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Thursday, October 06, 2011

Earth's Water Delivered By Comets?

Comet Hartley 2
Well, why not?

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.

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.
Given the amount of water on earth, the delivery schedule must have been a long one.


  1. When you have, as Carl Sagan actually didn't say, billions and billions of years, FEDEX (formerly Flying Tiger)rates of delivery aren't quite necessary.
    Pretty cool stuff. This is the sort of thing we covered in a planetary geology course I once took which also included such wild images as icebergs of rhyollite floating inlunar seas of molten basalt, hot ehough to glow blue.
    One of the guys who taught the course recently died, so to Dr. Mulberger, rest well, Marine.

  2. Anonymous9:38 AM

    A typical comet contains some 10 to the 13th power kg of water ice. How many comets would have to strike the Earth in order to account for the roughly 2 X 10 to the 21st power kg of water presently found on our planet? If this amount of water accumulated over a period of 0.5 billion years, how often would Earth be hit by a comet during that time?
    Divide the mass of the Earth's water by the mass of a typical comet: (2 X 10 to the 21st power kg) / (10 to the 13th power kg) = 2 X 10 to the 8th power comets.
    Divide this number by 5 X 10 to the 8th power years to get 0.4 comets hitting per year, or one comet every 2.5 years