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From: http://www.sciencemag.org/cgi/content/abstract/1181471

Space-based spectroscopic measurements provide strong evidence for water on the surface of the Moon.

Since the first sample return missions of the 1960s, lunar scientists have operated under the presumption that the Moon is entirely dry. Three papers in this week’s Science Express challenge that notion: Infrared spectroscopic measurements of the lunar surface from spacecraft provide unambiguous evidence for the presence of hydroxyl (OH) or water (1–3). Early inspection of the lunar samples returned by the Apollo missions revealed none of the water-bearing primary minerals that are common in Earth rocks (4); instead, all the rocks examined were composed entirely of anhydrous minerals. Some Moon rocks also contain primary igneous metallic iron, which is extremely susceptible to alteration by
water. No such alteration was found. Even in the case of coarse-grained plutonic rocks (which form from the cooling of molten rock below the surface), where volatiles in the
magma chambers would have had ample opportunity to react with minerals, no hydrous phases were observed. This perceived dryness of the Moon stands in stark contrast to the
abundant water found at Earth’s surface and throughout its crust.

Recent reports of a few parts per million water detected in lunar glasses and phosphates with more sensitive methods suggest that the deep lunar interior may contain much more water than previously assumed (5, 6). However, the spatial pattern of OH and other volatiles in volcanic glasses suggests a diffusion profile (5), perhaps established while these erupting glasses lost water in flight through the lunar vacuum following eruption. Calculations of the original water contents suggest water abundances near 0.1% by weight, similar to those of terrestrial mid-ocean ridge basalts, indicating that the deep Moon may be as rich in water as is the deep Earth.

The new spacecraft measurements shift the focus of lunar water from the interior to the surface of the Moon. Infrared spectroscopic measurements from three spacecraft have
unambiguously detected absorptions near 3 m on the lunar surface that are almost certainly due to hydroxyl or water, or both (1–3). Three micrometers is the position of the
fundamental vibrational absorption of the OH group and so is an extraordinarily sensitive indicator of the presence of water (or hydroxyl). Preliminary modeling by Sunshine et al. suggests a few tenths of a percent by weight water in the optical surface.

All three experiments show that the water-related absorption increases toward the lunar poles, although this observation does not necessarily mean that the concentration of a water-bearing phase increases toward the poles. Infill of the absorption feature by emitted thermal radiation will cause the depth of the apparent absorption to be temperature
dependent. However, preliminary modeling by Pieters et al. takes this effect into account, yet the authors still observe a residual poleward increase in absorption depth, suggesting that the abundance of the host of the water or hydroxyl also increases with proximity to the lunar poles. Pieters et al. also report local spatial variation in the
strength of the 3-m absorption associated with geologic features, especially areas rich in plagioclase feldspar, and suggest that these variations may be indicative of the deep water detected by Saal et al. (5). Sunshine et al. report that the spectral feature varies in strength with time, and suggest that the water is in a process of rapid and dynamic migration across the lunar surface.

These reports of lunar surface water coincide with intense interest in water at the poles of the Moon. Because the Moon’s rotation axis has only a 1.5° tilt with respect to its orbit around the Sun, many polar craters are permanently shaded and should be at very low temperatures (some models suggest as low as 40 K) (7–9). These cold surfaces should trap volatile material migrating near the polar regions. That notion has given rise to several space-based remote-sensing experiments aimed at detecting possible polar water (or other volatiles). On 9 October of this year, the LCROSS (Lunar Crater Observation and Sensing Satellite) mission will try to detect water by deliberately crashing a large spacecraft into a polar cold trap, thereby lofting lunar soil into sunlight for inspection by other spacecraft and ground-based observatories (10).

The results of the present studies lend credence to the lunar polar water hypothesis by providing a proven source of water on the surface of the Moon. Several extra-lunar sources are possible for polar water (11), but the latest observations provide a proximate source to supply a polar water deposit. What is the ultimate source of the water detected? Important continuous sources of water include reduction of lunar divalent iron in minerals to metallic iron by solar-wind hydrogen, producing water, and liberation of water from the impact of interplanetary dust and small meteoroids. Water may also be implanted episodically by large comets or asteroids, but these require a mechanism for retention of A Lunar Waterworld
Paul G. Lucey
Hawaii Institute of Geophysics and Planetology, University of Hawaii, 1680 East West Road, POST 504, Honolulu, HI 96822,
USA. E-mail: lucey@higp.hawaii.edu