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  3. Determining the Age of the Planets and the Universe P1: As the solar nebula, a large rotating cloud of interstellar dust and gas

Determining the Age of the Planets and the Universe P1: As the solar nebula, a large rotating cloud of interstellar dust and gas

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问题 Determining the Age of the Planets and the Universe
P1: As the solar nebula, a large rotating cloud of interstellar dust and gas, contracted, the vast majority of materials collected in the center to form the sun. The remainder formed a flattened, spinning disk. Within this spinning disk, matter gradually formed clumps of material that collided, stuck together and grew into planets, which revolve in the same direction and in orbits that lie in nearly the same plane. This is strong evidence that the planets formed simultaneously from a single disk of material that rotated in the same direction as modern planets.
P2: Scientists generally agree that the answer to the riddle of the age of the earth is carefully concealed within the earth’s crust. Since the earth’s surface is subject to inevitable erosion and igneous and metamorphic processes, with huge amounts of material blown or washed out to sea each year, it seemed very likely that the rocks and other material measured might not be quite as old as the earth itself. In fact, the current estimate of the earth’s age derives from extraterrestrial rocks—meteorites—which collided with the earth and escaped the erosion at its surface.
P3: Since meteorites come in all sizes, from miniscule particles to the "small" planets known as asteroids, much of meteorite classification is based, in fact, on the kind of substances a specimen primarily contains rather than its scale. The largest group of meteorites is stony meteorites, which once formed part of the outer crust of a planet or asteroid. Iron meteorites are made of pure nickel and iron, with some occasional impurities such as graphite and the mineral troilite, which may originate from within the metallic cores of asteroids. Stony-Iron meteorites are almost even mixes of metallic and rocky material. They were probably formed by mixing between metal cores and the rocky magma within asteroids. This makes them extremely rare because there is only a small region inside asteroids where metallic and stony material can mix.
P4: The age of the chemical elements within a meteorite can be estimated using radioactive decay to determine how old a given mixture of atoms is. The most definite ages that can be determined this way are measurements of time elapsed since the solidification of rock samples. When a rock solidifies, the chemical elements often get separated into different crystalline grains in the rock. When this method was applied to many different meteorites, it was gratifying to find that the oldest ages obtained for rocks gathered on the surface of the moon tend to cluster around 4.6 billion years. This makes for a very convincing measurement of the age of the solar system, for the simple reason that as no water exists on the lunar surface, which varies from that of the earth, ancient rocks found on the moon should be of the same age as the solar system.
P5: Determining the age of the universe has been far more complicated because the universe is comprised of many enormous disk-like galaxies, each including innumerable stars. What’s more, the universe is constantly expanding: according to the Big Bang theory, the matter that forms galaxies has been spreading in all directions ever since the explosive event occurred several billion years ago. It is not the galaxies themselves that are expanding, but rather the space between them. What is happening is analogous to inflating a balloon with small coins attached to its surface. The coins behave like galaxies: although they do not expand, the space between them does.
P6: In the 1910s, astronomers noticed that the light from nearly every galaxy was redshifted, which means light waves passing through those galaxies were stretched toward the red end of the visible spectrum of wavelengths. In 1929, American astronomer Edwin Hubble matched up these redshifts with distance estimates to these galaxies and uncovered something remarkable: the farther away a galaxy, the faster it is receding. Hubble had stumbled upon a startling truth: the universe was uniformly expanding. What came to be known as the cosmological redshift was the first piece of the Big Bang theory and ultimately a description of the origin of our universe. Calculations based on these redshifts indicate that about 13.7 billion years ago all of the galaxies would have been at one spot, the site of the big bang. This, then, is the approximate date of the big bang and the age of the universe.
P3: ■ Since meteorites come in all sizes, from miniscule particles to the "small" planets known as asteroids, much of meteorite classification is based, in fact, on the kind of substances a specimen primarily contains rather than its scale.■ The largest group of meteorites is stony meteorites, which once formed part of the outer crust of a planet or asteroid. ■ Iron meteorites are made of pure nickel and iron, with some occasional impurities such as graphite and the mineral troilite, which may originate from within the metallic cores of asteroids. ■ Stony-Iron meteorites are almost even mixes of metallic and rocky material. They were probably formed by mixing between metal cores and the rocky magma within asteroids. This makes them extremely rare because there is only a small region inside asteroids where metallic and stony material can mix.
The word "classification" in the passage is closest in meaning to
选项 A、categorization
B、grade
C、layer
D、hierarchy
答案A
解析 【词汇题】classification意为“分类”。
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