Academic Reading — Test 154

Reading passage

Among the most striking sights in the Solar System are the broad, flat rings that encircle Saturn. Although the four giant planets each possess a ring system of some kind, those belonging to Jupiter, Uranus and Neptune are faint and difficult to observe, whereas Saturn's are bright enough to be seen through a modest telescope. The rings are not solid sheets, as early observers once assumed, but vast swarms of separate particles, ranging in size from specks of dust to lumps several metres across. The overwhelming majority of this material is water ice, which gives the rings their characteristic brightness, with only a small proportion of rocky or dusty matter mixed in. Each particle travels along its own orbit, circling the planet in obedience to the same gravitational laws that govern the moons and the planet itself. The question of how such rings come into being has occupied astronomers for well over a century. One long-standing explanation holds that a ring forms when a moon or a passing comet strays too close to its parent planet and is torn apart by tidal forces. Within a certain distance of a large body, known as the Roche limit, the difference between the gravitational pull on the near side of an object and that on its far side becomes so great that the object cannot hold itself together, and it disintegrates into fragments. These fragments then spread out into a thin disc. A competing idea proposes that the rings are instead primordial, consisting of debris left over from the cloud of gas and ice out of which Saturn and its moons originally condensed, material that never managed to gather into a moon in the first place. The two explanations are not necessarily in conflict, and it is possible that different rings, or different parts of a single ring, owe their origin to different processes. A persistent puzzle concerns the apparent youth of Saturn's rings. Spacecraft measurements suggest that the ring material is remarkably clean, showing little of the dark contamination that would be expected to accumulate as interplanetary dust rained down upon it over billions of years. This relative purity has led some researchers to argue that the rings are far younger than the planet, perhaps only a few hundred million years old, and that they may even be a temporary feature that will eventually disperse. Other scientists remain unconvinced, pointing out that the rate at which material is gained and lost is poorly understood, and that a young age is difficult to reconcile with the absence of any obvious recent catastrophe capable of producing so much ice. Left to themselves, the particles in a ring would tend to spread outwards and inwards over time, gradually smearing the sharp edges that telescopes and spacecraft actually observe. That such crisp boundaries persist points to the influence of small moons embedded within or beside the rings. These bodies, often only a few tens of kilometres in diameter, are known as shepherd moons because they appear to herd the ring particles and keep them confined. A shepherd moon orbiting just outside a ring travels slightly more slowly than the particles at the ring's outer edge; its gravity tugs on those particles and nudges them inwards. A second moon orbiting just inside the ring moves slightly faster and pushes the inner particles outwards. Acting together, a pair of such moons can hold a narrow ring in place between them, confining material that would otherwise drift away. The clearest example of this mechanism is found not at Saturn itself but at Uranus, where the narrow Epsilon ring is bracketed by two small moons, Cordelia and Ophelia, which together maintain its sharp edges. At Saturn, the slender F ring lies near two moons, Prometheus and Pandora, whose gravitational influence helps to shape its strands, although the behaviour of this ring is more complicated and less fully understood than the simple shepherding picture might suggest. Other moons leave their mark in different ways. The moon Pan, for instance, orbits within a gap in the main rings and sweeps that gap clear of particles, much as a snowplough clears a road. Resonances, in which a ring particle completes a whole number of orbits in the time a moon completes another whole number, can also carve out gaps and sharpen edges at specific distances from the planet. The study of Saturn's rings therefore offers more than a catalogue of a beautiful object. Because the same physical principles that govern rings also operated in the disc of material from which the planets themselves formed, the rings serve as a natural laboratory in which processes that once shaped the entire Solar System can still be watched at work today.