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Exercise Four
Recently some scientists have concluded that meteorites found on Earth and long believed to have a Martian origin might actually have been blasted free of Mars's gravity by the impact on Mars of other meteorites. This conclusion has led to another question: whether meteorite impacts on Earth have similarly driven rocks from this planet to Mars.
According to astronomer S.A. Phinney, kicking a rock hard enough to free it from Earth's gravity would require a meteorite capable of making a crater more than 60 miles across. Moreover, even if Earth rocks were freed by meteorite impact, Mars's orbit is much larger than Earth's, so Phinney estimates that the probability of these rocks hitting Mars is about one-tenth as GREat as that of Mars's rocks hitting Earth. To demon- strate this estimate, Phinney used a computer to calculate where 1,000 hypothetical particles would go if ejected from Earth in random directions. He found that 17 of the 1,000 particles would hit Mars.
1. The passage is primarily concerned with
(A) presenting an argument to support a particular hypothesis
(B) suggesting an answer to a theoretical question
(C) questioning the assumptions of a research project
(D) criticizing experimental results
(E)explaining the origin of certain scientific data
2. According to the passage, which of the following events may have initiated the process that led to the presence on Earth of meteorites from Mars?
(A) A meteorite struck the Earth with tremendous velocity.
(B) A meteorite collided with Mars.
(C) Approximately 1,000 rocks were ejected from Mars.
(D) The orbits of Earth and Mars brought the planets to their closest points.
(E)Rocks from a meteorite impact broke free of Earth's gravity.
3. The passage suggests that which of the following is true concerning the probability that a rock, if ejected from Mars, will hit the Earth?
(A) The probability is increased when particles are ejected from Mars in random directions.
(B) The probability is increased by the presence of large craters on the surface of Mars.
(C) The probability is decreased when Mars's orbit brings the planet close to Earth.
(D) The probability is GREater than the probability that a rock from Earth will hit Mars.
(E)The probability is less than the probability that a rock from Earth will escape Earth's gravity.
4. Which of the following, if true, would cast most doubt on Phinney's estimate of the probability of Earth rocks hitting Mars?
(A) Rather than going in random directions, about 25 percent of all particles ejected from Earth go in the same direction into space.
(B) Approximately 100 meteorites large enough to make a noticeable crater hit the Earth each year.
(C) No rocks of Earth origin have been detected on Mars.
(D) The velocity of rocks escaping from Earth's gravity is lower than the velocity of meteorites hitting the Earth.
(E) No craters more than 60 miles across have been found on Mars.
Exercise 4 1. B 2. B 3. D 4. A