The interesting hole in Yucatan

British scientists are hoping to answer the riddle of whythe dinosaurs died out by plumbing the depths of a huge Mexican crater. Lucy Hodges reports.

This month a group of British scientists is setting out on an extraordinary expedition to try to solve the riddle of how the dinosaurs became extinct. They are leading an international effort to measure a huge crater among the Mayan ruins of the Yucatan peninsula, Mexico, the biggest scar on the earth's surface, stretching from the land into the ocean and measuring well over 150 kilometers in diameter.

Studying the hole will help to shed light on the 65-million-year-old mystery of how so many species died out at the end of the Cretaceous period. The vast majority of scientists are agreed that the Chicxulub crater which encircles the town of Merida was caused by either a comet or a meteorite, but they disagree about its size. When the hole is measured they will know. They should also know whether one meteorite could cause enough damage to the air, the climate and the food chain so as to lead eventually to the end of the dinosaurs, or whether several such impacts were necessary.

Scientists who believe the sci-fi scenario, that a large extraterrestrial object hastened dinosaur extinction, think it happened thus. Large amounts of atmospheric dust were thrown up, blocking out sunlight for a number of weeks, and chucking rock - carbonate and sulphate particles - into the air which mixed with moisture to form acid rain. That rain destroyed vegetation over many years, changing the face of the earth and making life very difficult for many species.

"There is good evidence, and increasingly good evidence, that the timing of the impact occurred absolutely at the time that large numbers of dramatic extinctions occurred," according to Mike Warner, reader in geophysics at Imperial College, London, who together with his colleague Joanna Morgan, is the moving force behind the expedition. "That link in time is as close as anything ever is in geology. Large impacts like this happen only once every 100 million years or so and mass extinctions happen only once every 100 million years or so. For two of those very unlikely events to happen in the same period of 100,000 years, which is as close as we can get, is so absurdly coincidental that there must be a causal link between the two."

The Pounds 500,000 expedition to Mexico, which is funded partly by the oil industry and partly by the Natural Environmental Research Council as well as by US money from the National Science Foundation, will help to unravel the nature of that causal link. It should throw up clues as to what kind of object was responsible for the big hole, whether it accounted for all the extinctions that happened at the time and whether another impact might have been needed to cause the damage to life that occurred.

Two groups of scientists have long been at loggerheads over the size of the crater that first became known to western scientists about six years ago. One group, based in Canada, looks at minute variations in the gravity field across the crater: the other, from the Lunar and Planetary Institute in Houston, Texas, consists of experts on impacts around the solar system. The Canadian group thinks the crater measures 180 kilometres across, whereas the group from Texas believes it is bigger - more like 280 kilometres. The expedition is expected to clear up the argument.

What matters here is the amount of dust and other material that was injected into the atmosphere. According to Dave Snyder, a geophysicist at Cambridge University, who will also be on the expedition, the volume of material thrown into the air by an object making a smaller diameter crater would not have been enough to cause a global climate change, whereas a larger object making a bigger crater would.

If it were a comet - and most scientists seem to think it was - it is estimated to have been around 15 kilometres in diameter. If it were a stony meteorite, it would have been about ten kilometres across. Had the comet struck central London, it would have covered most of the centre, with the crater stretching from Oxford to Canterbury. That would have been equivalent to many Hiroshimas. "If the impact did occur on London tomorrow, let's say, there's no doubt that almost everyone in the British Isles would be dead," according to Snyder. "The question is would people in Siberia and South Africa be dead as well or would they die a few thousand years later or a few months later because of the acid rain?" A handful of scientists, notably Charles Officer of Dartmouth College in the US, take issue with the whole theory that the extinction of the dinosaurs is linked to a meteorite-like impact. They favour the idea that life was snuffed out by huge volcanic eruptions like those that built the Decan Traps in India and which threw up enough dust and gas to change the climate over half a million years. But this thesis has not convinced others because, for example, Officer was not able to explain the existence of iridium - a substance rarely found on earth, but common in meteorites or comets - in the Chicxalub crater.

Snyder is part of the Institutions Reflection Profiling Syndicate in Cambridge which will be hiring a special boat stuffed with seismic equipment of the kind used to prospect for oil in the North Sea. The boat will let off a series of compressed air explosions every 30 seconds to complete a profile of the crater. Seismometers laid on the ocean floor and seismic stations on land will compile a profile of underground formations from sound reflections.

"We will have a cross-section through a crater much as you would have if you could do a topographic profile across, say, the Copernicus crater on the Moon," says Snyder. "This is the first time we have been able to do this on earth because all the other major impact craters are much older and have been eroded away, so there is very little left. This one in Mexico has been fossilised and preserved beneath these sediments that fell on top of it." The disadvantage is that there is more than a kilometer of sediment on top of the crater now. That means it cannot be excavated, though drillholes have been sunk at several points onshore.

There were in fact three holes dug by the Mexican oil industry when no one had any idea there was a crater in the Yucatan. That was back in 1952. At the time the oil drillers assumed the area was an oil basin, which was a good guess, but it turned out to be wrong. In the 1970s, they had another go, and again gave up because of the strange rock they were running into.

Not until 1990 was it realised by the wider world that the area was a crater. Some Mexican scientists thought it was a crater in the 1970s and published a little in Spanish and a one-liner in English in 1980, but no one noticed. "In 1990 it was rediscovered almost," says Warner. "It was clear there was a big crater there and it was clear it may be the big crater everyone has been searching for ten years."

Warner is confident that the scientists will find a causal link between the crater and dinosaur extinction. "I think we're going to go find that this event undoubtedly led to extinctions. If it's the small size we would expect [ a meteorite impact] to have happened five or six times since we have good fossil evidence, and if it's the larger size we would expect it to have happened only once."

The question Warner poses is this. Were the dinosaurs desperately unlucky to have died out as a result of one huge comet smashing into the earth? Was it a one-off event - bad for dinosaurs because it killed them off but good for the later humans because it enabled them to breed and evolve in a new environment conducive to the survival of homo sapiens? Or is the hurtling of extraterrestrial objects into the earth's crust a process that occurs every so often in evolution and resets the evolutionary clock. If the crater turns out to be the small size, then we can be fairly sure, according to Dr Warner, that every 60, 70 or 80 million years the earth is going to be hit by a comet or asteroid. "Since we have good fossil evidence - since there's been modern life about really - this will have happened several times, five, six or seven times," says Warner. "If it's the big size, it's so rare that it will have probably only have happened once."

If the crater turns out to be the larger size, we can sleep easy in our beds at night. If not, we have another few million years to go before I