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影像资料表明火星表面曾存在大量液体水。

Photographic evidence suggests that liquid water once existed in great quantity on the surface of Mars.

可以看到两个流动性特征：径流河床和溢流河床。

Two types of flow features are seen: runoff channels and outflow channels.

在南部高地可以找到径流河床。

Runoff channels are found in the southern highlands.

这些流动性特征分布广泛，有时总长度达到数百千米，交错曲折的河道看似要汇入更大更宽的河道。

These flow features are extensive systems—sometimes hundreds of kilometers in total length—of interconnecting, twisting channels that seem to merge into larger, wider channels.

它们跟地球上的河流系统非常相似，地质学家们认为它们是消失很久的干涸的河道，火星上的雨水曾通过这些河道从山上流到山谷里。

They bear a strong resemblance to river systems on Earth, and geologists think that they are dried-up beds of long-gone rivers that once carried rainfall on Mars from the mountains down into the valleys.

径流河床存在于40亿年前（火星高地时期），那时候火星空气更稠密，表面更暖和，液态水分布更广泛。

Runoff channels on Mars speak of a time 4 billion years ago (the age of the Martian highlands), when the atmosphere was thicker, the surface warmer, and liquid water widespread.

溢流河床很可能是很久以前火星上灾难性洪水的遗迹。

Outflow channels are probably relics of catastrophic flooding on Mars long ago.

它们仅出现在赤道地带，没有形成广泛交错的河道网络。

They appear only in equatorial regions and generally do not form extensive interconnected networks.

反而，他们可能是大量的水从南部高地流向北部平原的河道。

Instead, they are probably the paths taken by huge volumes of water draining from the southern highlands into the northern plains.

由泛滥的洪水而产生的激流可能会形成奇怪的泪滴状小岛（就像是在低潮时海滩上露出的湿沙的缩小版本），可以在溢流河床末尾处的平原上被看到。

The onrushing water arising from these flash floods likely also formed the odd teardrop-shaped “islands” (resembling the miniature versions seen in the wet sand of our beaches at low tide)  that have been found on the plains close to the ends of the outflow channels.

从河道的宽度与深度来看，当时水的流速非常大——可能是亚马逊河流速105吨/秒的一百倍。

Judging from the width and depth of the channels, the flow rates must have been truly enormous—perhaps as much as a hundred times greater than the 105 tons per second carried by the great Amazon river.

大约在30亿年前，洪水塑造了溢流河床，几乎在同一时期北部火山平原形成了。

Flooding shaped the outflow channels approximately 3 billion years ago, about the same time as the northern volcanic plains formed.

一些科学家猜测在早期的很长一段时间里火星表面分布着河流，湖泊，甚至是大海。

Some scientists speculate that Mars may have enjoyed an extended early period during which rivers, lakes, and perhaps even oceans adorned its surface.

一张2003年火星全球勘探者号获取的图片显示出程扇形网络状的河道沉积物，也就是河流注入更大水域的遗迹，比如南部高地的一个火山口湖泊，任务专家们认为这片扇形区域可能是三角洲。

A 2003 Mars Global Surveyor image shows what mission specialists think may be a delta—a fan-shaped network of channels and sediments where a river once flowed into a larger body of water, in this case a lake filling a crater in the southern highlands.

其他研究人员做了更大胆的猜测，他们认为数据提供了证据表明火星表面早期存在过大量的水。

Other researchers go even further, suggesting that the data provide evidence for large open expanses of water on the early Martian surface.

一张电脑合成的火星北极区域的图片表明古老的海洋可能覆盖了北部低地的大部分地方。

A computer-generated view of the Martian north polar region shows the extent of what may have been an ancient ocean covering much of the northern lowlands.

赫拉斯盆地大约有3,000公里范围，在它的边缘有一块比它底9公里的平地，这里很有可能曾经是远古海洋。

The Hellas Basin, which measures some 3,000 kilometers across and has a floor that lies nearly 9 kilometers below the basin’s rim, is another candidate for an ancient Martian sea.

支持者们指出照片里显示的台地“海滩”可以是由湖泊或者海洋蒸发干涸之后或者海退之后形成的。

Proponents point to features such as the terraced “beaches” shown in one image, which could conceivably have been left behind as a lake or ocean evaporated and the shoreline receded.

但是批评者坚持这些台地可能是由地质活动导致的，可能跟地质力有关，它让北半球远比南半球低，这样的话这种地貌就与火星水源无关系。

But detractors maintain that the terraces could also have been created by geological activity, perhaps related to the geologic forces that depressed the Northern Hemisphere far below the level of the south, in which case they have nothing whatever to do with Martian water.

此外，火星全球勘探者号2003年公布的数据显示火星表面几乎没有碳酸盐岩层——含有碳化合物与氧化合物的岩层——它本应该在远古海洋中大量形成。

Furthermore, Mars Global Surveyor data released in 2003 seem to indicate that the Martian surface contains too few carbonate rock layers—layers containing compounds of carbon and oxygen—that should have been formed in abundance in an ancient ocean.

它们的缺失支持这样一种观点：火星是寒冷干燥的，它从来没有经历过能形成湖泊海洋的气候温和阶段。

Their absence supports the picture of a cold, dry Mars that never experienced the extended mild period required to form lakes and oceans.

