AN important task for four of the 11 instruments on board Chandrayaan-1 is to look for confirmation of the presence of water-ice in the permanently shadowed regions of the south and north poles of the moon, according to Dr. M.Y.S. Prasad, Associate Director, Satish Dhawan Space Centre, Sriharikota.
The conventional thinking is that water or ice cannot exist on the moon because the moon does not have an atmosphere. So when the sun beats down on the lunar surface, the temperature rises to 120 degrees Celsius, which would cause water to evaporate. The only possible locations, therefore, where water-ice can exist on the moon are in its south pole and north pole, which are permanently shadowed regions, says Prasad.
A paper published in the November 1996 issue of the journal Science indicated the presence of water-ice in the moons South Pole. The paper was based on the findings of the U.S. Clementine spacecraft, which was operational in 1994. The U.S. Lunar Prospector mission in 1998 looked for hydrogen on the moon. If hydrogen were to exist on the moon, water-ice could be there. Experiments conducted by the Lunar Prospectors instruments revealed that both poles of the moon had distinct features that indicated the presence of hydrogen. It was argued that water-ice could exist about half a metre below the surface of the permanently shadowed regions of the moons poles.
Chandrayaan-1 has an important scientific payload called Mini Synthetic Aperture Radar (MiniSAR) of the U.S., which is an S-band radar. S-band signals are the best signals to find water-ice in soil. It will reveal how deep within the soil water content is there. Since Chandrayaan-1 will go round the moon in a polar orbit, it will image the north pole and the south pole in every orbit, says Dr. Prasad.
In addition to MiniSAR, the Hyper Spectral Imager (HySI), the Smart Near Infrared Spectrometer (SIR-2) and the Moon Mineralogy Mapper (M3) will also look for signatures of water-ice. If we can carefully synthesise the data from these four instruments and observe them consistently, we can definitely search for water-ice in the moons poles, says Prasad.
NASA will send the Lunar Reconnaissance Orbiter (LRO) to the moon in 2009. The Lunar Crater Observation and Sensing Satellite (LCROSS) on it will be crashed into a deep crater in the moons South Pole, and the dust particles kicked up on impact will be imaged by about 50 telescopes around the world. The particles will be analysed for the presence of water-ice or hydrogen.
T.S. Subramanian
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