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Earth's surface is dominated by oceans. But where did all that water come from?
Asteroids and comets smashing into the early Earth have long been thought to be a promising source. But measurements of Halley's Comet and five others threw cold water on the idea that comets provided a large share of the oceans. Because the chemical signatures of the comets did not match the oceans. Specifically, the ratio of heavy hydrogen, or deuterium, to regular hydrogen was too high in the comets.
But now astronomers have gotten a glimpse of a comet with a different origin, and it matches the oceans much better. They used the Herschel space telescope to examine Comet Hartley 2, which originated in the Kuiper Belt. Halley and its ilk came from a distant swarm of comets called the Oort Cloud.
At a planetary science meeting last week in France, and in a study in Nature, the researchers announced that Comet Hartley 2 has the same deuterium-to-hydrogen ratio as ocean water. [Paul Hartogh et al., "Oceanlike water in the Jupiter-family comet 103P/Hartley 2"]
So maybe comets did play a major role in delivering Earth's oceans. Something to ponder next time you have a day at the beach.
—John Matson
[The above text is a transcript of this podcast.]
[Scientific American is part of the Nature Publishing Group.]
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11 Comments
Add CommentDo all comets in the Kuiper Belt have the same deuterium-to-hydrogen ratio as ocean water?
Reply | Report Abuse | Link to thisOcean water having identical deuterium-to-hydrogen ratio to Kuiper Belt comets supports ONLY that ALL of Earth's water came from Kuiper Belt comets! That the ocean's ratio might match this one comet and not others could be coincidental.
"Do all comets in the Kuiper Belt have the same deuterium-to-hydrogen ratio as ocean water?"
Reply | Report Abuse | Link to thisProbably not. Other articles mentioned that this was the first one they discovered with this ratio (other comets had a different ratio). If other comets have different ratio it is highly likely that some of them are in the Kuiper belt.
Thanks! In that case, I fail to see how this one comet's deuterium-to-hydrogen ratio provides any evidence of the source of Earth's water!
Reply | Report Abuse | Link to thisIt provides evidence just not conclusive evidence. The fact that two separate water reservoirs have the same isotope distribution suggests that they have the same source. That is evidence. You can debate the relative merit but you can't debate whether or not it is relevant.
Reply | Report Abuse | Link to thisThen every comet that does not have the same isotope distribution provides evidence that they do not have the same source.
Reply | Report Abuse | Link to thisYes, or that their contribution is relatively small. There are different types of comets that formed at different times in different places so their relative contribution to earth's oceans may also be different. Water got here somehow, was it part of the accretion disk or was it deposited later? I guess the problem with it being part of the accretion disk is that the early hot earth would have boiled it off so the comet hypothesis has some potential. With this discovery they now have a potential source.
Reply | Report Abuse | Link to thisAs I understand, much of the Earth's water is currently contained within its mantle, not its thin atmospheric shell or its denser coating of oceans and lakes.
Reply | Report Abuse | Link to thisI also understand that during the first billion years of Earth's existence the Sun was only 70% as luminous as it is today. By that time Earth's magnetic field had developed, providing increased protection for its volatile atmosphere and water from the Solar wind.
Again, unless it's proposed that this one comet provided all the water on Earth or all Kuiper Belt comets have the same isotope distribution, that it matches the Earth's may be purely as statistical coincidence.
After all, this evidence also supports the assertion that this comet originated from Earth!
IMO the Earth's specific deuterium-to-hydrogen ratio is most like the product of an intermixing of water from several sources that may have had their own distinct ratios of deuterium-to-hydrogen.
You seem to be employing an appeal to ridicule in order to back-pedal from your earlier statement that you did not see this finding as evidence of the source of earth's water.
Reply | Report Abuse | Link to thisI don't understand the point of your first two paragraphs. If they were intended to refute my guess as to why the hypothesis that Earth's water did not originated from the accretion disk I think they are way off the mark. The heat of the early earth was due to friction not just heat from the sun. But perhaps you intended something else. Again, I am just guessing (I used that word BTW). It has been a long time since I modelled solar system formation.
Your statement that one must either conclude that earth was seeded with water by this one comet or conclude that this is only a coincidence goes to my first point. It is a ridiculous statement and you should be embarrassed for making it. Once again, the findings in this article are not conclusive but they are, in fact, evidence unless you are asserting that you know for a fact that there is only one comet in the solar system with the indicated isotope distribution and we just happened to find it, otherwise it is reasonable to assume there may be others and so continue the search.
Your assertion that the evidence supports that this comet originated from earth also goes to my first point. If that were true then the mineral composition would also have the same isotope distribution. This is how we determine the origin of meteorites. Regardless, it is irrelevant what other hypotheses this finding supports.
