评论
打印
Scientists formerly believed that the rocky planets—Earth, Mercury, Venus, and Mars—were created by the rapid gravitational collapse of a dust cloud, a deflation giving rise to a dense orb. That view was challenged in the 1960s, when studies of Moon craters revealed that these craters were caused by the impact of objects that were in great abundance about 4.5 billion years ago but whose number appeared to have quickly decreased shortly thereafter. This observation rejuvenated Otto Schmidt's 1944 theory of accretion. According to this theory, cosmic dust gradually lumped into ever-larger conglomerates: particulates, gravel, small and then larger balls, planetesimals (tiny planets), and, ultimately, planets. As the planetesimals became larger, their numbers decreased. Consequently, the number of collisions between planetesimals decreased.
The passage provides evidence that Schmidt would be likely to disagree with the theory presented in the first sentence over
A.the length of time it took for the rocky planets to form.
B.the most likely causes of the Moon's impact craters.
C.the importance of cosmic dust as a seminal material in planetary formation.
Which of the following best describes the “observation” (line 6) referred to in the passage?
A.The rocky planets were created by the rapid gravitational collapse of a dust cloud.
B.Certain features on the Moon's surface are impact craters caused by collisions with objects such as planetesimals.
C.The rocky planets were formed by a slow accretion of cosmic dust into increasingly larger bodies.
D.The number of objects colliding with the Moon appears to have been high for a while and then rapidly diminished.
E.There are far fewer planetesimals in existence today than there were about 4.5 billion years ago.
