杏吧原创

EAGER to begin the search, John Schutt jumps from the helicopter while the
blades are still turning, while the pilot Barry James and I are still struggling
with our seat belts. Schutt has dubbed this escapade 鈥渁 big Easter egg hunt鈥�,
but what we鈥檙e really looking for are rocks from outer space. Here, up on the
windy Antarctic plateau, is the best place on Earth to find them. 鈥淭here are
meteorites around here,鈥� Schutt declares. 鈥淚 can smell them!鈥�

We have landed on the edge of the East Antarctic Ice Sheet鈥攁 vast cap
of ice thousands of kilometres across and thousands of metres deep. Off in the
distance are the Allan Hills, molehills of dark brown rock that are really the
uppermost peaks of huge mountains up to their necks in ice. Around us, scattered
stones and rocks stand out against the bluish white. Most are Earth rocks, blown
from the hills. But some could have fallen from the sky.

鈥淗ere, look at these,鈥� says Schutt, and we gather expectantly. In his hand
are two small stones. One is pale with very fine crystals, the other darker,
packed with large ones. 鈥淭he meteorites we鈥檙e looking for won鈥檛 look anything
like these,鈥� he says, and tosses them over his shoulder.

He鈥檚 right. On the way here, over the crackly helicopter headsets, Schutt
told us what to expect. Our best chance, he said, is to find the most common
kind of meteorite, an 鈥渙rdinary chondrite鈥�. These have medium-sized crystals, in
between those we鈥檝e just seen in the terrestrial stones. What鈥檚 more, they
contain tiny white spheres called chondrules. No one knows the origin of
chondrules, but they probably started out as globs of molten rock in the cloud
of dust and gas that gave birth to the Solar System.

Four a half billion years ago, planets and asteroids, large and small, formed
by sweeping up debris from this cloud. The constituents of the more massive
bodies鈥� planets and larger asteroids鈥攈eated up and separated. Iron
moved towards the core, leaving behind a rocky outer mantle. But the smaller
bodies remained pristine. Plenty of asteroids still circle in the belt between
Mars and Jupiter, and all it takes is a chance collision for a chip off one of
these ancient blocks to come heading our way.

It鈥檚 the smaller, pristine asteroids that are the most plentiful, and these
give us the ordinary chondrites. More rarely, you can find achondrites, which
contain no chondrules and come from the larger bodies鈥攖he ones that have
been remodelled by heat. Some are from the biggest asteroids, others from the
Moon. And occasionally, mysterious rocks鈥攃alled shergottites, nakhlites
and chassignites鈥攈ave turned out to be messengers from Mars, the first
pieces of the Red Planet that we could hold in our hands.

There are still stranger meteorites, such as the irons and stony irons. These
come from large asteroids that have been transformed into rocky mantles and iron
cores. The mantles have then been blasted away by collisions in space, leaving
behind a lump of metal. Finding a chunk of iron hacked from one of these could
help us to understand how our own planet separated into core, mantle and
crust.

We鈥檇 all like to find one of these rarer beasts, but we鈥檇 be happy to find
any meteorite. Even an ordinary chondrite can yield new insights into how the
Solar System formed. With this in mind, the three of us start casting about
among the stones. Suddenly Schutt drops full length onto the ice, lifting his
glasses to peer intently at a small pebble. James and I hold our breath. 鈥淣ope,鈥�
says Schutt. 鈥淭hat鈥檚 a leavitrite.鈥� He stands up and dusts himself off. 鈥淟eave
it right there?鈥� James hazards. 鈥淵ou got it,鈥� Schutt replies briskly, and sets
off again.

Schutt is a meteorite hunter extraordinaire. His cold-weather gear marks him
out as a seasoned campaigner鈥攁 patchwork of glorious technicolour that
puts my standard-issue black windpants and red parka to shame. This is the 17th
year that he has come to Antarctica as a hired hand to find samples for the
US-funded Antarctic Search for Meteorites, a programme now run by meteorite
researcher Ralph Harvey from Case Western Reserve University in Cleveland, Ohio.
In the 22 years since it began, ANSMET people have found more than 8000
meteorites. Add these to the ones found over the same period by people in
Japanese and European programmes and the Antarctic total is around 16 000. Even
assuming that many of these are fragments of the same rocks, that鈥檚 more
meteorites in two decades than have been found in the rest of the world in the
past two centuries.

One obvious reason why the pickings are so rich in Antarctica is that there
are no trees, plants, roads or soil to obscure the view. On the ice, everything
stands out. And the meteorites that land there can stay deep-frozen and
unchanged for hundreds of thousands, even millions of years (unlike, say,
London, where they would disintegrate in just a few decades). But there鈥檚
another, more important reason, figured out largely by Bill Cassidy, a meteorite
researcher from the University of Pittsburgh in Pennsylvania and a former leader
of ANSMET.

Antarctic ice has an in-built mechanism for concentrating meteorites. Over
thousands of years meteorites fall here, are buried by snow and then
concentrated as they are compressed together hundreds of metres below the
surface, where the fluffy snow turns to dense ice. Next, ice from different
regions sweeps together as it flows to the outer edges of the ice sheet,
concentrating the meteorites further. Around most of the sheet, the snow, ice
and meteorites fall straight into the sea. But in some places鈥攈ere for
instance鈥攖he ice runs against a mountain range, and is forced to the
surface in tell-tale blue patches (see Diagram). Strong winds
gradually ablate the ice to leave its harvest of meteorites ready to be
gathered.

