Opinion Asking for the moon

Samples of lunar rock brought back to earth by moon missions such as Apollo and Luna never showed any signs of water,leading scientists to presume that the moon was dry. However,in the last two decades,various missions to the moon indicated that perhaps it has water. In 1994,NASA’s Clementine spacecraft that orbited the Moon picked up signals that suggested the presence of icy materials. The spacecraft beamed radio signals into shadowed craters in the polar regions,and the reflections received back appeared as if it came from water in its icy state. Two further missions,Cassini and Deep Impact,which did a fly-by manoeuvre around the moon,also found signals that indicated possible water in the polar regions.

These findings,though surprising,match current understandings of the formation of terrestrial planets and moons. Early earth was so hot that water molecules would have disassociated and broken up into oxygen and hydrogen. Thus internal sources could not account for much of the water on earth’s surface. Astronomers now postulate that comets and asteroids were the cosmic carriers of water. During the period of “late heavy bombardment” 3.9 billion years ago,thousands of comets and asteroids impacted on earth,each bringing in trillion litres of water for millions of years. As the moon shares the same area of space as the earth,it should have also received some share of water from comet impacts. However,since its gravity is very low compared to earth,and in the absence of atmosphere,water would have evaporated and drifted off into space. In fact not just water,the moon’s surface is largely devoid of even volatiles. In time,the moon lost its supply of water,perhaps nearly all,but not all of it. Scientists postulated that some remnants of icy water may be lurking in dark,cold,permanently shadowed craters at the moon’s south pole. Scientists thought that the signals detected by Clementine and other spacecraft were indeed this polar ice.

Inspired by this astonishing finding,in 1998 NASA sent another spacecraft,the Lunar Prospector,to verify the finding and to specifically explore polar regions. It had a state-of-the art device called a neutron spectrometer,which could detect hydrogen-rich minerals. Once again the poles yielded intriguing signals indicating hydrogen-rich deposits. Could this be H2O (water),HO (hydroxyl —a form of water) or some other minerals rich in hydrogen? The lunar prospector could not give a clear answer.

It was in this background that M3 was included in the payload of Chandrayaan-1. M3 is a state-of-the-art imaging spectrometer that could specifically look for signals of water (H2O — two atoms of hydrogen bonded with one atom of oxygen) and hydroxyl molecule (OH — a water molecule with a hydrogen atom stripped off). It was one of the eleven scientific instruments onboard Chandrayaan-1; the instrument itself came from NASA — a fine case of international collaboration and universal science.

How the M3 detects water on the moon is similar to how we see the grass as green. The blade of the grass absorbs all other colours of VIBGYOR of sunlight,except green. The same way,water or hydroxyl molecules absorb infrared light,specifically at 2.8 microns,roughly three to four times the wavelength of light our eyes can see. Therefore when sunlight falls on the surface of the moon and is reflected back,wherever water or hydroxyl molecules are present,2.8 micron wavelength would be missing in the reflected light. M3 could detect the absence of this wavelength,the chemical signature of water and hydroxyl.

Now it has been confirmed not only by NASA’s M3,but also by ISRO that the moon impactor,India’s space probe that crashed on the moon in June 2009,had also detected telltale signs of water.

Is this the water that scientists were searching for — left over from the comet impacts over millions of years? Apparently not. Careful observations for many months indicated that the amount of water detected by M3 changed over the month-long lunar day. There was more in the local morning,and less at noon a week later. This seems to imply that the sun has a hand in the water seen on the surface. Scientists postulate that solar wind,which is teeming with hydrogen,slams into the oxygen-rich lunar surface soil called regolith,resulting in the formation of water molecules or hydroxyl molecules. However when it is local noontime,the sunlight breaks up the water molecule,again reducing the levels. So we see the most water in the morning,which dries by noon. This process of water formation on the moon’s surface is far removed from what was postulated until now —- icy remnant of comets,stored in the dark polar craters.

So,Chandrayaan-1 has demolished the long-held view of the moon’s surface. While it’s clear that water has been found,it’s a long way off from real estate agents developing land and selling plots on the moon. Nevertheless,it has injected new life into fanciful lunar dreams. In any case,our nearest celestial neighbour seems to have many more surprises in store,and the only way to unravel them to pay a visit. I say we do it.

The writer is principal scientific officer,Vigyan Prasar