"IMO the Earth's specific deuterium-to-hydrogen ratio is most like the product of an intermixing of water from several sources that may have had their own distinct ratios of deuterium-to-hydrogen." that's excellent. Now perhaps you can show us the science you did to prove your hypothesis. Or are you suggesting that your uneducated opinion should be given a higher weighting than the actual science referenced by this article?
You seem to not understand what the word evidence means. It does not mean that it establishes the hypothesis conclusively. It does not mean that it does not support any other hypothesis. It simply means that it supports the hypothesis and does not refute it. Your attempt to deny that fact by offering ridiculous arguments once again shows how little you understand science.
We may want to consider a third possible reservoir of comets from the Inner Oort Cloud (IOC) with a lower than telluric D/H value that might require a small amount of Outer Oort Cloud (OOC) 'frosting' to even things out.
Reply | Report Abuse | Link to this"The primary δD values of the biotites and hornblendes in granitic batholiths are remarkably constant at about −50 to −85". (Hugh P. Taylor Jr, 2002)
My contention is that our solar system began as a triple star system inside the orbit of Mercury. The smaller tertiary stellar body (TSB) bled angular momentum from the larger binary pair causing them to merge in a luminous red nova (LRN) at about 4.567 Ga. The LRN created the short-lived radionuclides (SRs) of the early solar system in a shock wave bubble that expanded and stalled at about 1 light year to form the OOC comet clusters. The drag on the TSB during the red giant phase of the LRN caused it to spiral in and merge with the sun in a smaller secondary LRN that stalled at the closer distance of the IOC to form the IOC comet clusters.
The Oort Cloud comets differentiated from SR heating and chemoautotroph-microbe oxidation to form minerals that lithified into solid rock cores: gneiss/schist/dolomite in OOC comets and granite/greenstone in IOC comets. The more abundant IOC comets were greatly over represented in the late heavy bombardment (circa 3.9 Ga), caused perhaps by close encounters with sister stars when our birth star cluster broke up. Then perhaps in the Late Proterozoic our sun had a close encounter with a passing star or large molecular cloud that set us on our present inclined orbit around the galactic core. And this inclined orbit causes preferential tidal disruption of the OOC comets twice each galactic year (1 galactic year = 225-250 Ma) when our sun crosses through the galactic plane. This 'recent' over sampling of OOC comets has resulted in the recent increase in the gneiss, schist and sedimentary rock of the continental 'platforms' and has raised the D/H to modern values with the heavy-water frosting on the cake.
I assure you I intended nothing personal in my comment.
Reply | Report Abuse | Link to thisI simply cannot understand how the discovery that a single comet originating from the Kuiper Belt has the same deuterium-to-hydrogen ratio as Earth's oceans can be taken as evidence that Earth's water originated in Kuiper Belt objects.
For this to be true, all Kuiper Belt objects that delivered water to Earth must have had the same deuterium-to-hydrogen ratio and Earth could neither have been accreted from any significant amount of water bearing minerals (like many asteroids) or received any significant amount of water from any other source that had a different deuterium-to-hydrogen ratio.
As I said, IMO the Earth most likely contains water from several sources, each with differing deuterium-to-hydrogen ratios that, now blended together, coincidentally match the ratio found on cComet Hartley 2.
Again, you don't seem to understand the word evidence. Two water reservoirs with the same isotope distribution is unlikely if they had different origins. Therefore it is suggestive of the hypothesis that either one comes from the other or they have the same source. Keep in mind that the Kuiper Belt objects are still there because there is no planet to sweep them up. In the early solar system, objects with the same composition may have been common throughout the solar system. Only once the planets formed did they disappear from within the paths of the planets. Those comets may have condensed from the accretion disk and therefore had the same average isotope ratio. So again, the fact that there is a comet with the same isotope ratio as the earth is evidence that comets brought water to earth. It may prove not to be the case. But if it isn't then there will be additional evidence to rule out that possibility which we currently do not have. So, for now this comet does not contradict that hypothesis. It may also support other hypotheses but that is irrelevant. The fact that you can imagine other hypotheses is irrelevant. Your hypothesis that a random mix of different sources of water coincidentally matches the comet is even less likely but even so, it does not mean that this comet is not evidence. And you still have to prove that those bodies existed with that random mix of isotopes that when combined was the same as the earth and this comet. Occam's razor cuts your hypothesis down to size. You seem to think that your made up "evidence" is somehow better than actual scientific measurements. So what I cannot understand is how non-existent evidence about some unproven objects is proof that earth's water came from a mix of these imaginary sources.
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