April 20, 2010 - FOR IMMEDIATE RELEASE

Public affairs office:
 Maria Martinez
  Southwest Research Institute, P. O. Drawer 28510, San Antonio, TX 78228-0510. 
  Tel.: 1-210-522-3305; Email: maria.martinez@swri.org

 Karen Randall,
   SETI Institute, Carl Sagan Center, 515 N. Whisman Road, Mountain View, CA 94043.
  Tel.: 1-650-9604537; Email: krandall@seti.org


 SOURCE OF ZODIAC GLOW IDENTIFIED

 The eerie glow that straddles the night time zodiac 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.

 "This is the first fully dynamical model of the zodiacal cloud," says planetary
 scientist David Nesvorny of the Southwest Research Institute in Boulder, Colorado.
 "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."

 This result confirms what meteor astronomer Peter Jenniskens of the SETI Institute
 had long suspected. An expert on meteor showers, he had noticed that most of our
 showers consist of dust moving in orbits similar to those of Jupiter Family comets,
 but without having active dust oozing 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 have in common that
 they broke violently at some point in time in the past few thousand years, creating
 a massive dust cloud that now has evolved into Earth's path.

 Nesvorny and Jenniskens, with the help of Harold Levison and Bill 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, now have demonstrated that these comet disruptions can account for the 
 observed thickness of the dust layer in the zodiacal cloud.

 In doing so, they solved another mystery. It was long known that snow
 in Antarctica is laced with tiny micro-meteorites, some 85 percent of which have a 
 peculiar primitive composition, rare among the larger meteorites collected at the Earth's 
 surface that are known to originate from asteroids. Instead, Nesvorny and 
 Jenniskens are now suggesting that most antarctic micro-meteorites are tiny pieces 
 of comets. According to their calculations, cometary grains dive into the earth 
 atmosphere at entry speeds that are low enough for them to survive, reach the 
 ground and be later picked up be a curious micro-meteorite hunter.

 This work was funded by the NASA Planetary Geology and Geophysics Program 
 and the NASA Planetary Astronomy programs. 



Figure caption Asteroid_Comet_Illus_Final.jpg
The dust between the planets that scatters sunlight our way is not from the asteroid belt (depicted here in green), but from periodically disrupting comets that spend much of their time near the orbit of Jupiter ("V"), according to calculations by Nesvorny and Jenniskens. 

Illustration: SWRI/SETI Institute (Andrew Blanchard, David Nesvorny and Peter Jenniskens)

This and other illustrations are posted at: http://cams.seti.org