown University, Rhode Island, visited the Peru site with other geologists. They concluded that the size and composition of the meteorite blew away established impact models. “The Carancas impact should not have happened,” says Schultz.
All of this is prompting a rethink about what kinds of objects pose a threat to the Earth’s surface. The question is increasingly important, coming two years after a mandate from the US Congress that NASA embark on a search for all near-Earth objects more than 140 metres in diameter – a big drop from the original limit of 1 kilometre. Researchers have already found hundreds of objects down to just a few metres in size, and in the years to come they will find tens of thousands more. Such objects had previously been thought incapable of causing major impacts.
Impact craters are exceedingly rare on Earth. Unlike the moon, Mars and Mercury, with their heavily pockmarked surfaces, our planet carries only about 190 confirmed craters, most of them large and ancient. That’s because plate tectonics and erosion have wiped away most crater traces, and the majority of Earth’s surface is covered by water. Then there is the dense atmosphere: while objects from space strike the Earth constantly, the vast majority burn up before reaching the ground – some 100 tonnes’ worth every day.
Throughout human history, in fact, there have been only a handful of events suspected of being major extraterrestrial impacts (see Map). These have tended to follow clear patterns. Broadly speaking, meteorites come in two types – chondritic (grainy and rocky) and metallic – and each leaves a signature trace upon impact. There had never before been a proven case of a crater formed by a chondritic meteorite, presumably because such objects are too fragile to reach the surface in chunks large enough to leave craters. All known craters were thought to be caused by rarer but more durable impactors made of nickel and iron.
So it came as a major surprise that the Peru impactor was identified as an ordinary chondritic meteorite. What’s more, based on the size of the crater, the impactor was estimated at between 0.5 metres and 2 metres across. According to conventional impact models, this should have been too small to survive passage through the atmosphere. “The fragile, stony meteorite should have fragmented during entry,” says Schultz. That would have left a region peppered with widely-strewn pieces, but nothing big enough to leave a hole in the ground, much less a 13-metre crater. “So now we have to reconsider entry models for fragile objects,” he says.
Besides Schultz’s team, geologists from Peru, Bolivia, Uruguay and Canada have examined the site directly, and several kilograms of the shattered meteorite have been collected (a few pieces are even on offer on eBay). So far, however, the impact has raised more questions than it has answered. Chief among these is how the impactor managed to make it through the atmosphere. Schultz thinks it is possible that a fragile body might reshape itself into a form that reduces atmospheric drag.
This puzzle could have major implications. “Suppose an object is discovered that is headed for an impact in 10 years. The canonical interpretation is that anything smaller than 50 metres across is harmless,” says Clark Chapman, a specialist in asteroid and comet impacts at the Southwest Research Institute in Boulder, Colorado. “If astronomers predict something 20 metres across is going to hit, do you just tell people to go in the house, or do you evacuate to 100 kilometres?”
“If a 20-metre object is going to hit, do you just go in the house, or evacuate to 100 kilometres?”
Such a 20-metre impactor is likely to arrive every century or so on average, Chapman says. “What’s the threshold of what’s safe?” The Peru crater shows that the answer is more uncertain than ever – but studying it should help to pin down what the crucial factors are. That’s why, despite the initial scepticism, impact researchers have been fascinated by the accounts from Peru. “It does suggest that fragments of this size may be relatively successful at making it through the atmosphere,” says Alan Harris of the Space Science Institute in La Canada, California.
In time, the impact in Peru may provide important insights. By accumulating local accounts, it may be possible to pin down the angle and direction of the object’s entry, and that might even tell us which region of space it originated from. The team who studied the site in December interviewed eyewitnesses. “We have an idea of the impact speed and trajectory,” Schultz says, but he won’t reveal their findings until next month’s in Texas.
So after decades of using computer simulations, lab tests and analyses of larger impact structures, planetary scientists can now hope to calibrate the relationship between craters and the impactors that created them. The Peru case should give us a better estimate of the sizes and types of object that can make it through the atmosphere to wreak havoc on the ground. “This will be a kind of yardstick,” says Benny Peiser, an anthropologist at Liverpool John Moores University in the UK who specialises in the consequences of natural disasters.
Now the hunt is on to see if there are other sites like it. “There may be other Carancas-scale craters out there,” Schultz says. “The trouble is that stony meteorites will be pulverised at impact and hard to recognise. We need to get clever.”
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Extraterrestrial impacts in human history
1. Sikhote-Alin
Siberia, 1947
There has been only one previous case of a well observed, crater-forming impact that nobody disputes. It happened in 1947 in the Sikhote-Alin region of eastern Siberia (see “The hits keep coming”). Hundreds of people saw the incoming fireball streaking across the sky, followed by a huge plume of debris that came spewing out after the impact. An artist at the scene depicted the event immediately afterwards in a detailed painting. Hundreds of meteorite fragments were recovered and more than 100 craters identified, the largest measuring some 28 metres across.FIG-mg26412101.jpg
The meteorite was an iron type, a rarer kind than the stony rock that recently fell in Peru, but much more likely to make it through the atmosphere in big enough pieces to cause craters. “These events are so rare,” says Clark Chapman of the Southwest Research Institute in Boulder, Colorado. Well-recorded sightings like Sikhote-Alin, he adds, are all the information we have about what really happens in extraterrestrial impacts.
