托福阅读真题解析：火星上的流水

　　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 oceanevaporated and the shoreline receded. But detractors maintain that the terraces could also havebeen created by geological activity, perhaps related to the geologic forces that depressed theNorthern Hemisphere far below the level of the south, in which case they have nothing whateverto do with Martian water. Furthermore, Mars Global Surveyor data released in 2003 seem toindicate that the Martian surface contains too few carbonate rock layers―layers containingcompounds of carbon and oxygen―that should have been formed in abundance in an ancientocean. Their absence supports the picture of a cold, dry Mars that never experienced theextended mild period required to form lakes and oceans. However, more recent data implythat at least some parts of the planet did in fact experience long periods in the past duringwhich 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 theunproven hints of ancient oceans, the extent of the outflow channels suggests that a hugetotal volume of water existed on Mars in the past. Where did all the water go? The answer maybe 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.

　　Paragraph 1 Photographic evidence suggests that liquid water once existed in great quantityon the surface of Mars. Two types of flow features are seen: runoff channels and outflowchannels. Runoff channels are found in the southern highlands. These flow features areextensive systems―sometimes hundreds of kilometers in total length―of interconnecting,twisting channels that seem to merge into larger, wider channels. They bear a strongresemblance to river systems on Earth, and geologists think that they are dried-up beds oflong-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.