Canadian Rockhound: Meteorites from Antarctica

The Search for Shooting Stars:
Hunting for Meteorites on the
Antarctic Plateau
By Rick Hudson

– Meteorites are samples from parts of the Solar System astronauts may never be able to visit, or that would cost a great deal to explore. They have been dubbed 'the poor man's space probe'. – 'Meteorites' by Robert Hutchison and Andrew Graham, Sterling Publishing, NY.

The wind is gusting at 40 knots from the north-west, the air temperature is a bracing -20 centigrade, the sky is clear and sunny. In a land devoid of shape, or form, or colour, there is nothing to see in any direction. Antarctica is truly the last place on Earth: remote, aloof, yielding its secrets only after the most back-breaking effort.

Yet today this great continent is witnessing a remarkable race, ever since a Japanese scientist picked up 9 rocks from the ice's surface near the Yamato Mountains in 1969. Months later, startled researchers realized what he had found - a collection of rare meteorites.

In 1975, a Japanese team visited the same area again, and this time returned at the end of the summer program with a staggering 663 samples! The international race was on, to collect, classify and store these important finds. The USA quickly formed the Antarctic Search for Meteorites group (ANSMET). During the following 20 years, over 17,000 meteorites have been found, tripling the known number available to science.

The Source

What's happening here? To understand the process, we need to go back in time, a long way back. Asteroids are the building blocks of our early solar system, orbiting the sun between Mars and Jupiter. A fragment, or meteoroid, occasionally escapes the asteroid belt, and wanders into an Earth orbit. Some time later, these fragments of space flotsam enter our atmosphere as meteorites or "shooting stars". If observed and measured upon entry, they are called falls; if discovered later, they are finds. Most are finds. After a particularly famous fireball was photographed (by accident) in 1959, both Canada and the USA established a Camera Network to scan the skies and record trajectories. Only when a meteorite's trajectory is precisely known can its origin be calculated. To date, each country has successfully photographed only a single meteorite. In both cases, the source was found to be the asteroid belt.

The largest meteorite found to date is the 65 ton Hoba meteorite in Namibia, south-west Africa (which may have been as large as 120 tons before weathering), but many are tiny by comparison. Small or large, together they form a rain of dust which results in an amazing 10,000-30,000 tons of debris raining down on us every year. (Time to wear a hard hat, when leaving the house!) While most fall into the ocean (the oceans cover 72% of the Earth), others come down on mountains, deserts and forests, where the chances of recovery are slim indeed. The Canadian Camera Network estimates over 26,000 fragments over 100 grams (about 3.5 oz) arrive each year. Since the late 1970s, specially equipped NASA aircraft have been flying at over 60,000 feet, where sticky panels on the wings trap micrometeorites and other space dust for subsequent analysis.

Why Antarctica?

Why, then, are so many meteorites being found in Antarctica? No more are falling there than elsewhere. In fact, quite the opposite. Most meteors, comets and asteroids orbit in the same plane as the planets, and rotate about the sun in the same direction as the planets. A a result, they tend to converge with the Earth, rather than slamming head-first into us on diametrically opposing paths. Further, the focussing effect of the Earth's gravitational field tends to draw more meteorites in towards the equatorial regions, with the result that there is a slight preference for falls to occur away from the polar regions.

But Antarctica is unique because it is encased in glacial ice. This frozen cover moves steadily outwards toward the coast, carrying with it anything that has been deposited on it. Remember, of course, that meteorites, being more dense than ice, will slowly sink into the glaciers. But as these rivers of ice approach the coast mountains, they start to be thrust upwards by the underlieing slopes. The ice begins to sublimate (change from a solid to a gas state, without going through a liquid phase), aided by fierce katabatic winds that roll down off the high ground (over 2000m on the polar plateau). This process scours the ice away at a rate of about 5cm per year, and slowly, gently, the meteorites are exposed. These zones are known as 'stranding surfaces'.

Such a process serves to concentrate the deposits. By plotting the positions of the meteorites when found, and determining the age of the associated ice, scientists can calculate how long a stone has been buried, and hence when it entered our atmosphere.

A further plus to this deep-freeze process is that the meteorites are kept, quite literally, in cold storage, so they are often in excellent condition when found, with almost no corrosion, oxidation or physical damage done to them after their arrival. The same cannot be said, obviously, for material coming down in deserts or jungles.

The Search

How do scientists find them? Each year, small, mobile groups are dropped off in areas which have been determined to be good sites. Remember, they are looking for zones where there is bare ice, just upstream of mountains or nunataks, which force the sea-bound glaciers to rise and dissolve. The team usually searches in a grid pattern, on snowmobiles, often just 25 metres apart. Every rock on the surface must be examined, and a trained eye quickly tells whether the object is a shooting star or of local origin.

When a meteor is found, its exact position is noted using a Global Positioning System (satellite positioning). It is photographed, and placed in a specially decontaminated bag, for analysis later. Because of this low pollution process, some important discoveries have been made. In 1980 the ANSMET team returned from the Elephant Moraine area near McMurdo Base with what turned out to be a very exciting sample. EETA79001 was an achondrite (stony meteorite) with abundant melted rock on its exterior. Inside, chemists later discovered traces of noble gases that were identical to those measured by the Viking Lander in the Martian atmosphere.

Another meteorite, found in a crevassed glacier field while scientists were taking a break from meteorite hunting, turned out some years later when it was thawed (all samples are kept frozen back at the Johnson Space Center in Houston, TX) to contain feldspar glass and oxygen isotopes which confirmed its Martian origin. More importantly, there appeared to be extremely tiny structures resembling bacteria. Who knows? The debate will continue for years, but there are exciting possibilities ahead.

How many meteorites are found these days? Typically, a group can work in the field for about 7 weeks during the so-called 'summer' (Dec/Jan). Bad weather (high winds or low cloud) will take up as much as 10 days of that precious time. A team tries to cover about 150 square kilometers per season. A typical season yields 100-200 finds; the best single day was close to 40.

Studying Them

The study of meteorites is called meteoritics, and there are three broad types: stony, stony-iron, and iron. The former are the most common, and are divided into chondrites and achondrites. While meteorites heat up tremendously as they enter the Earth's atmosphere, their cores remain supercooled to near absolute zero, so only a narrow 1-2 mm surface usually melts, often forming a fusion crust of black glass. Other common features are the surface dimples, pits and flow features.

By studying these specimens, that have been 'on ice' both in space and the Antarctic plateau, we are able to see back in time to view primitive matter from early solar nebulae, or from a pre-planetary era, or more recently from our own solar system. To find meteorites, therefore, is to travel back in Einsteinian time to our very distant past and in so doing, gain a glimpse, however fuzzy, of our future.

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