The history of life on Earth is marked by occasional mass extinctions, events wiping out huge numbers of species. The most famous of these killed off all the dinosaurs (or at least those that hadn’t evolved into birds) 65 million years ago. But the mass extinction that ended the Permian period 250 million years ago was even more dramatic, killing off 90 percent of all species in an astonishingly short amount of time. As yet, the cause of this devastation is unexplained.
Mass extinctions have happened at least five times. (A sixth great extinction currently in progress, but that’s an anomaly because humans are responsible.) Some researchers have tried to figure out whether they’re periodic, recurring after specific time intervals. If they truly do repeat regularly, maybe there’s a common cause for them.
That’s the premise behind a recent paper by New York University geologist Michael R. Rampino. He implicates dark matter, the invisible particles that make up 85 percent of all the mass in the cosmos. We know there’s a lot of dark matter in the Milky Way, and it’s possible dark matter isn’t evenly distributed but occurs in dense clumps. Maybe our solar system passes through clumps of it periodically. If those clumps are dense enough to wreak havoc, they could knock comets loose and cause collisions, maybe, or heat Earth’s interior and cause massive volcanic eruptions, or somehow set loose all sorts of other species-obliterating disasters.
It’s a … provocative idea. It depends on many things all working in concert. First, extinction events on Earth must be truly periodic, and they must happen when the solar system crosses the densest part of the galaxy. Next, according to his theory, the dark matter density has to be higher where the stars in the galaxy are more concentrated. And finally, dark matter particles must be drawn in by Earth’s gravity and annihilate each other in the right way to heat up Earth’s core just enough to cause massive eruptions, but not too much to melt the solid part of Earth’s core, which we know hasn’t happened. If any of these premises fail, the whole theory totters.
As it turns out, all of these premises are sketchy. With so many shaky aspects, it’s almost too tenuous a hypothesis to care about. However, it was published in a reputable scientific journal—the Monthly Notices of the Royal Astronomical Society—and got sympathetic coverage in Science and a few other outlets. (Scientific American ran a slightly more critical take.)
The new proposal is yet another version of the “nemesis hypothesis,” the idea that some astronomical influence causes destruction on Earth periodically, which dates back at least to the 1970s. That’s another reason it’s worthwhile to look at all the pieces to understand why we can probably rule out dark matter as the nemesis of mass extinctions, even though we still don’t fully understand either dark matter or the death of huge numbers of species.
The first problem with these theories is that death comes unexpectedly. The evidence for periodic extinction events is controversial at best. Based on the geological record, for example, astrophysicists Fabo Feng and Coryn Bailer-Jones found that randomly occurring mass extinctions matched the data as well as regular events. Similarly, researchers have failed to demonstrate any clear repeating pattern of asteroid impacts from craters on Earth and the moon or strong changes in Earth’s atmosphere over time that would indicate recurring volcanic events. These failures don’t entirely rule out the periodic catastrophe hypothesis, but they weaken Rampino’s case.
Another iffy aspect is the theory’s requirement that mass extinctions coincide with the sun crossing the thickest part of the Milky Way disk. Most of the Milky Way’s bright stars, gas, and dust lie in a relatively thin disk, which also contains the spiral arms. Our sun and other stars orbit the galactic center, and as they do, they also move up and down through the disk.
Both the orbits and the up-down cycle are nearly regular, but they have noticeable variations, thanks to the gravitational influence of nearby stars and other fluctuations in the neighborhood. With those variations in mind, one cycle of the up-down boogying takes roughly 64 million years, with two crossings of the disk happening during that time. (Interestingly, other researchers have proposed that mass extinctions occur when the sun is farthest from the thick part of the galactic disk, when we are most exposed to harmful cosmic rays.) But the correlation between the disk crossing and mass extinctions is no better than chance. Rampino’s scheme also requires dense clumps of dark matter concentrated in the galactic disk. However, most of the dark matter in the galaxy isn’t there: From the motion of stars and gas, astronomers have shown that dark matter forms a vast, nearly spherical halo at least three times the diameter of the visible part of the galaxy. But there’s still a small amount of wiggle room—if some dark matter is made up of an exotic type of particle.
