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.

　　2017年2月26日-托福听力Lecture1真题

　　话题分类：天文学

　　内容回忆：明信片的prehistoric value，一开始有些学者认为没有这个价值，并不能作为研究史前的工具，因为很多并没有包含多少内容且卖明信片的公司会为了盈利，使明信片上的图片更好看，导致失真，而且会做其它处理。另外，因为postcard是到处寄的，找不到original place很正常(我们在研究Statue的时候如果是不动的，就好研究，但是大部分是移动的，所以有困难)。但是后来学者们发现可以研究明信片中的个人情感和文字，可以反映意识形态，这个领域后来大火。

　　1. What is the lecture mainly about?

　　The value of postcards as primary historical sources

　　2. What point does the professor make when she discusses the mass production of postcards?

　　Mass-produced objects often do not provide the kinds of information historians consider to be important.

　　3. According to the professor, what did postcard companies do in order to appeal to customers?

　　Click on 2 answers.

　　Alter photographs after they were taken

　　Create images not based in reality but on consumers’ expectations

　　4. Why does the professor mention a statue?

　　To explain why original settings are significant to historians

　　5. What point does the professor make when she discusses the 1960s and 1970s?

　　Understanding popular culture became more important to historians.

　　6. Why does the professor say this:

　　"They argued that the staged scenes that I mentioned earlier could be a window into people’s perceptions of the time."

　　To point out that even the lack of authenticity of some postcards has historical value

　　2017年3月4日-托福听力Lecture1真题

　　话题分类：生物类

　　内容回忆：并非所有动物迁徙都是为了更舒适的气候，举例一种叫 GS 的蜻蜓。关于它们的研究还没确切证据，但值得一看。它们会从印度到印度洋中的马尔代夫岛，再到北非。但是马尔代夫岛上几乎没有地上淡水，而蜻蜓需要在淡水中产卵。而实际上这些蜻蜓在岛上树林里寻找雨后的水塘产卵。另外从印度到 M 岛是向南飞，而风是向北吹，蜻蜓如何实现逆风飞行呢?它们会提高飞行高度，在一定高空上，空气流向就反了，会帮助它们飞行。它们的飞行距离非常远，需要 4 代才能完成，如果理论被证实是对的，那么它们就超过一种蝴蝶成为世界上迁徙最远的昆虫。

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