What is it that is so special about the ratio of isotopes in our oceans that other sources of water would not be expected to have similar ratios?
I'm not sure 'special' is the right word, but past measurements of the abundance of elements in water in a few comets have been made, and the majority of the results have indicated about twice as high a deuterium to hydrogen ratio in comets as is found in the oceans on earth. By contrast the water in meteorites has turned out to have ratios matching that found on earth's oceans. So, possibly water may have arrived on meteorites and asteroids rather than comets.
So instances of comets with similar ratios to that found in the earth's oceans are interesting because they suggest that perhaps the water on earth might come from comets. So far only 1 or 2 comets have been found that might have isotopic hydrogen ratios like the water found on earth.
Why could it not have formed in situ ??
Most of the lighter stuff like hydrogen and water got moved towards the outer part of the solar system during formation resulting in rocky inner planets and gaseous/icy outer planets. If this is correct, then we need an explanation for why there is so much water on earth.
Edited by NoNukes, : No reason given.
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