6.3.1 Water in lunar minerals
While the first evidence for small amounts of water in lunar rocks came from analyses of volcanic glass beads, there are also signs of water in the minerals of crystallised Moon rocks, because some minerals that have been found in Moon rocks are capable of holding small amounts of water as structurally bound hydroxide (OH−) ions. The most suitable mineral candidates are apatite (phosphate of calcium) and amphibole because both minerals contain OH− (derived from water) as an essential structural constituent. While amphibole is extremely rare in lunar rocks, apatite is more common.
Apatite contains structurally bound water (H2O) in much higher concentrations than the glass, as much as 0.7%. The amounts of water in apatite are much higher than in the glass, so scientists are able to make different kinds of measurements and explore the reasons why the water is there and where it came from. This is a very exciting area of lunar science, and recently Open University researchers have measured the hydrogen isotope ratios, specifically the deuterium : hydrogen ratio in apatites. (Deuterium is hydrogen with an extra neutron, written 2H. Ordinary hydrogen, when viewed as an isotope, is written 1H.) They have discovered that Moon apatites have very similar 2H : 1H ratios to their terrestrial counterparts. This is significant because the ratio is different from that in samples from other Solar System bodies, such as meteorites.
This is an area of science that is currently changing very rapidly as new discoveries are made, but if confirmed, these findings may indicate that the water in lunar rocks came from the early Earth. The most likely scenario is that this occurred during the giant impact Moon-forming event. The new discoveries appear to be tying the Earth and Moon together even more strongly, making it a truly unique pairing in the Solar System. This doesn’t mean the Earth’s oceans are leaking water to the Moon; remember that this event happened just a few million years after the Earth formed, and the unaltered lunar rocks in which these measurements have been made are older than any rock at the Earth’s surface.