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Comets Seeded Earth’s Early Atmosphere

The ratio of nitrogen isotopes in several comets almost exactly matches the ratio on Earth, implying that our early atmosphere probably came from a cometary bombardment.

Astrobiologists have long puzzled over the origin of Earth’s oceans. But they’ve dwelt a little less long over a related question: where does the nitrogen in our atmosphere come from?

Now a new analysis by Damien Hutsemekers and pals at the Universite de Liege, in Belgium, suggests an answer to both questions.

One of the most attractive theories of the origin of our water is that Earth was once bombarded by icy comets that left a watery residue. The trouble is that the ratio of deuterium to hydrogen in water on Earth is much lower than it is in the few comets we’ve been able to measure it in (i.e., Halley, Hyakutake, Hale-Bopp, and C/2002 T7 LINEAR). So if these types of comets, which we know came from the Oort Cloud, did supply Earth’s water, it must have mixed with water already on Earth that had a very low deuterium content.

Now Hutsemekers and co have put a different kind of constraint on the cometary contribution by measuring nitrogen isotopes. They say that comets must deliver water and nitrogen together (although they don’t say why), so a comparison of nitrogen isotopes can also place limits on the amount of water that must have been delivered.

Their conclusion is that “no more than a few percent of Earth’s water can be attributed to comets.”

But that’s not the end of the story. Interestingly, they say that the ratio of nitrogen-14 to nitrogen-15 in cyanide and hydrogen cyanide in comets almost exactly matches that on Earth. “A significant part of Earth’s atmospheric nitrogen might come from comets,” they conclude.

That’s any exciting result because it implies a dual origin for our oceans and atmosphere.

Of course, the huge fly in this hypothetical ointment is that there may well be other types of comet out there that have hydrogen to deuterium ratios as well as nitrogen isotope ratios that more closely match our own.

But in the meantime, the idea that comets gave us our early atmosphere is cool enough to keep us a-wonderin’ for a while.

Ref: arxiv.org/abs/0906.5221: New Constraints on the Delivery of Cometary Water and Nitrogen to Earth from the 15N/14N Isotopic Ratio

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