Telltale signs

The moon impact probe, with the tricolour painted on it.-PICTURE COURTESY: ISRO

SCIENTISTS of the Space Physics Laboratory (SPL), Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, the lead institute for one of the experiments on the Moon Impact Probe (MIP) of Chandrayaan-1, obtained very clear signature of H2O (18 atomic mass unit, or amu) intensifying in the thin atmosphere of the moon as the MIP, carrying a very sensitive mass spectrometer called ChACE (Chandras Altitudinal Composition Explorer), raced towards the lunar surface for more than 20 minutes on November 14, 2008, according to Professor Dr R. Sridharan, Director of the SPL. Our conclusion is that there is water present in the thin atmosphere of the moon, he asserted. Consistent signature was revealed by ChACE, which was developed as a payload by the SPL. We got the spectrum covering a mass range of 1 to 100 amu all through. For us, the finding of water vapour in the moons thin atmosphere is very important, he added.

The MIP was one of the 11 scientific instruments on board Chandrayaan-1, Indias first scientific mission to the moon. The MIP sat like a hat on top of the spacecraft. A Polar Satellite Launch Vehicle (PSLV) of the Indian Space Research Organisation (ISRO) lifted off from the spaceport at Sriharikota on October 22, 2008, and put Chandrayaan-1 in its initial orbit. A series of tricky manoeuvres saw the spacecraft reaching the lunar orbit at an altitude of 100 km on November 14.

After Chandrayaan-1 reached its lunar orbit, the MIP, with the Indian flag painted on its sides, hived itself away from the orbiter, raced towards the moon and crashed near the Shackleton Crater. Its journey towards the moon lasted more than 20 minutes. It had three instruments on board: ChACE, to sniff the moons very thin atmosphere as it descended towards the moons surface; a video camera (VIS) to take pictures of the lunar surface during its descent; and a radar altimeter to measure instantaneously the MIPs altitude from the moon as the payload flew towards the lunar surface.

Sridharan dispelled misconceptions that the mass spectrometer detected water on the surface of the moon. It found water vapour in the moons thin atmosphere, he said. The purpose of the mass spectrometer was to analyse the gaseous constituents of the moons tenuous atmosphere. It was capable of analysing how much [of gas] was present and what it is composed of, he said. It did this in latitude and altitude after the MIP split away from Chandrayaan-1 and raced towards the moons surface. The mass spectrometer was sensitive enough to analyse the presence of H2O in the form of gas in the form of water vapour. It provided this data on November 14, [2008], the SPL Director said.

Scientists of the SPL went ahead and analysed the constituents of the lunar atmosphere from the data given by the mass spectrometer. Sridharan said, This was the first time that any composition measurement was done on the sunlit side of the moon as the earlier attempts by the Apollo missions did not succeed owing to saturation effects. Any sensitive instrument such as the mass spectrometer is vulnerable to contamination, especially water vapour. So it becomes extremely important to glean out the signatures that belong exclusively to the lunar ambience. The SPL scientists, therefore, went about differentiating the water vapour present in the latitude and altitude in the lunar atmosphere from possible instrument degassing.

Sridharan further explained: That is why we took time to announce that there was water vapour present in the thin atmosphere of the moon. But there was one clue and we had to use it judiciously. If the water vapour was of instrument origin, it would have continued all through. But it cannot show the variations in latitude and altitude as the source itself is within. However, we had seen very clear signatures of H2O intensifying as the MIP raced towards the lunar surface. The Moon Mineralogy Mapper [M3] of NASA, on board Chandrayaan-1, has also inferred [water molecules on the moons top surface]. There is a consistency [in the findings of the mass spectrometer and the Moon Mineralogy Mapper]. Our conclusion is that there is water vapour present in the thin lunar atmosphere. We got the signatures all through.

Any substance exists in three phases: solid, liquid and vapour. If a substance has one of these phases, the other phases will coexist depending on the ambient conditions. Further, owing to the large thermal cycling of the lunar surface, the lunar atmosphere went through cycles as it breathed in and out, Sridharan said. But the moons atmosphere is so thin that the breathing effect turns out to be phenomenal. While it breathed out, all gases get released owing to desorption, said the SPL Director.

Asked how significant the success of the MIPs mass spectrometer and the finding of water on the moon by M3 were, Sridharan said: As a person who was involved in this mission, it was a phenomenal success. For us, the whole experiment was a phenomenal success.

The SPL team was on the job of analysing the data from the mass spectrometer. The team had to publish its findings and the world should accept it, he said. The MIP mission was over long ago. While the world might have forgotten about it, we cannot. We are on the job of analysing the data. Maybe we will be working on the data for several years to come. For us, detecting water vapour in the moons thin atmosphere is very, very important.

Y. Ashok Kumar, Project Director of the MIP, is also thrilled about the success of the MIP. The video camera on board the probe took about 3,000 pictures of the lunar surface right from the time it separated from the orbiter and crashed on the Shackleton Crater, he said. The video camera clicked away all through its descent. In fact, the MIP was switched on soon after the spacecraft entered the lunar orbit and the camera took pictures when the spacecraft was in this orbit itself, he said.

On whether it was true that it was A.P.J. Abdul Kalam, rocket engineer and former President, who suggested that ISRO keep MIP on board Chandrayaan-1, Ashok Kumar replied that in a way, it was the brainchild of Mr. Kalam. The former President suggested that since ISRO was sending a spacecraft all the way to the moon, why not we have something that can crash on the moon. An instrument, with Indias flag painted on its side, could drop on the moon and it would convey the message that India has arrived on the moon, Kalam had said.

The MIP landing on the moon was both symbolic and significant, Ashok Kumar said. It was significant because both the M3 and ChACE had revealed the presence of water on the moon. India, in its very first attempt, could put a spacecraft in the lunar orbit; it could land a payload on the moons surface; and the mass spectrometer was able to detect water vapour in the moons atmosphere. All these are unique successes of the Chandrayaan-1 mission, Ashok Kumar said. The 3,000 pictures of the lunar surface taken by the MIPs video camera would help ISRO determine where the lander-cum-rover of Chandrayaan-2 mission should soft-land on the moon, he added.