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Physicist Rob Sheldon on the Rosetta/Rosina experiment

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Dr Sheldon
Rob Sheldon

Sheldon tells us, re this Rosetta finding: Earth’s water not from comets):

I had a small part in building the Rosetta/Rosina experiment, the two mass spectrometers delivered by the University of Bern that measure the mass of water molecules, and hence the heavy isotopes of hydrogen that are described in the D/H ratio. (We had an internal debate on whether we should fly a magnetic sector or a time-of-flight mass spectrometer, and in the end, flew both.)

This reminds me of a recent paper in Science: Cleeves, L. I. et al., 2014, Science, 345(6204), 1590-1593, 26 September 2014, The ancient heritage of water ice in the solar system

The authors look at all the different reservoirs of water in the solar system, and try to explain why the D/H varies as it does. D or deuterium, is a very fragile isotope, and is the one most easily burnt up in the Sun. Oxygen–needed to make hydrogen into water–is not primordial, not made in the Big Bang, but made in stars like our Sun. So the paradox is that water should not have much deuterium in it, because the oxygen is processed through a star, but the deuterium cannot be. Hence, the models usually have the D coming from some “pristine” and primordial source like comets, while the O comes from exploding stars via dust clouds.

Well all this mix-and-match is hard to do quickly, but given lots of dust and time to blend, our proto-solar nebula could end up with with the requisite water. So one would think that all the planets would have exactly the same origin, and exactly the same D/H ratio. But they don’t.

So now our story gets even more complicated. It seems that after the mix-and-match condensation of our Solar System that makes the original water, there must be a “distillation” event. Since D makes water heavy (e.g. heavy water), it boils at a slightly higher temperature. So as the sun condensed and started burning some 5 billion years ago, it boiled off all the water on Mercury, most of the water on Venus, and sent it into the far reaches of the Solar system, where water condensed onto Jupiter and Saturn. Thus, the argument goes, Earth’s oceans should be enriched in D/H, having boiled off half their water, while Jupiter and Saturn are depleted in D/H, having acquired everyone else’s light water.

Not so, said the cometary contingent, but the Earth must have boiled off *all* its water, and the oceans were recreated by collecting icy, pristine comets from the far reaches of the solar system. Since the mix-and-match of cometary ice probably is different from the planets, then the D/H ratio of earth’s oceans reflects comets rather than the Sun.

Perhaps it is both boiling and comets, said a third contingent. Well this is now getting complicated enough that, oh, 20 scientists can make a nice living modelling comets and transport and boiling and planet-building etc, and of course, every one of the models tells us about how life came to be on our planet. (sarcasm alert)

The Cleeves paper pulls in some data on D/H ratios seen in distant stellar “planetary systems” by telescope–a tour de force actually–where it is much higher than anything in our solar system, and matches the highest cometary D/H ratios we’ve ever measured.

This resent Rosina result confirms this extra-solar-system result, finding D/H about 3X higher than the Earth’s oceans. So this means, what?

Well, not much actually. D/H ratios vary from comet to comet–which pretty much trashes the idea that the entire solar system dust cloud began with the same D/H mix-and-match ratio. But if all comets were like this one, then the Earth’s oceans weren’t made simply from comet water, and would require an admixture.

On the other hand, perhaps it tells us a great deal about comets. Comets are boiling off their ice as they approach the sun. Jupiter-class comets such as the one visited by ROSETTA are deflected by Jupiter, and go around the sun every 6 years. That’s a lot of boiling. So perhaps their D/H ratio is simply a result of boiling off most of their water. I’ve five or six papers on the theory of “wet comets” that would explain this ratio as this sort of fractionation, but having left Bern 18 years ago, I’m afraid I’ve lost my contacts with that scientific community.

In any case, the many recent comet missions are going to provoke a reanalysis of the Big Bang Nucleosynthesis paradigm that predicts low D/H and no O. Expect many surprises. As someone said, “comets are like cats–no two of them alike.”

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