June 1, 2012
A recent article tells how Soviet scientists studying soil samples returned from the Moon in 1976 by the unmanned Luna 24 mission first discovered lunar water. This assertion is based on a paper published in the Russian journal Geokhimiia (vol. 285, p. 285-288, February 1978). The measurement used infrared absorption spectroscopy to look for the “water band” centered around 2.8 microns, the same technique used recently by several groups to map the water band on the lunar surface regionally from flyby (Cassini and EPOXI) and orbital (Chandrayaan-1) spacecraft. The Soviet paper claimed to detect water at a level of about 0.1 weight percent. This high concentration level of water raised my antennae.
The discovery of significant amounts of water would tell us about lunar processes and history as well as provide evidence that water might be manufactured on the Moon to support future exploration. The first lunar samples returned to Earth in 1969 by the Apollo 11 mission were intensely scrutinized for water content. Besides being exceedingly dry, the chemistry of the Apollo samples suggested they were created in a completely anhydrous, reducing environment. Samples from subsequent missions confirmed and extended this initial impression to the point where talk of water on the Moon was mostly dismissed.
A rock returned in 1972 by the Apollo 16 mission displayed visible brownish splotches which turned out to be “rust” in the form of the mineral akaganeite, an iron-hydroxyl phase, with minor amounts of chlorine. This mineral could have formed by the aqueous alteration of the iron-chlorine mineral lawrencite found in some meteorites. However a source of water is still needed to create the “rust,” so for several years the source of the water and the nature of the alteration were debated. Did water come from the inside of the Moon or from an impacting comet? Did the oxidation occur on the Moon or was it caused by the exposure of the highly reduced lunar sample to humid air (from inside the returning Apollo command module or the Houston summer humidity)? Different workers had a variety of opinions but with no resolution, interest faded.
But a few inquisitive types didn’t forget it. Jim Arnold, a chemist from UC-San Diego, resurrected an old idea about permanent cold and dark areas near the lunar poles. He concluded that over the course of history these areas were cold enough and old enough to have accumulated significant amounts of water from meteorites and comets. Groups studying the regolith (soil) from the Apollo missions measured variable amounts of hydrogen on dust grains; when heated, hydrogen in that dust reacted with metal oxides in the soil producing native metal (iron) and water vapor. Although done in the laboratory, it was shown that the process could occur naturally on the Moon during the impact of a micrometeorite, whose energy is mostly dissipated as heat. This heat and the hydrogen on dust grains could “reduce” the soil, creating measurable water release.
During the lunar “wilderness years” (i.e., 1976-1994, when no one was going to the Moon) all we could do was speculate and analyze existing samples. In 1982 a meteorite from the Moon was discovered in Antarctica. Lunar meteorites provided a new source of samples but even though all had significant exposure to the terrestrial hydrosphere, none of them showed evidence for water-bearing phases. Attempts were made to map the poles of the Moon from Earth using optical and radar telescopes but poor viewing geometry led to uncertain conclusions.
Two events re-ignited the water debate. The 1994 Clementine spacecraft probed the south pole of the Moon and found evidence for coherent backscatter near the dark areas. The team interpreted this as indicating the presence of water ice. Following Clementine, the Lunar Prospector neutron detector found elevated amounts of hydrogen near both poles of the Moon, resulting in new interest about the possibilities for water on the Moon. In recent years, a variety of robotic missions, carrying instruments designed to address the lunar water question one way or another, found large amounts of water in a variety of different forms, locations and concentrations. We are just beginning to decipher the origins, cycles, and eventual fate of this water.
So what can we say about the Soviet results published in 1978? No other scientist or group has repeated this measurement on the Luna 24 samples to confirm its validity. Under a reciprocal exchange agreement with the Soviet Union in the late 1970s, others studied the Luna 24 samples but none reported any traces of water in their samples. No one in Russia has studied the Luna 24 samples in years (at least to my knowledge), although they still exist and presumably are available for analysis. The spectral detection of water in the Luna 24 soil should be repeated and then followed up with analyses by other techniques to confirm the water’s presence and to cross-check the amounts claimed. The published value of 0.1 weight percent (1000 part per million) water seems very high for lunar soils from equatorial and mid-latitudes; typically, such soil contains 10-50 ppm hydrogen, almost two orders of magnitude less than the 1978 reported result. Finally, even if the old analysis is confirmed, questions about its source are still pertinent; we are still arguing about the origin of the water that made the rust in “Rusty Rock.”
If you’ve stayed with me this far, I hope that if nothing else, this brief history of a lunar controversy has shown that it is difficult (I would say impossible) to assign “credit” to any one paper or worker or group for the discovery of water on the Moon. In science we always proceed from the knowledge gained by previous work. Sir Isaac Newton put it well when he famously said that he saw more clearly because he stood on the shoulders of giants. A lunar scientist’s goal is to study, document and explain, thereby contributing to and advancing our knowledge and understanding of the Moon.
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