From: The Antarctic Sun
Posted: Saturday, October 30, 2004
By Emily Stone Sun staff
The Antarctic meteorite hunters knew they'd found something good when they spotted the crusty black rock on a Miller Range ice field last year.
"The field notes said, 'this is very, very, very sexy.' Three verys," said Ralph Harvey, head of the U.S. Antarctic Search for Meteorites program (ANSMET).
The hunters had to wait months before learning what they had discovered. The fist-sized rock first had to be carefully collected by the 4-person team, shipped frozen to NASA's Johnson Space Center in Houston, and then split so that a small chunk could be sent to the Smithsonian Institution for analysis.
In July, the team learned they'd found a piece of Mars. The piece is a 1.3 billion year- old volcanic meteorite, weighing 715 grams.
Martian meteorites are rare finds. Eleven of the 31 known Mars meteorites were found here.
This meteorite is particularly valuable because it belongs to a group of Mars meteorites known as nakhlites. This is the seventh nakhlite on record. All the nakhlites are believed to have come from the same volcanic event and are among the oldest known rocks from Mars. They are named after the Egyptian city of Nakhla, where the first meteorite of that type was retrieved in 1911 after, as legend goes, fragments struck and killed a dog there.
The older the rock, the more it can tell scientists about Mars' past, explained Harvey, a geology professor at Case Western Reserve University in Cleveland.
"The rock has potentially recorded not only a volcanic event 1.3 billion years ago, but all of the ensuing activity on Mars," he said, like the rock's interactions with the planet's fluids and atmosphere. "What you've got is a little recorder, if you will. That's what us geologists do – we play that back."
Eighty-five scientists have requested a small piece of the rock to use in their own experiments, according to Timothy McCoy, the curator in charge of the national meteorite collection at the Smithsonian.
The scientists will analyze the samples with different goals, such as searching for evidence of life, or learning more about Martian volcanoes. Others might dissolve a piece and measure the different isotope levels inside.
"There are a lot of little signs there that lead us toward a picture of the environment of Mars over the last billion years, and that's pretty cool," Harvey said. "Thirty chunks of rock from 90 billion miles away is really more than we could ask for. It's a dream."
McCoy, who was in charge of classifying the meteorite, said he could tell it was a nakhlite as soon as he looked through a microscope at a one-inch long, hair's width thick piece known as a thin section. Nakhlites are full of minerals that crystallized in the rocks as they hardened. Under the microscope, they look like bits of bright stained glass against a dark background, or like the view through a colorful kaleidoscope.
"The texture of this type of rock is so distinct that you can't possibly mistake it for something else," McCoy said. "They're really pretty."
McCoy said this meteorite has clearly interacted with liquid in its lifetime, but it's not yet clear what kind of liquid. He is one of the people who requested a sample to analyze more fully. McCoy plans to compare the new nakhlite to volcanic rock from a lava flow in Ontario, Canada. That flow is one of the few places on Earth that may be similar to the flow on Mars where the nakhlite originated. McCoy hopes the comparison will help determine at what depth in the flow the meteorite originated.
ANSMET teams have been searching for meteorites in Antarctica since 1976. Meteorites fall evenly all over Earth, but Antarctica is a particularly good place to look for them. Antarctica's advantage is two-fold, Harvey explained.
"If you want to find things that fall from the sky, lay out a big white sheet," he said. "And Antarctica is a big white sheet 3,000-miles across."
The second reason has to do with the way ice flows across the continent. The meteorites get sprinkled across the ice, and many end up getting buried over time. The ice is slowly moving toward the sea, but it gets blocked in places by mountains and forms ice cul-de-sacs. Over millions of years, the ice surface at those bends evaporates, exposing more and more meteorites, Harvey said. ANSMET targets these areas for their hunts.
ANSMET sends out two teams a year, a 4-person reconnais - sance team and an 8-person systematic search team. The reconnaissance team usually spends a couple days to a week at each site, checking to see if it's worth returning there. If it's deemed a good spot, a systematic search team will go there another year and spend the whole season collecting meteorites.
This year the reconnaissance team will work at a number of ice fields throughout the mid-range of the Transantarctic Mountains, from the Zanefeldt Glacier in the south to Buckley Island in the north. The systematic search team will work at the LaPaz ice fields, approximately 400 km from the South Pole Station.
Last year's reconnaissance team found the nakhlite on Dec. 15, the third day of its six-week season. The group used snowmobiles to scan the blue ice field in the Miller Range of the Transantarctic Mountains, about 750 km from the South Pole.
"We just knew it was something unusual," said Nancy Chabot, a research scientist at Case Western who was on the reconnais - sance team.
The rock was particularly black and quite shiny, Chabot said. The black crust, called a fusion crust, is created because much of the meteorite's exterior gets burned off when the meteorite enters Earth's atmosphere traveling at 10 km to 20 km per second. It also appeared very crystalline, which Chabot said usually indicates that the rock came from a planet.
The two ANSMET teams collected a record 1,358 meteorites last season, Harvey said, including not only the nakhlite but a couple new moon rocks and some interesting rocks from asteroids as well.
Chabot said there's little doubt about whether it's worth sending the systematic search team back to the site.
"I can say that definitely with this discovery and the other things we found at the Miller Range, the answer is yes."
Since 1969, more than 16,000 meteorites have been found in Antarctica, more than doubling the number available for study. For those working at or visiting McMurdo Station, there are three types of meteorites on display at the Crary Laboratory.
NSF-funded research in this story: Ralph Harvey, Case Western Reserve University, http://geology.case.edu/~ansmet/
// end //