Matter | Pages 88-89 | See Linked Version | |||||||||
arise. Models of solar-system formation suggest that dense Earth-like planets probably form in the warm environment close to the parent star. Gas giants, whose elements are more prone to being boiled off by a hot young star, prefer a cooler setting farther away. Recent discoveries of Jupiter-sized planets orbiting close to other stars may conflict with that model. Alternatively, it could mean that over time giant planets can migrate inward toward their parent stars. When a planet coalesces from the debris around a star, it starts as a thorough mixture of the matter in that part of the disk. But the densest matter quickly sinks to the center of the new planet, a process similar to pulp settling to the bottom of a glass of freshly squeezed orange juice. In Earth's case those materials were iron and nickel. This segregation of dense elements released a tremendous amount of heat, keeping Earth molten for much of its early history. The less dense elements, such as oxygen, stayed near the surface. They eventually formed the floating slabs of rocky crust on which we live. As evidence of that process, Earth's solid surface is nearly half oxygen by weight. Another source of heat within Earth remains important today: radioactivity. For quantum mechanical reasons, most combinations and sums of protons and neutrons in an atomic nucleus are unstable. Atoms containing such combinations will eject a helium nucleus of two protons and two neutrons (an "alpha particle") or change a neutron into a proton and an electron (a "beta particle"). These decays are mediated by the weak nuclear force. Over time they transmute a radioactive substance into other elements, atom by atom. For example, radioactive uranium decays to lead, a stable element, via a chain of other radioactive elements, including thorium, radium, and radon. If dangerous levels of radon gas build up in your basement, you know that this process is happening in the soil and rocks around your home. There's also a clear benefit from radioactivity: Without it, Earth's interior would be much colder. The planet would be less active and, perhaps, less hospitable to life. We cannot predict when a particular radioactive atom will decay. Rather, we use statistical averages to calculate overall rates of decay for large numbers of atoms. The time it takes half of any given amount of an element to decay is called its half-life. After two half-lives have passed, one-quarter of the original element is left. After 10 half-lives, only a thousandth remains. These intervals of time can range from fractions of a second to billions of years, depending on the element. | |||||||||
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