KentuckyFC writes: "One of the biggest puzzles of astrobiology is the origin of the Earth's oceans and atmosphere. One favoured theory is that our water is the leftovers from a bombardment of comets early in Earth's history. But the ratio of hydrogen and deuterium in the oceans doesn't match the ratio in the four comets measured so far (Halley's, Hyakutake, Hale-Bopp and C/2002 T7 LINEAR). Now a new analysis of the ratio of nitrogen-14 and 15 isotopes in these comets and on Earth places new limits on how much of our environment could have come from comets. On the one hand, the astronomers who did the work say that no more than a few percent of Earth's water could have come from comets. But on the other, they say that the ratio of nitrogen isotopes in these comets almost exactly matches the ratio in Earth's atmosphere. That suggests that while Earth's oceans must have come from somewhere else, Earth's early atmosphere was probably seeded by comets."