WATER (OH, H2, H2O) IN AND ON THE MOON: FROM "BONE DRY" TO WET AS BONE

Soon after the return of the first samples from the Moon, lunar rocks had been reported as "Bone Dry." Compared to rocks from the Earth, they were completely devoid of water. H was present from solar wind, but there were no hydrous mineral phases and ferric iron was nearly absent. The idea of a dry Moon persisted for nearly 40 years until 2007, when two independent research groups dared to ask the question, is there something we missed in regards to lunar water? The answer to the question surprised many and sparked a revolution in the way that we look for water in the Solar System.

The presence of water on the Moon was first confirmed in volcanic glass beads at the 1-10’s ppm level in 2008. About a year later, OH/H₂O molecules were remotely sensed on the lunar surface by the M³ instrument on the Chandrayaan-1 spacecraft. In 2010, the presence of OH was confirmed in apatite from several lunar samples. Later in 2010, NASA impacted the LCROSS probe into a permanently shadowed crater, finding “buckets of water” in the impact plume. In 2011, melt inclusions in lunar olivines were shown to have 100-1000 ppm H₂O, indicating that portions of the Moon’s interior are as wet as the Earth. Consequently, over a five-year period, the Moon went from “bone dry” to nearly as “wet” as the Earth, but how did we miss the water for nearly 40 years?

Advances in technological development of analytical instrumentation have lowered detection limits by a few orders of magnitude, but this only accounts for part of the observation. The low oxygen fugacity of lunar rocks (below that of the iron-wüstite buffer) causes H₂ to be the main stable form of H during magmatic degassing as opposed to H₂O, which is commonly degassed from terrestrial systems. Therefore, planetary scientists needed to recalibrate their thinking on the textural and chemical evidence for hydrous magmatism for reducing conditions. This recalibration has led to finding H in other “dry” bodies like Mars and the asteroid 4-Vesta.