WUSTL

Cosmic voyager has a layover in St. Louis

WUSTL geochemist helps analyze rare and beautiful meteorite found by a Missouri farmer
By Diana Lutz

Dave Gheesling

Randy Korotev, PhD, research professor in earth and planetary sciences in Arts & Sciences, with a new Missouri meteorite the owners allowed him to hold briefly on their way through town last January. Korotev’s lab analyzed the olivine crystals in the meteorite to help identify its parent body.

 

Last January, two amateur meteorite hunters dropped by Randy Korotev’s office at Washington University in St. Louis to show him their latest purchase, a 17-kilogram pallasite meteorite found in 2006 near Conception Junction (population 202) in northwest Missouri.

Korotev, research professor in earth and planetary sciences in Arts & Sciences and an expert in lunar meteorites, identified the stone as of a fragment of an asteroid.

His lab also analyzed crystals within the rock to help identify its body of origin, eventually referring the meteorite hunters to the University of California, Los Angeles (UCLA), for analysis of the metal in which the crystals are embedded.

The meteorite is a pallasite, a type of meteorite named for Peter Pallas, a German naturalist who first described one in 1749.

These meteorites consist of green olivine crystals embedded in an iron-nickel matrix like cherries in a pie, a rock type so odd that it was the first to be identified as extraterrestrial.

Not only are they beautiful, they are rare. The Conception Junction meteorite is only the 20th pallasite found in the United States so far.

And, like all meteorites, it carries within it a bit of the history of the early solar system. Pallasites are thought to be fragments of asteroids large enough to produce sufficient heat early in their history to partially melt and separate into a metal core and a rocky exterior.

Pallasites come from the lower mantle of these differentiated bodies and contain both metal from the core and olivine from the mantle. As such, they are miniature models of planet formation that provide clues to what lies beneath our feet.

The meteorite’s story

What a website had to do with it
“We wouldn’t have been involved in the validation of the Conception Junction meteorite,” Korotev says, “if Karl hadn’t found my site.”

Korotev still has heartburn over the Milton pallasite. Found before Korotev’s website had much of a presence, this pallasite was shunted off to the University of New Mexico for analysis and verification.

“When I read the abstract a New Mexican colleague wrote about this unusual meteorite, I thought, ‘How did that thing ever end up going to New Mexico?” Korotev says. As a point of pride, he didn’t want that to happen again.

Korotev’s website began as an online list of lunar meteorites first posted in 1994. Soon, a Canadian pilot found the list and asked Korotev to identify a crater he had just flown over. Then, queries started to come in about all sorts of funny-looking rocks that might be meteorites.

Before he knew it, Korotev was single-handedly trying to stem a rising tide of industrial slag, sedimentary rock, volcanic rock, and just plain unidentifiable something or other. In self defense, he started to explain, and soon he was the webmaster of a large and highly entertaining educational site about meteorites

Almost all the meteorites people submit to him turn out to be what he calls “meteorwrongs.” The meteorwrong pictured above is one of his favorites. It is actually a complex nodular concretion, probably composed of chert, and most definitely not extraterrestrial.

But roughly one in 300 candidate rocks submitted for his inspection is actually a meteorite.

The site also puts Korotev in touch with a community of amateur meteorite hunters such as Karl Aston. As the roots of the word amateur suggest (it comes from amour, French for love), these are people who do science for the love of it. Not a bad crowd to know.

And some visitors do come away knowing more about meteorites. “You have the coolest website!” one of them wrote. “I am a little chapped, though. Before I found your website, I had six meteorite treasures that I had found. Now, thanks to your ‘It’s probably not a meteorite,’ I have NONE!”

Karl Aston, a St. Louis chemist with an interest in meteorite collecting, spotted Korotev’s extensive meteorite website in 2007 and got in touch. Aston told Korotev he was trying to collect samples of all the 20-some meteorites that have been found in Missouri.

“I’m working on the St. Louis meteorite,” he said. “I’ve got some small samples; would you like them for your museum?’”

Of course, Korotev said “Yes, please.”

The St. Louis meteorite, a stony meteorite, fell through the roof of a convertible being driven down Florissant Avenue on Dec. 10, 1950. Contemporary accounts of the meteorite fall report a fireball some residents took to be an “A-bomb explosion.”

But Aston also was interested in new finds, and following in the footsteps of  a legendary early 20th-century meteorite hunter named Harvey Harlow Nininger, he hunted for them by advertising in local newspapers for funny-looking stones.

He got lots of bum leads. “He’d call, I’d ask what was new,” Korotev says, “and he’d say, ‘Oh, you know how it is. I went out and I found another hematite concretion today.” (Hematite concretions, which are sedimentary rocks, are one of the stones most commonly mistaken for meteorites.)

But in 2009, Aston heard from a farmer who had found an unusually heavy stone buried deep in a hillside near Conception Junction in 2006.

The farmer, who has asked to remain anonymous, had sawed off the end of the stone, revealing an interior impossible to mistake for that of a terrestrial rock.

Dave Gheesling

The rusting fusion crust on the outside of the Conception Junction meteorite disguises it as just another rock, but one glimpse of the interior gives the game away. The olive-green crystals set in lustrous metal are unique to pallasites.

