Knock 'em dead: how does one extinguish life on Earth? Let me count the ways

Natural History, May, 2005 by Neil deGrasse Tyson

Ever since dinosaur bones were discovered, scientists have proffered no end of explanations for the disappearance of the hapless critters. Maybe a torrid climate dried up the available sources of water, some say. Maybe volcanoes covered the land in lava, poisoned the air, and brought on an ice age. Maybe too many early mammals dined on too many dino eggs, or the meat-eating dinosaurs are up all the vegetarian ones. Maybe the need to find water led to massive migrations that spread diseases. Maybe the real problem was a reconfiguration of landmasses, caused by tectonic shifts. All these crises have one thing in common: the scientists who came up with them were well trained in the art of looking down.

Other scientists, however, trained in the art of looking up, began to make connections between Earth's surface features and the visits of vagabonds from outer space. Maybe meteor impacts generated some of those features, such as a bowl-shaped depression nearly a mile wide in the Arizona desert. In the 1950s at the big bowl, the American geologist Eugene M. Shoemaker and his associates discovered a kind of rock that forms only under extremely high pressure--the kind of pressure only a meteor can deliver. Geologists could finally agree that an impact caused the bowl (now called Meteor Crater). Shoemaker's discovery also resurrected the nineteenth-century concept of cat-astrophism--the idea that changes to our planet's skin can be caused by brief, powerful, destructive events.

Once the gates of speculation opened, people began to wonder whether the dinosaurs might have disappeared at the hands of a similar, but bigger, assault. Meet iridium: a metal rare on Earth but common in metallic meteorites, and conspicuous in a 65-million-year-old layer of clay that occurs at scores of sites around the world. That clay, dating to about the same time as the dinos checked out, marks the crime scene: the end of the Cretaceous. Now meet Chicxulub Crater, a 120-mile-wide depression at the edge of Mexico's Yucatan Peninsula. It, too, is about 65 million years old.

Case closed? Perhaps not.

Scientific inquiry shouldn't stop just because a reasonable explanation has apparently been found. Some paleontologists and geologists remain skeptical about assigning Chicxulub the lion's share--or even a substantial share--of responsibility for the dinos' departure. Some think Chicxulub may have significantly predated the extinction. Furthermore, Earth was volcanically busy at about that time. Plus, other waves of extinction have swept across Earth without leaving craters and rare cosmic metals as calling cards. And not all bad things that arrive from space leave a crater. Some explode in mid-air and never make it to Earth's surface.

So, besides impacts, what else might a restless cosmos have in store for us? What else could the universe send our way that might swiftly unravel the patterns of life on Earth?

Several sweeping episodes of mass extinction have punctuated the past half-billion years on Earth. The biggest are the Ordovician, about 440 million years ago; the Devonian, about 370 million; the Permian, about 250 million; the Triassic, about 210 million; and, of course, the Cretaceous, about 65 million. Lesser extinction episodes have taken place as well, at timescales of tens of millions of years.

Some investigators pointed out that, on average, an episode of note takes place every 25 million years or so. People who spend most of their time looking up are comfortable with phenomena that repeat at long intervals, and so astrophysicists decided it was our turn to name some killers.

Let's give the Sun a dim and distant companion star, a few up-lookers said in the 1980s. Let's declare its orbital period to be about 25 million years and its orbit to be extremely elongated, so that it spends most of its time too far from Earth to be detected. This companion would discombobulate the Sun's distant reservoir of comets whenever it passed through their neighborhood. Legions of comets would jiggle loose from their stately orbits in the outer solar system, and the rate of impacts on Earth's surface would vastly increase.

Therein was the genesis of Nemesis, tile name given to this hypothetical killer star. Subsequent analyses of the extinction episodes have convinced most experts that the average time between catastrophes varies too greatly to signify anything truly periodic. But for a few years the story was big news.

Periodicity wasn't the only intriguing idea about death from outer space. Pandemics were another. The late English astrophysicist Sir Fred Hoyle and his longtime collaborator Chandra Wickramasinghe, now at Cardiff University in Wales, pondered whether Earth might occasionally pass through an interstellar cloud laden with microorganisms, or be on the receiving end of similarly endowed dust from a passing comet. Such an encounter might give rise to a fast-spreading illness, they suggested. Worse yet, some of the giant clouds or dust trails might be real killers--bearing viruses with the power to infect and destroy a wide range of species. Problem is, it's not clear how an interstellar cloud could manufacture and carry something as complex as a virus.