argument50

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TOPIC: ARGUMENT50 - From a draft textbook manuscript submitted to a publisher.

"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."

In this argument, the writer asserts that the water in Earth's oceans must have originated from comets. This argument develops cogently and seems no logical flaws in it. But the writer's assertion is based on two assumptions he makes, and these assumptions may not hold.

First, the writer considers that at the early time that the Earth formed, all water has evaporated. It is true that according to our knowledge, water will evaporate when the temperature up to 100 centigrade. The extreme high temperature during the early stage of the Earth is certainly to make water evaporated. But to notice, 100 centigrade to evaporate is under the precondition that it is under standard atmosphere pressure. And water may stay liquid under high temperature as long as it is under enough pressure. At that time, the collision force may provide enough pressure some liquid water may exist in the inner of the Earth. If this is the truth, the writer's first assumption that all water had evaporated is not hold. If there were some water exists when the Earth formed, the conclusion that the writer come from comet is precarious.

Second, the writer makes another assumption that water contained in comets was vaporized when comets hit the surface of the Earth. The writer says that comets water vaporized to become the atmosphere of the Earth. It means that comets melted first, and then there was atmosphere. And we know that the heat caused by friction between atmosphere and comets is the reason that comets melt. Since there was no atmosphere before, how the comets melt? So this assumption is not grounded.

In addition, even if comets can reach the surface of the Earth and form the atmosphere, there are some other celestial bodies, such as meteoroids or other planets may also have collided with the Earth and bring water to the Earth. Without ruling out this passable reason, although previous two assumptions hold, the writer’s conclusion is not convincing yet.

The argument seems plausible at first glance, but these two assumptions in the argument lack evidences to support. And the writer also ignores some other passable reason, like other celestial bodies may bring water to the Earth. So to sum up, the argument is warranted.