Physics trumps biology
Or, perhaps more precisely, Did dark matter kill the dinosaurs?, which is the way that an article in ScienceNOW put it.
Readers of PT doubtless know that there have been a half-dozen or so mass extinctions in the history of the earth, and they appear with a periodicity on the order of 30 million years. You can see an early graph here. The vertical arrows are separated by approximately 30 million years. Not every vertical arrow points to a mass extinction, so it might be better to say that the first harmonic of the data set is 30 million years; that is, if the periodicity is real, it sometimes skips a beat.
What is interesting is that some of the extinctions appear to have been caused by collisions with an asteroid, whereas others may be the result of long periods of extreme volcanism – yet all the extinctions occur with the same period of 30 million years.
According to the ScienceNOW article by freelance journalist Sid Perkins, scientists have speculated that the mass extinctions occur when the solar system in its orbit around the galactic center crosses the galactic plane. At that time, presumably, the earth is bombarded by comets or asteroids, and these account for at least some of the mass extinctions. It is hard to see, however, how such a bombardment would trigger a long period of volcanism.
Michael Rampino of New York University has thrown dark matter into the mix. Good form requires me to confess that we do not know what dark matter is, but there appears to be a lot of it, and it is affected by gravity, if nothing else. Dark matter is thought to be concentrated in the plane of the galaxy. According to Mr. Perkins, if we think of it as a thin disk, then there ought to be an areal density of roughly one solar mass per square light year. Professor Rampino suggests that the dark matter, besides possibly perturbing comet orbits, may well penetrate the earth and heat the core to the point where it rips the crust of the earth or causes a long period of extreme volcanism before the core cools. If the dark matter is unevenly distributed, like ordinary matter, then we would not necessarily expect a mass extinction every time the Earth passes through the plane of the galaxy, and we do not see one.
Far-fetched? Maybe, but the hypothesis has at least one thing going for it. Mr. Perkins quotes Dennis Kent of the Lamont-Doherty Earth Observatory to the effect that many large impacts did not cause mass extinctions, which suggests that there is something special about crossing the galactic plane – either the collisions and the dark matter are both required, or the collisions are incidental and dark matter caused the extinctions. Indeed, some scientists are apparently questioning whether the asteroid killed the dinosaurs, or whether it was extreme volcanism instead.
As for me, I haven’t foggiest idea; as a physicist, though, I rather like the dark-matter hypothesis. But then that is to be expected: according to the link immediately above, paleontologists generally prefer volcanism, physicists prefer asteroid impacts, and geologists are evenly split. Maybe dark matter will bring the physicists into the paleontologists’ camp.
Acknowledgment. Thanks to Sid Perkins for illuminating (sorry) the distribution of the dark matter.