但是，最近的数据表明至少火星表面的某些地方确实曾经很长一段时间内有液体水。

However, more recent data imply that at least some parts of the planet did in fact experience long periods in the past during which liquid water existed on the surface.

除了2000年发现的小规模的、不足定论的河道，天文学家现在还没有直接证据表明火星表面有液体水，并且火星大气的水蒸气含量微乎其微。

Aside from some small-scale gullies (channels) found since 2000, which are inconclusive, astronomers have no direct evidence for liquid water anywhere on the surface of Mars today, and the amount of water vapor in the Martian atmosphere is tiny.

但是就算是不考虑远古海洋存在的未经证实的痕迹，溢流河床的规模也表明了火星上曾经存在大量的水。

Yet even setting aside the unproven hints of ancient oceans, the extent of the outflow channels suggests that a huge total volume of water existed on Mars in the past.

答案可能是火星上的水现在被锁在了地表下的永冻层里，并且在火星极地地区含量更多。

The answer may be that virtually all the water on Mars is now locked in the permafrost layer under the surface, with more contained in the planet’s polar caps.

What does the discussion in paragraph 1 of runoff channels in the southern highlands suggest about Mars?

In paragraph 2, why does the author include the information that 105 tons of water flow through the Amazon river per second?

According to paragraph 2, all of the following are true of the outflow channels on Mars EXCEPT:

All of the following questions about geological features on Mars are answered in paragraph 3 EXCEPT:

According to paragraph 3, images of Mars’ surface have been interpreted as support for the idea that

What can be inferred from paragraph 3 about liquid water on Mars?

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

According to paragraph 4, what do the 2003 Global Surveyor data suggest about Mars?

Look at the four squares  that indicate where the following sentence could be added to the passage. These landscape features differ from runoff channels in a number of ways.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Running Water on Mars? Photographic evidence suggests that liquid water once existed in great quantity on the surface of Mars. Two types of flow features are seen: runoff channels and outflow channels. Runoff channels are found in the southern highlands. These flow features are extensive systems—sometimes hundreds of kilometers in total length—of interconnecting, twisting channels that seem to merge into larger, wider channels. They bear a strong resemblance to river systems on Earth, and geologists think that they are dried-up beds of long-gone rivers that once carried rainfall on Mars from the mountains down into the valleys. Runoff channels on Mars speak of a time 4 billion years ago (the age of the Martian highlands), when the atmosphere was thicker, the surface warmer, and liquid water widespread. Outflow channels are probably relics of catastrophic flooding on Mars long ago.  They appear only in equatorial regions and generally do not form extensive interconnected networks.  Instead, they are probably the paths taken by huge volumes of water draining from the southern highlands into the northern plains.  The onrushing water arising from these flash floods likely also formed the odd teardrop-shaped “islands” (resembling the miniature versions seen in the wet sand of our beaches at low tide) that have been found on the plains close to the ends of the outflow channels.  Judging from the width and depth of the channels, the flow rates must have been truly enormous—perhaps as much as a hundred times greater than the 105 tons per second carried by the great Amazon river. Flooding shaped the outflow channels approximately 3 billion years ago, about the same time as the northern volcanic plains formed. Some scientists speculate that Mars may have enjoyed an extended early period during which rivers, lakes, and perhaps even oceans adorned its surface. A 2003 Mars Global Surveyor image shows what mission specialists think may be a delta—a fan-shaped network of channels and sediments where a river once flowed into a larger body of water, in this case a lake filling a crater in the southern highlands. Other researchers go even further, suggesting that the data provide evidence for large open expanses of water on the early Martian surface. A computer-generated view of the Martian north polar region shows the extent of what may have been an ancient ocean covering much of the northern lowlands. The Hellas Basin, which measures some 3,000 kilometers across and has a floor that lies nearly 9 kilometers below the basin’s rim, is another candidate for an ancient Martian sea. These ideas remain controversial. Proponents point to features such as the terraced “beaches” shown in one image, which could conceivably have been left behind as a lake or ocean evaporated and the shoreline receded. But detractors maintain that the terraces could also have been created by geological activity, perhaps related to the geologic forces that depressed the Northern Hemisphere far below the level of the south, in which case they have nothing whatever to do with Martian water. Furthermore, Mars Global Surveyor data released in 2003 seem to indicate that the Martian surface contains too few carbonate rock layers—layers containing compounds of carbon and oxygen—that should have been formed in abundance in an ancient ocean. Their absence supports the picture of a cold, dry Mars that never experienced the extended mild period required to form lakes and oceans. However, more recent data imply that at least some parts of the planet did in fact experience long periods in the past during which liquid water existed on the surface. Aside from some small-scale gullies (channels) found since 2000, which are inconclusive, astronomers have no direct evidence for liquid water anywhere on the surface of Mars today, and the amount of water vapor in the Martian atmosphere is tiny. Yet even setting aside the unproven hints of ancient oceans, the extent of the outflow channels suggests that a huge total volume of water existed on Mars in the past. Where did all the water go? The answer may be that virtually all the water on Mars is now locked in the permafrost layer under the surface, with more contained in the planet’s polar caps.