|Comet Hartley 2|
Interesting report from BBC News Comet's water 'like that of Earth's oceans':
Comet Hartley 2 contains water more like that found on Earth than all the comets we know about, researchers say.Given the amount of water on earth, the delivery schedule must have been a long one.
A study using the Herschel space telescope aimed to measure the fraction of deuterium, a rare type of hydrogen, present in the comet's water.
Like our oceans, it had half the amount of deuterium seen from other comets.
The result, published in Nature, hints at the idea that much of the Earth's water could have initially come from cometary impacts.
Just a few million years after its formation, the early Earth was rocky and dry; most likely, something brought the water that covers most of the planet today.
Water has something of a molecular fingerprint in the amount of deuterium it contains, and only about half a dozen comets have been measured in this way - and all of them have exhibited a deuterium fraction twice as high as the oceans.
Asteroids, by contrast, give rise to the meteors and meteorites that arrive on Earth, making their deuterium fraction more well-established.
Meteoritic material has roughly the same proportion of deuterium that the Earth's oceans contain, and so the assumption has been that if water arrived from elsewhere, it came from asteroids.