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Yet the rubble-pile hypothesis is conceptually troublesome. The material strength of an asteroid is nearly zero, and gravity is so low you are tempted to neglect that, too. What’s left? The truth is that neither strength nor gravity can be ignored. Paltry though it may be, gravity binds a rubble pile together. And anyone who builds sand castles knows that even loose debris can cohere. Oft-ignored details of motion begin to matter: sliding friction, chemical bonding, damping of kinetic energy, electrostatic attraction and so on. (In fact, charged particles from the sun can cause dust at the surface to levitate.) We are just beginning to fathom the subtle interplay
of these minuscule forces.
(考过的狗主人说第二段重点考,请大家注意!)
The size of an asteroid should determine which force dominates. One indication is the observed pattern of asteroidal rotation rates. Some collisions cause an asteroid to spin faster; others slow it down. If asteroids are monolithic rocks undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with a statistical “tail” of very fast rotators. If nearly all asteroids are rubble piles, however, this tail would be missing, because any rubble pile spinning faster than once every two or three hours (depending on its bulk density) would fly apart. Alan Harris of the Jet Propulsion Laboratory in Pasadena, Calif., Petr
Pravec of the Academy of Sciences of the Czech Republic in Prague and their colleagues have discovered that all but five observed asteroids obey a strict rotation limit [see illustration on page 48]. The exceptions are all smaller than about 150 meters in diameter, with an abrupt cutoff for asteroids larger than about 200 meters.
The evident conclusion—that asteroids larger than 200 meters across are multicomponent structures or rubble piles—agrees with recent computer
modeling of collisions, which also finds a transition at that diameter. A collision can blast a large asteroid to bits, but those bits will usually be moving slower than their mutual escape velocity (which, as a rule of thumb, is about one meter per second, per kilometer of radius). Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile [see illustration above]. Because collisions among asteroids are relatively frequent, most large bodies have already
suffered this fate. Conversely, most small asteroids should be monolithic, because impact fragments easily escape their feeble gravity.
2.3.6 Moon-earth 生命体
是讲Moon-earth的,说是很多学者认为在宇宙中存在和地球类似的星球,所以一定有有智生命体的存在。[本文作者很固执的认为由于地月系统的形成是极具偶然性的,尽管其他星体也有可能形成卫星系统,但是没有一个能像地月系统这么特殊,所以作者否定了外星生物的存在性。]
第一段说以前很多的学者认为外星系有生命存在,因为太阳系和很多其他星系一样,有很多planet云云,但是为什么外星系至今没有发现呢?说地球与月球两者之间的联系和对地球生命出现的作用,说生命体出现是需要Moon-Earth这种条件的星球的。
第二段是到底是地月系的什么不同导致地球有生物呢?是地球和月球的质量比。说地月系的特殊性,文章承认有可能存在这种双球体系的星球,但月球轨道很特殊,其他类似的双球体会相撞。好像是说,地月系的质量对比非常好所以有生命体,与太阳系的另外一个有卫星的是木星(还是土星?)比较,对比的那个星系因为卫星太小(只能当作asteroid来看)没有办法使得生命存在。(这个地方有个题是问,那月球对于地球而言有好处是为什么,我选的是因为它的质量相对够大。(分析是,对比对象因为卫星太小没办法有生命体,那么月球的质量相对够大所以才能有生命体))同时这个比例还使得月亮量对地球的引力产生了潮汐作用,从而为地球生物进化创造了条件。
第三段大概说了什么地球和月球是“capture”还是。。好像一个是吸引,一个是分离出来的,还有一个什么的。。。怎样怎样。。。真的忘了;
第四段反正最后是又一次强调正是这个质量比例导致了地球有生物;而这种比例关系在千千万万个其他类似系统中比较难得,所以也就至今还没有发现其他星球上有生物,云云。(最后这段高亮全段,问作者用什么方法来写这段:举个例子来说明?这种类型的问题,具体真的忘了)[说到叻月球形成原因]
题目:
1、文章最后一句highlight,问证明了什么
2、全文最后一段高亮,问论证手法。
举个例子来说明?
[因为缺少观察到的数据,所以一个generation 的可能性几乎没有]
3、有问文章结构题:
作者第一段提出问题,为什么别的地方没有生命呢?第二段和第三段都是解释月球对我们多么重要。
4、好像是说为什么地月系统很特殊
A. 因为地球上有水
B. 因为月亮的shape
C. 。。。
D。。。
E。因为月球的size(我选的这个) |
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