Origins of meteorites traced back billions of years

The North Bay meteorites collide with the Earth's atmosphere and break up in spurts, generating bright flashes of light in 2012. The North Bay meteorites collide with the Earth's atmosphere and break up in spurts, generating bright flashes of light in 2012. Photo: Robert P. Moreno Photo: Robert P. Moreno Image 1 of / 8 Caption Close Origins of meteorites traced back billions of years 1 / 8 Back to Gallery

The space rocks that fell on a Novato rooftop and other North Bay sites two years ago are relics of a monster collision with Earth that formed the moon more than 4.5 billion years ago, scientists believe.

The six battered meteorites called metallic chondrites landed in the North Bay after a spectacular fireball that blazed across Bay Area skies on Oct. 17, 2012. Their fall from high in the atmosphere was captured by NASA cameras so accurately that scientists could trace their origins back both in space and time.

The tumultuous journey of the six objects began in a crash between the Earth and some unknown Mars-size object when the Earth was young, said astronomer Peter Jenniskens of the SETI Institute in Mountain View.

"I like to think that the objects formed by that Earth-moon impact so long ago have now come back to impact the Earth," he said.

That crash so long ago not only formed the moon, scientists believe, but also must have blasted a shower of giant rocks into what astronomers call the asteroid belt - a vast ring of tumbling objects as small as dust and as large as small moons that still circle the sun between the orbits of Mars and Jupiter.

Moon-forming impact

Debris from that moon-forming impact smashed into one of the larger asteroids and broke it up billions of years ago, Jenniskens said. Eventually, one of those asteroid pieces itself was broken apart, producing the meteorites that rained down on the North Bay.

The first one to be recovered, weighing 2 ounces, had tumbled off the roof of Novato resident Lisa Webber's home. Five others were later picked up by meteorite hunters nearby and in Sonoma County vineyards.

Jenniskens is the leader of an international consortium including more than 52 meteorite experts from eight nations who have analyzed the objects. They are reporting their conclusions online this week in the journal Meteoritics and Planetary Science. Jenniskens is the principal author of one report.

Cosmochemist Qing-zhu Yin of UC Davis is the principal author of the second report. He and his colleagues dated the ages of the meteorites and calculated that one major object produced by the debris impact in the asteroid belt must have broken into fragments many millions of years after the first giant crash - about 470million years ago, he said.

"We now suspect that the moon-forming impact must have scattered debris all over the inner solar system and hit the parent body of the Novato meteorite," Yin said.

Telescope network

NASA has long had a ground network of telescopes and cameras across the country known as Cameras for Allsky Meteor Surveillance, or CAMS. The network, which includes cameras in Sunnyvale and at the College of San Mateo, detects fireballs in the sky that are caused by flashes from meteors before they hit the ground.

Those cameras first spotted the fireballs of the North Bay meteorites, and from their trajectories, Jenniskens was able to calculate their precise orbits and trace them all the way back from their origins within the asteroid belt.

One major group of those asteroids includes what scientists call the Gefion family, and its members are known to exist in the middle of the asteroid belt. Jenniskens said he found that when he calculated the approach orbit of the North Bay meteorites, they tracked back to have formed within the Gefions.

The space rocks that fell on the North Bay could have been buried inside a much larger object until about 1 million years ago, when that object broke apart, said Kunihiko Nishiizumi, director of the cosmochemistry lab at the Lawrence Berkeley National Laboratory.

'Best-studied ever'

That object would finally form the North Bay meteorites, and before it ultimately hit the Earth's atmosphere and broke up into fragments dotted with metallic particles, it was a single chunk only about a foot in diameter weighing more than 175 pounds, according to scientists at NASA's Ames Research Center.

Its final collision with the atmosphere generated the brilliant fireball that the cameras photographed.

"The photographs show that this meteorite must have broken up in spurts, and each one created a flash of light as it entered the atmosphere," Jenniskens said. "They are exceptional objects, and they're now the best-studied meteorites ever."