The National Museum of Natural History's support center in Suitland, Md., contains some of the rarest and most precious objects owned by the American people: 17,000 rocks. They represent the bulk of the nation's Antarctic meteorite collection, an assortment that includes pieces of other planets, shrapnel from the collisions that shaped the solar system, rubble older than anything on Earth and crystals possibly older than the sun.
Retrieved from the bottom of the world and stored for decades in inert nitrogen gas, the collection offers clues to some of the biggest mysteries of existence.
“Each meteorite is a piece of the bigger puzzle about how our solar system formed,” said Cari Corrigan, the Smithsonian geologist who oversees the collection. “They can tell us where we came from.”
Shuffling around the lab in a white gown, hair net and blue cloth bootees (you can't do anything but shuffle in bootees) Corrigan looks like a grim character in “The Andromeda Strain” — until she runs down the row of tanks and high-fives the rubber gloves sticking out of them.
She fits her hands in a pair of gloves, reaches inside a tank, pulls a tub off a shelf, passes it through the air lock, then yanks open the door. Sifting through the meteorites, each encased in its own protective plastic bag, Corrigan explains what they indicate about the origins of our world.
The oldest rocks are chondrites — meteorites that clumped together in the swirl of dust and gas that surrounded the sun as the planets began to form. Some include pale flecks called “calcium aluminum inclusions” that are thought to be the most primitive substances in the solar system — some of their crystals may predate the sun.
At a venerable 4.5 billion years old, chondrites are as old as our planet and substantially older than anything else on it. Tectonic activity on Earth means that most material is churned back into the interior before it gets too old — the most ancient rock known to science was formed 4 billion years ago. These space rocks offer insight into the conditions that created our planet that can't be found anywhere else on Earth.
Iron meteorites come from the heavy cores of asteroids or long-vanished planets. The smallest of these have the surprising heft of a paperweight; the larger ones feel like a cannon ball.
“People study these to figure out what's going on at the center of the Earth,” Corrigan said. “We are never going to get samples from the core of the Earth," — no human drilling operation has even gotten halfway through the crust to the mantle — but these are the next best thing.
Corrigan specializes in the rocks that result from collisions between asteroids, planets and other bits of space junk. The melt patterns on these meteorites hint at a period called the “Late Heavy Bombardment,” 3.9 billion years ago, when a mysterious gravitational disturbance swung through the solar system and sent rocky bodies slamming into each other with cataclysmic results.
Rarest of all are meteorites from other, known bodies in our solar system, like the moon and Mars. “It's like it's own planetary mission every time we get one of those,” Corrigan said. “You can learn what the climate was like, the temperature, the history of the surface ... all from one rock you can hold in your hand.”
Though meteorites fall all over the planet — on cities, on deserts, on cars, on the hips of women asleep on their couches — Antarctica is far and away the best place to look for them. The flow of ice across the continent sweeps the rocks into piles. Meanwhile, the cold, dry conditions keep the rocks pristine.
“When the only things around you are white snow and blue ice, and then you see that black and brown rock, it's exciting,” said Corrigan, who has spent two field seasons working on the frozen continent with the Antarctic Search for Meteorites (Ansmet). “You're the first person ever to see a piece of another planet.”
Twenty years ago this August, NASA scientists announced a “startling discovery”: A Martian meteorite collected near the Antarctic coast held small structures that looked like the fossilized forms of tiny microbes. It also contained organic molecules that are almost always the result of biological processes.
The rock was named Allan Hills 84001 for the spot (Antarctica's Allan Hills) and year (1984) in which it was found. Encased in a glass sphere that Corrigan stores in a hard-shell black briefcase, the small, dark rock is among the Ansmet's most precious finds. It formed during the first few hundred thousand years of Mars's history, was blasted off the surface during an impact 16 million years ago and fell to Earth at the end of the last ice age. It sat unnoticed in the meteorite collection for years, until scientists realized it came from Mars. Then geologist Dave McKay spotted the strange, wormlike structures buried in the rock.
“Today, Rock 84001 speaks to us across all those billions of years and millions of miles,” then-President Bill Clinton said at a news conference the day of NASA's announcement. “It speaks of the possibility of life. If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered.”
Corrigan, then a graduate fellow at NASA's Johnson Space Center, witnessed the tense excitement that gripped the space community. This was the kind of discovery that many scientists spend their lives dreaming about, sure to win a Nobel Prize and change the world. And, taken together, the lines of evidence defined by the researchers made a compelling case for life. But there was no unequivocal confirmation that living beings once dwelled in the rock. It ran up against the scientific truism that “extraordinary claims require extraordinary evidence.” Did NASA really have the extraordinary evidence required?
One by one, each of the lines of evidence thought to point to life inside Allan Hills 84001 were refuted. The structures that looked like microfossils might have been introduced when the meteorite was treated in the lab or they could have been caused by chemical reactions with no life required. The organic compounds embedded in the rock could have come from the exhaust of the snowmobiles the original collectors were driving. Within a decade, scientists had more or less settled the issue: the odd forms inside the meteorite almost certainly are not Martians.
But they were left with a new question — one that many researchers had not previously considered: If a meteorite containing Martians did fall to Earth, would we even recognize them as such?
“You could argue that the whole field of astrobiology came out of this,” Corrigan said, gesturing toward the unlikely chunk of rock secured inside its glass case.
Partly in response to the Allan Hills debate, biologists and planetary scientists began working together to figure out how organisms could live on Mars and what they might look like. In 1999, NASA's Mars Global Surveyor began mapping the planet and found suggestions of liquid water on its surface. It was followed by Spirit, Opportunity and Curiosity, rovers whose mission was to seek out signs of habitability on the Red Planet. Closer to home, biologists began to find more and more organisms living in the darkest caves, the depths of the oceans, wisps of cloud and newly formed rocks still hot from the planet's interior.
There may not have been organisms in Allan Hills 84001, but they were almost everywhere else on Earth.
“It opened up so many new questions and lines of study we didn't even know existed then,” Corrigan said. “You can do a study and have it not necessarily be correct in the end ... but you end up changing the face of science.”
In November, six of Corrigan's colleagues headed to Antarctica for their 40th season of meteorite collecting. Bundled up against the wind and brutal cold, they'll spend weeks out on the ice, scouring the blue and white landscape for tiny bits of black. They go back year after year, retrieving rock after rock, because there's no knowing which one will be the next to change everything.
Or perhaps, like Allan Hills, that rock is already sitting in the Smithsonian's collection, locked in a tank and preserved in nitrogen, waiting for someone to reach in and grab it.
Tales From the Vault: Science museums are home to vast research collections, most of which the public never gets to see — until now. Once a month, Speaking of Science will go behind the scenes at our favorite museums to introduce readers to the fascinating objects and people we find there. Read past installments here.