托福阅读真题+题目+答案：Was There Water on Ancient Mars?

托福阅读真题+题目+答案：Was There Water on Ancient Mars?

Despite Mars's bleak,cold climate today,there is abundant geologic evidence that liquid water once flowed on the Martian surface (and even recent evidence that some water still periodically flows today).The evidence of past flows includes river channels,now dry,that were once carved by powerful foods,valley networks of branching streams and tributaries indicating precipitation falling over wide areas,river deltas and layered sediments suggesting flow into standing bodes of liquid water,and salt and other minerals that dissolved in liquid water and were left behind when water evaporated. When that ancient wet period existed and when it ended is difficult to say.

1.The word “periodically” in the passage is closest in meaning to

O occasionally

O constantly

O possibly

O surprisingly

2.All of the following are mentioned in paragraph 1 as evidence that water once flowed on Mars EXCEPT

O networks of channels on the surface of the planet indicating the presence of rivers and streams in the past

O minerals that would have been left behind when water dried up

O different layers of sediment indicating alternating wet and dry periods

O deltas that would have formed where rivers flowed into standing bodes of water

The best method for dating a surface uses radioactivity,the process by which certain elements decay over time.One collects rocks from the surface and measures their radioactive elements to see how much of the decay products have accumulated in the rock since it formed.Transferring this age to the age of the surface can be tricky unless one knows where the rock came from.And one needs to collect the samples and bring them to a well-equipped laboratory.

The other way to date a planetary surface,which is used when rock samples are unavailable,Is by counting craters.The longer a surface has been exposed to meteorite bombardment,the more craters it will have.This gives the relative age,and allows one to determine which surfaces are older and which surfaces are younger. To get the actual age one needs a standard.This was established for the Moon when lunar samples were returned to Earth as part of the United States Apollo program.The crater counts of the areas of the Moon from which the samples came were carefully noted.The samples were dated in terrestrial laboratories using radioactive decay,and the relation between absolute age in years and crater density--numbers per unit area-was determined.This relation is valid for the Moon,and it is possible that it is valid for Mars as well. Then counting craters on a Martian surface should yield its actual age. It is possible,however,that the number of impacting objects is different for Mars.The problem has been carefully studied,and adjustments have been made,but the general conclusion is that a wet period occurred early in Mars's 4.5-billon year history.

3.Paragraph 3 suggests that,without a standard of known crater density dating a planet's surface by counting craters

O requires exact calculations of the sizes of the craters

O can be used only on surfaces containing radioactive elements

O can show only whether the surface is older or younger than a surface it is compared to

O is valid only for planets that received very heavy meteorite bombardment

4.Which of the following is "The problem" referred to in paragraph 3?

O The fact that the craters on Mars are older than the craters on the Moon.

O The fact that the Moon's actual age is known,while the age of Mars is not.

O The likelihood that some of Mars's impact craters are no longer visible.

O The possibility that the Moon and Mars experienced different numbers of impacts.

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