The primary method for investigating where the water on Earth comes from is by measuring the deuterium to hydrogen (D/H) ratio of water from different sources. The problem with measuring that ratio in water on Earth is that almost all water that is readily available has been recycled and mixed with materials on the Earth surface or atmosphere. Recent lava flows in Canada and Iceland brought primordial material up from deep within the Earth. Researchers measured the D/H ratio of water trapped in this material and found the D/H ratio to be lower than anything else measured apart from the Sun. This suggests that this water did not come from comets nor meteorites, but from the dust cloud from which the Earth formed (from the research paper summary):
Where did Earth's water come from? Lavas erupting on Baffin Island, Canada, tap a part of Earth's mantle isolated from convective mixing. Hallis et al. studied hydrogen isotopes in the lavas that help to "fingerprint" the origin of water from what could be a primordial reservoir. The isotope ratios for the Baffin Island basalt lavas suggest a pre-solar origin of water in Earth, probably delivered by adsorption onto dust grains.
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You don't need alien asteroids, you just need a hydrogen-rich atmosphere and liquid hot magma:
A new research model shows that Earth's oceans could have formed from interactions between a hydrogen-rich early atmosphere and oxygen within the planet's magma.
The study from the multi-institution AETHER project also demonstrates why Earth's core is lighter than it should be, owing to the presence of gaseous hydrogen.
Edward Young, professor at the University of California Los Angeles, and colleagues propose that one of the protoplanets involved in the formation of Earth was heavier than thought. By maximizing its size to more than a fifth or third of Earth, the researchers show there would have been enough gravity to make the hydrogen-rich atmosphere hang around long enough to interact with the magma ocean, according to a paper published in Nature this week.
Prevailing theories explaining the abundance of water on Earth – oceans make up around 70 percent of the planet's surface – depend on the impacts of water-carrying asteroids.
[...] In a statement coinciding with the publication, co-author Anat Shahar, staff scientist and deputy for Research Advancement Earth and Planets Laboratory at Carnegie Science, said the inspiration for the new model came from studies of planets forming outside the solar system.
(Score: -1, Offtopic) by Anonymous Coward on Sunday November 29 2015, @03:02AM
World war 2 was faked! Wake up!
(Score: 3, Funny) by aristarchus on Sunday November 29 2015, @03:15AM
So, extra-solar system source for water, eh? Thus, the water on the Earth is an illegal immigrant! It's foreign water!! Some of it probably has thighs the size of cantelopes, some of it (hey, who are we fooling! All of it) is murderous and rapy, and some of it is probably ISIS!!! Just think, Mandrake, did you ever see a Commie drink water?
(Score: 1) by nitehawk214 on Sunday November 29 2015, @07:30AM
I thought deuterium is stable. Why would there be different ratios of it anywhere?
Or is it just that since it is heavier than hydrogen, things like D2O have a slight tendency to sink, and therefor the Earth's core has more D compounds than H?
"Don't you ever miss the days when you used to be nostalgic?" -Loiosh
(Score: 1) by nitehawk214 on Sunday November 29 2015, @07:32AM
Oh wait, the deep water has a lower concentration of deuterium. The interesting part is that it is different from comets, but not why it is different. But I am still confused as why.
"Don't you ever miss the days when you used to be nostalgic?" -Loiosh
(Score: 4, Informative) by hubie on Sunday November 29 2015, @07:28PM
Deuterium and hydrogen can be created or destroyed (for deuterium) via cosmic ray interactions in space (or on the Moon), so cometary or meteroite D/H ratios are probably different than from what the Earth accreted out of, and any water at the Earth's surface has mixed with the atmosphere and most likely had its D/H ratio changed via cosmic rays or other chemistry. These guys are arguing that they've found a source of water that has been contained inside the Earth since the beginning, so this D/H ratio is representative of what the Earth formed out of.
A related 2005 paper [rice.edu] gives a lot of nice background explanation for this, and it would seem that these new observations are consistent with the speculation in the 2005 paper.
(Score: 1, Interesting) by Anonymous Coward on Sunday November 29 2015, @10:17PM
If the water did not come from comets, it follows that either comets do not contain much water or we need to revise the estimated number of past impacts.