Source of Night Sky's Cosmic Zodiacal Glow Explained

The origin of a mysterious glow that stretches across the nighttime sky has been identified by scientists who examined the particles that make up the luminous dust cloud.

Called zodiacal light, the faint glow is caused by millions of tiny particles along the path followed by the sun, moon and planets across our sky, also known as the ecliptic.

The faint, whitish glow, which can be seen best in the night sky just after sunset and before sunrise in the spring and autumn, was first correctly identified by Joshua Childrey in 1661 as sunlight that is scattered in our direction by dust particles in the solar system.

Yet, the source of the thick cloud of dust has been a topic of debate.  

In a new study, David Nesvorny and Peter Jenniskens found that more than 85 percent of the zodiacal dust originated from Jupiter family comets (so-called because their orbits are modified by their close passage to the gas giant Jupiter), rather than asteroids, as was previously thought.

"This is the first fully dynamical model of the zodiacal cloud," said Nesvorny, a planetary scientist at 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."

The researchers identified the dust coming from Jupiter family comets after examining the shape of the zodiacal cloud, said Nesvorny.

"With other comets, like the Halley-type comets, they have large orbital inclinations," Nesvorny told SPACE.com. "They are coming into the inner solar system from all directions, so if these comets were producing the zodiacal cloud, it would be almost a ball, and not a disk. Telescopes, like Spitzer, all show that the zodiacal cloud is a disk. This points best to Jupiter family comets, which have more moderate inclinations."

These results confirm what Jenniskens, an astronomer with the SETI Institute in Mountain View, Calif., had long suspected. As an expert on meteor showers, Jenniskens had noticed that most consist of dust moving in orbits similar to those of the Jupiter family comets, but without having dust-shedding comets associated with them.

Instead, Jenniskens discovered a dormant comet in the Quadrantid meteor shower in 2003 and has since identified a number of other such parent bodies.

While most are inactive in their present orbit around the sun, all broke apart violently at some point in time in the past few thousand years, creating debris in the form of dust streams that have now migrated into Earth's path.

Nesvorny and Jenniskens, with the help of Harold Levison and William Bottke of the Southwest Research Institute, David Vokrouhlicky of the Institute of Astronomy at Charles University in Prague, and Matthieu Gounelle of the Natural History Museum in Paris, demonstrated that these comet disruptions can account for the observed thickness of the dust layer in the zodiacal cloud.

In doing so, they also solved another mystery.

It was long-known that snow in Antarctica is laced with micrometeorites, some 80 to 90 percent of which have a peculiar primitive composition, rare among the larger meteorites that originated from asteroids.

"These micrometeorites are small meteorites that are about 100 microns in size," Nesvorny explained. "These micrometeorites are collected in the Antarctic ice, and it's been puzzling as to why they have a different composition than the large meteorites that are collected elsewhere."

Nesvorny and Jenniskens suggest that most Antarctic micrometeorites are actually fragmentations of comets, which explains the different composition from other meteorites that come from the asteroid belt. According to their calculations, cometary grains dive into Earth's atmosphere at entry speeds low enough for them to survive and reach the ground.

The study will be detailed in the April 20 issue of The Astrophysical Journal.

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