Day 27

Passage 27

Water on Venus, Earth and Mars

① Venus, Earth and Mars, all at approximately the same distance from the Sun, are predicted to have formed out of the same material and had approximately the same initial temperatures 4.6 billion years ago. Back at the time, the three planets probably all had moderate enough temperatures and water suitable for life. However, Venus is now much too hot while Mars now much too cold, with no water found on both. What happened to these two planets and where all the water has gone are major puzzles for planetary scientists. The answer, as it turns out, lies not in their approximately same distance from the Sun so much as in their subtly-diff erent distances from it.

② Scientists are not certain if Venus had large amounts of water like Earth after it formed. In the wet scenario, Venus was originally cooler than what it is now and it had a greater abundance of water several billion years ago. ■ Also, most of its carbon dioxide was locked up in the rocks. ■ Because Venus was slightly closer to the Sun than the Earth, its water never liquified and remained in the atmosphere. ■Ultraviolet light from the Sun then broke down the water vapor in the atmosphere into hydrogen and oxygen. ■The lighter hydrogen atoms escaped into space, carried off by the solar wind, while the heavier oxygen atoms reacted with minerals in the crust. The carbon dioxide formed during the reaction began to heat up the planet in the greenhouse heating process. As Venus heated up, some of the carbon dioxide in the rocks was “baked out”. This added even more carbon dioxide to the atmosphere and further enhanced the greenhouse heating, which baked more carbon dioxide out of the rocks (as well as any water), and a runaway positive feedback loop process occurred. This huge and continuous greenhouse eff ect finally raised Venus's surface temperature to a point where the oceans boiled away entirely.

③ As for our mother planet Earth, since it is a little farther from the Sun than Venus, most of its water was liquid from the beginning. The rest of its water vapor works with carbon dioxide in the atmosphere to create a small greenhouse eff ect, raising the temperature to about 34°C. This natural greenhouse effect, in turn, makes it warm enough on the surface for liquid water to exist. Besides making life possible, the liquid water also helps to keep the amount of atmospheric carbon dioxide from getting too high. The temperature on Earth can remain generally stable because of a negative feedback process that cools the Earth. The rate at which carbon dioxide is removed from the atmosphere depends on the temperature, in such that the higher the temperature, the higher the rate that carbon dioxide is removed. If the Earth warms up, there will be more evaporation and rainfall, resulting in a greater removal of atmospheric carbon dioxide. The reduced atmospheric carbon dioxide leads to a weakened greenhouse effect that counteracts the initial warming and cools the Earth back down. If the Earth cools off, the rainfall decreases, resulting in less removal of atmospheric carbon dioxide. The atmospheric carbon dioxide level will build backup because of the outgassing of volcanoes. A strengthened greenhouse eff ect counteracts the initial cooling and heats the Earth back up.

④ Contrary to what happened to Venus, Mars had a reverse greenhouse effect, called a runaway refrigerator. Since Mars was slightly farther away from the Sun than the Earth, its initial temperature was slightly lower. This meant that all water vapor condensed to form a liquid water layer on the surface. Gaseous carbon dioxide dissolves in liquid water and can then be chemically combined with rocks. This would have happened on Mars long ago. The removal of some of the carbon dioxide caused a temperature drop further from the reduced greenhouse eff ect, which caused more water vapor to condense, leading to more removal of atmospheric carbon dioxide and more cooling, and so on. This feedback process finally froze the planet and leftMars's now frozen water in a permafrost layer below the surface. Much evidence has proved the existence of running liquid water on Mars in its past. Some geologic features look very much like the river drainage systems on Earth and other features point to huge floods. The Mars Pathfinder studied martian rocks in the summer of 1997 and found some rocks are conglomerates (rocks made of pebbles cemented together in sand) that require flowing water to form. Abundant sand also points to widespread water long ago.

——2012年7月28日北美机经

Look at the four squares [■] in Paragraph 2 that indicate where the following sentence can be added to the paragraph.

Through a process called a runaway greenhouse, Venus heated up to its present blistering hot level.

Where would the sentence best fit?

核心词汇：

词汇练习：

阅读下列句子，用所给单词（或词组）的正确形式填空：

ultraviolet scenario reverse raise positive permafrost moderate feedback gaseous evaporation enhance drainage crust condense atom abundance

1. This____ begins with the planting of hyper accumulating species in the target area, such as an abandoned mine or an irrigation pond contaminated by runoff.(TPO-5:Minerals and Plants)

2. Some scientists have proposed that the presence of increasing levels of____ light with elevation may play a role, while browsing and grazing animals like the ibex may be another contributing factor.(TPO-1:Timberline Vegetation on Mountains)

3. The same minerals that occur in refractory inclusions are believed to be the earliest-formed substances to have____ out of the solar nebula.(TPO-22:The Allende Meteorite)

4. To compare the compositions of a meteorite and the Sun, it is necessary that we use ratios of elements rather than simply the abundances of____ .(TPO-22:The Allende Meteorite)

5. All exhibited____ growth rates in the course of the century (Denmark the highest and Sweden the lowest), but all more than doubled in population by 1900. Density varied greatly. (TPO-18:Industrialization in the Netherlands and Scandinavia)

6. Thus the seed's chances of survival are greatly____ over those of the naked spore. (TPO-9:The Arrival of Plant Life in Hawaii)

7. This “negative, ” as Talbot called it, could then be used to print multiple____ images on another piece of treated paper.(TPO-22:The Birth of Photography)

8. Shortwave radiation from the Sun passes through the pollution dome more easily than outgoing longwave radiation does; the latter is absorbed by the____ pollutants of the dome and reradiated back to the urban surface.(TPO-23:Urban Climates)

9. The picture of Teotihuacán that emerges is a classic picture of positive____ among obsidian mining and working, trade, population growth, irrigation, and religious tourism. (TPO-8:The Rise of Teotihuacán)

10. At the city of Coba the Maya built dikes around a lake in order to____ its level and make their water supply more reliable.(TPO-14:Maya Water Problems)

11. The Earth's____ is thought to be divided into huge, movable segments, called plates, which float on a softplastic layer of rock.(OG:Geology and Landscape)

12. The oxygen isotope ratio of the ocean changes as a great deal of water is withdrawn from it by____ and is precipitated as snow to form glacial ice.(TPO-19:Discovering the Ice Ages)

13. The____ process brought young men to Britain, where many continued to live after their 20 to 25 years of service, and this added to the cosmopolitan Roman character of the frontier population.(TPO-19:The Roman Army's Impact on Britain)

14. Geologists think that this fluidized ejecta crater indicates that a layer of____ , or water ice,lies just a few meters under the surface.(TPO-25:The Surface of Mars)

15. Such anomalies are due to the relative____ of the “isotopes” or varieties of each element.(TPO-16:Development of the Periodic Table)

16. If no____ system exists, the water table rises, bringing dissolved salts to the surface. (OG:Desert Formation)

参考答案：

1. scenario 2. ultraviolet 3. condensed 4. atoms 5. moderate 6. enhanced 7. positive 8. gaseous 9. feedback 10. raise 11. crust 12. evaporation 13. reverse 14. permafrost 15. abundance 16. drainage