A Wetter Moon Impacts Understanding of Lunar Origin
Is there water on the Moon?
Is there water on the Moon?
We know now that the answer to that question is a resounding Yes! As information continues to emerge from a wide range of studies, it’s evident that we’ve just begun to understand the process of the creation, movement and history of water on the Moon and its prevalence.
A paper recently published in the Proceedings of the National Academy of Sciences describes lunar samples containing the calcium-phosphate mineral apatite. Using a sensitive technique, they detected water (in the form of its ion hydroxyl, -OH) within the crystal structure of this mineral. Moreover, these hydroxyl-bearing apatite grains are found in several different rocks from a variety of geological settings. This indicates that the presence of water in the lunar interior is not some fluke, but a general property of the Moon. So the story of water on the Moon advances.
Why did scientists believe for so long that the Moon was bone dry? Largely because the samples returned from the Apollo missions contained no obvious hydrous phases, such as mica or amphibole, common water-bearing minerals in terrestrial igneous rocks. In addition, the chemical composition of lunar samples indicated that they formed under very reducing conditions, indicating very low partial pressures of oxygen. Typically, water oxidizes metals in magma on Earth, creating minerals that contain ferric iron (Fe3+); the exclusive presence of ferrous (Fe2+) iron in lunar samples indicates that no water was present during their formation. Finally, there is the extremely reducing nature of the current surface environment of the Moon, in which solar wind protons (hydrogen ions) continuously impinge upon the surface and reduce metal oxides in the soil. This hydrogen reduction creates “free” metallic iron (0Fe) and hydroxyl ions, most of which are lost to space through a variety of mechanisms. But at least some of these ions migrate to cooler regions of the Moon, the poles.
We now have found traces of water in volcanic glasses, mare basalts and an alkali highland rock. All these samples are very different types of material, formed from different parent materials at different places within the Moon at different times and under different conditions. The rocks possess apatite grains that show evidence for the presence of water at the time of their formation. One reason that we are finding this water in lunar samples now is that the technology of laboratory instrumentation has vastly improved since lunar samples were first studied 40 years ago.
The ion microprobe used in the study by McCubbin and others resolved a spot size on a mineral grain about 8 microns (about 100th of a single millimeter) in diameter. Additionally, the composition of this spot was resolved with extremely high precision, measuring the presence of water at about 3 parts per million or better. We now have at our disposal a variety of brand new lunar samples in the form of meteorites – rocks that were blasted off the Moon during impact events. More than 130 individual lunar meteorites are now known; one of the samples in this new study is a piece of a lava flow (mare basalt), found as a meteorite from Northwestern Africa.
The results of the new discoveries indicate that water is (or at least was) present in the deep lunar interior. This water probably existed as gas as the pressures and temperatures within the Moon do not permit the existence of liquid water. The total amounts of water implied by this work are still very low; the bulk lunar water content is estimated to be between 0.064 to 21 parts per million, a low amount by almost any standard – except when compared to the previous estimate for the Moon, which was less than one part per billion of water. Thus, the new estimate suggests water contents inside the Moon that are several orders of magnitude higher than previously thought.
So what does all this mean? For models of lunar origin, some mechanism preserved primordial water inside the Moon immediately after it formed. It had been thought that if the Moon originated during a collision of a planet-sized object with the proto-Earth 4.5 billion years ago (the currently favored model), the high temperatures extant during such an event would “boil away” most, if not all, volatile substances. Indeed, the near absence of volatile substances in the Moon has long been cited as prima facie evidence for a high-energy environment of lunar formation, such as would be expected from a giant impact. It now appears that regardless of high temperatures prevalent during this time, some water was incorporated into the Moon. Does this make the giant impact model less likely? Perhaps. Clearly we do not fully understand the conditions created by such an event. Work continues on the problem of lunar origin with the handicap that a planetary-scale collision is something well beyond human experience or observation.
Some of this endogenous lunar water may have found its way into one of the permanently dark, extreme cold “traps” near the poles of the Moon. Thus, in addition to the water made on the Moon from solar wind reduction and deposited on its surface by the collision of water-bearing asteroids and comets, we must also consider the addition of water from the deep lunar interior. Considering that our estimate of the abundance of internal lunar water is still very low, it is likely that the vast bulk of the water found at the poles is of external origin. Therefore, the possible finding of indigenous “Moon water” in the polar areas makes detailed study and examination of the poles even more attractive.
The Moon continually surprises us as she reluctantly (but always provocatively) reveals her secrets. In recent months, a wholly new and totally unexpected picture of the processes and history of our nearest planetary neighbor has emerged. We are in the midst of a renaissance of lunar science.