Some of the most convincing evidence yet for the presence of water on the moon has been collected by physicists, and it could be relatively accessible. The discovery has implications for potential lunar missions and exploration of deeper space.
With no substantial atmosphere isolating it from the sun's radiation, it had been believed that the moon's surface was dry. But by the 1990s, orbital satellites detected traces of ice in wide and inaccessible craters at the poles of the moon.
Then in 2009, onboard India's Chandrayaan-1 spacecraft, imaging spectrometers observed signatures consistent with water in light reflected off the surface of the moon. Technical constraints, however, meant that it was difficult to know if this was actually H2O water or hydroxyl molecules composed of one atom of oxygen and one atom of hydrogen in minerals.
By analyzing the wavelengths of sunlight bouncing off the moon's surface, Casey Honniball at NASA's ASA Goddard Space Flight Center in Maryland, USA, and colleagues have found a chemical signature that is distinctly H2O. The Stratospheric Observatory for Infrared Astronomy (SOFIA), a modified Boeing 747 carrying a 2.7-meter reflecting telescope, collected the data.
The water was found at high latitudes in the relative abundance of around 100 to 400 parts per million H2O near the south pole of the moon.
For future lunar missions, the presence of water has implications since it could be processed and used for drinking, separated into hydrogen and oxygen for use as a spacecraft propellant; and it could be used for breathing with oxygen separated.
However, it would be a risky endeavor to extract it from dark, steep-walled craters where the temperature barely rises above-230C, which is where the majority of any frozen water is believed to lie.
However, questions persist. One is the form in which the water lies. An explanation is that inside lunar glass it is dissolved, formed when meteorites strike the surface of the moon. Alternatively, within grains of lunar dust, tiny ice crystals may be dispersed.
Another is how deep this recently verified source of water extends. If it were limited to the uppermost microns or millimeters, its practical significance would be marginal, while interesting theoretical concerns regarding how it got there would still be posed.
Going to the moon and begin drilling is the only possible way to find out. Maybe this isn't far off. By 2024, NASA's Artemis mission hopes to send a male and female astronaut to the moon. And as part of a Russian mission planned for 2025, British scientists are also designing a robotic drill to take samples of lunar soil from depths of up to one meter.
