Comet Hartley 2 has been thrust into the limelight during the last couple of weeks after a report on it published by Nature. The comet has caused a bit of a problem for NASA scientists.

And the problem ….

Well, the water it contains is more like Earth’s water than water on other comets. The problem is deuterium, the heavier isotope of hydrogen.

The handful (six or so) of comets that have had their water content analysed seem to have twice as much deuterium. Asteroids on the other hand, seem to have the same deuterium content as the earth. The early Earth had very little water and so it needed to come from somewhere. The assumption was that it had therefore come from asteroids.

This latest discovery has left researchers scratching their heads and looking for answers to the question, where did the water on earth come from?

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