2016年5月7日托福阅读真题答案及解析(宁波新东方)

　　5月7日托福阅读真题第二篇

　　题材划分：地质类

　　主要内容：火星是否存在生命体

　　火星探索发现表面没有水，减少了有生命的可能。很久以前，一颗火星的陨石飞到了地球的极圈，因为很冷所以能基本无污染的保存下来。科学家打开来检查，发现有种长得像地球远古生物(虫?)的生物，但是其他科学家说这个结构太小了，不可能有生物的基本功能。还发现了carbon的元素，这是地球生命产生的必须物质，但又有指出说这是地球生物的必须，还是不能证明火星能有生命。

　　相似TPO练习推荐：

　　TPO-8 Running Water on Mars

　　TPO-41 Climate of Venus

　　相关背景资料：

　　The current understanding of planetary habitability—the ability of a world to develop environmental conditions favorable to the emergence of life—favors planets that have liquid water on their surface. Most often this requires the orbit of a planet to lie within the habitable zone, which for the Sun extends from just beyond Venus to about the semi-major axis of Mars. During perihelion, Mars dips inside this region, but the planet's thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water demonstrates the planet's potential for habitability. Some recent evidence has suggested that any water on the Martian surface may have been too salty and acidic to support regular terrestrial life.

　　The lack of a magnetosphere and the extremely thin atmosphere of Mars are a challenge: the planet has little heat transfer across its surface, poor insulation against bombardment of the solar wind and insufficient atmospheric pressure to retain water in a liquid form (water instead sublimes to a gaseous state). Mars is also nearly, or perhaps totally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals and minerals between the surface and interior of the planet.

　　Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there remains unknown. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites and had positive results, including a temporary increase of CO2 production on exposure to water and nutrients. This sign of life was later disputed by some scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life. A re-analysis of the Viking data, in light of modern knowledge of extremophile forms of life, has suggested that the Viking tests were not sophisticated enough to detect these forms of life. The tests could even have killed a (hypothetical) life form. Tests conducted by the Phoenix Mars lander have shown that the soil has a alkaline pH and it contains magnesium, sodium, potassium and chloride. The soil nutrients may be able to support life, but life would still have to be shielded from the intense ultraviolet light.