The identity of dark matter is one of the biggest mysteries in cosmology today. We know that dark matter isn’t the same stuff as normal matter: atoms and the like. But that doesn’t mean we don’t know anything about it. Astronomical observations even tell us roughly where it is in the Milky Way and other galaxies. Mysterious as it is, it is responsible for the shape and structure of galaxies and helps govern the evolution of the entire universe.
From observations and experiments, we can also place strong limits on how much dark matter interacts with ordinary matter and with itself. Every interaction between particles—physics-speak for “collisions”—exchanges energy, causing particles to speed up or slow down. The more particles interact, the more likely they are to form disks: That’s how gas atoms made the Milky Way we see, with stars forming where the gas is. Because dark matter particles make a halo instead of a disk, physicists conclude they doesn’t interact very much either with atoms or with themselves.
Rampino isn’t the first to implicate dark matter in periodic extinctions: Harvard physicists Lisa Randall and Matthew Reece proposed that perhaps 5 percent of dark matter is made up of a type of particle that interacts just a little more strongly, and that stuff could form a disk. They built their model in part to see if such a dark matter disk could shake loose comets from the Oort cloud to explain periodic mass extinctions, if they actually are periodic. But as Randall—who is a highly respected physicist and popular author—points out, the current evidence for her own idea is shaky at best. The Chicxulub asteroid that at least helped end the age of dinosaurs is the only extraterrestrial impact we can associate with any mass extinction, and most researchers think there must be alternate explanations for the other major events.
Rampino uses the dark matter disk hypothesis in another way. In his model, Earth passes through a particularly high concentration of dark matter every 30 million years or so. Those dark matter particles sink to the center of Earth, where the extra density helps particles collide with each other, annihilate, and heat the molten rock there. The extra heat propagates up through the mantle to the crust, where it shakes the continents around, causing violent earthquakes and volcanic activity.
As geologist Brian Balta of the University of Pittsburgh points out, “a 30-million-year timescale as proposed is much slower than the time it takes the mantle to really move.” To make it worse, the mantle is quite dense and tends to dissipate heat pretty well: Changes in temperature can take 50 million years or more to travel from the core to the surface. Even if something managed to heat the core up, we might never notice it on the surface, and certainly it wouldn’t have an immediate effect. Apart from the problems of heating the planet’s interior, Randall notes that a dark matter disk isn’t enough to create the necessary dark matter density: Rampino “needs extremely dense clumps in the dark disk. But if they are so dense they carry a lot of mass, and then they are dispersed too much (so far as I can tell) for the Solar System to pass through at the necessary rate.” She concludes: “It's a very cute idea to connect annihilation to tectonic motion but I don't see how to get it all to work.”
Institute for Advanced Study astrophysicist Jo Bovy, who is one of the world’s experts on the structure of the Milky Way, is even more skeptical: “Statistically there is only a few percent chance that such a thin disk of dark matter exists.” (In scientist-speak, that’s your mom saying, “We’ll talk about it.” Meaning: no way.) We can explain the motion of the sun, other stars, and gas in the Milky Way without the need to hypothesize extra dark matter—much less very dense clumps of it—in the galaxy.
With so many uncertainties about both the nature of dark matter and the cause of mass extinctions, it’s no wonder some scientists—including sensible ones—are casting around for ideas to explain them. Definitively connecting dark matter to the death of dinosaurs would be a phenomenal scientific achievement (besides making everyone’s inner 10-year-old giddy with excitement). But there’s really no evidence to support the notion that these mysteries are connected in any way; without that evidence, we should remain skeptical no matter how much a theory sounds cool and dramatic.