Marvels and Mysteries of the World Around Us [antikvár]

Chapter One EARTH'S ANCIENT DRAMA Though details of the early days of our planet are obscured by time, modern science has revealed how the Earth was created, how old it is and how life may have started The Earth's Beginning By Gerald S. Hawkins In the beginning. A scientist cannot continue this sentence with absolute certainty. It would be like asking a child to give an account of his birth or a description of his conception. Religious scriptures explain the creation of the Earth in compelling ways, but no two accounts agree exactly. Some of them, however, do come quite close to the scientist's idea of creation—or, at least, to his reading of the evidence lodged in the Earth's ancient rocks. In exploring the origin of the Earth we must at the same time try to explain the beginning of the Solar System, for the Earth's past is intimately tied to the history of our nearest neighbors in space. In 1755 the German philosopher Immanuel Kant published his theory of the heavens, postulating that in the beginning there was an immense, cold whirling cloud of dust and gas. This suggestion is accepted readily by astronomers today. Their extremely powerful modern telescopes show remote, dark clouds of dust floating between distant stars —clouds that must even now be similar to the local, swirling cloud that Kant had in mind. In 1796 Kant's contemporary, the French mathematician Pierre Simon Laplace, took his idea a step further by suggesting how the Solar System might have formed from such a cloud. The immense mass was set spinning by cosmic forces, Laplace hypothesized. At the same time it began to shrink in size under the gravitational pull of its own matter. At intervals, the contracting cloud shed veils of particles into space, which eventually condensed into the planets. Shrinking under the force of its own gravity, meanwhile, the central mass became the Sun. As potent as Laplace's concept was, it fell victim to fundamental physical laws of more recent discovery. Calculations based on these laws show that a shrinking Sun would spin faster and faster as it grew smaller and smaller, until today it would be rotating at a far greater speed than it actually is. After Laplace's brilliantly imaginative picture was shown to contain flaws, several other seemingly plausible suggestions were put forward by astronomers. One theory assumed the formation of the Sun first, with no planets. Then, a second star passing close by in space tore out a long stream of material. The planets, it was suggested, might then have condensed around the Sun, with the passing star continuing on its way. Unfortunately, calculations show that such hot material from the Sun would disperse, rather than form planets. Even if by some unknown process planets were to condense, their orbits would be much more irregular than those found in the Solar System today. Another theory held that in the distant past of the cosmos, or universe, the Sun had a twin companion, and a passing star collided with its twin. Out of the debris resulting from such a collision, planets might possibly form in orbits around the single remaining sun. But the great distances at which the stars are scattered in space make collisions of this type most unlikely. If such a catastrophe did occur, it seems impossible that planets could form directly from the intensely hot and volatile material of the exploding stars. Both the "close encounter" theory and the "collision" theory fail on one further count; neither explains how most of the planets have obtained moons. Today, cosmologists have gone back to the suggestion of Kant, careful to avoid the pitfall of Laplace. A With the Milky Way as a backdrop, the Solar System is born from a spinning cloud of gas and dust. While a young Sun glows yellow at the center, the dust-veiled planets are settling into their present orbits. The Earth is third from the Sun. 10