Stardust and Sunbreath in the Sutter's Mill Meteorite

On Sunday morning, April 22, 2012 a bright fireball streaked westward over the Reno-Tahoe area, rattling homes with a sonic boom. Shortly afterward, residents of El Dorado County heard the whistling sounds of falling meteorites. Weather radar tracked the hail of stones in midair, and the experts outlined a probable strewnfield centered on the site of Sutter's Mill, the place that launched the California Gold Rush in 1848. Soon a meteorite rush began as the story hit the news. Today, eight months later, the journal Science has published intimate details about this leftover piece from the solar system's infancy.

Meteorites are samples of the asteroids, which orbit the sun in the gap between Mars and Jupiter. Today, after more than 4 billion years of collisions with each other, they are mostly broken chunks of rock and iron. A handful of the largest asteroids still have their original form of little planets, with iron cores and rocky mantles like Earth or the Moon, although they're pretty banged up.

Back in April it made me thrill to hear that the Sutter's Mill rocks were of the type called carbonaceous chondrite ("CON-drite"). This is older stuff that escaped processing in a mini-planet asteroid. It lets us peek beyond the birth of the planets to the original cloud of interstellar gas and stardust—the nebula—that first formed the sun and solar system.

According to the Science paper, the Sutter's Mill meteorite started out as a rock the size of a minivan, 2 to 4 meters across. It entered the atmosphere at 28.6 kilometers per second, twice as fast as most meteorites. The rock shattered from the shock at an altitude of 30 miles, producing an explosion of about 4 kilotons yield. Thousands of pieces quickly slowed down to ballistic speed, getting a thin blowtorched crust during the second or two they were supersonic.

Scientists from the SETI Institute and NASA Ames Research Center, led by meteor specialist Peter Jenniskens, made an aerial survey of the strewnfield, looking for craters, and found none. For that they hired the airship Eureka in one of the Bay Area zeppelin's most unusual missions. (Alas, the Eureka is no more.)

Carbonaceous chondrites look a lot like asphalt, and some are soft enough to cut with a knife. The Sutter's Mill stones are harder than that, though—more like old asphalt. So far, less than a kilogram of meteorite material has been collected. The biggest piece weighs 205 grams, or just over 7 ounces.

The tarry black color marks it as carbonaceous (it's about 2.5 percent carbon), and the little round grains inside are chondrules, tiny droplets made of the minerals olivine and pyroxene that condensed out of the hot nebula. Those are what define a chondrite.

The Science paper describes the Sutter's Mill meteorites as typical of their class, the CM class. The chondrules have been partly erased by reactions with water. These reactions produced minerals that sound surprising to a geologist: clays, dolomite, calcite, serpentine. These tend to disappear when things get rough, geologically speaking, and are not the silicate minerals and iron metal you find in most meteorites. The fact that these minerals are preserved means that the material made it through the entire history of the solar system without being heated beyond about 500°C.

The same is true of the carbon-bearing fraction; indeed, some bits of rock appear never to have gotten hotter than about 150°C, or the heat of a moderate oven. Other parts hold high-temperature grains. About 2 percent of the carbon appears to be in microscopic grains of diamond and silicon carbide: ancient stardust created in supernova explosions. Other parts of the carbon fraction hold a whisper of neon and argon from the sun, blown into it by the solar wind as the asteroid drifted in space.

The stone is made of broken fragments (a breccia) that was once the outer surface (regolith) of a small asteroid. Between the fragments is very fine grained material. The image below gives a sense of how complex this stuff is.

Two specimens were collected before a rainstorm passed through. This pristine material proved unexpectedly rich in calcium sulfide (CaS), which disappeared after a single rainfall. Formate, acetate, sulfate and chloride likewise were quickly washed out. Another delicate fraction is the natural, non-biological amino acids that carbonaceous chondrites are famous for. In the Sutter's Mill stones the amino acids were less abundant than in other CM-class meteorites.

These results don't tell a big new story or topple old theories; they're simply more data for the experts to chew on. But the paper is impressive for the state-of-the-art lab techniques used on these little stones. The text alone (not counting the 72-page supplement) cites results from X-ray tomography, helium pycnometry, magnetic susceptibility tests, backscattered electron mapping, neutron tomography, whole-rock chemistry, thermoluminescence, Raman spectrometry, powder diffractometry, Fourier transform ion cyclotron resonance mass spectrometry, nuclear magnetic resonance spectroscopy, ion chromatography, gas chromatography mass spectroscopy, and liquid chromatography with fluorometric detection and time-of-flight mass spectrometry. The paper has 71 authors from 44 different institutions who call themselves the Sutter's Mill Meteorite Consortium.

Working backward from the impact, the authors used radar frames, infrasound evidence, and photos taken by witnesses to estimate the size and orbit of the original rock. Its off-center orbit had carried it out near Jupiter at one end, where it must have originated as part of an asteroid, and close to Mercury at the other. There are some asteroids out there with light signatures close to that of the Sutter's Mill object. One of them is 1999 JU3, the target of the upcoming Hayabusa-2 asteroid sampling mission.