Science Question, where did the water that is on the earth come from?

• Since the earth cooled from a hot ball of fire, and there was no Atmosphere, how or where did the water come from, and what kept it trapped in to make the atmosphere, what kept the water vapor here in the first place. To me it's kind of like the chicken and the egg thing. Maybe some of you smart Science people can shed some light on this for me, I saw a show on the history channel, but there was no real conclusion, so what can you add to this? thanks

• Answer:

  One of the websites linked in a former answer says this: > Now there are some scientists who suggest that water was dumped on Earth by a zillion comets billions of years ago. This view is intriguing because the Earth's distant brother, the planet Mars, or even our Moon, contain no large quantities of water. ... Then it goes on to say that this question will continue to confound scientists and the only realistic explanation is that God did it just like it says in Genesis. Well, comets ARE the scientifically accepted explanation. The Earth was not a ball of fire, but it was very hot due to the gravitational energy of all the rocks falling from space. Eventually, we pretty much ran out of big rocks in our neighborhood, and things calmed - and cooled - down. But comets continued to collide with the Earth. Since comets are made up of ices of methane, ammonia, and water, that's what they brought. Our first atmosphere was methane and ammonia, and water vapor,just like that of Jupiter, Saturn, Uranus, and Neptune. The comets kept coming - and are doing so even today - and the water cooled the Earth. In the process, it boiled, then rained, and so on. At first there were no oceans, but lots of rain. The rain eroded the hot land and dissolved minerals from it. This explains why the ocean is salty. So why didn't the atmosphere and oceans just fly off into space? Because Earth has enough atmosphere to keep most of it in. (Our atmosphere used to be thicker.) Mars is smaller, and has collected and kept less atmosphere. The moon is much smaller, and has practically none. Jupiter, Saturn, Uranus, and Neptune are very large and heavy and have very thick atmospheres. (Venus is about the same size as Earth but has a much thicker atmosphere. That's because it's mostly carbon dioxide and sulfuric acid - much heavier molecules than oxygen and nitrogen.) After the Earth cooled enough for the rain to puddle, the oceans began to grow and the atmosphere to become more dense. Life in the oceans eventually created free oxygen as a byproduct of metabolism, and that replaced the methane and ammonia atmosphere. Now as to the mystery of which came first: That website's argument is that the atmosphere had to be there before the oceans or it would evaporate into space. Well, why doesn't the atmosphere itself evaporate into space? After all, it gets thinner and thinner the higher you go, and there's no fence or airlock. The answer is twofold: Gravity keeps most of it in, and some of it DOES escape. Are we then running out of air and water? No, because the comets keep bringing in more material. Just because someone doesn't understand something doesn't mean that nobody does. Just because nobody fully understands something doesn't mean that the only alternative is magic.