Aston didn’t have the money to purchase the stone, so he brought in a partner, David B. Gheesling of Atlanta. Gheesling’s friend, Robert Ward, of Prescott, Ariz., also joined the endeavor, directing a systematic search of the area where the stone was found, on the chance that it had broken up on entry and there were more pieces.

On their way back from Conception Junction with the newly purchased stone, Gheesling and Aston stopped by Korotev’s office so that he could examine it.

Naming the stone

The meteorite hunters were eager to learn whether they had a new stone or a fragment of a known stone.

If it was a unique stone, it could be submitted for naming to the Meteoritical Society, a nonprofit organization founded in 1933 for the study of extraterrestrial materials. Naming, which follows chemical characterization and comparison to other meteorites, amounts to acceptance by the meteoritical community of the stone’s validity and uniqueness.


adapted from Google Earth

Only 20 pallasites have been found in the U.S., but six of these were found in Missouri and Kansas. (Only 19 yellow dots appear on this map because the 20th meteorite was found in Alaska.) By longstanding convention, meteorites are named after the place that they fall or are found. The Brenham pallasite, the first pieces of which were found in Kansas in 1890, is famous for its heft — fragments found to date tip the scales at more than 4.3 metric tons. The Milton pallasite, a Missouri pallasite found in 2000, was chemically very different from other pallasites. The town of Milton is close to Conception Junction, but analysis showed the meteorites found near the two towns came from different asteroids.

Korotev said, “Well, that’s where we might be able to help you. The composition of the olivine might tell you.”

Most of the 20 pallasites found in the U.S. belong to what is called the pallasite “main group” because the elemental composition of their olivine is similar.

But, as Korotev was aware, a pallasite found by a farmer near Milton, Mo., in 2000 is “ungrouped,” meaning it has a chemical composition unlike that of the main group pallasites.

“So they gave us a sample,” Korotev says, “and Ryan Zeigler, PhD, then a research scientist in earth and planetary sciences, prepared it and analyzed the olivine’s elemental composition.


Randy Korotev

Light shines through the olivine crystals of the Conception Junction meteorite in the WUSTL geology museum. The crystals in this pallasite are smaller than usual, so the stone must be sliced very thin before the crystals penetrate its width and become translucent.

“We got the numbers and looked at them,” Korotev says, “and it was clear the stone was a main group pallasite. That meant it could be established as a unique stone only by studying the composition of the metal matrix.”

Because WUSTL didn’t have the tools or expertise to analyze the metal properly, Korotev referred the meteorite hunters to UCLA’s John Wasson for that work.

Wasson concluded that the new stone didn’t match any of the other palllasites he’s analyzed — “and he’s analyzed practically all of them,” says Korotev. So Conception Junction was unique.

UCLA became the repository for the “type specimen” of the new meteorite. Before the Meteoritical Society will recognize a meteorite, Korotev says, a representative specimen must be deposited in a museum or other institutional collection that routinely makes material available for scholarly research.

The Nomenclature Committee of the Meteoritical Society assigned the stone the name Conception Junction on Aug. 27, 2011.

A wandering bit of the solar nebula

Of 1,000 meteorites that strike the Earth, 998 are from asteroids, one is from the Moon and the other one is from Mars, Korotev says.

Conception Junction was once part of an asteroid that circulated in the asteroid belt between the planets Mars and Jupiter. The gas giant Jupiter played havoc in this zone, preventing material from the primordial solar nebula from coalescing into planets.

Dave Gheesling

Meteorites on display and for sale
Karl Aston (above), the first hunter in the field, has donated a polished slice of the Conception Junction meteorite to the Department of Earth and Planetary Sciences. It is on display in the department’s museum, together with several other Missouri meteorites also donated by Aston.

Much of the Conception Junction meteorite will be institutionally exchanged, but some slices are still for sale, as is a charming monograph Gheesling wrote about the meteorite. The monograph is modeled on another monograph written about a meteorite found in Taney County, Mo., in 1857.

Much of the asteroid belt’s original mass has been lost since the solar system formed, with some fragments, like the Conception Junction meteorite, finding their way into Earth-crossing orbits.

Most meteorites from asteroids are from undifferentiated asteroids. But some, including Conception Junction, are from asteroids large enough that they produced sufficient internal heat to partially melt and separate into a metal core and a rocky exterior.

Meteorites blasted out of large asteroids can consist just of metal, just of stone, or — more rarely — mixtures of stone and metal, called stony irons.

The subgroup of stony irons called pallasites are thought to represent material from the boundary between the asteroid’s metal core and the olivine of its lower mantle.

The boundary between Earth’s mantle and core is probably similar.

“We can’t break the Earth open,” Korotev says, “we can’t go down there and sample the rock, but we’ve got these pieces of broken asteroids that land on Earth, and they’re made of the same stuff, they’re just a lot smaller. “

 

MEDIA CONTACTS
Diana Lutz
Senior Science Editor
(314) 935-5272
dlutz@wustl.edu
EXPERTS @ WUSTL
Randy L. Korotev
Research professor of Earth and planetary sciences
314-935-5637
korotev@wustl.edu