I see you're confused.
"So far, astronomers have measured the deuterium/hydrogen ratio 11 comets. Only one, Comet 103P/Hartley 2, has a deuterium/hydrogen ratio matching the composition of Earth’s water. The other comets measured show a wide range of values."
-- http://earthsky.org/space/rosettas-comet-fuels-debate-on-origin-of-earths-oceans
"So far, astronomers have measured the deuterium/hydrogen ratio 11 comets. Only one, Comet 103P/Hartley 2, has a deuterium/hydrogen ratio matching the composition of Earth’s water. The other comets measured show a wide range of values."
-- http://earthsky.org/space/rosettas-comet-fuels-debate-on-origin-of-earths-oceans
I believe that the best matches for the terrestrial water are the asteroids.
Has asteroid water been tested the way comet water has been tested?
Did you figure out how deuterium is formed yet?
God poofs it into existence.
Nope.
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[12] As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. However, some comets may have a higher dust content, leading them to be called "icy dirtballs"
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Anyone who is interested can take a closer look at that:
http://kgov.com/origin-of-comets
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Only if you do not want accuracy.
There is no explanation how the very long period comets could have come back quite so quickly
That's the killer for Walt's story. How do we see a comet with a period of millions of years, returning in a solar system only a few thousand years old?