Unambiguous evidence of water on Moon found

Published in The Hindu on September 24, 2009

Earth-based radar image of the North Pole of the Moon, showing the position of the crater Erlanger (arrow). — Photo: Arecibo Observatory and NASA

The moon does contain water on its surface. This has been the conclusion of three studies published online in the journal Science. One of the papers is based on data gathered by the Moon Mineralogy Mapper, a NASA instrument, on Chandrayaan-1.

Unlike what was inferred by studying the rocks retrieved from the moon by the lunar missions in the 1960s, these three papers clearly show that moon is a lot wetter. Water exists as a thin film.

The results of three papers come from studying the data generated by three spacecraft — Chandrayaan-1, Cassini (1999) and Deep Impact (June 2009).

The Moon Mineralogy Mapper on Chandrayaan-1 “show small amounts of OH/water on the uppermost surface of the Moon,” notes the paper.

Unlike what was observed earlier, all the three papers provide unambiguous evidence that water is present not in the interior but on the surface of the moon. Also, the lunar polar have greater chances of having water or hydroxyl than elsewhere.

The results provide more credence to the lunar polar water hypothesis.

The proof of water on moon comes from measurements taken using infrared spectrometer. “Infrared spectrometer measurements of the lunar surface from spacecraft provide unambiguous evidence for the presence of hydroxyl (OH) or H2O,” notes the Perspective piece in the same issue of the journal.

Measurements from all the three spacecraft showed that there was an unambiguous absorption near the 3 micrometer on the lunar surface that are almost certainly due to hydroxyl or H2O, or both.

“Three micrometers is the position of the fundamental vibrational absorption of the OH group and so is an extraordinarly sensitive indicator of the presence of water (or hydroxyl),” notes the Perspective.

According to C.M. Pieters, the first author of the paper dealing with the data gathered from Chandrayaan-1, the formation and retention of hydroxyl and water is an ongoing surficial process. “OH/H2O production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration.” Dr. Pieters is from Brown University, U.S.

The spectrometer on Chandrayaan-1 provided sufficient evidence to show that absorption near the 3 micrometer is “observed systematically across the Moon.”

Data from the Deep Impact spacecraft found that the moon surface was hydrated during some portions of the lunar day. Hydroxyl and water absorption in the near infrared were strongest near the North Pole. It also found that the hydration varied with temperature, rather than cumulative solar radiation.

“Our results suggest that it is likely that all areas of the Moon, regardless of composition or location, exhibit similar maximum hydration during the night,” notes the paper by Jessica M. Sunshine, the first author of the paper dealing with the Deep Impact data. Dr. Sunshine is from the University of Maryland, U.S.

But the source of the water is still debatable. It could come from a variety of sources. Continuous source of water on the Moon is now a given thing. Pieters et al suggest that the hydrated phases seen at the poles could be “endogenic to the Moon and freshly exposed by craters in ancient highland terrain, or they may from during an impact event by a water bearing comet or asteroid.” But there should be a mechanism for retaining the water during such impact by comets or asteroids.

So the the most probable source according to Pieters et al could be: “surficial processes involving the solar-wind are the most likely explanation of our observation.” The solar-wind possibility is backed by Sunshine et al.

“Rapid photodissociation of water and a return to steady-state abundance during the day requires a ready source, which would be consistent with a solar wind origin,” they note.

Sunshine et al go even further to state that “such hydration via solar wind is expected to occur throughout the inner Solar System on all airless bodies with oxygen-bearing minerals on their surfaces.”

The Perspective piece suggests that water could have come from the “reduction of lunar divalent iron in minerals to metallic iron by solar-wind hydrogen, producing water, and liberation of water from the impact of interplanetary dust and small meteoroids.”

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