Comet water in lunar rocks


Comet Hale-Bopp.Image via Wikipedia

There are two main hypotheses about the origin of Earth’s oceans: that they are filled with water that was locked in the meteoritic matter that initially accreted to form the Earth, or ocean water was delivered by massive comet bombardment in the first half billion years of the Earth’s history. It hasn’t yet been possible to decide whether one of these, or both were  involved, but the Moon might give a clue, even though until very recently it was regarded as being bone dry (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). The ratio between deuterium and hydrogen (D/H) gives a clue to the origin of water, in which both hydrogen isotopes occur (Greenwood, J.P. et al. 2011. Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Nature Geoscience, v. 4, p. 79-82). Using an ion microprobe to analyse the water in apatite, its dominant host in lunar rock samples, the authors were able to report two things. First, there is water in magmatic rocks of all ages found on the Moon: the earliest anorthosites of the lunar highlands and the younger basalts that fill the dark maria. Secondly, the water has D/H ratios significantly outside the terrestrial range. In detail, apatites with the greatest enrichment of deuterium relative to hydrogen are found in the maria basalts which fill enormous basins thought to have formed around 4 Ga ago as a result of cometary impacts. The D/H ratios are lower in apatites from the lunar highland anorthosites, which probably formed through flotation of low density calcium-rich feldspar as the Moon’s initially molten mantle crystallized not long after its formation through the impact of a small planet with the Earth. The highland D/H values are not wildly dissimilar from those found on Earth, yet those found in the mare basalts match the admittedly less well-constrained levels determined from comets hale-Bopp, Hyakutake and Halley. Because the Earth’s mass would ensure that it would corral 15 times more incoming extraterrestrial matter than would the Moon, the argument goes that if the Moon captured cometary water then Earth did so in trumps. The difference is that the Earths greater gravitational pull and thick atmosphere allowed it to retain gaseous and liquid water, while the Moon’s lower escape velocity let them leak away so that only mineralogically bound water could be retained.


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