Passage 87

　　What accounts for the low-lying, flat surface of Mars’s north? On Earth’s surface, higher- and lower-lying areas have different types of crust: one, thin and dense, is pulled toward Earth’s center more strongly by gravity, and the planet’s water naturally comes to sit over it, creating oceans. The processes that generate this oceanic crust drive plate tectonics.

　　Is Mars’s north similarly characterized by a sort of crust different from other areas of the planet? Some researchers do see signs of tectonic activity surrounding the northern basin that suggest that it was created through the formation of new crust, like ocean basins on Earth. However, McGill points to Northern bedrock structures that predate the features said to mark the start of the tectonic process. McGill instead believes that through some novel mechanism the ancient surface sank to its current depth as a single unit. This would explain why features around the basin’s edge, which would have formed as the surface dropped, seem to be younger than structures at its floor.

　　The third possibility is that the northern lowlands result from impacts. Some researchers suggest they formed as a series of big overlapping impact craters. Others arguing that the odds against such a pattern of impacts are large, postulate a single event-the impact of an object bigger than any asteroid the solar system now contains.

　　1. The primary purpose of the passage is to

　　A. explore an analogy between aspects of the geology of Mars and the geology of Earth

　　B. describe how a certain feature of Mars’s surface formed

　　C. point out the effect that new data has had on a scientific investigation

　　D. summarize potential explanations of a large-scale geological feature

　　E. present the rationale for a scientist’s theory and expose some of its weakness

　　2. Which of the following can be inferred from the passage about geological features on Earth

　　A. the relative elevation of the lowest-lying regions of the crust arises in part from forces generated within the planet

　　B. the difference in elevation between the ocean basins and their surroundings is greater than the difference between Mars’s northern basin and its surroundings

　　C. the formation of low-lying areas proceeds by a different process than the one that created Mars’s northern basin

　　D. the weight of the oceans does not affect the depth of the ocean basins

　　E. the proportions of the crust that is oceanic crust is increasing

　　3. As presented in the passage, McGill’s account of the formation of Mars’s northern basin differs from the other mentioned in that it alone

　　A. explains the formation of certain northern bedrock features

　　B. does not specify the force that caused the northern basin to be lower than its surroundings

　　C. takes the northern basin to be a landform that is not analogous to any found on Earth

　　D. denies that features around the northern basin are the result of tectonic activity

　　E. attributes the creation of the northern lowlands to processes occurring within the planet