One theme of my posts that I have raised more than once is that while scientists are very good at collecting information and of measuring things, this leaves the problem of interpreting what it means. Scientific theory is based on either propositions or statements. A proposition is of one of two forms:
(1)  If theory P is true, then you will observe A
(2)  If and only if theory P is true, then you will observe A
Failure to observe A falsifies either proposition, but if you observe A, all you can say about (1) is the theory is in play. As Aristotle noted over two millennia ago observing A can only prove P if (2) applies, and it is the "only" condition that is difficult to validate. A statement (and an equation is a statement) carries the implied proposition that it is true.
 
What brought this thought on was one paper (Science, 345: 1590 – 1593) that has had quite some publicity, even in the public news media. What it claims is that at least some of the water we have is older than the solar system. What does that mean? First, it was deuterium/hydrogen ratios that were measured. We also note the authors were astrophysicists, and I quote: " our emphasis is on the physical mechanism necessary for D/H enrichment: ionization." As stated, that is an "only" statement, but I consider the "only" condition is unjustified. However, before getting to that, all hydrogen and deuterium was made in the Big Bang, and all oxygen atoms were made in supernovae. Water is made in space by oxygen and hydrogen reacting, usually on dust. Deuterium enrichment can arise because the O – D bond is stronger than the O – H bond, mainly because the latter has the larger zero point energy, so any process that breaks an O – H bond, particularly if it just does so, may increase the D/H ratio in what remains. It also arises through sublimation equilibria of ices in space, as heavier molecules sublime slightly less easily and under equilibrium conditions, they become enriched. Under these conditions, the D/H ratio of all water remains constant, and if ice gets enriched in deuterium, the vapour becomes depleted.
 
What they note is that the highest levels of D/H in water occurs in interstellar ices, and that Earth's oceans have a significant deuterium enhancement over solar hydrogen levels and are similar to comets from Jupiter's orbit, and a little less than that of interstellar water. They then model what they believe happened in the solar accretion disk and note that the deuterium levels we see are inconsistent with the disk physics/chemistry leading to the observed enhanced, with respect to solar, deuterium levels. What they then conclude is that comets could comprise either 14% or up to 100% of accreted interstellar ice, and ~7% or up to 30-50% of earth's oceans originated as interstellar ices. Why the "either" options? Largely because while they have a ratio for interstellar ices, they also have a water signal from the disk of a protostar. In short they believe the nature of the original water may vary from star to star. However, that is irrelevant to their claim that our water predates our solar system formation. They then conclude that provided the formation of our solar nebula was typical, then interstellar ices from the molecular cloud core should be available to all young planetary systems.
 
The last conclusion seems obvious. If there is water and ice in the cloud, which would be expected as long as the carbon levels do not consume all the oxygen, then the water ice should persist at least to the outer parts of the accretion disk, and indeed my theory of formation of the gas giants relies on this being so, so in one sense the paper supports my theory. On the other hand, provided there were water ices in the cloud, what could possibly happen to them until they reached the ice sublimation temperature, given that the disk is opaque so while the star is forming, ionizing radiation should be absorbed much closer to the star? It is here that they seem to have overlooked that there are three important hydrogen sources: interstellar ice, interstellar water vapour, and hydrogen. The latter is about four orders of magnitude higher than anything else, and determines the initial deuterium levels in the star. Nuclear burning will then decrease the stellar deuterium levels.
 
However, the conclusion that Earth's water reflects the deuterium content of the water as it was accreted is an "only" statement, and it is not true. There is a further possible mechanism: as water travels through hot rock, and current volcanism shows it does, it may oxidize any reduced species, and in many cases liberate hydrogen, which may then escape to space. Such reactions involving the breaking of the O-H bond will also be affected by the chemical isotope effect, with O-D bonds reacting significantly more slowly, and that in turn will lead to deuterium enrichment. That is my explanation for the Venusian atmosphere, where there is a hundred-fold enrichment of deuterium (Science 216: 630-633). The reactions include water reacting with carbon or carbides as the original source of the carbon dioxide in the Venusian atmosphere. As I showed in Planetary Formation and Biogenesis by reviewing a number of papers, either the gases were emitted from the earth, in which case they had to be accreted as solids, or they were delivered from space, but if the latter were the case, each rocky planet had to be struck by completely different types of bodies, and the Moon, quite remarkably, be struck by only trivial amounts of any of the volatile containing bodies. Note that most asteroidal bodies contain negligible volatiles.
 
So what do I make of this? Of course water arrived from interstellar space, and this work at least supports my concept of ice accretion. On the other hand, the presence of ices in the disk is generally held to be the reason why the giants form, so in another sense this paper simply supports what was long assumed. I am not convinced it warranted the media attention it received.