Academic Reading — Test 19

Reading passage

Of all the places on Earth where fragments of space rock come to rest, none has proved more rewarding to scientists than Antarctica. Meteorites fall more or less evenly across the surface of the planet, yet they are recovered in far greater numbers from the frozen continent than from anywhere else. This is not because more of them land there, but because the Antarctic environment is uniquely suited to preserving and concentrating them. A dark stone resting on a vast white sheet of ice is comparatively easy to spot, and the extreme cold, combined with the dryness of the polar air, slows the chemical weathering that would otherwise corrode such objects within a few thousand years in a warmer, wetter climate. The way in which the ice itself gathers meteorites is equally important. When a stone falls onto the high interior of the continent, it is gradually buried beneath accumulating snow and becomes locked within the slowly moving ice sheet. Over many millennia the ice flows outward towards the coast. Where this flow is obstructed by buried mountain ranges, the ice is forced upwards and is then stripped away by fierce, persistent winds in a process known as ablation. As the surface ice evaporates, the meteorites that were trapped within it are left behind and accumulate at the surface. These regions, called blue-ice areas because of their distinctive colour, act as natural collecting points where specimens from a wide area and a long span of time are brought together. Organised expeditions have systematically searched such zones since the 1970s, recovering tens of thousands of fragments. The scientific value of these objects lies in their age and their composition. The great majority of meteorites are pieces of asteroids that never formed into planets, and many of them have remained essentially unchanged since the Solar System took shape roughly 4.6 billion years ago. Because Earth itself has been continually reshaped by volcanic activity, erosion and the movement of its crustal plates, almost no rock of that great antiquity survives on its surface. Meteorites therefore provide a kind of time capsule, preserving material from the earliest moments of planetary formation that is otherwise inaccessible. By measuring the relative quantities of certain radioactive elements and the products of their decay, researchers can date these fragments with considerable precision. Among the most informative specimens are the so-called primitive meteorites, which contain tiny spherical grains known as chondrules. These grains are thought to have formed when dust in the disc of gas and debris surrounding the young Sun was rapidly heated and then cooled, perhaps within minutes, before clumping together. The chemistry of chondrules and of the fine material that surrounds them offers a direct record of the conditions in that early disc. Some meteorites also contain microscopic mineral grains that pre-date the Solar System entirely, having been forged in older stars and carried into the cloud from which the Sun was born. Such grains allow astronomers to study processes occurring in stars that died long before our own planetary system existed. Antarctic meteorites have also widened the known range of where such material can originate. While most come from the asteroid belt, a small number have been identified as fragments of the Moon, blasted from its surface by ancient impacts and eventually swept up by Earth. Rarer still are meteorites whose trapped gases match the composition of the Martian atmosphere as measured by spacecraft, indicating that they too were thrown into space by collisions and travelled across the Solar System before landing on the ice. These rare visitors are studied intensely, for they are at present the only samples of Mars available to laboratories, and they carry clues about whether that planet once held conditions suitable for life. The cold of Antarctica confers a further, subtler advantage. Because the recovered stones have been kept frozen and dry, often for hundreds of thousands of years, they retain delicate substances that would have been altered or lost in a temperate setting. Certain meteorites are rich in carbon-bearing compounds, including some of the organic molecules that are the building blocks of living things. The survival of these fragile materials makes Antarctic specimens especially valuable to those investigating whether the raw ingredients of life were delivered to the early Earth from space. To preserve this quality, collected meteorites are kept frozen and handled under carefully controlled, sterile conditions so that they are not contaminated by terrestrial material before they can be examined. In this way a continent that supports almost no life of its own has become one of the richest sources of evidence about the origins of the Solar System, and perhaps of life itself.