Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

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Archive for October, 2014
Recently, there have been two themes regarding the Moon's origin. Some unexpected but now well-known results from the samples returned by Apollo included:
1. the rocks were remarkably dry,
2. the isotopes of oxygen and some other elements were essentially the same as those of Earth whereas these isotope ratios differ from other samples, such as chondrites, and from Mars,
3. there was considerable anorthosite, which is a feldspathic mineral, present. Earth is the only planet that we know of that has extensive feldspar. (Mars has a limited amount of plagioclase, but no known extensive granite. Venus may have two granitic cratons, Ishtar and Aphrodite Terra, but we have no means of knowing.)
 
This information was most easily accommodated by postulating a Mars-sized impactor, Theia, colliding with Earth and sending up massive amounts of silica vapour, from which the moon condensed. Various computations have shown this was possible, it explained the dryness, provided the bulk of the mass came from Earth it explained the isotope levels and the composition, so it became the established theory. Because the condensate was from Earth's surface, radioactivity levels would be low, which explains why the moon has been essentially dead for a long time. The major activity has been considered to have involved massive impacts during the so-called late bombardment. There was always one problem: in detail most impactor computations require much of the moon to have come from Theia, in which case Theia, coming from somewhere else, should have a different isotope and mineral composition. Also, the relative velocity of Theia on impact should not significantly exceed the escape velocity, which means, at a distance where Earth's gravitational field becomes insignificant, it should have little excess energy. There is one largely overlooked option from Belbruno and Gott that I prefer: Theia accreted at one of the Lagrange points to the Earth/Sun system. If we assume the isotope composition of the accretion material is dependent on the solar radial distance, then the composition similarity follows automatically, while there is no problem with the collision energy. If my theory outlined in Planetary Formation and Biogenesis is correct, maximum rates of initial accretion occurred for rocky planets at the Earth distance from the star, and there would be enhanced accretion of calcium aluminosilicates (because as cements, they caused the accretion) and this would explain/require the enhanced anorthosite.
 
The standard picture is now starting to show signs of wear. First, the moon did not die quite so rapidly, and certain small volcanic areas on the Moon appear to have had eruptions within 100 My BP (before present), and possibly up to 50 My BP. Further, while the Procellarum region has been interpreted as an ancient impact basin of approximately 3,200 km diameter, gravity anomalies show that the region is essentially a massive lava outflow, consistent with the higher concentrations of the heat producing elements uranium, thorium and potassium in the rock. These elements are readily concentrated if the body has melted, because they tend to be the last to crystallize out. But that requires fluid, such as from a magma ocean. Even if the impactor did not form a vapour, a magma ocean still remains very probable. The magma ocean also favours the formation of the aluminous crust, as it would float on basalt. (Interestingly, one review noted that plagioclase only floats on dry magma; where this came from is unclear because basalts usually have a density of about at least 0.7 units greater.)
 
The issue of whether the moon condensed from vapours is unclear. There is a lack of fractionation among refractory elements, which is strong evidence that the moon did not form by condensation of vapours, yet the moon is depleted in volatile elements such as potassium, which is generally considered to indicate that there were vapours, BUT it turns out the isotopes of potassium are the same as on earth and other solar system bodies, which counts against vapour condensation. Even more suggestive, lithium isotopes are the same as on Earth. Thus it is generally concluded that the Moon has little trace of the impacting body, even though models show the impactor makes a significant contribution to the putative proto-lunar disk. To summarize, the formation of the Moon requires a highly energetic origin, it carries the elemental signature of Earth, but it is depleted in volatile elements and water. Of course, if Theia accreted at the Lagrange point, then the resultant collision will still have been energetic, but maybe not quite as energetic. There may have been sufficient energy to lead to extensive dehydration and moderate loss of potassium, but essentially as a single event, which would not lead to significant isotope fractionation, as opposed to equilibration between vapour/liquid, which would.
 
References:
Andrews-Hanna, J. C and 13 others. 2014. Structure and evolution of the lunar Procellarum region as revealed by GRAIL gravity data. Nature 514: 68 – 71.
Belbruno, E., Gott, J.R. 2005. Where did the Moon come from? Astron. J. 129: 1724–1745.
Braden, S. E., and 5 others, 2014. Evidence for basaltic volcanism on the Moon within the past 100 million years. Nature Geoscience : doi:10.1038/ngeo2252
Taylor, S. R. 2014. The Moon re-examined. Geochim Cosmochim. Acta 141: 670–676.
Posted by Ian Miller on Oct 26, 2014 10:00 PM GMT
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.
Posted by Ian Miller on Oct 13, 2014 1:57 AM BST