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The February issue of Nature Geoscience features a couple of interesting articles on the origin of water in the Earth-Moon system. By way of an introduction, Robert (2011) reviews the known ratios of deuterium (heavy hydrogen; one proton, one neutron) to hydrogen (one proton) of various planetary bodies in the solar system: the proto-Sun, Earth, and Moon, along with carbonaceous chondrite meteorites and comets (Fig. 1).

Robert (2011)

Fig. 1. Deuterium/hydrogen ratios of the proto-Sun (peach), Earth (blue), Moon (red), carbonaceous chondrites (black), and comets (green). The D/H ratio is multiplied by 10^6 reflecting parts-per-million quanities of deuterium with respect to hydrogen. Adapted from Robert (2011).

As you can see, there are a couple of interesting isotopic associations. The Earth overlaps strongly with carbonaceous chondrites, while the Moon spans a range of D/H values, potentially indicating a significant affinity with comets.

Why might this disparity between the Earth and Moon exist? The prevailing hypothesis for the formation of the Moon is the impact of Theia with Earth, so within this framework, it stands to reason that water on Earth appeared after the formation of the Moon via significant contributions of water from carbonate chondrites.

By comparison, Greenwood et al. (2011) discovered that abundant lunar water exists bound up within the hydrous mineral apatite, which represents a mafic phase within the mare basalts and anorthositic highlands. The interesting thing about the Moon, however, is that a number of sources are identified including solar protons, the lunar mantle, and comets (D/H ratios increasing respectively, with the solar fraction as the lightest). These different sources potentially explain the wide range of D/H ratios observed in lunar rocks.

This appears to be a tidy hypothesis, but as Robson (2011) hints, how do you explain the prominent influence of comets on lunar water, and its apparent absence in terrestrial water? The Earth and Moon are next-door neighbours in the context of the solar system, and Greenwood et al. (2011) predict a likewise cometary bombardment of the Earth at this time.

So where is the terrestrial D/H isotopic signature reflecting this cometary bombardment interval? Or is it there, but just obscured by the lighter, carbonaceous chondrite fraction? That may be the case following the research of Kulikov et al. (2006) which indicates that the relatively high D/H ratio on Venus arises from the equivalent loss of a terrestrial ocean; something which most certainly did not occur on Earth.

References

Greenwood, J.P., Itoh, S., Sakamoto, N., Warren, P., Taylor, L., and Yurimoto, H., 2011: Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Nature Geoscience, vol. 4, p. 87-92.

Robert, F., 2011: Planetary science: A distinct source for lunar water? Nature Geoscience, vol. 4, p. 74-75.

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