Geoscientist Online

Ted Nield* thinks it's ’s time to reassess our attitude to cosmic impacts. Meteorites are our friends.

Geoscientist Online October 2008

One hundred and sixty million years ago, before the dinosaurs have evolved, somewhere out in the asteroid belt between Mars and Jupiter a random collision sent a certain asteroid out of its harmless orbit in towards the sun. Ninety-three million years later, after countless harmless orbits, that single cannon in the great cosmic pool game sent the same rock hurtling into the Earth’s gravitational field – for us, the luckiest eight-ball of all time.

We know it happened, but just how probable was the K/T impact? Even though our Sun sits at the centre of a bird’s nest of intersecting orbits of similar asteroids with the potential to hit the Earth, in the vastness of space our planet is a tiny object. Even if the orbit of an errant asteroid and that of our planet eventually intersect – a process of orbital precession that can take tens of millions of years - for those two bodies to collide, both will also have to be in the right point on their orbits at the right time.

Things move exceedingly fast in space. Cosmic velocities are so huge (in the order of 30 to 40 kilometres per second) that in this case, our planet and the asteroid whose impact marked the end of the Cretaceous Period, “the right time” actually means “within seven and a half minutes”. If that bolide had been running any earlier or later on that Thursday afternoon, 65.5 million years ago, then Earth and asteroid would have grazed past one another harmlessly. Out of four and a half billion years of Earth history, only 450 seconds made the difference between a modern world populated by the descendants of dinosaurs, and one populated by us.

In 1980, when physicist Luis and geologist Walter Alvarez and their co-workers revealed that the end of the Cretaceous Period was marked all over the world by a thin layer rich in the rare element iridium, the conclusion that a gigantic impact had finished off the dinosaurs was much more than just a fascinating tale. If it happened before, it will happen again. Awareness of a fact imposed the need to do something.

Fortunately, incoming asteroids are a potential global threat for which we can at least imagine plausible solutions. If we can see them, we can deflect them, or blow them up, or something. So, the first job for astronomers was to find out what was up there. Just how probable was another strike of similar magnitude within the foreseeable future?

In 1991 an international working group sponsored by NASA tried to answer this question. It defined the “Spaceguard Survey”, whose main aim would be to identify Near Earth Objects (NEOs) with diameters of >1km. The idea was eventually put into effect in 1998, with the aim of finding 90% of such objects within the decade. That time is now up. How did they do?

By June this year they had found 742 – probably only about 79% of the total. Yet of the objects discovered, not one was found to have more than a negligible chance of hitting Earth in the next 50 years. Since the risk of death from smaller impacts was itself rather small (roughly equivalent to any other natural disaster), thanks to this discovery the global risk of dying from impacts of all kinds now stands at a negligible 1 in 720,000.

The average chance for all humans of dying in an earthquake is about 1 in 130,000. The global risk of dying in an airplane accident – including people who never set foot in an airport – is 1 in 30,000. However, the global odds of dying in an automobile accident are three orders of magnitude greater, at 1 in 90 – and remember that goes for every member of the remotest Amazonian tribe who will never even see Chevy Malibu. So if you are a commercial traveller working out of Detroit, your personal risk is – well, astronomical. Yet you and your insurance company readily accept it every day. Thanks to Spaceguard we know that our chances of death from the skies now stand on a par with death by firework (1 in 600,000). We can afford to relax.

In fact, I believe we should stop worrying, and learn to see the meteorite in a wholly new and more benevolent light. Recent work by geologist Birger Schmitz (University of Lund, Sweden) has raised the possibility that meteorite impacts might be good for living things. In an amazing series of discoveries, he has found that about 470 million years ago the chances of being hit by a meteorite went up considerably.

About then, in an event akin to that which set the dino-killing impactor on its deadly course, a massive collision involving an asteroid about 185km in diameter, created the Flora family of asteroids and a wealth of space debris known to meteoriticists as the L-chondrites - which still comprise as much as 20% of all recorded meteorite hits today. However, when they first started arriving, barely a million years after the collision, they came in much greater numbers.

Schmitz and his co-workers have described fossil meteorites from limestones in southern Sweden. These limestones were laid down very slowly - and thanks to having been deposited in one of the Earth’s most geologically stable areas, they are still almost as fresh and flat as they were 470 million years ago, in the Ordovician Period. They comprise the greatest concentration of fossil meteorites ever described.

Because the area of the quarries in which they are found, the age of the rocks and the rate of sedimentation are all known, Schmitz has been able to calculate that during this time the Earth was being bombarded by twice more meteorite material than today. Even more intriguingly, this period of intense bombardment coincides exactly with one of the most sudden periods of evolutionary diversification in the history of life – the so-called Great Ordovician Biodiversity Event (GOBE).

Palaeontologists have puzzled about the GOBE for years, but although many theories have tried to explain why so many species and genera of marine life originated at this time, none seems able to explain its sheer suddenness. Schmitz now thinks he may have the answer. Perhaps intense but globally sub-lethal impacts by kilometre-sized impactors repeatedly sterilised areas of the Earth, allowing opportunistic species to move in and fill the ecological niches made vacant.

Such an ecological effect is a short step from the sort of diversity increases we see today when, for example, a gravel pit or quarry disturbs the natural order, increases the number of habitats available for living things, and so raises local biodiversity. In nature, the same thing happens without our help – for example, in shallow coral reefs, where storm waves do the work. Increased biodiversity in space translates into greater speciation through time.

It really is time to stop worrying and learn to love the meteorite. They are little threat to us, so long as we keep our eyes open. Only one mass extinction event, at the K/T Boundary, has ever been unequivocally linked to an extraterrestrial impact - and dating work in the Deccan of India is placing the maximum volume of volcanic outpouring there at just before the K/T impact - suggesting that the dinosaur extinction would probably have happened without it.

Although they have taken out a few animals (and at least one Chevy Malibu) no humans have ever been killed by meteorites. The famous Nakhla stone - a very rare lump of Mars, bounced to Earth by an impact that struck the red planet - is reputed to have killed a dog when it landed in Egypt in 1911 (which would be the only instance of an earthling dying at the hands of a martian). But even that’s probably a myth.

Meteorites are the oldest objects it is possible to hold in your hand. They deserve respect, but not out of fear.

* Dr Ted Nield is Editor of Geoscientist magazine and Geoscientist Online . His book Supercontinent – 10 billion years in the life of our planet” is published by Granta (UK) and Harvard University Press (USA); also by Kunstmann (Germany) and Paidos (Spain). He is currently working on a book about asteroids and meteorites, and humans’ attempts to derive meaning from them, called "Incoming".