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 “” 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 (IRAS) telescope revealed in the 1980s, it is also fluffed up for tens of millions of kilometres on either side.
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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 . 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, 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.”
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 EH1was , 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 EH1 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.
By Jove!
Since then, he has discovered several other undead, “zombie” comets commuting between the realms of Jupiter and Earth, and with orbits that seemed to coincide with meteor streams. Might these “Jupiter family” bodies be sources of the zodiacal dust as well? Armed with the IRAS map as a reality check, Jenniskens and Nesvorný clubbed together with four other astronomers to find out. They developed a model that followed individual dust grains as they are dragged in by the Poynting-Robertson effect and tweaked by the gravity of the planets.
They found that asteroid dust, whether from continual grinding or one-off collisions, does not cut the mustard as a source of the zodiacal cloud. Asteroid dust would stay close to the plane of the solar system where it is made, producing a disc-like cloud much thinner than the actual cloud. Long-period comets – bodies that swing by the inner solar system at intervals ranging from hundreds to millions of years – don’t work either. They plunge in from any old direction, and their dust should form an almost featureless spherical blob, much fatter than the real cloud.
In true Goldilocks fashion, however, dust from Jupiter-family comets is just right. This dust usually starts out on orbits close to the plane of the solar system, but over time Jupiter’s gravity whisks it up. After a few hundred thousand years of Jovian meddling, much of the dust from these comets is moving in highly tilted orbits. The model showed it would account beautifully for the fluffed-up dust cloud – or at least 90 per cent of it, with the rest from asteroids and long-period comets ().
The result is more than a mere curiosity. Almost 4 billion years ago, upheavals in the outer solar system fired a fusillade of comets at the inner planets, an event known as the late heavy bombardment. With such large quantities of cometary material around, the zodiacal cloud would have become much denser, meaning that a lot of the organic material that allowed life to get going could have been brought in by its dust floating down to Earth.
The cloud would also have been much brighter – about 10,000 times brighter than it is today, according to the team’s calculations. The finding could have implications for the dynamics of other planetary systems where astronomers have seen bright discs of warm dust. The assumption so far has been that these discs are the signs of a very early stage of solar-system evolution, but perhaps some of them are actually comprised of cometary dust from events similar to our late heavy bombardment. If that is so, these solar systems could be more grown-up than we thought.
Zodiacal dust is still settling on Earth today, although it is generally difficult to discern, mixing with sea or soil and becoming lost to science. In Antarctica, though, it is much easier to spot embedded in the snow. Work by Jenniskens and Nesvorný can explain why micrometeorites collected in Antarctica seem so peculiar chemically: “Eighty-five per cent of them have a primitive composition,” says Jenniskens. That is easy to explain if the zodiacal dust is primarily pristine material from comets that originated around Jupiter sprinkling down onto the Antarctic snows.
The mysteries of the zodiacal light are not quite all resolved yet. Even if the explosions of dormant comets are to blame for the dust – and the zodiacal light it spawns – we still don’t know what causes them to explode. Oddly, it doesn’t seem to be the sun’s heat. “Break-ups seem to happen all over the place, not just near to the sun,” says Jenniskens.
Are comets being torn apart by rapid spin? Or exploding due to pockets of heated ice? By identifying more cometary corpses, Jenniskens hopes to work out what re-animates them. Then, perhaps, we can finally finish the story of the false dawn.
