Shining zombies (Image: ESO/Y. Beletsky)
TRAVEL out into the primal darkness, far away from the bright lights of civilisation, and, on a clear night in the hour before dawn, you may see a great pyramid of light rising up into the sky from the eastern horizon.
Back in the 11th century, the Persian poet and astronomer Omar Khayyam alluded to this "false dawn" in his Rubaiyat. Known as zodiacal light because it follows the same broad band of the sky as the constellations of the zodiac, the phenomenon is caused by sunlight reflected off dust in space. This explanation goes back to a suggestion made by the English astrologer and archdeacon Joshua Childrey in 1661. But till now, where that dust comes from, and how it stays there, has had us stumped.
The zodiacal dust forms a vast, diffuse cloud that extends all the way from the sun to beyond the orbit of Mars (see diagram). It is densest in the orbital plane of the Earth and the other inner planets, but as the Infrared Astronomical Satellite (IRAS) telescope revealed in the 1980s, it is also fluffed up for tens of millions of kilometres on either side.
This dust can't just be stuff left over from the creation of the solar system. Dust grains would orbit the sun indefinitely like minuscule planets, were it not for a peculiar force called Poynting-Robertson drag. As a dust grain zooms along, it ploughs through the stream of sunlight that pervades the solar system. This slight photon headwind gradually robs the grain of its angular momentum, making it spiral slowly inwards.
Something, then, must be constantly replenishing the zodiacal cloud. The prime suspects have been asteroids, the rocky objects that occupy a wide belt between the orbits of Mars and Jupiter. In 1984, Stanley Dermott at the University of Florida in Gainesville suggested that continual grinding between colliding asteroids could account for most or all of the dust that forms the zodiacal cloud.
But since then, David Nesvorný of the Southwest Research Institute in Boulder, Colorado, has found evidence that occasional one-off collisions in three asteroid groups only account for three dense bands of dust, not all of it.
Even so, most people thought asteroids were an important source of dust, perhaps the predominant one. "That was the prevailing opinion," says Nesvorný. "And mine too."
Peter Jenniskens of the SETI Institute in Mountain View, California, suspected otherwise. In 2003 he was looking into the origin of meteor showers, which arise when Earth passes through streams of space junk, causing pieces of rocky detritus ranging from a few millimetres to a few centimetres across to rain down on us. Some streams were known to originate in the rocky exhaust left behind by comets as a result of evaporating volatile compounds heated by the sun. Most, however, had no such obvious source.
Then Jenniskens made a discovery that changed all that. He found that a small, dark body called 2003 EH
1 was following an orbit similar to that of the Quadrantid meteor stream, which causes an intense meteor shower when Earth crosses its path in early January each year.
This object is a dormant comet whose icy interior has long since ceased to be troubled by the sun's heat - either because its volatile ices are exhausted, or because they are trapped beneath a dusty crust impervious to the sun's rays. It was not obvious how 2003 EH
1 could have produced the Quadrantid stream, but Jenniskens had an idea: perhaps it was part of a larger body that had suddenly broken up, releasing a swarm of gravelly debris.
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