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电梯直达

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发表于 2010-8-19 21:21:26 |只看该作者 |倒序浏览

"As Earth was being formed out of the collision of space rocks, the heat from those collisions and from the increasing gravitational energy of the planet made the entire planet molten, even the surface. Any water present would have evaporated and gone off into space. As the planet approached its current size, however, its gravitation became strong enough to hold gases and water vapor around it as an atmosphere. Because comets are largely ice made up of frozen water and gases, a comet striking Earth then would have vaporized. The resulting water vapor would have been retained in the atmosphere, eventually falling as rain on the cooled and solidified surface of Earth. Therefore, the water in Earth's oceans must have originated from comets."
WORDS: 460
TIME: 00:24:10
DATE: 2010/8/19/星期四 20:49:13

In this argument, the notion that the water in Earth's oceans must have originated form comets seems at first glance to be an obvious conclusion. However, upon a second consideration, the conclusion that comet's ice vaporized and then falling as rain on the surface of the earth mask other (and potentially more significant) explanations.

The threshold problem of this argument is the author assumes that a comet stroke Earth and then ice would have vaporized. Although it is entirely possible, the author lacks evidence to confirm this assumption. It is quite likely that the Earth at that time was no in a molten status, as we know, Earth has experienced the Ice Age and other extremely cold weather. The author cannot prove the time when the ice struck the earth or provide the relevant record or information about that period. Until the author can provide further evidence to exclude all these concerns, it is unfounded to reach this conclusion involved in this argument.

The argument is also weakened by the assumption that the resulting water vapor would have been retained in the atmosphere. There is no guarantee that it is a necessary case and it is quite possible that the Earth has not yet developed enough gravitation to hold gases and water vapor. As the author provided above, the Earth needs to approach its size to the current one and then is able to be strong enough to retain gases and water. If it is not that strong, all the water would have evaporated, so how could the comets' water be evaporated. Without further evidence ruling out all these and other alternative explanations, it is reasonable to cast considerable doubt on the assumption.

Finally, even the author can substantiate all of the foregoing assumptions, his assumption that the water in the Earth's oceans originated from comets is still unwarranted. Because no compelling evidence is provided to affirm his assumption. There are several ways to form seas and oceans. It is common sense that numerous rivers and streams would converge and then develop into the sea, the same explanation may well applied in the coming of oceans. Even though it originated from collisions, it may be from other planets such as mars, sun , or others. Therefore, under any scenario, adopting the author's conclusion without critical thinking would astray others.

To sum up, the argument lacks credibility because the Earth may be cold or icy when the comet hit, and it has not yet increased its gravitation to hold water and gases at that time. And the seas and oceans may have other originations such as convergence or other planets collision. Thus, if the author had considered the facts given above, the argument could have been more thorough and logically acceptable.