Peter Jenniskens, a meteor astronomer at NASA Ames Research Center and… (Eric James / NASA )
The fireball that streaked through the skies on April 22 and exploded with the equivalent of four kilotons of TNT and fell around Sutter’s Mill, birthplace of the California Gold Rush. Scientists and meteor hunters alike quickly hunted down a few of the fallen fragments just before rain hit, allowing them an unprecedented look at the most pristine sample of a rare type of carbon-rich asteroid yet found.
Now a study published in Friday’s edition of the journal Science shows that all that hustle paid off. The Sutter's Mill meteor shows chemical evidence of a complex formation history -- evidence that would have been wiped out by the rain, said lead author Peter Jenniskens, a meteor astronomer at the NASA Ames Research Center and the SETI Institute in Mountain View.
Meteorites are chunks of the asteroids that fall to Earth, and they give planetary scientists a look back into the solar system’s history. Scientists think these far-out bits of rocky debris may have originally brought organic molecules, the building blocks for life, to Earth.
The Sutter’s Mill meteorite, which came in at a blazing 28.6 kilometers per second -- nearly 64,000 miles per hour -- weighed roughly 44 tons just before burning up and exploding in the Earth’s atmosphere. Tracked using Doppler weather radar imaging, it rained in countless pieces over an area roughly 7 kilometers by 4 kilometers, Jenniskens said.
“It was like a hailstorm of stones over gold country,” Jenniskens said.
Looking for these bits of rock is rather like searching for a needle in a haystack: Thus far, only 77 pieces totaling 943 grams of that original mass have been recovered. Jenniskens had been searching for fragments around Henningsen Lotus Park in El Dorado County, growing increasingly frustrated, when he arrived back at the parking lot. There, something caught his eye – velvet-black rock that had been crushed by a tire.
The crushed rock was a treasure. It revealed evidence of oldhamite, a mineral so reactive that the moisture from a human breath could destroy it, Jenniskens said. This was surprising because oldhamite is the type of mineral scientists expect to see in meteorites closer to Earth, not in more primitive, carbon-rich asteroids that generally are farther out in the solar system.
The well-mixed rock “indicates there were collisions happening between really primitive materials and really evolved materials,” Jenniskens said.
By examining how long the meteorite had been exposed to cosmic rays, the researchers found that the rock hadn’t split from its parent body too long ago. Using the photos and video of the fireball’s path, they traced its trajectory back to a Jupiter-family of asteroids.
The researchers have only begun to analyze the dozens of rocks they recovered from around Sutter’s Mill, he added.
“The meteorites themselves look beautiful, they’re just gorgeous-looking,” Jenniskens said, “but from a scientist’s point [of view], they are really stunning.”
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