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  3. Massive projectiles striking much larger bodies create various kinds of crat

Massive projectiles striking much larger bodies create various kinds of crat

游客2024-01-10  47
问题     Massive projectiles striking much larger bodies create various kinds of craters, including multi-ring basins–the largest geologic features observed on planets and moons. In such collisions, the impactor is completely destroyed and its material is incorporated into the larger body. Collison’s between bodies of comparable size, on the other hand, have very different consequences: one or both bodies might be entirely smashed, with mass from one or both the bodies redistributed among new objects formed from the fragments. Such a titanic collision between Earth and a Mars-size impactor may have given rise to Earth’s Moon.
    The Earth-Moon system has always been perplexing. Earth is the only one of the inner planets with a large satellite, the orbit of which is neither in the equatorial plane of Earth nor in the plane in which the other planets lie. The Moon’s mean density is much lower than that of Earth but is about the same as that of Earth’s mantle. This similarity in density has long prompted speculation that the Moon split away from a rapidly rotating Earth, but this idea founders on two observations. In order to spin off the Moon, Earth would have had to rotate so fast that a day would have lasted less than three hours. Science offers no plausible explanation of how it could have slowed to its current rotational rate from that speed. Moreover, the Moon’s composition, though similar to that of Earth’s mantle, is not a precise match. Theorizing a titanic collision eliminates postulating a too-rapidly spinning Earth and accounts for the Moon’s peculiar composition. In a titanic collision model, the bulk of the Moon would have formed from a combination of material from the impactor and Earth’s mantle. Most of the earthly component would have been in the form of melted or vaporized matter. The difficulty in recondensing this vapor in Earth’s orbit, and its subsequent loss to the vacuum of outer space, might account for the observed absence in lunar rocks of certain readily vaporized compounds and elements.
    Unusual features of some other planets might also be explained by such impacts. Mercury is known to have a high density in comparison with other rocky planets. A titanic impact could have stripped away a portion of its rocky mantle, leaving behind a metallic core whose density is out of proportion with the original ratio of rock to metal. A massive, glancing blow to Venus might have given it its anomalously slow spin and reversed direction of rotation. Such conjectures are tempting, but, since no early planet was immune to titanic impacts, they could be used indiscriminately to explain away in a cavalier fashion every unusual planetary characteristic; still, we may now be beginning to discern the true role of titanic impacts in planetary history. [br] The passage suggests that which of the following is true of the cited compounds and elements?
选项 A、They were created by reactions that took place during a titanic collision.
B、They were supplied by an impactor that collided with Earth.
C、They were once present on the Moon but were subsequently vaporized.
D、They are rarely found on planet-size bodies in our solar system.
E、They are present on Earth but not on the Moon.
答案 E
解析 这些成分和元素就是月球和地球之间不同的那部分成分，而且根据黑体字所在句子的the observed absence in lunar rocks可知，这些成分月球没有。那么就是地球有，因此选项E正确。A、B项：根据第二段第九句，这些成分本就属于地球，而大冲撞发生在地球形成之后，因此不可能是大冲撞所导致的反应形成的，也不可能是撞击者带来的。C项月球形成之前，这些成分就在撞击后蒸发掉了。D项文中未提及。
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