Academic Reading — Test 117

Reading passage

Every year, fragments of rock and metal survive the fiery passage through the atmosphere and land on the surface of the Earth. These survivors, known as meteorites, are among the oldest solid objects available for study, and many of them formed at roughly the same time as the planets themselves, some 4.6 billion years ago. Because most meteorites originate in the asteroid belt that lies between the orbits of Mars and Jupiter, they offer astronomers a rare opportunity to examine, in the laboratory, material that would otherwise remain hundreds of millions of kilometres away. The challenge for scientists is twofold: first, to organise this varied material into a coherent classification, and second, to work out which body in the solar system each specimen came from. The most fundamental division separates meteorites into three broad groups according to their composition. Stony meteorites, which are by far the most common, consist largely of silicate minerals and resemble ordinary terrestrial rock to the untrained eye. Iron meteorites are made chiefly of an alloy of iron and nickel, and they are unusually dense and heavy for their size. The third group, the stony-irons, contains roughly equal proportions of metal and silicate and is comparatively rare. This simple three-part scheme, although useful as a starting point, conceals a far richer structure, because each group is subdivided into numerous classes and subclasses based on finer chemical and mineralogical differences. A more revealing distinction is the one drawn between chondrites and achondrites. Chondrites take their name from the chondrules they contain: tiny, roughly spherical beads of once-molten material that solidified in the early solar system before the planets had assembled. Crucially, chondrites have never been melted as whole bodies, so their overall chemical make-up has remained close to that of the original solar nebula. This makes them invaluable as a chemical baseline against which other materials can be measured. Achondrites, by contrast, come from bodies large enough to have undergone melting and differentiation, a process in which heavier elements such as iron sink towards the centre while lighter minerals rise. As a result, achondrites lack chondrules altogether and have a texture that records this later, more violent history. To assign a specimen to its correct class, researchers rely on a battery of laboratory techniques rather than appearance alone. The proportions of different oxygen isotopes are especially informative, because material from a single parent body tends to share a distinctive isotopic signature that is preserved regardless of subsequent heating. Mineral composition, the abundance of trace elements, and the degree to which a sample has been altered by heat or water are all measured and compared. Together these measurements allow a meteorite to be placed within the established framework with considerable confidence, even when two specimens look superficially identical. Classifying a meteorite is only half the task; linking it to a specific parent asteroid is considerably harder. One approach compares the spectrum of sunlight reflected from an asteroid's surface with the spectrum produced in the laboratory by a meteorite. When the two patterns match, it suggests, though it does not prove, that the asteroid and the meteorite are made of the same material. By this method, the large asteroid Vesta has been connected to a whole family of achondrites, a conclusion later supported by a spacecraft that orbited Vesta and studied its surface directly. A second approach depends on tracking the path of a meteorite through the sky before it lands. Networks of automatic cameras now photograph bright fireballs from several locations at once, and from these images the original orbit can be calculated by working backwards. In a handful of celebrated cases, scientists have both recovered the fallen stone and reconstructed the orbit it followed, allowing them to point to the region of the asteroid belt from which it most probably came. Despite these advances, the great majority of meteorites in the world's collections cannot be matched to a named parent body. The asteroid belt contains an enormous number of objects, many of them too small or too faint to have been catalogued, and collisions between them over billions of years have thoroughly mixed and scattered their fragments. Nevertheless, each meteorite that is successfully traced adds a fixed point to an expanding map of the solar system's history. By combining careful classification with spectral comparison and orbital reconstruction, astronomers are gradually transforming a drawer full of anonymous stones into a detailed record of how the planets and their smaller companions were built.