Sutter’s Mill Meteor Fastest, Most Diverse Ever

One of the Sutter’s Mill meteorites demonstrates a crust fused by the heat of atmospheric entry but a pristine interior (Credits: Robert Beauford, University of Arkansas).

On April 22, 2012, the western United States witnessed a fireball streaking across the sky, followed by a sonic boom. Within hours, professional and amateur collectors were out hunting for the valuable carbonaceous chondrite meteorite fragments that littered the ground across the Sierra Nevada mountains. Now, the first paper has been published on the meteorites and the meteor has been declared the fastest ever observed at 28.6 km/s.

“We want to learn about this asteroid,” Peter Jenniskens, lead author and a scientist at the Carl Sagan Center at the SETI (Search for Extraterrestrial Intelligence) Institute and the NASA Lunar Science Institute said at the time. “This is scientific gold.” Well now he has, just eight short months after the event – record time in scientific circles. Jenniskens is one of 71 authors from 41 institutions calling themselves the Sutter’s Mill Meteorite Consortium who participated in the research and produced the paper – along with its 72 page supplement. The paper was submitted in July and approved for publication in November 2012.

The consortium accomplished the feat through what Jenniskens describes as a crowdsourcing effort. Volunteers called in sightings, brought in suspected meteorites, and provided video, all of which helped Jenniskens and his team collect samples and get to work – fast. “It’s all very important, and it’s fantastic how this came together,” he said.

The article was published in the journal Science on December 21. Jenniskens and the team at the Carl Sagan Center gathered 77 fragments totalling 0.9 kg. They suspect that the meteor massed 40,000 kg before it hit the atmosphere, and broke off of a larger asteroid before that. The meteor is estimated to be 50,000-90,000 years old.

These meteorite fragments were run over by a car before being rescued by Peter Jenniskens (Credits: Peter Jenniskens/SETI Institute and Eric James/NASA Ames).

Not all of the fragments have been analyzed yet, but those that have were revealing. Using state of the art analyses including various forms of tomography, spectrometry, and chromatography to name just a few, scientists have established the composition and structure of these rocks. While CM carbonaceous chondrites are not very abundant, the exciting thing about these meteorites is their freshness. Since everyone saw the meteor, the length of time the meteorites lay on the ground is known exactly, unlike rocks that are discovered without their meteors being observed. The freshness of the samples make a big difference – scientists saw significant divergence in composition even between samples found right away and those found a couple days later after a light rain. The rain washed away calcium sulfide, formate, acetate, sulfate, and chloride. These chondrites also have fewer amino acids than has been observed in other meteorites of the same family and a more complicated, layered structure that seems to be lost quickly following atmospheric exposure. “It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface,” said the authors in their abstract. “That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.”

The meteorite may provide a preview of the samples expected from the Japanese Hayabusa-2 asateroid sample return mission. The target of that mission, ateroid 1999 JU3, demonstrates a similar light signature to the Sutter’s Mill meteorites.

See the search for meteorites by airship and some of the subsequent testing that resulted in today’s paper: