Water on the Moon
Moon occupies a special place for humans. As the closest celestial object to Earth and with the recent advances made in lunar exploration, Moon will continue to be an important object of study. The early Moon landings during the Apollo era were followed by a lull in lunar studies until the early 1990s with the launch of Clementine (USAF/NASA) followed by Lunar Prospector (NASA), SMART-1 (ESA), Kaguya (Japan),Chang’e missions (China), Chandrayaan-1 (India), Lunar Reconnaissance Orbiter (NASA), etc.
A decade ago, ISRO launched our first mission to the Moon, Chandrayaan-1. It was a unique mission in many respects with the key focus to search for evidence for water on Moon, to understand the origin of the moon from mineral and chemical composition studies, map the lunar surface in greater detail and to detect and identify the presence of atomic species in the thin atmosphere of the Moon.
Chandrayaan-1 was unique in inviting international partners for joining our lunar science investigation through contributed complementary instruments or jointly developed experiments. The mission carried instruments from USA, Europe and Bulgaria. These were optimally chosen to expand the scope and capability of Chandrayaan-1 science goals which included the search for water on Moon.
Chandrayaan-1 data showed evidence for water in the exosphere of Moon, on the surface of Moon and also sub-surface (tens of meters deep). In the thin atmosphere of Moon, the mass spectrometer experiment (CHACE on Moon Impact Probe) showed evidence for water even within its limited operational time, as the Moon Impact Probe was deployed on its destructive, ballistic trajectory to the south polar region. The Polar region of the moon is believed to host volatiles like water. Water is expected from primordial origin (~3-4 billion years ago) which remained preserved due to the unique geometry of solar illumination which prevents direct sunlight from entering craters in polar regions. Water and other volatiles are also expected to be enhanced at the polar region from migration from lower latitudes.
Evidence for surface water came from the Moon Mineralogy Mapper (M3) experiment on Chandrayaan-1. Initially it showed the presence of water on the sunlit side using water/ice spectral signature (2- 2.5 microns) in the reflected sunlight. Very recently on Aug 21st, it was reported in the Proceedings of the National Academy of Sciences (USA) that M3 also discovered evidence for water at the lunar poles where the signature arose from nearly dark regions but faint spectral features in the reflection of sunlight that entered craters nor directly but from multiple reflections from crater walls. The constraint from limited wavelength coverage of M3, remained a worry. In parallel, the team pursued laboratory experiments which showed complementary evidence of signal absorption by water, in the part of the spectrum formally covered by M3 spectrometer (0.7 to 3.0 microns). The key evidence derived from laboratory studies was to search for simultaneous occurrence of absorption features at 1.3, 1.5 and 2 microns which provided clear evidence for water-ice. The localisation of this new signature coincided with permanently shadowed craters on the north and south poles of the Moon. Since M3 infra-red reflectance spectroscopic signal arises from only the top few mm of the lunar surface, this was evidence for surface water in polar craters. This further strengthened earlier evidences from the laser altimeter and the UV spectrometer experiments on Lunar Reconnaissance Orbiter (LRO).
Evidence for subsurface water (tens of meters deep) emerged from the Synthetic Aperture Radars deployed on Lunar mission including those on Chandrayaan-1 and LRO. The mapping was most intense at the poles, yielding evidence for subsurface water-ice.
To add strength to the overall story of water on the Moon, in mid-2017, a core group from the same team that reported the recent M3 findings, had studied volcanic rocks from Apollo 15 and 17 using very sensitive instruments and reported the larger than anticipated abundance of water in these rocks which emerged from the lunar interior. Numerous years of pursuing laser ranging of the Moon using earth-based powerful lasers which were reflected off the retroreflectors left on the Moon by Apollo (11, 14, 15) astronauts and the Lunokhod (1 and 2) landers, showed evidence for a liquid core.
The comprehensive evidence for lunar water coming from surface, sub-surface, deep interior and the exosphere is most exciting as one looks at future space exploration and travel. The ready access to water at the poles has both scientific and utilitarian interest. A sample of primordial water would be key towards addressing the origin of water on Moon as well as earth and may have more to say on the story of water in the solar system. As we begin a wider exploration of space and the solar system, Moon could form the base for fuel and oxygen and other critical raw materials. If Moon can be considered a pit-stop for resources including water, space transportation could be more affordable as some studies have shown.Chandrayaan-2 begins its lunar studies in early 2019 with an orbiter that carries a wider range spectrometer that goes upto 5 microns to clearly provide the water signature. The global map from this experiment is expected to yield the firmest conclusions on the distribution of water on the Moon’s surface. The dual frequency SAR experiment on Chandrayaan-2 will further refine sensitivity to sub-surface water. Along with a mass spectrometer that can study the exosphere for much longer durations, Chandrayaan-2 truly has a unique opportunity to provide major findings on the important subject of water on the Moon.