What's the lunar water source?

Mooning over H2O: Chandrayaan-1 credits Sun for creating liquid


The most likely explanation of lunar water is Moon’s exposure to the solar wind which carries hydrogen ions to oxygen-rich lunar minerals on the surface. Both react to form OH and H2O, which are weakly held to the surface. Image courtesy of University of Maryland

“There are several possibilities,” said J N Goswami, director of the Physical Research Laboratory at Ahmedabad and Indian leader of the Chandrayaan-1 scientific team.

Water could be intrinsic to the Moon or imported from external sources like comets. It could also be products of interaction of solar wind ions with lunar rocks and soils. An indirect proof was found last year by US researchers after studying moon rocks brought by the Apollo-15 and Apollo-17 missions. They discovered volcanic glass spherules — glass pebbles — in lunar samples. Since the very formation process of these spherules requires water, it provides evidence that water is intrinsic to the Moon.

Chandrayaan-1 gives a direct evidence and indicates that the Sun is responsible for creating water on the Moon.

Since the rocks and soils contain about 45 percent oxygen, mostly combined in silicate phases, the question before researchers is where the hydrogen component of the water came from. In this case, scientists believe the hydrogen may have come from an astronomical phenomenon called the solar wind.

As the Sun undergoes nuclear fusion, it constantly emits a stream of particles, mostly protons, which are positively charged hydrogen atoms. On the Earth, the atmosphere and natural magnetism prevent the planet from being bombarded by these protons. But as the Moon lacks that protection, lunar surface is constantly pounded by hydrogen ions, moving at velocities of one-third the speed of light. The number of such ions heating the lunar surface is quite high — about a hundred million ions per square centimetre per second.

The energy of the solar wind hydrogen ions allows them to penetrate one-tenth of a micrometer of the lunar rock or soil surface where they can pick up an oxygen ion to create a strongly bonded hydroxyl (OH) species. Addition of another hydrogen ion of solar origin will lead to the formation of a water (H2O) molecule.

Data obtained by the Deep Impact mission corroborates the theory. “The OH-Water is prevalent across the Moon, almost as a soft blanket of signal from the upper few mm. It seems to be associated with the highlands rather than the maria (basaltic plains),” Lawrence Taylor at the University of Tennessee and one of the scientists involved in deciphering the NASA instrument data on board Chandrayaan-1, told Deccan Herald.

While lunar craters are suspected to have ice, Chandrayaan-1 demonstrated wider availability of water on the satellite. But again the evidences are indirect.
To find out more about the water ice in lunar poles, another NASA probe, LCROSS (Lunar Crater Observation and Sensing Satellite), is all set to crash a large spacecraft deliberately into a polar cold trap, thereby lofting lunar soil into sunlight for inspection by other spacecraft and ground-based observatories. The ambitious experiment is expected on October 9.

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