BOULDER, Colo., April 16 (UPI) — An eerie, greenish nighttime glow in the sky known as the zodiacal cloud comes from comets, not asteroids, U.S. planetary scientists say.
The pancake-shaped glow, seen along the same plane as the orbit of the planets, was first explained by astrologer Joshua Childrey in 1661 as sunlight scattered in Earth’s direction by asteroid dust particles in the solar system.
The source of the dust, which has particles 1 to 300 micrometers in diameter, was long debated.
But more than 85 percent of the dust actually comes from Jupiter family comets, David Nesvorny and Peter Jenniskens argue in a paper published in the peer-reviewed Astrophysical Journal.
Jupiter family comets are comets with orbital periods of less than 20 years that travel to the outer planets such as Jupiter. Better-known Halley family comets have orbital periods of 20 to 200 years.
“This is the first fully dynamical model of the zodiacal cloud,” said 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,” he said. “Only the dust of short-period comets is scattered enough by Jupiter to do so.”
The research was funded by the NASA planetary geology and geophysics and planetary astronomy programs.
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