2. Tunguska
Siberia, 1908
Even the best-known case in human history of a large object impacting from space remains shrouded in mystery. Tunguska’s effects were observed in most of the northern hemisphere: a loud boom was heard throughout most of Asia and Europe, and the sky lit up, even thousands of kilometres from the impact site. Detailed studies of the region, where trees were flattened in a pattern radiating outwards from the centre of the blast (pictured, right), uncovered no signs of a meteorite.
Eventually it became clear that the impactor had exploded high in the atmosphere, producing a blast 1000 times as powerful as the Hiroshima bomb. Estimates of the size of the object vary. It was originally thought to be about 60 metres across, but recent calculations based on the most detailed computer simulation yet suggest it may have measured just 30 metres across (żěè¶ĚĘÓƵ, 18 December 2007, p 10).
There was no crater – or so we thought for nearly a century. Then last year, a team led by geologist Luca Gasperini of the University of Bologna, Italy, reported that a lake in the vicinity might be the remnant of a sizeable impact crater (). Some 500 metres across and 50 metres deep, Lake Cheko is located about 8 kilometres from what is believed to be the epicentre of the blast. It bears an unusual funnel shape that suggests an impact crater, and radar measurements reveal the presence of an anomaly many metres below the lake bottom that may even be the impactor itself. The team plans to return this year to make further measurements and do some excavating.
That might resolve the question of whether the impactor was a comet or an asteroid. Asteroid material is well documented from meteorite falls. On the other hand, cometary material, which is richer in volatile chemicals and thought to be much more fragile, has only been studied in the form of microscopic fragments brought back by NASA’s Stardust mission. A sizeable chunk of comet, if that’s what the Tunguska impactor turns out to be, would be a treasure trove: a time capsule from the earliest days of the solar system.
3. Sirente
Italy, c. 300
A possible historical impact site is in central Italy, near the village of Sirente. It consists of a field of craters, including a water-filled hole 120-metres across. Carbon dating indicates that it formed in the fourth century AD. Local records from the time give detailed accounts of a bright object in the sky, followed by a loud impact that shook the earth so violently that people were thrown to the ground.
Jens Ormö of the National Institute for Aerospace Technology in Spain thinks this may have been the event that the Roman emperor Constantine saw in the sky in the year 312, and that triggered his conversion to Christianity. Since identifying the main crater in the late 1990s, Ormö has mapped more than 20 surrounding craters and found magnetic anomalies and other indications of an impact, though he has yet to discover fragments of the impactor itself. Whether the Sirente craters were formed by meteorite impacts at all remains in dispute, but Ormö’s team plans to continue excavating in the hope of recovering the culprit.
Though the recent Peruvian crater is much smaller than the main one at Sirente, it is similar to some of the smaller craters around it. This could make the Peru crater a useful reference in the event that the Sirente crater field can be shown to have resulted from an impact, says Ormö. What’s more, the Peru crater has an unusual feature that might undermine an argument put forward by some of Ormö’s critics. They say Sirente’s craters cannot be the result of an impact as they do not have a significantly raised rim – but then neither does the Peru crater. This similarity might be because the surface of Sirente, like the Peruvian plain, is composed of loose soil rather than rock.
4. Campo del Cielo
Argentina, c. 2000 BC
A field of impact craters in northern Argentina, the largest of which is about 50 metres across, was formed at a time when there were almost certainly people around to witness it. The 22 craters go by the name Campo del Cielo (“Field of the Heavens”), and nearly 100 tonnes of iron-nickel meteorites have been recovered from the site. Although the dating is uncertain, it is thought to be about 4000 years old. “There were oral legends about cosmic events,” says Peter Schultz of Brown University, Rhode Island, who has carried out research at the site.
Early Spanish colonists in the area were led to the craters by locals who claimed that a great mass of iron had once fallen from the sky. Although the site has been mined for its metals since antiquity and studied by scientists for decades, it is still yielding new insights. Since 2004, researchers have found two craters not previously identified and excavated others to reveal new aspects of their original shapes. They have also recovered a huge 15-tonne meteorite buried under the surface. Research at the site is expected to yield important new clues about exactly how low-angle impacts can scar the Earth’s surface.
5. North America
c. 11,000 BC
The further back in time you go, the harder it becomes to pinpoint what happened in a given impact event – or whether it even was an impact. Last May, a highly controversial report presented at an American Geophysical Union meeting in Mexico (żěè¶ĚĘÓƵ, 26 May 2007, p 8) made headlines around the world, and it remains the subject of intense debate.
The authors claim that a massive comet struck somewhere over North America about 13,000 years ago, perhaps shattering high in the atmosphere. It showered much of the continent with debris, started wildfires that consumed much of the vegetation, and led to the extinction of most of the continent’s large animals at around the time that humans were first settling into the area. The evidence came mostly from a thin layer of charcoal found in sediments from that period, which incorporated tiny glassy spheres that may have been molten debris from the impact ().
Many researchers don’t buy the idea. “How do you get something to have all these effects and not leave a crater?” says Chapman. “Until I’m hit over the head with strong evidence, I’m going to doubt that it all hangs together.” He is not alone. “It seems incredibly implausible that an impact of the scale they propose would have happened that recently,” says Alan Harris of the Space Science Institute in La Canada, California.
Schultz, a co-author of the report, does not see the absence of a crater as a problem. He points out that when Luis Alvarez and colleagues first presented evidence that an impact was responsible for killing off the dinosaurs 65 million years ago – at the border of the Cretaceous and Tertiary periods – their idea gained credibility despite there being no known crater at the time to support it. “This is a non-argument,” he says.
