Science Briefs

Galactic Dark Matter May Be Hazardous to Your Health

Whatever it was that ended the long reign of the dinosaurs 65 million years ago is less of a scientific mystery nowadays. We have learned, to our astonishment, that a giant impact crater, over 100 miles across, formed suddenly in Yucatan, Mexico, around that fateful date. Two craters almost as large are known from another age of mass die-offs of species, 35 million years ago; one of these craters lies at the bottom of Chesapeake Bay and the other is visible in Russia. Still older craters punctuate the geologic record, more than 1000 million years into the past.

What kinds of extraterrestrial bodies could have created these enormous impact craters — not to speak of the host of smaller ones found scattered across Earth's ancient landscape? Comets crashing in from the farthest reaches of the Solar System probably formed the biggest of them. Asteroids, mostly small rocky bodies spiraling in from the orbital gap between Mars and Jupiter, would have produced most of the smaller ones.

Picture the havoc! If a giant meteoroid smashes on land, the dust thrown up darkens the sun for months, shutting down photosynthesis in plants. Animals are thereby deprived of food. If it falls into the sea, tsunamis raised up swarm over coastlines rich in life, drowning them. Crushed and heated

Frozen in space and time, Lake Manicouagan, Québec, with a diameter of 60 miles, is the ancient scar of a giant impact 214 million years ago. (NASA photograph)
rocks lofted into the atmosphere fall back blazing hot, setting off vast wildfires and creating whirlwinds of superheated steam. Then, smoke, acid rain, and carbon dioxide flood the atmosphere with a toxic pollution that takes years to dissipate.

In the early 1980s, a GISS scientist, Richard Stothers, discovered that the impact cratering record on Earth displays a certain regularity. Wholesale bombardments by large meteoroids seem to have recurred at intervals of 30 to 35 million years. He also noticed that this magic number seems to be close to the length of time the Solar System needs to move up from and back down to the plane of our disk-like galaxy of stars, the Milky Way. Astronomers have known for a long time that, in its revolution around the central mass of our galaxy, the Solar System performs such an up-and-down motion quite regularly, like a horse on a merry-go-round. Material lying mostly near the galactic plane tugs with its gravitational force on objects only loosely bound to the Solar System, like the distant comets that occupy a vast halo far beyond the orbit of Pluto. Many of these shaken comets then rain down in a shower toward the Sun, much as ripe apples drop from a tall tree during a windstorm. Our Earth intercepts some of the infalling comets. The biggest gravitational disturbances of the outer comet halo are believed to be made by close encounters with very massive objects, like the huge clouds of gas and dust that lie between the stars.

To predict the length of time between the Solar System's crossings of the galactic plane, astronomers have had to gauge the up-and-down motion statistically, using the numbers and the velocities of many sample stars distributed above and below the plane. The mathematical analysis then yields as a final result both the period of the up-and-down motion and the space density of matter in the flat galactic disk. In the same way that a pendulum swings much faster on Earth than in a low-gravity environment, such as an Earth-orbiting spacecraft, the surprisingly large space density found in the galactic disk gives the Solar System an unexpectedly short up-and-down period of 30 to 35 million years. This new measurement agrees uncannily well with the known impact cratering period on Earth. And it is likely that another big impact on Earth will happen sometime soon, at least within the next million years, because the inner Solar System seems to be in a comet shower now.

Why, though, is the inferred space density of galactic matter so large? It is apparently 50 percent larger than what we can count in the visible stars, gas, and dust. The nature of the galactic disk's dark matter remains a profound mystery in astronomy. Maybe it comprises ultracold, and therefore dim, gas and dust clouds, or else very faint brown dwarf stars. It may just be something else. In any event, its gravitational presence must be making disturbances of the Solar System's outer halo of comets much more frequent than would otherwise be the case. It is perhaps doubling the risk of a catastrophic impact on Earth!


Stothers, R.B. 1998. Galactic disk dark matter, terrestrial impact cratering, and the law of large numbers. M. Notices Royal Astron. Soc. 300, 1098-1104.