Computer simulations have revealed what no telescope can see: Earth’s orbit is collecting the debris of asteroids and comets.
There is a ring of dust around the sun, and we are in it. The ring is a couple of hundred thousand miles thick and about 30 million miles wide from its inner to its outer edge. Earth is embedded in the inner edge. But the dust is so diffuse--on average the ring contains less than one particle per cubic mile--that it is almost impossible to see in a night sky full of stars. In fact, the ring was discovered only recently--in computer simulations done at the University of Florida and the Johnson Space Center.
Astronomers have long known that the inner solar system in general is dusty. Some of the dust is scattered by passing comets, and some comes from the asteroid belt between Mars and Jupiter, where billions of small rocks are constantly bumping into and pulverizing one another. As sunlight strikes these particles, which are only a thousandth of an inch or so across, it slows them down. That causes the width of their orbits around the sun to shrink by about 4,000 miles per year. At certain times of the year, just after sunset or just before dawn, it is possible to see the faint glow of sunlight bouncing off interplanetary dust particles; the glow is called zodiacal light.
In 1989 Al Jackson and Herb Zook of the Johnson Space Center first calculated that Earth’s gravitational pull should trap some of the infalling dust particles in a ring around the sun. But Jackson and Zook’s computer simulation tracked the behavior of fewer than a hundred particles- -which was not enough to convince all astronomers that the ring was real. Now Stanley Dermott, Bo Gustafson, and their colleagues at the University of Florida have used a supercomputer to simulate the trajectories of thousands of individual dust grains. Their simulations prove not only that the ring exists but that it has a peculiar structure. As Earth patrols the inner edge of the ring, it carves out a small, traveling niche in the dust, leaving a gap in front of the planet and concentrating dust in its wake.
The cause of the dust ring is a phenomenon called resonance: dust particles are trapped whenever their orbital period and Earth’s fall into a ratio of whole numbers. If the ratio is, say, 5 to 6, a dust grain orbits the sun five times for every six Earth orbits. On every fifth orbit, it passes near Earth and gets accelerated by the planet’s gravity. This provides just enough of a boost to offset the drag of sunlight on the particle. Instead of falling into the sun, the particle remains in a more or less stable orbit.
Because the particles in the dust ring gain their biggest boost when they approach Earth from behind--in which case Earth tugs them in exactly the direction they are already going--they tend to collect in the planet’s wake. Conversely, Earth pulls back on any particle that is directly in front of it, thereby adding its own drag to that exerted on the particle by sunlight. No dust can linger there without falling toward the sun, and so the result is a gap in front of Earth.
As it happens, there is evidence that the gap and thus the ring itself actually exist in the real world and not just in computers. In the 1980s a space probe called the Infrared Astronomical Satellite found that the zodiacal light was 1 or 2 percent brighter in the direction trailing Earth in its orbit than in the direction ahead. The observation was so puzzling that some people didn’t believe it. Some scientists thought it was a calibration problem, says Gustafson. But here for the first time we have a scientific explanation for it. The explanation is simple: there is more zodiacal light coming from Earth’s wake because there is more dust there.
Besides clearing up a puzzle about zodiacal light, the Florida group’s work has another interesting ramification. Some of the dust particles in Earth’s wake, says Dermott, should occasionally get pulled into the atmosphere. These dust particles might strike Earth at such low velocities that they could settle to the planet’s surface without burning up in the atmosphere. A gentle rain of carbon-rich asteroid dust, Dermott thinks, coming early in Earth’s history, could have played a role in the origin of life. This would be a means of getting life-forming material onto the surface of Earth after it cooled, he says. One way that has been discussed is comets, but this gives us another mechanism.