The Extinction Oscillator

The Big Idea / by Adrian Melott /

Sometimes, something kills nearly all life on the entire planet. But is there a regular cycle to this creation and destruction of Earth’s biodiversity?

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Back to my sleepless nights. When a long, nearly regular cycle is found, an astronomical event or interaction may be the source, because orbits under gravity usually maintain regular rhythms for very long times. Some cycles of the Earth’s motion around the Sun are already known. But these have periods of hundreds of thousands of years, possibly a million or two, and no more. Nothing known in the motion of the Earth itself can make a 62-million- year cycle. Further, the laws of celestial mechanics rule out any object orbiting the Sun with such a long period; it would be so distant that the gravity of other stars would pull it away. But other astronomical cycles are still in play.

It takes about 200 million years for the Sun to complete one orbit around the center of our Milky Way galaxy. Moreover, the galaxy is a thin disk, and there is also a motion along a vertical direction. As our solar system slowly orbits the Milky Way’s center, it oscillates through the galactic plane with a period of around 65 million years. When we move up in the disk, we are pulled back down by gravity, coasting past the midpoint, then rising back up again, akin to a weight bobbing up and down on a spring.

Was this the missing mechanism? In fact, Rohde and Muller had considered this and dismissed it, for the same reason almost anyone would: One would think that any effect would occur when we passed through the disk of the galaxy, or perhaps when we got very far away from it. But that would happen twice per cycle, every 30 million years or so, which doesn’t explain the 62-million-year signal.

Trying to understand all this, I did something that in retrospect is fairly obvious: I looked at the phase. That is, how did the cycles of biodiversity and the Sun’s bobbing motion correspond? People had already computed the history of the Sun’s galactic orbit. It turns out that the biodiversity minima of the 62-million- year cycle happens when the Sun is “bobbed up” on only one side of the galaxy, when the solar system is on the disk’s upper, “north” side. So I visited my colleague Barbara Anthony-Twarog in the office next door. She has a beach ball painted with constellations, the Milky Way, and astronomical coordinate systems. It confirmed what I recalled: The galaxy’s north side lies toward the constellation Virgo, as well as the largest concentration of mass in our neighborhood, the Local Supercluster some 60 million light-years away. This supercluster is so massive that its gravity pulls our galaxy toward it at a velocity of about 200 kilometers per second.

This realization was the key for what follows, which I developed with my collaborator Mikhail Medvedev. The space between galaxies is not empty. It’s actually full of rarefied hot gas. As our galaxy falls into the Local Supercluster, it should disturb this gas and create a shock wave, like the bow shock of a jet plane. Shocks in hot gas at such high speeds generate cascades of high-energy subatomic particles and radiation called “cosmic rays.” These should be showering the north side of the galaxy’s disk. We are protected by the galactic magnetic field, much as the Earth’s magnetic field protects our planet. When we rise to the north side, we are less protected—and the ensuing flux of cosmic rays contains particles of such energy that they can reach the Earth’s surface.

So what’s the harm? Of course, radiation can be dangerous. It can lead to mutations, most of which are detrimental, often fatal, to organisms carrying them. The cancer rate would likely rise. Although it takes many mutations to do this, the rate of evolution of new species might rise too, thus assisting the rapid diversifications seen after major extinctions. We also know that cosmic rays ionize the atmosphere, knocking electrons out of atoms, and this as well might have detrimental effects, like enhanced cloud formation and depletion of the ozone layer. More clouds make the Earth more reflective, reducing the amount of solar heat reaching the ground and making the biosphere less productive. Ozone protects us from a dangerous form of ultraviolet light from the Sun. So the Earth would lose a lot of its sunblock, inducing cancers as well as killing off many small organisms at the base of the food chain, potentially leading to a population crash.

We don’t know how severe these effects would be; our research group is actively involved in making better estimates. Cosmic rays may provide only a slow decline, which would still show up in the power spectrum of biodiversity. But even if they are not of extinction-level intensity alone, there is ample evidence that stresses on the biosphere can amplify the negative effects of other events. For instance, a constant low-grade stress, like global warming or an ice age, when combined with a sudden catastrophic event, like massive volcanic eruptions or a large asteroid strike, could result in a mass extinction when neither force alone would be sufficient.

Now the natural question is, where are we in this putative cycle of extinction? Our solar system has just passed the midplane of the Milky Way, on its way up. If the past is any guide, we are on the downside of biodiversity, a few million years from hitting bottom. Our cosmic-ray hypothesis may not be the right mechanism, but it should be testable by simply looking for specific kinds of radiation from gas clouds just on the galaxy’s northern side. We now must try to understand the 62-millionyear cycle itself by seeking correlations with things like the rate of seabed fossil formation or the rates of species origination and extinction. Only by gathering these clues can we fathom the diversity recession that seems to lie in our geological future.

—Adrian Melott is a professor of astronomy and physics at the University of Kansas.

Originally published June 29, 2009

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