Science: The end of the world as weknow it

Forget nuclear war, global warming and the threat of melting ice- caps - a large lump of rock could well do the final deed. With thousands of asteroids crossing the path of the Earth's orbit, a near-miss meteoric collision last year and a possible return of cyclical cosmic rains, things are looking far from safe. Norman Miller investigates
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For people living on a planet under bombardment, everyone is being very calm. The threat comes not from any tentacled aliens or nasty cyborgs but something more impassive yet potentially just as deadly - asteroids. At least 100 space rocks large enough to cause global devastation are now known to intersect the Earth's orbit, but astronomers believe that there may be around 2,000 big enough to cause massive damage. And an asteroid has never met a planet it wasn't attracted to, as a black astronomical joke goes. This negative outlook is balanced by the cosmic timescale of the attack, with massive asteroids working on a scale of tens of millions of years, but smaller cosmic rains of terror may operate every few thousand years. And chance means that the next Armageddon asteroid could as easily swing into view tomorrow as in a million years.

New evidence just published suggests that the most recent significant disruption of the Earth was only about 4,000 years ago, when the Bronze Age civilisations around the world were devastated by a series of meteorite impacts. A study of sediments from three regions of the Middle East by French scientist Marie-Agnes Courty turned up evidence of abrupt climactic changes at the time, along with tiny spheres of a calcite material unknown on Earth but found in meteorites, and signs of widespread fires which cannot be explained by volcanic activity. There is also historical evidence of violent cultural upheaval at the same period at over 40 sites, including the collapse of civilisations in Mesopotamia, India's Indus Valley and Egypt. The suggestion is that Earth was hit several times by debris, probably from a comet which fragmented.

Going further back, scientific evidence increasingly suggests that the dinosaurs' long, successful reign on Earth came to an abrupt end when a massive meteorite (an asteroid that hits the Earth) terminated the Cretaceous period 65 million years ago when it ploughed into what is now Mexico's Yucatan peninsula and exploded with a force estimated at 5 billion times that of the Hiro- shima bomb, leaving a crater (now underwater) around 100 miles wide. A planetary insurance claim for the resulting damage would have included compensation for a nine-mile-deep hole, enough tons of pulverised rock thrown into the atmosphere to put 15 noughts on the end of a one, worldwide blazes ignited by the fiery ash, and six months of total darkness as dust blocked out the Sun's rays. In the darkness and near freezing temperatures, plants and other micro-organisms would have been unable to create the energy they needed to survive, triggering a collapse of the food chain and the extinction of between 60 and 85 per cent of life on Earth.

The forensic evidence behind this scenario for Bad Day With Big Rock begins with a thin layer of clay from the time which contains levels of the element iridium 30 times higher than normal - concentrations common in meteorites but found on Earth only deep within the planet. Against scientists who argue that massive volcanic eruptions known to have occurred in India around the time could have brought up such high levels of iridium, the cosmic catastrophe case is bolstered by further clues. One is tiny glass spheres called tektites created when basalt from the Earth's crust was melted by a blast of heat from an impact big enough to fling them as far as Belize, Haiti and Florida. Then there is the presence radiating from the Yucatan area of shocked quartz, which is caused by sudden enormous pressures - previously found near underground nuclear test sites, the impact of a six-mile-wide asteroid would be far more likely to produce shocked quartz than volcanic eruption.

Bad though it was for the dinosaurs the end-Cretaceous extinction was not the worst such event in Earth's history. In fact, the planet has suffered several major mass extinctions in which a large number of unrelated animal and plant families disappeared from the fossil record at the same time. In addition to the end-Cretaceous extinction, other "big ones" took place at the end of the periods known as the Permian (245 million years ago), Devonian (360 million years ago), Ordovician (438 million years ago) and the Cambrian (510 million years ago).

It was the end-Permian mass extinction which saw the greatest culling of life on Earth, with an estimated 90 per cent of species becoming extinct. Unlike the end-Cretaceous event, the most likely cause appears to be a combination of something hot (volcanic eruption) and something cold (glaciation). Both these events could be linked since massive volcanic activity would produce large ash clouds that would ultimately cool the planet.

While extinction is extinction for its victims, palaeontologists distinguish between the processes involved. For example, extinctions can strike everywhere, hit either land or sea more, or affect animals more than plants (in fact, plants tend to be highly resistant to mass extinctions). An important distinction must also be drawn between what experts call catastrophic agents (such as meteorite impact) and Earth agents (volcanism, for instance). Proponents of chaos theory have suggested that normal planetary events or species interactions could unexpectedly magnify into a catastrophic cascade of events ripping through ecosystems - a process analogous to chaos theory 's image of the flap of a butterfly's wings triggering a chain of events culminating in a hurricane thousands of miles away.

