【凤于九天作文小组第八次作业】Argument050 by Linty

lintyhuangUID: 2464885

"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: 337          TIME: 00:46:04         

The arguer draws a conclusion that the water in Earth's oceans must have originated from comets. The evidence of this conclusion is that the water in comet which collided the Earth would be retained in Earth, after the Earth approached its currents size. As first glance, the conclusion is reasonable, however, it is actually inconvincible. Although the comet would bring some water to Earth, it is unfair that all the water in Earth's oceans must have originated from comets.

First of all, before Earth's gravitation became strong enough to hold gases and water vapor around it, there is still some water retained in Earth, which can not gone off into space. For example, the water inside the Earth can not vapor and go off into space through the surface. Nowadays, there is plenty of water vapor, when the volcano erupts. This is evidence that the water is still conserved inside of the Earth and the only way the water can come to the Earth's surface is volcanic eruption. After the Earth approached its current size, the volcanic eruptions happened thousands of times, which would bring a lot of water to the Earth's surface. Finally, this water would flow into oceans.

Second, at the instant when the Earth's gravitation became strong enough to maitain an atmosphere, there was still a lot of water vapor around the Earth's surface. The water vapor was from the original space rocks which formed the Earth. This part of water was not originated from comets and finally flow into oceans too.

Furthmore, water can come from other different ways, but not comets. For example, when the Earth's gravitation became strong enough, the Earth could capture the water vapor of the space; some plants and animals could produce water; human can produce water through chemical reaction. After millions of years, the water came from other  ways is a large part of oceans.

In a word, the above conclusion is not convincible. The arguer should find some other valid evidence to support his conclusion.