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Antarctic Meteorites Reveal Solar System History

by Lysa Chizmadia

glaciers

Schematic diagram showing the Antarctic glaciers encountering the Transantarctic Mountains on their way to the ocean. The mountains slow down the glaciers, enabling the winds to erode them down into their ancient blue ice interiors. Courtesy Dr. Ralph Harvey, Case Western Reserve.

Because of its unique properties, Antarctica is the source for over 90 percent of the meteorites in scientific collections. Antarctica has Earth's oldest ice sheet, which has formed from the accumulation of snow near the South Pole over several million years. As the snow piles up, the snow at the bottom is squeezed into ice that forms huge glaciers that flow downhill toward the ocean. When meteorites fall, they are incorporated into the glaciers just like a piece of ice would be.

As the glaciers progress toward the ocean, some of them encounter the Transantarctic Mountains, which act as a speed bump that slows the glaciers and allows the fierce winds to erode the glacier down into the very old blue ice, which has been squeezed into a different crystal structure by the weight of the ice and snow above it. The meteorites in the glacier do not erode like the ice does, so they pile up on these blue ice fields.

Members of the Antarctic Search for Meteorites (ANSMET) hunt for meteorites in areas that have been identified as blue ice fields by satellite and airplane photos. We search for the black rocks on the light blue ice field visually while riding on snowmobiles. When a meteorite is spotted, we photograph and measure it, and record our initial observations. These observations include the amount of fusion crust present, colors of the minerals, and whether any chondrules can be seen. The fusion crust is the glassy outer layer that results from heating when the meteorite comes through Earth's atmosphere. Chondrules are very small (millimeter-size) spheres that formed early in the history of the solar system and are indicative of chondritic meteorites.

meteorite hunters

ANSMET scientists collecting a meteorite from the blue ice fields of Antarctica. Courtesy Dr. B. A. Cohen, University of New Mexico.

After these observations are made, the meteorite is picked up with sterile tongs and placed into a sterile Teflon plastic bag that is sealed for the journey back to Johnson Space Center (JSC) in Houston. The meteorites are kept frozen until they reach JSC, where they are thawed in a pure nitrogen atmosphere so that any ice will sublime directly into a gas. This is done to prevent the ice from melting, since the liquid water would dissolve minerals inside the meteorite and leave salts on the outside as the water evaporates, compromising the signatures from processes that occurred early in the history of the solar system.

Meteorites contain the chemical and textural record of the processes that occurred in the early solar system, such as core formation, alteration by water, collisions, and heating. For example, meteorites have revealed that very early in the history of our solar system some asteroids became hot enough to melt. The metal elements went to the asteroid's center of gravity, forming a molten core just like Earth has. Radioactive elements can be used to date these processes, and they tell us that these asteroids differentiated only a few million years after the formation of the first solids in our solar system.

Another group of meteorites comes from asteroids that never became hot enough to melt. These meteorites, called chondrites, contain the first solids that formed in our solar system, calcium-aluminum-rich inclusions (CAIs). When dated, these CAIs give us the age of our solar system--4.55 billion years. (The oldest rocks on Earth are only 3.9 billion years old.) Chondritic meteorites also tell us about secondary processes such as the melting of water ice into liquid water, which reacted with the original minerals before coming to Earth. Chondritic meteorites still contain some of this water in the form of hydrated minerals, which have water incorporated into their crystal structures.

largest meteorite The largest meteorite discovered in Antarctica is about 2 feet by 2 feet by 1.5 feet. Due to its size it was not able to be thawed in the 100% nitrogen atmosphere and therefore the ice inside melted. The liquid water dissolved minerals inside the meteorite, and when it evaporated, white salts were left on the surface of the meteorite. NASA Lyndon B. Johnson Space Center, Houston, TX.

http://geology.cwru.edu/~ansmet/
http://meteorites.wustl.edu/id/regmaglypts.htm

Lysa Chizmadia, a postdoctoral fellow with the UH NASA Astrobiology Institute, is a geochemist who studies the effect of liquid water on asteroids before they were broken up into meteorites found on Earth.