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【分享】Howdoscientistsknowthecomposition of Earth's interior?【已搜无重复】
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How do scientists know the composition of Earth's interior? —J. Gerber Arthur Lerner-Lam, associate director for seismology, geology and tectonophysics at the Lamont-Doherty Earth Observatory at Columbia University, explains: Because we cannot extract samples directly from the deep Earth, we must deduce its composition. Scientists look at the clues hidden in rocks that are igneous (volcanic) or metamorphic (those that have changed after exposure to high pressures and temperatures underground). They also examine proxies for composition and structure, such as the three-dimensional variation of the velocity of seismic waves produced by earthquakes, which are sampled by networks of seismometers on the surface of the planet. The late Francis Birch, the eminent Harvard geophysicist, and his colleagues and students worked out the basic methodology that brings these distinct observations together. Birch showed how the stiffness of rocks changes under the extreme conditions of pressure and temperature deep within planets. Because the speed of seismic waves depends on the stiffness of the medium through which they propagate, it is possible to calculate temperature and composition from maps of seismic velocity. Most current research is based on Birch's work, and it has even been extended to the most extreme temperature and pressure conditions of Earth's core. For example, much of our understanding of the large- and small-scale convection patterns driving plate tectonics has come about by using Birch-type proxies for temperature and composition. How can we improve our knowledge of the other planets? Manned and unmanned missions to the moon and Mars deployed seismometers, which provided tantalizing, but ultimately limited, information before they stopped operating (although the Spirit and Opportunity rovers continue to transmit chemical analyses and pictures of the Red Planet back to Earth). Almost all planetary landing missions now in the design stage include seismological instrumentation, and some even plan to return rock samples to Earth. We hope the best geoscience is yet to come. How does decanting red wine affect its taste? And why not decant white? —J. Eastley, Pittsburgh, Pa. Andrew L. Waterhouse, a professor in the department of viticulture and enology at the University of California at Davis, provides an answer: Decanting—simply pouring wine into another container—can mellow the flavor of harsh younger red wines by exposing them to oxygen; it also serves to remove sediments in older vintages. White wines, which are aged for shorter periods, do not benefit from decanting. Some young red wines—between three and 10 years old—can be astringent if consumed directly after opening the bottle because they are maintained in an environment relatively free of oxygen during aging. Over time, these conditions result in a “closed” character for the wine that comes from the accumulation of particular compounds. A wine's aroma changes during the first 10 to 30 minutes after opening. Decanting accelerates this so-called breathing process, which increases the wine's aroma from natural fruit and oak, by allowing a few volatile substances to evaporate. Decanting also appears to “soften” the taste of the tannins that cause harshness and astringency, although chemists have not detected noticeable changes to the tannins. Less dramatic changes result from just uncorking the bottle 15 to 60 minutes before pouring. In older wines, decanting also serves to remove sediments, which are harmless but make wine look cloudy and taste gritty. Unlike younger wines, older wines should be served immediately after decanting—the “bottle bouquet” can be fleeting. The author would like to thank Kay Bogart for her help in preparing this answer. |
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