Zodiacal Light’s Mystery Solved

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.

Zodiacal light from La Silla Observatory
The zodiacal light is towering and unmistakable in this September 2009 image taken minutes after sunset from the European Southern Observatory's La Silla Observatory in Chile. Click here for a larger view.
ESO / Y. Beletsky
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.

Zodiacal light from Clementine spacecraft
By taking images when the Sun (dot at center) was blocked by the Moon's limb, the Clementine spacecraft captured the true extent and shape of the zodiacal light inside Earth's orbit. (Colors indicate intensity.) The combined light from all this glowing dust would outshine Venus by dozens of times.
Joseph Hahn
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.

Sources of the zodiacal light
A computer-simulated zodiacal light created primarily from asteroid-derived dust (solid line in upper panel) is a poor fit to the latitudinal distribution observed (dashed line). Instead, the dust must be derived almost entirely from short-period comets (solid line in lower panel). Gray bars indicate interference from the Milky Way.
D. Nesvorný & others
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!

7 thoughts on “Zodiacal Light’s Mystery Solved

  1. Grant Martin

    It’s surprising that this comes as a surprise. When we see the tails streaming from comets, it seems obvious that this would be the source of the dust that reflects the zodiacal light.

  2. J Mahoney

    Has anyone given thought that the dust may be from our proximity to the galactic equator? It seems to me that the equatorial regions of our galaxy would be “dirtier”, or perhaps we are moving through a bit of a molecular cloud?

  3. J Mahoney

    Has anyone given thought that the dust may be from our proximity to the galactic equator? It seems to me that the equatorial regions of our galaxy would be “dirtier”, or perhaps we are moving through a bit of a molecular cloud?

  4. J Mac

    One would expect that if interstellar galactic dust was the dominant source of the zodiacal light, the light curve would show a uniform rather than a unimodal distribution.

  5. D Wang

    Why is the dust aligned with the ecliptic as opposed to the solar equator ?

    After all the dust shouldn’t have much to do with the ecliptic (earth orbit – sun) plane as much as the invariable plane or the solar equatorial plane. The ecliptic is tilted about 7.155 degrees to the solar equator – enough to show up in the plot if it was there……

    Have a look at the Wikipedia entry on the ecliptic and planets.

  6. Brian Tung

    @D Wang: The ecliptic may be used as a proxy for the invariable or invariant plane: the plane representing the average orbital inclination of the planets–”average” being weighted by the planets’ masses. The difference between the two is about a degree and a half, which might not be large enough to discern in a graph of the data, although my guess is that it would show up in the actual data.

    But another possibility is that ecliptic latitude is being used because we’re viewing the zodiacal light from within. If the results are averaged throughout the year, it makes sense that the distribution of dust, as seen from our vantage point, would be symmetrical around the ecliptic plane. If the “pancake” of dust were aligned on a plane inclined to the ecliptic, we might be above that plane at some times, and below it at others, but on average, we would be on it, and the intensity distribution would reflect that.

  7. Brian Tung

    @D Wang: The ecliptic may be used as a proxy for the invariable or invariant plane: the plane representing the average orbital inclination of the planets–”average” being weighted by the planets’ masses. The difference between the two is about a degree and a half, which might not be large enough to discern in a graph of the data, although my guess is that it would show up in the actual data.

    But another possibility is that ecliptic latitude is being used because we’re viewing the zodiacal light from within. If the results are averaged throughout the year, it makes sense that the distribution of dust, as seen from our vantage point, would be symmetrical around the ecliptic plane. If the “pancake” of dust were aligned on a plane inclined to the ecliptic, we might be above that plane at some times, and below it at others, but on average, we would be on it, and the intensity distribution would reflect that.

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