Hydroxyl and water molecules
Source: By Sohini Ghosh: The Indian Express
In what can be called a path breaking discovery, the Indian Space Research Organisation’s (ISRO) homegrown instrument aboard Chandrayan-2 has detected the unambiguous presence of hydroxyl and water molecules on the Moon with the precision of differentiating between the two. The findings also strongly suggest that the presence of these correlates with mineralogy and latitudinal location, according to a paper published in the latest issue of fortnightly journal Current Science.
Imaging infrared spectrometer (IIRS), an imaging instrument that collects information from the electromagnetic spectrum for understanding the mineral composition of the lunar surface with each element possessing a ‘spectral signature’ unique to itself, was developed by Ahmedabad-based unit of Space Applications Centre (SAC) of ISRO.
Capable of operating in the wavelength of 0.8 to 5 micrometre, the relatively broader range within which the indigenous instrument for water detection can image, is a first for IIRS capabilities on India’s second moon mission. The discovery is being hailed as critical for future planetary exploration and resource utilisation.
While the first Moon mission of Chandrayaan-1 in 2008 carried a similar instrument called Moon Mineralogy Mapper (commonly known as M3) capable of detecting water, the range of detection was lower — between 0.4 to 3 micrometre — and was also developed by NASA’s Jet Propulsion Laboratory and was not indigenous to ISRO.
The higher wavelength range of IIRS permits for better accuracy in results. In September 2009, results published of the M3 instrument data had shown detection of absorption features on the polar regions of the surface of the moon “usually linked to hydroxyl- and/ or water-bearing molecules,” as NASA states. A 2017 research article by researchers from the Brown University had noted, “…the wavelength range of M3 is too limited to accurately determine the full shape and maximum absorption point within the 3-μm (micrometre) region, making it difficult to differentiate OH (hydroxyl) from H2O (water), particularly if both species are present.” Notably, the human eye is capable of detecting wavelengths in the range of 0.3 and 0.7 micrometre.
Prakash Chauhan, director at IIRS Dehradun, in response to queries said in a written statement that since the spectral coverage of M3 instrument was limited upto 3 micrometre, distinction between hydroxyl, water and water ice/frost was not possible.
“Remote detection of water and/or hydroxyl signatures on the lunar surface is of significant importance as it provides clues to understand the various sources and water production mechanisms. Accurate detection of water and hydroxyl on the Moon in the three micrometre wave band region requires a thermal correction to reflected sunlight from Moon due to high daytime temperature…which contributes an additional thermal signal in the reflectance spectra…,” the statement said.
As more data from the mission is made available in the future, the scientists are hoping that they will be able to learn more about hydroxyl and water production and hydration processes on the moon.
As per a research article in the August issue of Current Science, three strips on the Moon’s surface were analysed by IIRS sensor for hydration presence and as reported, the initial analysis “demonstrates the presence of widespread lunar hydration and unambiguous detection of OH (hydroxyl) and H2O (water) signatures on the Moon.” It was also observed from the data that the brighter sunlit highland regions at higher latitides of the Moon were found to have higher hydroxyl or possibly water molecules that are enhanced hydration, compared to the large basaltic plain regions where hydroxyl appeared to be dominant, especially at higher surface temperature.
As the paper authored by scientists from Indian Institute of Remote Sensing (IIRS) in Dehradun, SAC in Ahmedabad, UR Rao Satellite Centre in Bengaluru and ISRO headquarters in Bengaluru note, the most common and widespread process for the formation of hydroxyl and water on the Moon is considered to be due to interaction of solar winds with the lunar surface, a process termed as ‘space weathering’. Space weathering along with the impact from small meteorites often lead to chemical changes on the surface of the moon ultimately leading to formation of either the volatile and reactive hydroxyl molecules or the more stable form of water molecules.
On the future scope of this initial analysis the paper concludes, “The proper interpretation of hydration feature through spectral analysis is significant as it provides important inputs regarding geology and geophysics of the mantle (of the moon) in terms of their mineralogy, chemical composition, rheology and solar–wind interaction. This is also significant for future planetary exploration for resource utilization.”
Chandrayaan-2 was aimed to widen the scientific objectives of Chandrayaan-1 by way of soft landing on the Moon and deploying a rover to study the lunar surface, as ISRO states on its website. While the soft landing failed, thus losing the lander and rover and the associated five payloads, the orbiter which was carrying eight scientific payloads for mapping the lunar surface and to study the outer atmosphere of the Moon has successfully continued to send back crucial scientific data. The mission life of the Orbiter was planned to be of one year but has exceeded expectations and is expected to serve for seven years as per a statement by Minister of State in Prime Minister’s Office Dr Jitendra Singh in reply to a Lok Sabha starred question in March this year.
