As my S&T colleague Tony Flanders describes in his observing blog, this is a particularly good time of year for northern skygazers to seek the night-sky glow known as the zodiacal light.
Eerie and elusive, it appears after evening twilight as a towering but feeble cone of light that, under ideal, ultradark circumstances, can be traced far along the ecliptic.
The zodiacal light arises from sunlight scattering off countless tiny flecks of dust drifting through the inner solar system. Imagine an enormous, swollen pancake of tenuous dust with the Sun at its center, and you'll have the right idea. Over the years many scientists have taken a stab at explaining this phenomenon — some more fanciful than others. My 40-year-old college astronomy textbook says it's likely due to a dusty tail that trails Earth as it orbits the Sun. (Not!)
Since the glow is brightest along the ecliptic, it's logical to assume that asteroids play a major role in its formation, and that's what theorists believed in the mid-1990s. More recently, however, they've come to realize that cometary dust must play a role, though the exact mix has been largely guesswork.
Last year a five-member team of dynamicists, led by David Nesvorný (Southwest Research Institute) decided to tackle the zodiacal light's origin from first principles. They modeled what would happen to dust released from various sources — asteroid collisions, comets arriving on random orbits from the Oort Cloud, and especially "Jupiter-family comets" (orbital periods of less than 20 years) — and kept track of what went where.
Then, like any good chef, they tinkered with the recipe until their model matched the zodiacal light's true appearance. It wasn't enough to match the visible-light glow in the pre- and post-twilight sky, which comes mostly from particles inside Earth's orbit that scatter sunlight strongly in our direction. The model also had to match the sizes and concentrations of dust lying outside Earth's orbit — a diffuse cloud of grit with a distinct infrared signature that's been recorded by a host of spacecraft.
At the outset, Nesvorný felt he could get a good match by combining dust from asteroids (to match the glow's peak along the ecliptic) and comets from the Oort Cloud (to explain its vertical breadth).
But the model provided a very different answer: virtually all the dust must be coming from short-period comets, with a little contribution from Oort Cloud comets. No more than 10% of it can be coming from the asteroid belt. Moreover, these Jupiter-family comets don't just sprinkle fairy dust along their orbits — more likely, they cough up pulses of debris by breaking up repeatedly, dozens of times, over their lifetimes.
The curves at right tell the story: asteroidal dust is a poor match to reality, whereas cometary dust gives a near-perfect fit. All told, Nesvorný and his team estimate that there must be some 20 trillion tons of dust in the zodiacal cloud (twice the mass of the Martian moon Phobos), and that 100,000 tons of the stuff falls to Earth every year.
The team's exhaustive analysis even considers what the situation must have been like billions of years ago, when the solar system teemed with comets. The answer is that the zodiacal light would have been hundreds or thousands of times brighter than it is now. Imagine trying to stargaze with all that "natural" light pollution in the sky!