Earth's water: from where?
One thing that brings joy to someone who engages in theoretical work is to find observational evidence that supports a theory that contradicted "standard theory" that everyone accepted when the theory was presented, which in this case was in 2011, in my ebook Planetary Formation and Biogenesis. That fact that nobody else takes any notice is irrelevant; the feeling that your theory alone actually meets the conditions imposed by nature is great.
The relevant part involves the formation of the rocky planets. The standard theory is that these formed from the collision of planetesimals (bodies up to 50 km in size, which were formed by some totally unknown process), and the volatiles came from a subsequent bombardment of carbonaceous chondrites, or something like them. The review I gave of this process (the ebook has over 600 references) shows a number of reasons why this should be wrong, mainly in the form of a whole lot of other things that should have accompanied the water, and clearly did not in the right ratios, but the theory was held onto because it was perceived that there was no alternative. When the rocky planets accreted, it was too hot for water to accrete at those pressures by any reasonable physical process.
My answer was that Earth formed by chemical processes. Very specifically, in the early stages of the accretion disk, there were temperatures where calcium aluminosilicates could phase separate out of melt-fused rocks, and when the disk cooled, collisions made dust, the dust adhered to rock and collected water vapour from the nebula to set the cements into effectively concretes. These were strong enough to permit them to survive the milder collisions, and they would rapidly accrete small material, effectively growing more by monarchic growth than the usually assumed oligarchic growth. Accordingly, the water that set the cements would be primordial, and this would be the source of Earth's water.
The good feelings I am sharing come from a recent paper by Hallis et al. (Science 350: 795 – 797) that reports the deuterium/hydrogen ratios in some primordial rock samples originating in the deep mantle. These lavas, found in Baffin Island and Iceland, have 3He/4He ratios similar to primordial gas (and up to 60 times higher than atmospheric helium) and have Pb and Nd isotopic ratios consistent with primordial ages (4.45 – 4.55 Gy). They also contain water, and the deuterium levels of the water indicate that the water almost certainly had to be primordial, from the accretion disk itself and not from chondrites. You can see why I am happy.
Posted by
Ian Miller on Nov 16, 2015 10:12 PM Europe/London