Few experts dispute what kind of things can cause mass extinction even if they wrangle over which ones occurred when. There is, however, heated debate about claims of a clear-cut cycle to these events. The main proponents of the idea that mass extinctions occur at regular intervals are David Raup and John Sepkowski, palaeontologists at the University of Chicago, who used graphs based on marine extinctions to come up with an extinction cycle with a period of around 26 million years. If this were true it would suggest that the primary mechanism for these events is cosmic, since only astronomical agents are known to operate so precisely over such a time scale. Indeed, Berkeley physicist Richard Muller has even suggested that a distant companion star to the Sun could be an agent for periodic catastrophe. This so-called "Nemesis" or "death star", says Muller, could exist in an eccentric orbit that, every 26 million years, takes it through a huge cloud of comets discovered by Dutch astronomer Jan Oort in 1950 far beyond the orbit of Pluto. As Nemesis sweeps through the Oort Cloud its gravity sends a shower of comets into the inner solar system, greatly increasing the chance of the Earth being hit. However, an infra-red search of the sky in the early Nineties found no sign of Nemesis, though Muller is continuing to search.

Undaunted, other astronomical catastrophists have suggested supernovae as agents of extinction, since a nearby exploding star would cause massive damage to the Earth's ozone layer, allowing deadly UV rays to bombard the surface. Most recently, astrophysicist Stephen Thorsett from Princeton has come up with an even more exotic suggestion in the form of colliding neutron stars. A collision between two such strange, massive objects could occur within 200 light years of Earth every 100 million years or so, according to statistics, and this would bombard the Earth with several centuries worth of gamma radiation in a single day, blitzing life on the surface. Indeed, the Compton Gamma Ray observatory detects similar gamma bursts from distant parts of the galaxy on a regular basis, so Thorsett's theory can't be dismissed.

Though the end-Cretaceous was a spectacular score for the catastrophists, the gradualist school can claim more definite scores. The extinction which saw the Ordovician period give way to the Silurian about 440 million years ago was almost certainly caused by massive glaciation and a dramatic shrinking of the oceans - home to most life at the time - as glacial deposits found in the Sahara desert show. As large amounts of water became tied up in ice sheets, the sea-level lowered, severely reducing ecospace. This combination of overcompetition for space and global cooling is believed to have wiped out over 90 per cent of marine life.

Species which survived must have felt a grim sense of deja vu when, around 80 million years later, glacial deposits in north-ern Brazil suggest it all happened again with the end-Devonian extinction. This time the land-based newcomers of the Devonian - amphibians, insects and the first true land plants - came through relatively unscathed, but at least a third of all animal families are believed to have checked out. Glaciation is also believed to be the culprit for the oldest mass extinction of all at the boundary between the Cambrian and the Ordovician 510 million years ago. Against this, the cosmic catastrophists point to iridium anomalies similar to the end-Cretaceous in sediments from the Petinian-Triassic and Ordovician-Silurian boundaries which could level the scores.

If mass extinctions mark different volumes in the book of the Earth's history, smaller extinctions have been like chapters punctuating the story more frequently, with global cooling again the dominant agent. The glaciation 650 million years ago which decimated Precambrian flora and fauna may seem very distant but the most recent beat in Earth's pulse of extinctions took place just 11,000 years ago when 39 animal classes - including sabre-toothed cats, ground sloths and mammoths - were wiped out either by global cooling, over-hunting by humans or a combination of the two. Some ecologists also argue that human involvement in this first extinction of our present period, known as the Holocene epoch, is an ongoing extinction process now continuing through a blend of deforestation, poor agricultural practice, overhunting and pollution which has extinguished thousands of species, most of which have never been documented by science or ever will be.

What an irony, then, if humanity, now lording it over Earth's other species, were to become victims in the same way as the poor old dinosaurs. With over 2,000 Earth-crossing asteroids (ECAs) believed to be speeding in our vicinity, this cosmic version of Rus-sian roulette delivered a serious warning shot in the middle of last year when asteroid 1996JA1 flashed into the sights of startled astronomers a few days before missing the Earth by 280,000 miles - an astronomical hair's-breadth. A third of a mile across, moving at 58,000mph, an impact would have caused an explosion roughly equivalent to lighting all the world's nuclear bombs at once.

The devastating global effect of a major collision means experts such as David Hughes, a physicist at Sheffield University, put the risk of dying due to a killer asteroid at one in 20,000 - only 10 times less likely than dying in a car accident. Nasa has even called for urgent funding to create an early warning system to spot any ECA on a collision course. Work is also being done into possible defence systems, probably based on nuclear weapons.

Despite the evidence of history, however, funding authorities such as the US Congress continue to react to the threat of sudden mass extinction with what famed asteroid-discoverer Eugene Shoemaker has dryly referred to as "the giggle factor". But does anybody hear the dinosaurs laughing? !