托福机经：2013年3月16日托福阅读机经

据话题对应，可重点结合TPO11 Orientation and Navigation; TPO15 A Warm-blooded Turtle; TPO16 Planets in our Solar System三篇文章阅读。

　　1、我们如何计算太阳系的年龄

　　如何计算地球的年龄，地球表面由于水和风华作用影响底层，不易推算，但是可以通过研究meteor三种分别是rocky，irony，和rocky-irony。介绍了他们的来源，大部分是裂解的大型陨石。

　　我们推断出地球大约形成在64million年前左右，同时科学家还发现了月球的一个meteor致是这个时间，而由于月球表面不容易风华，这个时间应该是准的。

　　最后我们都通过检测其他星球传来的wavelight的波长推算出宇宙在膨胀，星系之间的距离在变远。

　　AGE OF THE EARTH[ //pubs.usgs.gov/gip/geotime/age.html]

　　So far scientists have not found a way to determine the exact age of the Earth directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed by the process of plate tectonics. If there are any of Earth's primordial rocks left in their original state, they have not yet been found. Nevertheless, scientists have been able to determine the probable age of the Solar System and to calculate an age for the Earth by assuming that the Earth and the rest of the solid bodies in the Solar System formed at the same time and are, therefore, of the same age.

　　The ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and that decay with half lives of 700 million to more than 100 billion years to stable isotopes of other elements. These dating techniques, which are firmly grounded in physics and are known collectively as radiometric dating, are used to measure the last time that the rock being dated was either melted or disturbed sufficiently to rehomogenize its radioactive elements.

　　Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years). [See Editor's Note.] These ancient rocks have been dated by a number of radiometric dating methods and the consistency of the results give scientists confidence that the ages are correct to within a few percent. An interesting feature of these ancient rocks is that they are not from any sort of "primordial crust" but are lava flows and sediments deposited in shallow water, an indication that Earth history began well before these rocks were deposited. In Western Australia, single zircon crystals found in younger sedimentary rocks have radiometric ages of as much as 4.3 billion years, making these tiny crystals the oldest materials to be found on Earth so far. The source rocks for these zircon crystals have not yet been found. The ages measured for Earth's oldest rocks and oldest crystals show that the Earth is at least 4.3 billion years in age but do not reveal the exact age of Earth's formation. The best age for the Earth (4.54 Ga) is based on old, presumed single-stage leads coupled with the Pb ratios in troilite from iron meteorites, specifically the Canyon Diablo meteorite. In addition, mineral grains (zircon) with U-Pb ages of 4.4 Ga have recently been reported from sedimentary rocks in west-central Australia. The Moon is a more primitive planet than Earth because it has not been disturbed by plate tectonics; thus, some of its more ancient rocks are more plentiful. Only a small number of rocks were returned to Earth by the six Apollo and three Luna missions. These rocks vary greatly in age, a reflection of their different ages of formation and their subsequent histories. The oldest dated moon rocks, however, have ages between 4.4 and 4.5 billion years and provide a minimum age for the formation of our nearest planetary neighbor. Thousands of meteorites, which are fragments of asteroids that fall to Earth, have been recovered. These primitive objects provide the best ages for the time of formation of the Solar System. There are more than 70 meteorites, of different types, whose ages have been measured using radiometric dating techniques. The results show that the meteorites, and therefore the Solar System, formed between 4.53 and 4.58 billion years ago. The best age for the Earth comes not from dating individual rocks but by considering the Earth and meteorites as part of the same evolving system in which the isotopic composition of lead, specifically the ratio of lead-207 to lead-206 changes over time owing to the decay of radioactive uranium-235 and uranium-238, respectively. Scientists have used this approach to determine the time required for the isotopes in the Earth's oldest lead ores, of which there are only a few, to evolve from its primordial composition, as measured in uranium-free phases of iron meteorites, to its compositions at the time these lead ores separated from their mantle reservoirs. These calculations result in an age for the Earth and meteorites, and hence the Solar System, of 4.54 billion years with an uncertainty of less than 1 percent. To be precise, this age represents the last time that lead isotopes were homogeneous througout the inner Solar System and the time that lead and uranium was incorporated into the solid bodies of the Solar System. The age of 4.54 billion years found for the Solar System and Earth is consistent with current calculations of 11 to 13 billion years for the age of the Milky Way Galaxy (based on the stage of evolution of globular cluster stars) and the age of 10 to 15 billion years for the age of the Universe (based on the recession of distant galaxies).

　　2、lizard是一类冷血动物，他们的体温其实很高，也需要维持正常的生理生长活动，但是他们直接获取周围的热量而不是自身产热。接着对比了这一类冷血动物和能够自身产热的动物的优缺点。能产热的动物，需要觅食来获取热量，因此容易被天敌发现，一旦缺少食物会生长困难甚至死亡冷血动物可以适应突然的食物和环境变化，在低温时inactive，但是容易被吃掉

　　Are Lizards Warm Blooded or Cold Blooded?[ //www.animalquestions.org/reptiles/lizards/are-lizards-warm-blooded-or-cold-blooded/]

　　With a few exceptions all reptiles such as the lizard are cold blooded creatures.  The temperature of an animal’s blood (whether an animal is warm blooded or cold blooded) is related to its body temperature.

　　Cold blooded creatures such as the lizard take on the temperature of their surroundings.  In other words, they are hot when their environment is hot and cold when their environment is cold.  In hot environments, cold blooded animals can have blood that is much warmer than warm blooded animals.  Cold blooded animals are also much more active in warm environments and are very sluggish in cold environments.  This is because their muscle activity depends on chemical reactions which run quickly when it is and slowly when it is cold.  Cold blooded creatures such as the lizard can covert much more of its food into body mass compared to warm blooded animals.

　　Whereas many warm blooded animals sweat or pant to lose heat by water evaporation and can cool off by moving into a shaded area or getting wet, cold blooded animals such as the lizard often like to bask in the sun to warm up and increase their metabolism.  While basking, these reptiles will be found lying perpendicular to the direction of the sun to maximize the amount of sunlight that falls onto their skin.  In addition to this they will expand their rib cage to increase their surface area and will darken their skin to absorb even more heat.

　　When a lizard is too hot it will like parallel to the sun’s rays, go into a shady area, open its mouth up wide and lighten its skin color or burrow into the cool sand.  Most reptiles such as the lizard have been known to hibernate occasionally during the cold winter months.  Whereas warm blooded animals are able to remain active, seek food and defend themselves in a wide range of outdoor temperatures, cold blooded animals such as the lizard lack this ability and are only able to do so when they are warm enough.  A cold blooded animal’s level of activity depends upon the temperature of its surroundings.  A lizard will increase its body temperature before hunting and it is better able to escape from predators when it is warm.  These creatures also need to be warm and active in order to find a mate and reproduce.