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?

The Moon Re-examined.

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.
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 Europe/London

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