Academic Reading — Test 72

Reading passage

Each year, fragments of rock and metal fall to the Earth from space. Most burn up in the atmosphere or land in oceans and remote regions where they are never found. Antarctica, however, has proved to be an unusually productive hunting ground for these visitors. The continent's vast ice sheet, combined with its cold and dry climate, preserves fallen meteorites for long periods and concentrates them in places where searchers can recover them in large numbers. Since organised collecting began in the 1970s, tens of thousands of specimens have been retrieved from the Antarctic interior, transforming our understanding of the materials that make up the Solar System. The reason Antarctica yields so many meteorites lies in the behaviour of the ice itself. When a meteorite lands on the high inland plateau, it is gradually buried by accumulating snow and becomes locked within the slowly moving ice. Over thousands of years this ice flows outward towards the coast. Where the flow meets an obstacle such as a buried mountain range, the ice is forced upward. In these zones, fierce dry winds strip away the surface layers through a process of evaporation known as sublimation, in which ice turns directly into vapour without first melting. As the ice is removed, the meteorites it carried are left exposed on the surface. Areas where this happens are called blue ice fields, named for the colour of the dense, ancient ice revealed there. A single blue ice field may expose specimens that fell across an enormous span of time. Once collected, meteorites are sorted into broad categories according to their composition. The most common group, the stony meteorites, are made largely of silicate minerals and resemble ordinary terrestrial rock, although they often contain small amounts of metal. A second group, the iron meteorites, consists chiefly of an alloy of iron and nickel; these are dense, heavy and were once part of the molten cores of larger bodies that have since broken apart. The third and rarest group, the stony-iron meteorites, contains roughly equal proportions of silicate and metal. Within the stony group, scientists draw a further and important distinction between chondrites and achondrites. Chondrites are considered especially valuable because they have changed little since the Solar System formed, and so preserve a record of its earliest chemistry. Achondrites, by contrast, come from bodies that were once heated enough to melt and separate into layers. The origins of these objects can often be traced with surprising confidence. The majority are thought to be debris from the asteroid belt, the region of rocky bodies lying between the orbits of Mars and Jupiter. Collisions between asteroids over billions of years have chipped off countless fragments, some of which eventually cross the path of the Earth. A small but scientifically precious minority, however, did not come from asteroids at all. By comparing the chemistry of certain unusual specimens with measurements made by spacecraft and landers, researchers have established that some meteorites originated on the Moon, and others on Mars. These were blasted off the surface of their parent worlds by the impact of larger objects, drifted through space, and were finally captured by the gravity of the Earth. Such samples are extraordinarily useful, because they allow scientists to study the geology of other worlds without the expense of sending a mission to retrieve material directly. Antarctic meteorites carry a further advantage beyond their sheer numbers. Because they fall onto clean ice rather than onto soil, and because the polar climate is so cold and dry, they are far less contaminated than specimens found elsewhere. A meteorite lying in a temperate field is quickly invaded by water, plant roots, bacteria and minerals from the surrounding ground, all of which alter its chemistry. On the Antarctic plateau these processes are greatly slowed, so the recovered rock remains closer to its original state. For this reason, fragile compounds, including some organic molecules, can survive in Antarctic specimens that would have been destroyed in a warmer setting. To protect this purity, collectors handle the rocks with sterile equipment and keep them frozen during transport to the laboratory. The study of Antarctic meteorites continues to reshape planetary science. Because the recovered material spans such a wide range of types and ages, it offers a kind of natural archive of the Solar System, assembled at very little cost compared with a space mission. Researchers use these specimens to estimate the age of the Solar System, to investigate how planets formed and differentiated, and even to search for clues about the chemical ingredients from which life may have arisen. The frozen continent, hostile to almost all life, has thus become one of the most important laboratories for understanding the history of the worlds beyond our own.