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  3. During its formative years, the inner solar system was a rough-and-tumble place. There were a couple of hundred large objects fl

During its formative years, the inner solar system was a rough-and-tumble place. There were a couple of hundred large objects fl

admin2010-06-17  84
问题     During its formative years, the inner solar system was a rough-and-tumble place. There were a couple of hundred large objects flying around. Moon-size or bigger, and for millions of years they collided with one another. Out of these impacts grew the terrestrial planets—Mercury, Venus, Earth with its Moon, and Mars—and the asteroids.
    Scientists have thought of these collisions as mergers: a smaller object (the impactor) hits a larger one (the target) and sticks to it. But new computer modeling by Erik Asphaug and Craig B. Agnor of the University of California, Santa Cruz, shows that things weren’t that simple. "Most of the time, the impactor and the target go off on their merry ways", Dr. Asphaug said. About half the collisions are these hit-and-nm affairs. Now the two researchers and a colleague, Quentin Williams. have done simulations to study the effects of these collisions on the impactors. They are not pretty.
    "The impactors suffer all kinds of fates", Dr. Asphaug said. They undergo tremendous shearing and gravitational forces that can cause them to fracture into smaller pieces or melt, causing chemical changes in the material and loss of water or other volatile compounds. Or the crust and cover can be stripped off, leaving just an embryonic iron core.
    The researchers, whose findings are published in Nature, discovered that two objects did not even have to collide to create an effect on the smaller one from the gravitational forces of a near-collision during the simulations. Dr. Asphaug said, "We’d look and say, ’Gosh, we just got rid of the whole atmosphere of that planetoid: it didn’t even hit and it sucked the whole atmosphere off.’"
    The researchers suggest that the remains of these beaten-up, fractured and melted objects can be found in the asteroid belt. Dr. Asphaug said that could explain the prevalence of "iron relics" in the belt. Some of these planetoid remnants also eventually hit Earth: that would help explain why certain meteorites lack water and other volatile elements.
    The hit-and-run collision model also provides an explanation for Vesta. a large asteroid with an intact crust and cover. How did Vesta keep its cover while so many other objects were losing theirs? Dr. Asphang said it could be that Vesta was always the target, never the impactor, and was thus less affected. "It just had to avoid being the hitter", he said, "until bigger objects left the system".
选项 A、collisions of objects in inner solar system
B、the merging of a smaller object and a larger one
C、the impactor sticking to the target
D、chemical changes
答案A
解析 推理判断题。题目是要找出行星形成的起源，根据第一段，在太阳系形成阶段，太阳系各种物体相互碰撞，结果就形成了行星。"太阳系物体碰撞"，与文章信息吻合；"小物体与大物体融合"，第二段中指出这种观点是以前的看法，现在人们又发现大多数的碰撞不会产生融合，只是"hit-and-run affairs"；"碰撞物粘到目标物上"；"化学变化"，这不是本文的观点。
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