To squeeze it out

Published : Oct 23, 2009 00:00 IST

An image of the moons surface as seen in a short movie taken by the Terrain Mapping Camera of Chandrayaan-1.-PTI

An image of the moons surface as seen in a short movie taken by the Terrain Mapping Camera of Chandrayaan-1.-PTI

NOW that we have more definitive proof of the presence of water in the form of hydrated compounds (molecules chemically combined with water), the obvious question that arises is whether this locked-up water or ice can be extracted somehow. The observed data by the National Aeronautics and Space Administration (NASA) instrument M3 that was carried by Chandrayaan-1 (and other corroborating evidence from satellites of other missions) suggest that water on the moon exists only in minute quantities and that too in the form of hydrates compounds combined with one or many molecules of water in the form of X.nH2O, where X is any molecule of lunar surface materials. So any technique used has to be very efficient.

Lunar soil and rock would be largely made of silicates, which are compounds derived from silica or silicon dioxide (SiO2), the material of sand and quartz. Therefore water molecules on the lunar surface would be largely in the form of hydrated silicates. The concentration of water molecules in the lunar surface material is believed to be about 700 parts per million (ppm). In terms of weight, this is equivalent to a teaspoon of water in a few kilograms of lunar soil. But if this is squeezed out, it could be used as drinking water or as a source for rocket fuel by splitting water molecules into hydrogen and oxygen by electrolysis. One could think of the moon as a transit point for refuelling in interplanetary mission, and of taking off more easily, say, for Mars because of the weak lunar gravity.

According to a technique developed by Edwin Ethridge of NASAs Marshall Flight Centre and William Kaukler of the University of Alabama, Huntsville, in 2006, microwaves offered a simple way of harvesting this water. What they did was to make lunar soil in the laboratory and simulate the lunar environment by cooling the soil to extreme sub-zero temperatures of 150oC and keeping it in near-vacuum conditions. Lunar soil is actually in the form of regolith, loose heterogeneous material covering the solid rock beneath, forming a layer of about a few metres.

Ethridge and Kaukler found that if you heated the simulated lunar regolith to just about 50oC by irradiating it with microwaves, the water sublimated (became vapour directly from the solid ice phase). The vapour then was found to diffuse out through pores in the soil pores. This is because the vapour pressure of water in the soil is much higher than the vacuum above. The water vapour could be collected on a cold (below 50oC ) plate held close to the soil surface where it would deposit as ice and could be scraped off. They have demonstrated this technique with1 kW equipment. With a higher-power device, the extraction will be faster, Kaukler points out. Their experiments showed that about 99 per cent of the water could be sublimated in this fashion and 95 per cent of that could be collected.

According to Kaukler, using microwaves to heat the soil offers several advantages. As microwaves are not strongly absorbed by the regolith, they can penetrate a few metres into the soil and heat it. Heating, he says, is possible because the moons soil has about 5 per cent iron, similar to volcanic rock on earth, and microwave absorption is the most efficient method to heat large volumes of regolith or rock.

Most importantly, you do not have to dig the regolith and put it into a furnace for high-temperature baking. This avoids carrying heavy equipment to the moon and would leave the moon largely undisturbed. He feels that it is important to understand the electromagnetic properties of regolith at various microwave frequencies because altering those frequencies could allow deeper penetration into the surface if it is necessary to reach additional water.

R. Ramachandran
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