That lunar craters are not distributed evenly over the lunar surface is immediately obvious with even a small telescope. Craters also appear in a wide range of sizes from only a few meters across to hundreds of kilometers in diameter. The maria (the very large plains) are noticeably devoid of large craters. Any explanation of crater formation must clearly take these facts into account. There are two main theories concerning the origin of the lunar craters: one is that they were formed by the impact of meteorites, the other that they are due to some type of volcanic activity. According to the former theory, the large majority of the craters were formed during the early stages of the Moon's development when, as seems possible, the number of meteoritic bodies moving in orbits similar to that of the Earth-Moon system was quite large. Since the Moon, unlike the Earth, possesses no atmosphere, such bodies would reach the lunar surface at a higher velocity than they would reach Earth. They also would not be affected by frictional burning such as they would undergo when plunging through the terrestrial atmosphere. When a large body strikes the lunar surface, it is not stopped at the moment of impact but penetrates to a depth that depends on its mass, radius, and impact velocity.
If we are to explain the very large craters on the assumption that several meteorites fell close together (in space, although not necessarily in time), this would imply that such craters should have uneven floors, not only indented to varying degrees by the several impacts but strewn with boulders and debris from the breakup of the meteorites themselves. Observation, however, suggests that the floors of large craters are smoother than can be accounted for by this theory unless there is some other mechanism whereby the floor becomes appreciably smoother following the impact.
One mechanism that has been suggested is that those meteorites that formed the craters fell on the lunar surface when the Moon was in a semi plastic condition or, if they fell after the lunar crust had solidified, that the impact itself would liquefy the surface rock sufficiently to force it to flow rather like lava. This liquefying of the rock at the moment of impact has also been put forward to explain the presence in some craters of a central mountain peak since we know, from high-speed photography of drops falling into liquids that a jet is formed in the center of an impact region.
There is unfortunately, one very serious objection to this idea. When a body the size and shape of a meteorite strikes solid rock it is quite probable that no intermediate liquid phase of the surface rock is ever produced.
1. According to the passage, one hypothesis proposed to explain the smoothness of the floors of large craters argues that the smoothness is a result of the(A) disintegration of meteorites
(B) liquefying of surface rock
(C) numerous impact of meteorites
(D) polar concentration of craters
(E) frictional burning of matter
2. The purpose of the first paragraph is to (A) summarize the evidence supporting a scientific theory
(B) explain what is known about a scientific puzzle
(C) introduce a theory that has been proved correct
(D) describe phenomena to be explained
(E) clarify the meaning of an obscure statement
3. The author fails to suggest any explanation for which of the following statements? (A) There is little evidence of the breakup of meteorites on the floors of lunar craters.
(B) Large numbers of meteorites struck the lunar surface during the Moon's formation.
(C) Meteorites are likely to penetrate the lunar surface.
(D) Some craters contain a central mountain peak.
(E) The maria are devoid of large craters.
4. Which of the following statements about the meteorite-impact theory is most clearly supported by the information in the passage? (A) It is more acceptable to scientists than is the volcanism theory.
(B) It is able to explain some observed features of the Moon's surface.
(C) It is one of several theories all of which seem equally tenable to scientists.
(D) Its importance is primarily historical.
(E) It has been abandoned by most scientists.
5. The passage states that, in contrast to meteorites that enter the Earth's atmosphere meteorites striking the Moon are (A) likely to be almost entirely burned up
(B) likely to fall close together
(C) likely to liquefy immediately
(D) able to create craters of widely varying diameters
(E) able to hit the surface at greater speeds
6. The author suggests that the theory that has been presented to explain the existence of central mountain peaks in some craters is based on (A) the author's own observation of the behavior of lunar rock
(B) inadequate evaluations of the material structure of-the lunar crust
(C) inference from an apparently similar situation
(D) conjectures with which the author agrees
(E) information that has become outdated
7. The author considers an explanation for the formation of very large lunar craters and then evaluates the explanation as (A) confirmed by the characteristics of meteorites as they travel through space
(B) supported by observations concerning the liquefying of rocks
(C) incompatible, without further additions. with evidence of the smoothness of crater floors
(D) improbable because of a disparity with the known causes of analogous formations on Earth
(E) inconsistent if unmodified, with the facts about the distribution of the large lunar craters
8. It can be inferred from the passage that unevenness in the floors of craters (Highlighted) should have resulted from the fact that (A) the meteorites that formed the craters had different masses or impact velocities
(B) the meteorites that formed the craters were all traveling at the same speed
(C) the meteorites that formed the craters all reached the Moon's surface at the same time
(D) when the meteorites struck the Moon, its surface was in a semi plastic condition
(E) when the meteorites struck the Moon, its surface was already uneven