There seems to be a lot of confusion and conjecture in the comments about the grandiosity of the claim. This does not necessarily rule out all comets. Maybe an attempt at a better summary of the article would be helpful:
- Not all water is the same. Some water is heavier due to a presence of a certain amount of deuterium.
The general consensus is:
- When the solar system formed, the components for water were created.
- These components eventually formed with the early Earth and a water cycle was created.
- Yes, the early Earth was hot, but heat and elements were plentiful and Earth managed to hold onto some of these elements and would have had water evaporating and raining back down again.
- The planet Theia *collided* into the Earth. A certain amount of the debris coalesced into the moon. Imagine Pluto smashing into your house.
- The heat from the collision would have evaporated/released all elements lighter than X, which includes water. (ed: perhaps water on the moon is more closely related to early earth water coalesced and re-condensed?)
- Sometime later, the Earth received much more water than would have been sustained from such an impact.
- The weight (deuterium ppm) of this "new" water is different (much lighter) than the weight of "old" water, and generally any other water in the solar system.
So where did this "new" water come from?
This article suggests:
"We have light water in some comets and very heavy water in other comets. We have to assume the mixture of all these comets is something that is heavier than what we have on Earth, so this probably rules out Kuiper Belt comets as the source of terrestrial water."
And I believe this means:
It would have taken many of these Kuiper Belt comets to contribute a great deal of water to the Earth. If we use probe measurements to confirm other measurements and calculate the *average* weight of water on a number of Kuiper Belt comets (along the order of magnitude necessary be a main source of "new" water for the Earth), then we see that the amount of deuterium in Earth's water would have been much greater -- i.e. the water would contain an average weight of all impacts needed to saturate.
Thus this rules out Kuiper Belt comets being the main source of "new" water for Earth. Their water in general is simply too heavy on average. As soon as enough Kuiper Belt comets impact the Earth to come close to the amount of water needed, the calculations show that the level of deuterium would be much, much higher than what we see.
And the article itself turns to conjecture with:
So where do we look for lighter water? Maybe asteroids?