A new look at Moon rocks from the Apollo missions, along with a lunar meteorite show a much higher water content in the Moon’s interior than previously thought. Using secondary ion mass spectrometry (SIMS) which can detect elements in the parts per million range, scientists at the Carnegie Institution’s Geophysical Laboratory found the minimum water content ranged from 64 parts per billion to 5 parts per million—at least two orders of magnitude greater than previous results. The science team says their research suggests that the water was preserved from the hot magma that was present when the Moon began to form some 4.5 billion years ago. “The concentrations are very low and, accordingly, they have been until recently nearly impossible to detect,” said team member Bradley Jolliff of Washington University in St. Louis. “We can now finally begin to consider the implications—and the origin—of water in the interior of the Moon.”
The prevailing belief is that the Moon came from a giant-impact event, when a Mars-sized object hit the Earth and the ejected material coalesced into the Moon. In this new study of lunar samples, scientists determined that water was likely present very early in the formation history as the hot magma started to cool and crystallize. This result means that water is native to the Moon.
The SIMS technique measures hydroxyl by bombarding the grains of a type of phosphorous, water-bearing mineral called apatite with high-energy particles and counting the ions that are ejected. Based on the SIMS measurements, the scientists authors place the lower limit for the total lunar water at 100 times greater than previous estimates, and speculate that water may be “ubiquitous” in the moon’s interior.
The study could alter current theories about lunar magmatism (how igneous rock formed from magma), and how the moon formed and evolved.
Water is showing up in all sorts of unexpected places on the Moon. In September of 2009, a trio of spacecraft detected a ubiquitous layer of a combination of water (H2O) and hydroxyl (OH) that resides in upper millimeter of the lunar surface. It doesn’t actually amount to much; only about two tablespoons of water is believed to be present in every 1,000 pounds (450 kg). Then in October of 2009, the LCROSS impactor and spacecraft detected “buckets” of water in the permanently shadowed region of Cabeus crater near the moon’s south pole.
In 2008 water was found inside volcanic glass beads in Apollo Moon rocks, which represent solidified magma from the early moon’s interior. That finding led to this new study, using the SIMS. The scientists combined the measurements taken with the spectrometer with models that characterize how the lunar magma crystallized as the Moon cooled. They then inferred the amount of water in the apatite’s source magma, which allowed them to extrapolate the result to estimate the total amount of water that is present on the moon.
“For over 40 years we thought the Moon was dry,” said lead author of the new study, Francis McCubbin.
The research is published in the on-line early edition of the Proceedings of the National Academy of Sciences the week of June 14.