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Meteorites, Ice, and Antarctica: a Personal Account

Natural History, Sept, 2003 by Laurence A. Marschall

by William A. Cassidy Cambridge University Press, 2003; $30.00

Like a cruising car on a buggy summer night, the Earth, as it orbits the Sun, continually collides with small flying objects. The interplanetary debris is made up of fragments of disintegrated comets and shards of shattered asteroids, ranging from gnat-size specks of dust to house-size rocks and larger. Meteor watchers see the objects fleetingly as they enter the atmosphere, heating the air around them to incandescence before, in most cases, they are reduced to airborne ash. Out of an estimated hundred tons of meteoroids that collide daily with the Earth, only a minuscule fraction are large enough to make it to the ground before they burn up entirely. And of those, most fall unseen into the oceans or bury themselves in the ground, where they rapidly weather and become indistinguishable from terrestrial rocks.

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Meteorites that happen to fall on the Antarctic ice sheet, however, meet a different fate. Buried by annual snowfalls after impact, they eventually become embedded in the two-mile-thick frozen mantle that overlies the continent. The ice both protects them from weathering and concentrates them in certain areas. As the ice flows at, let's say, glacial speeds downhill to the sea, meteorites are carried along with the flow. When the ice meets barriers such as mountain ranges along the way, the meteorite flotsam it carries is cast on the foothills like so many piles of driftwood on a beach, readily distinguished against the background of windswept snow. In such places, geologists have discovered, you can stretch out your hand in almost any direction and pick up a rock from another world.

William Cassidy, emeritus professor of geology and planetary science at the University of Pittsburgh, was the founder, in 1976, of the U.S. Antarctic Search for Meteorites (ANSMET), a pioneering effort to mine Antarctica's extraterrestrial bonanza. In nearly three decades of bone-chilling expeditions, Cassidy's team, along with similar groups from Japan and Europe, have collected more than 30,000 meteorites from the ice fields of Antarctica, more than fifteen times the number that had turned up elsewhere in the preceding two centuries.

For the casual reader, Cassidy provides an exciting picture of what it's like to be a meteorite hunter on the world's cruelest continent--from tales about surviving in hurricane-force winds that tear tents to shreds, to wry ruminations about answering the call of nature when the outside temperatures are cold enough to freeze your spit before it hits the ground.

But Cassidy's book is also full of authoritative science. Nearly all Antarctic meteorites, he notes, are splinters ejected from collisions between asteroids--those miniature worlds that have accumulated mostly between the orbits of Mars and Jupiter. Because asteroids are thought to be remnants of the material that formed the planets some 4.6 billion years ago, Antarctic specimens may carry important clues about how the solar system formed. The new wealth of data, however, has raised as many questions as it answers: the great variety of rock types among meteorites, for instance, indicates that the early solar system was not as uniform as astronomers once thought, either in temperature or in chemical composition.

The most remarkable finds on the ice sheet are a handful of meteorites that appear to be chunks from other large bodies in the solar system. In most cases they are rocks chipped off the Moon. Amazingly, though, some of them come from Mars [see "Bolts from Beyond," by Donald Goldsmith, page 28]. Cassidy describes in convincing detail how geologists can infer the planetary origins of such specimens (and, in some cases, even the lunar or Martian crater from which they were once ejected). And he gives a brief account of the most famous, and most controversial, meteorite of all, ALH 84001. According to some geologists, ALH 84001 contains evidence of fossil life on Mars.

The controversy comes as no surprise: the study of meteorites preserved in ice fields is still a young science. So many specimens have accumulated so rapidly that analysis has yet to catch up with the available evidence. Yet Cassidy makes an excellent case for continuing the hunt for evidence. If a research enterprise can be measured by the excitement and beauty of its fieldwork, by the unique value of its data, and by the insights it yields into "big questions"--What are we made of? Where did we come from?--the study of Antarctic meteorites will remain a hot topic for many decades to come.

Meteorites, Ice, and Antarctica: a Personal Account

Natural History, Sept, 2003 by Laurence A. Marschall

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