That鈥檚 why we鈥檙e standing on ice right now, instead of snow. And that鈥檚 why
meteorite hunters seek out ice fields, pale blue against the white. Right across
the continent they lie, butting up against the Transantarctic Mountains. Some
are formed when the ice rears up over buried hills: anything you find there must
have fallen from the sky. But even here, where the hilltops are exposed, there
are so few rocks that it鈥檚 not too arduous to pick through them for aliens.

Today鈥檚 search is unusual: we have come just for the day to an old site and
are simply scouting around. More typically, a team of about six people arrives
at a fresh site ready to pitch camp for six weeks or more. Then, if the weather
cooperates, the team members arrange themselves in formation on skidoos, and
carefully sweep across the area, scanning for signs of meteorites. 鈥淚t鈥檚 a bit
like the Charge of the Light Brigade,鈥� says Schutt.

Though Harvey and Schutt are always on the team, the rest are usually
volunteers. But if you fancy applying, be warned鈥攊t鈥檚 no picnic. Today the
weather is extraordinary, a balmy 0 掳C with just a slight breeze and bright
sunlight. Usually at this time of year the temperature plunges to 鈥�30
掳C or less, and the wind slices through you, chilling you to the bone. 鈥淚t
can be brutal,鈥� says Schutt. This point is stressed on ANSMET鈥檚 website. 鈥淭hink
about it for a minute,鈥� writes Harvey. 鈥淒o you really want to freeze your rear
end off, living in a tent for 45 days?鈥� Even if you 鈥渇ail this intelligence
test鈥�, he adds, you still have virtually no chance of being accepted unless you
are a graduate student or researching into meteorites.

The search teams are very efficient and the site we鈥檙e now in鈥攖he main
ice field of the Allan Hills鈥攈as been picked over many times
(see Map). It
has already yielded a Martian find as well as countless less exciting ones. We
all know that we are highly unlikely to find anything that the previous teams
missed. But the wind has dropped to almost nothing, the sunlight is catching the
crests of ripples in the ice left by the wind, and we have high hopes.

We鈥檙e coming to the end of our assigned ground time and James and I are still
peering at rocks, with Schutt a tiny multicoloured figure in the distance.
Suddenly we see him beckoning. 鈥淐ome over here,鈥� his voice floats over the ice.
鈥淚鈥檝e found one!鈥� We run to meet him, skidding and sliding on the glassy
surface. 鈥淭here it is,鈥� he beams, gesturing at a small brown stone, 鈥渢he oldest
rock you鈥檒l ever see.鈥�

About 3 centimetres in diameter, our meteorite is an ordinary chondrite,
typical in every way. One end has been sheared off, revealing medium-sized
crystals and a few small white chondrules. The unbroken side is smooth and
rounded, looking for all the world like a pebble in a stream. This shape and its
dark brown matt covering鈥攐r 鈥渇usion crust鈥濃�攊s evidence of the
searing heat the object encountered as it tore through the Earth鈥檚 atmosphere.
Staring at this extraterrestrial visitor, seen by humans for the first time, we
are all three awestruck.

Because this is not an official collection trip, we can鈥檛 take the meteorite
with us. Instead we use an ice drill to erect a marker pole. Next season,
someone will return, armed with a sterile Teflon bag, and our find will be
sent鈥攕till frozen鈥攖o the Johnson Space Center in Houston. Once there
it will be dried, examined to determine its class and stored in a cleanroom.
Twice a year, descriptions of all new samples are circulated in the
Antarctic Meteorite Newsletter, and researchers from anywhere in the world
can ask to study them.

Now our blood is up and we spread out, scrambling to scan the remaining
rocks. But our enthusiasm is short-lived. We don鈥檛 want to risk overstaying our
allotted ground time, which could trigger a rescue mission from the Search and
Rescue team back at the main American base at McMurdo. Eventually, reluctantly,
we traipse back to the helicopter. Still, looking out as the rotors begin to
turn again, we can see the red and white marker standing out against the ice, a
testament to our success.

Back at McMurdo, Schutt shows me pictures of the famous meteorites found by
the ANSMET teams. He flips over pages in a folder and I see a rock studded with
large lumps of iron鈥攁 remnant of an asteroid鈥檚 core. Another is shot
through with creamy white fragments of broken rock. This was the first lunar
meteorite ever reported, and bears an uncanny resemblance to the samples brought
back by the Apollo astronauts. And there on the next page is a picture of
ALH84001, the most famous meteorite in the world. That鈥檚 the one that caused all
the furore鈥攖he one that bears possible signs of Martian life. Schutt
remembers the day the team found it, at an ice field in the Allan Hills just 50
kilometres from where we searched. Finds like that, he says, keep him in the
game. 鈥淚 had a thrill finding that meteorite out there today, even though it鈥檚
possibly my two-thousandth one,鈥� he says. 鈥淵ou never know what it will turn out
to be.鈥