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.”

The writer of the given argument suggests that the water present in oceans on the Earth’s surface has originated from comets. As Earth came into existence, its surface was very hot. Any comet, which was made up of ice and gases, would get evaporated into the space as soon as it hit the Earth’s surface. However, Earth developed its gravitational energy and water started retaining in its atmosphere. This water eventually rained over its surface when Earth cooled off forming the oceans present today. This seems to be a well-developed theory of the formation of oceans on Earth’s surface. However, when the argument is analyzed closely, it exposes the shortcomings that it has not been able to explain.

First of all, it would be inappropriate to consider that there was no possibility at all of water being present on Earth’s surface when it was formed. While it might be true that Earth was in a molten state and was very hot, it is possible that there were many parts of the Earth’s surface where the temperature was not too high. For water to get evaporated it requires that the temperature of the Earth’s surface be high. However, in parts of Earth where the surface temperature is lower than that, water would not get evaporated and would remain in liquid form. Therefore, we cannot ignore the possibility of existence of traces of water on the Earth’s surface from the time of its formation.

The argument further suggests that when atmosphere developed around the surface of Earth, it started retaining water vapor and gases. At that time when a comet hit its surface, its water would get evaporated into the atmosphere. However, this might not be true. Firstly, since atmosphere was just being formed gradually and Earth was developing its gravitational pull, it is possible that all these objects were not retained in its atmosphere. The impact of such a collision is very high and with the force that is created it is possible that pieces of ice and water vapor after the collision get thrown back into the space and the atmosphere is not capable to hold them. Secondly, it is possible that Earth had begun to cool off slowly. Hence, when it developed an atmosphere around itself, any comet that hit its surface would not get evaporated. Therefore, the broken pieces and leftovers of the comet would remain in that state or just melt on the surface of Earth, without getting evaporated. Therefore, there are possibilities other than those mentioned in the argument, which it pays no attention to.

While it is possible that the water present in the atmosphere came down as rain and added to the amount of water present on the Earth’s surface, it is far from one’s imagination that the oceans came into existence only due to this rain. Oceans are large water bodies covering 3/4th of the earth’s surface. Rainwater does add to this by flowing into the oceans but the fact that only rainwater contributed to the formation of oceans is not comprehendible. There could have been other ways of this water being collected on the surface of the earth. As discussed above, it is possible that water already existed on Earth. It is possible that when Earth cooled off to form the solid crust, water was formed due to some form of condensation. There could be some precipitation other than rain on a large scale that resulted in water on Earth. There could be other heavenly objects colliding with Earth like meteorites etc. that could have eventually added water to the Earth’s surface.

Hence, it can be seen that the argument is not completely convincing. It falls short of explanation in certain situations as discussed above, which makes it inappropriate and faulty.