Exploring and Using Lunar Polar Volatiles
Knowledge: What do we know about lunar polar volatiles?
The possibility of significant amounts of frozen water existing on the floors of shadowed craters near the Moon’s poles was first suggested by Watson et al. (1961) and Arnold (1979). The idea that water and other volatiles could exist in the lunar polar regions was based on a truly unique characteristic of the Moon, which is the very tiny angle between its equator and the orbital plane of the Earth-Moon system as it revolves around the Sun. Remarkably, the lunar obliquity is only 1.5º, which means that (1) sunlight always strikes the poles from a very low elevation above the horizon; (2) hills and craters produce persistent, and even permanent shadowed regions; and (3) mountaintops may be bathed in persistent, quasi-permanent sunlight. It is thus hypothesized that the polar shadowed regions can serve as “cold traps” for volatiles that migrate along the Moon’s surface.
However, human and robotic lunar exploration in the 1960s and 1970s focused on equatorial and mid-latitude regions, and evidence available from missions of that era contributed to an initial impression that the moon is “bone dry”. For example, examination of the Apollo rock samples returned to Earth revealed none of the water-bearing primary minerals that are common in Earth rocks. As a result, the potential for the Moon to host significant amounts of water or other volatiles was under-appreciated until a string of missions starting in the 1990s began to collect important new data from lunar orbit. While the evidence for lunar volatiles is continuing to accumulate, their form, abundance, and distribution is not well known.