Water from the Sun
What is the ultimate source of the water found at the poles of the Moon? A new study of some lunar soil samples suggests a surprising answer
New data returned from a fleet of orbiting satellites changes our perceptions of the history and processes of the Moon. Concentrated at both lunar poles, and to date the most striking discovery, is the documentation of the presence of large amounts of water. Though this water has been confirmed by several differing techniques (from multiple missions), we remain uncertain about its source. Two principal origins have been proposed: 1) water added by the in-fall of water-bearing meteorites and comets during the impact bombardment of the Moon; and 2) the manufacture of water from hydrogen implanted in the lunar soil by the wind from the Sun.
A recent discovery may shed some new light on the origin of lunar water. Researchers conducting detailed examination of tiny fragments of glass in soil returned by the Apollo astronauts found the molecule hydroxyl (OH) present in the glass. Interestingly, the isotopic composition of these OH molecules indicates the bulk of the hydrogen comes from the Sun, not from cometary and asteroidal impacts.
The Moon has no atmosphere and no global magnetic field. As a result, the solar wind – the stream of atoms and molecules constantly emitted by the Sun – directly impinges upon the lunar surface. Most of this solar wind consists of hydrogen, either in the form of neutral atoms or positively charged ions (i.e., protons). After it encounters the Moon, this spray of hydrogen has a complex fate, with at least some of it being implanted into the lunar dust. In a process called adsorption, many of the hydrogen atoms stick to the surfaces of the dust grains. The amount of adsorbed hydrogen varies by position and chemical composition around the Moon, but it can be present in quantities ranging from less than 10 to over 100 parts per million (ppm).
Impact glass is a major component of lunar regolith – up to 60% by weight of the soil at some landing sites. The constant bombardment of the lunar surface by microscopic meteorites crushes and grinds up the surface rock, continually mixing the outer layer of the Moon. When a micrometeorite strikes a rock, it forms a micro-crater (wholly melting the surface beneath this pit) and creates a clear, chemically homogeneous glass particle. However, when a micrometeorite strikes lunar soil instead of rock, its energy is converted mostly into heat. This flash heating creates a mixture of melt and mineral debris called agglutinate glass.
The new work details results of analyses of agglutinates returned from several lunar landing sites. Their study measured both the amounts of hydroxyl present and its isotopic composition. A normal atom of hydrogen is a single proton and an electron. But in a rare form of hydrogen, called deuterium, the nucleus contains both a proton and a neutron. The ratio of this form of “heavy hydrogen” to “normal” hydrogen is unique for different materials throughout the Solar System. By tracking the D/H ratio in the sample, one can assign a source origin to the measured hydrogen.
When the lunar agglutinate glasses were studied, it was found that their D/H ratios indicated that most of the hydrogen in the hydroxyl molecules came from the Sun and not from cometary or meteoritic sources. However, the source of the hydrogen is not completely solar, as the D/H ratios suggest some mixing with a subordinate component of either lunar or cometary origin. The authors of this study suggest that the hydroxyl found on the Moon was created when a small impact flash heated the soil, releasing the adsorbed hydrogen and chemically reducing the metallic oxides in the soil into native metal (found as extremely tiny grains on the surfaces of the agglutinates) and hydroxyl molecules. Multiplied by billions, such a process could account for the generation of water on the lunar surface. Subsequent migration of these molecules toward cooler-than-average areas of the Moon (i.e., the higher latitudes, up to and including the poles) may have created the polar ice deposits found by numerous techniques. In the view of the authors of this study, lunar water comes mostly (but not entirely) from the Sun. This constant process, occurring on the sunlit hemisphere of the Moon, could create an enormous reservoir of hydroxyl molecules (in motion due to their thermal instability), slowly but constantly moving toward the poles.
If such a process occurs on the Moon, one might expect the accumulation of water in every location where water is stable (i.e., within every permanently dark and cold region near both poles). But it appears that ice at the poles is not uniformly distributed, occurring in high concentration in some areas while absent in others. This pattern suggests that the source of polar water might be controlled by a non-equillibrium process, such as episodic bombardment by asteroids and comets. In fact, both solar wind-produced and cometary water may be present at the poles, but until the ice there is actually analyzed for its D/H content, we cannot be certain of its origin. Such a measurement does not require the return of a polar ice sample to the Earth. It could be made remotely in situ on the Moon with a properly instrumented robotic spacecraft.
It is important to emphasize that although the quantities of water generated by this process are potentially very large, the hydroxyl in agglutinate glass should not be considered an economic resource. These molecules occur globally but at very low levels of concentration (tens of ppm). Even if this water is the primary and ultimate source reservoir of lunar water, the migration of the molecules and their subsequent collection by the cold traps near the poles serve as a concentrating mechanism, where ice accumulates in large quantities, confined within small areas — the classic definition of an ore body.
What a change has occured in the mindset the lunar science community in the past few years! From a bone-dry lump of rock in space to a complex, still mysterious body with a dynamic hydrological cycle. It’s clear that many more discoveries about our Moon and its resources have yet to be revealed. The more we learn about the Moon, the greater the range of processes we must account for and the more subtle and complex its history becomes.