Where did earth s water come from

A.� Well there have been a few theories about the origin of earth's water. The most recent theory is that colliding comets that would have broken up on impact with the earth's surface dumped it. To find out how water got into comets in the first place, click here.

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This theory did fall out of favour because scientists pointed out that most comets have heavier water than that found on earth.

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Q.� Heavy water Is there more than one type of water

A.� Yes. Although all water is made up of two hydrogen atoms joined to one oxygen atom, there are different types of hydrogen atoms, known as isotopes, which have different weights due to extra components in their nuclei.

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Up until recently the analysis of the water found on comets was isotopically different to that found on earth.

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Q.� What happened recently to put this theory back in favour

A.� If you've read the articles on where ice in comets come from you'll know that comets formed on the outskirts of our solar system, where it is very cold, cold enough for ice to form. However it now seems that some comets formed nearer to the sun, although still far enough from the centre for ice to form. And scientists now think that ice held in these comets will be lighter than that found on other comets like Hale- Bopp, and Halley, which formed at the furthest reaches of our solar system.

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And so the theory that comets dumped all the necessary ingredients for life on earth water, amino acids and energy, is back in favour.

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Q.� Surely a comet collision would be cataclysmic, how could anything survive, let alone water and amino acids

A.� Scientists are quite confident that if a comet hit earth at a low angle then enough ice would survive. In fact in experiments the amino acids not only survived the impact but also polymerised into chains of two, three and four amino acids, so-called peptides, the first stage of the building blocks of life.

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Q.� Were comet collisions so frequent to provide all the water that makes up 70%of the earth's surface

A.� Fortunately for us comet collisions are quite rare events now. But at the time that the planets were forming, several billion years ago, there would have been a lot more debris flying around and comet collisions with earth would have been more frequent.

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Scientists suggest that a comet with a 1,000 kilometre diameter can hold up to 3 billion kilograms of water.

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Do you want to quiz The AnswerBank about our solar system Click here to ask your question.

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by Lisa Cardy