The possibility of the presence of water on the moon has been talked about since the 1970s, though the evidence at that time was very weak. In fact, the rock and soil samples brought back by the Apollo missions of NASA in the late 1960s and early 1970s suggested that moon’s surface was bone dry, bereft of any water.
In the 1990s, two NASA missions, Clementine and Lunar Prospector, picked up signals of water on moon. So did Cassini mission in 1998 which flew by the moon on its way towards Saturn.
But the clinching evidence of presence of water on moon was provided by two instruments on board Chandrayaan-1 which was launched in 2008. It has totally changed the way scientists now view the moon, and has led to a renewed interest in lunar exploration.
Presence of water is crucial for the hopes of using moon as a future launch pad to send probes deeper in to space. Such a facility would require human beings to use moon for extended period stays which is not possible in the absence of water. It is economically not feasible to transport water from the earth to sustain extended human presence on moon.
A lot of water is believed to be present in the polar regions of the moon, trapped as ice in deep craters. Some estimates put the amounts of ice in the polar region to be in millions to billions of tonnes.
Chandrayaan-2 would provide fresh evidence on the presence of water, especially in the polar region. It would also assess the abundance of water molecules on moon.
There is water on moon, but not in extractable form
There are several possibilities about the manner in which water detected on moon could have been formed. It is known that the lunar surface is full of oxides of different elements. These oxides could react with hydrogen ion in the solar winds to make hydroxyl molecules, which could combine again with hydrogen to make H20.
The water could also have come from external sources. Comets and asteroids that contain water vapour are known to have collided with the moon in the past, and could have transferred traces of this water to the moon, which could get trapped inside the extremely cold regions.
There are different implications if it is found that water on the moon was not formed in-situ but was delivered from outside, due to collision of comets or asteroids, for example. That would mean that the quantity of water was limited.