Antarctic meteorites offer insights into history of early solar system
Posted September 25, 2009
Field researchers in Antarctica have returned with more than 17,500 meteorites over the 30-plus years that the extraterrestrial material has been collected from the frozen continent.
Yet meteorite science is still in its infancy, and the collected rocks still hold plenty of surprises that could shape our understanding of the solar system, according to scientists involved in the search and characterization of the Antarctic meteorite collection.
Ralph Harvey, who heads the Antarctic Search for Meteorites (ANSMET) field program funded by the National Science Foundation (NSF), explained that much of the work by the scientific community revolves around building the catalog of meteorite material.
“It’s a young science in that respect. Any new specimen has the potential to shake the foundation of how these things are categorized. That’s why it’s still important to bring back more specimens,” explained Harvey, a researcher and professor at Case Western Reserve University. “There’s always the potential for something that’s going to change the whole way we see the solar system.”
A paper in the journal Nature earlier this year illustrates his point. Scientists who analyzed two meteorites collected by Harvey’s team during the 2006-07 field season in an area called the Graves Nunatak ice field reported that the rocks are unlike anything found before. The composition of the light-colored rocks has similarities to the Earth’s crust, which has implications for how some asteroids form and evolve.
The more meteorite samples available, the greater the chance of such discoveries, said Kevin Righter, Antarctic meteorite curator at NASA’s Johnson Space Center in Houston. He noted that the worldwide collection of meteorites is probably greater than 50,000 — with the finds in Antarctica driving the numbers — yet the two rocks from Graves Nunatak turned out to be unique.
Early speculation on their origin, he said, focused on planetary bodies like Venus or Jupiter’s moon Io. “A lot of people requested samples in case they turned out to be something interesting like that,” Righter said. “Figuring out the samples’ origin is an area of active research right now.”
Making discoveries possible
Such discoveries and research are possible thanks to the collaboration between the NSF, NASA and the Smithsonian Institution’s National Museum of Natural History over the last three decades.
NSF funds the field research carried out by ANSMET each year. The field teams, composed of different members from the planetary science research community each year, find at least a few hundred samples each season, but have also collected more than 1,300 in a single season since they started.
All those rocks, ranging in size from smaller than a marble to larger than a football, are bagged and boxed in Antarctica in their frozen state for shipment back to the United States. A cargo ship carries them from McMurdo Station to Port Hueneme, Calif. Still frozen, the rocks usually arrive at Johnson Space Center’s Antarctic Meteorite Laboratory by late March or early April.
The samples remain in a freezer until Righter and his team can process them. The curation team then thaws the rocks in a sterile dry nitrogen cabinet. The dry gas keeps the meteorites, which can contain metals and minerals, from rusting or from becoming contaminated.
“A lot of meteorites contain iron metal. It can rust pretty quickly. That’s why we keep them frozen,” Righter said.
The small staff in the NASA lab — part of the same facility used to house rocks from the Apollo moon missions — also photographs, weighs and describes each rock. The staff of the Smithsonian Institution makes the initial classification of the meteorites, most of which are stony chondrites.
“The other stuff is what people really want to study,” noted Tim McCoy, curator of the Smithsonian’s Antarctic collection, the largest museum collection in the world. The “other stuff” includes small chunks from Earth’s moon or Mars, or the oddball space rocks like the ones from Graves Nunatak.
McCoy’s team at the Smithsonian receives a small piece of each meteorite — or the whole rock it if turns out to be an iron meteorite — for analysis that allows a classification of each sample. A thin section less than the width of a human hair is glued to a piece of glass, and then studied under an electron microscope, which can provide information on an area one millionth of a meter wide.
Additional work by scientists in the community may involve isotopic analysis to help “fingerprint” the sample to determine its origin. For example, scientists can identify rare Martian rocks by analyzing the nitrogen and noble gases found in glass pockets in the rocks created by impacts. That information is matched against the data collected by NASA’s Viking program about Mars’ atmosphere.
“After a while, you get used to what they look like and can pick them out pretty reliably,” McCoy said.
NASA publishes a biannual newsletter with classification data on the latest rocks returned from Antarctica to alert scientists on what kind of samples are available for research. Righter estimates the active meteorite community probably consists of between 1,000 and 2,000 members.
His lab responds to a couple hundred requests for samples each year, he said. Long-term storage of samples is at the Smithsonian.
“It’s an interesting system. When they started collecting these samples 30 years ago, nobody had any idea it would continue for 30 years or it would be [17,500] samples,” Righter said. “It’s been great dealing with the Smithsonian and every institution that’s in the meteorite recovery program. It’s a fun project to be associated with.”
McCoy said about once or twice a year his lab will come across a meteorite that they’ve never seen before, like those from Graves Nunatak that took two years to classify.
“Those are really exciting,” he said. “They tell us about a place or a process — something we have even envisioned, thinking this kind of rock should exist but we haven’t seen it — and those rocks can help us fill in those gaps.”
While the asteroids that fall to Earth are from our solar system, some contain what scientists call “pre-solar grains” — bits of stardust that predate our solar system. That means the bits of dust are older than 4.65 billion years, though how much older is a matter of some debate, ranging from 40 million to a billion years.
“We can actually do astrophysics. We can look at the formation of other stars by studying meteorites,” McCoy said. “That’s a pretty remarkable finding over the last decade or so.”
Other areas of planetary study involving meteorites include speculation on whether Earth’s water originated here or was carried on a comet like an interstellar seed. “Water-bearing comets may have been building blocks for the Earth,” Righter said.
And, of course, lunar and Martian meteorites offer insight into those planetary bodies. In fact, while the Apollo astronauts returned from the moon with rocks samples, it turns out those are not very representative of most moon material that’s fallen to Earth, according to Cari Corrigan, a geologist at the Smithsonian involved in classifying the Antarctic meteorite collection.
“The lunar meteorites should be a random sampling of the whole moon. There are places on the far side, the south pole, or places we haven’t sampled by being there that these are giving us,” she said.
Noted Harvey about the utility of the Antarctic meteorite collection, “It’s amazing how many times a specimen 20 years back is still immensely valuable as people try to reconfigure again the catalog of material that are out there in the solar system.”
NSF-funded research in this story: Ralph Harvey, Case Western Reserve University, Award No. 0839168.
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