Sutter's Mill: A Meteoritic Gold Mine

Samples of the Sutter's Mill meteorite, found quickly after being dropped by a brilliant daylight fireball, contain pieces of primitive, carbon-rich matter like none ever studied before.

Peter Jenniskens is a cautious, soft-spoken researcher at the NASA-affiliated SETI Institute. But he morphs into an aggressive tiger when it comes to tracking down freshly fallen meteorites. Back in 2008, facing all odds, he spearheaded the effort to recover pieces of a small asteroid that broke apart over northern Sudan.

On April 22nd, luck was on his side when a big cosmic prize fell right into his lap. A dazzling fireball, peaking at magnitude –18 to –20, had streaked east-to-west above Nevada and California at 7:51 a.m. It was widely seen by astonished onlookers — and, more importantly, captured on the cameras of two of them. These records, combined with distinct echoes in Doppler-radar scans, strongly suggested that pieces of the object had fallen to Earth near the towns of Coloma and Lotus, just 120 miles northeast of Jenniskens' office in Mountain View, California.

All the evidence pointed toward a really large arrival — later reconstructions estimate that object was the size of a small car (2½ to 4 m across) and had a mass of perhaps 40 tons before it slammed into the atmosphere. Jenniskens wasted no time, quickly driving to the area in the hope of finding freshly fallen meteorites. He came away empty-handed after searching in a state park but eventually spotted a small 4-g sample — partly crushed from being run over — in a parking lot. By then other searchers were scouring the territory, and in fact professional space-rock hunter Robert Ward found a small fragment before Jenniskens did.

It's now called the Sutter's Mill meteorite, SM for short, named for the area landmark that triggered the 1848 California gold rush. April's interplanetary arrival triggered another gold rush of sorts, as a loosely knit consortium of scientists, meteorite hunters, and local hopefuls repeatedly combed the hilly terrain. At one point Jenniskens and some colleagues even took to the sky in Airship Ventures' Zeppelin to look for small impact craters pocking the Sierra foothills. They didn't find any, but so far collectors have located nearly 80 pieces, with a total mass of just over 2 pounds (943 g).

Critically, the two pieces found on April 24th by Ward and Jenniskens, plus a third spotted that same day by local sleuth Brien Cook, got snatched up before rain soaked the area. This meteorite's dark, crumbly texture was an immediate sign that it's very primitive, a C-class chondrite having fallen to Earth virtually unaltered since its formation 4½ billion years ago — and thus of special interest to planetary scientists. For example, geochemists studying these samples found abundant bits of the mineral oldhamite (calcium sulfide), so touchy that it decomposes after even the slightest exposure to water vapor. Score one for the quick recovery!

As Jenniskens and 69 coauthors report in Science for December 21st, the Sutter's Mill stones are a mash-up of rocks with varying degrees of exposure to heat and moisture. Planetary scientists term this a breccia, representing pieces of parent bodies with very different origins. "The SM meteorite demonstrates that the complexity of C-class asteroid surfaces is greater than previously assumed," conclude the analysis team. Some pieces have experienced temperatures of up to 400°F (300°C), unlikely to be the result of the flash-heating they got in Earth's atmosphere. Others contain clay and carbonate minerals (from contact with water somewhere in space) and traces of organic compounds.

The printed Science report is only five pages long, but in a 72-page supplement, the Jenniskens-led team details how they pieced together eyewitness reports, photographs, and radar signatures to deduce the incoming object's trajectory and orbit. It came from the asteroid belt, most likely injected directly onto an Earth-crossing path by Jupiter (evidence suggests that it had been exposed to space radiation for less than 100,000 years). Then it slammed into our atmosphere at 17.8 miles (28.6 km) per second, delivering the kinetic-energy equivalent of 4,000 tons of TNT before breaking apart at roughly 34 miles (55 km) up.

To learn more about these celebrated space rocks, see the press releases from NASA and the SETI Institute, or check out Jenniskens' web page dedicated to the impact and recovery. Also worthwhile is a 40-minute Google+ interview moderated by science writer Alex Witze.