S&T: Zodiacal Light's Mystery Solved

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bystander
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S&T: Zodiacal Light's Mystery Solved

Post by bystander » Thu Mar 11, 2010 4:53 pm

Zodiacal Light's Mystery Solved
Sky & Telescope - 2010 March 11
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.
http://lanl.arxiv.org/abs/0909.4322

Image
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. (ESO / Y. Beletsky)


Image
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)


Image
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 et al)

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SwRI: Source of zodiac glow identified

Post by bystander » Thu Apr 15, 2010 9:13 pm

Source of zodiac glow identified
Southwest Research Institute (SwRI) | 15 April 2010
The eerie glow that straddles the night time zodiac in the eastern sky is no longer a mystery. First explained by Joshua Childrey in 1661 as sunlight scattered in our direction by dust particles in the solar system, the source of that dust was long debated. In a paper to appear in the April 20 issue of The Astrophysical Journal, David Nesvorny and Peter Jenniskens put the stake in asteroids. More than 85 percent of the dust, they conclude, originated from Jupiter Family comets, not asteroids.

"This is the first fully dynamical model of the zodiacal cloud," says planetary scientist Nesvorny of the Southwest Research Institute in Boulder, Colo. "We find that the dust of asteroids is not stirred up enough over its lifetime to make the zodiacal dust cloud as thick as observed. Only the dust of short-period comets is scattered enough by Jupiter to do so."
Cometary Origin of the Zodiacal Cloud and Carbonaceous Meteorites. Implications for Hot Debris Disks

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NS: Zodiacal light: zombie comets to blame

Post by bystander » Tue Jul 06, 2010 10:14 pm

Zodiacal light: zombie comets to blame
New Scientist | Space | 06 July 2010
Image
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 EH1 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 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.
...
Breathtaking Images of Zodiacal Light

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