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Planetary News: Chandrayaan-1 (2009)

Indian Orbiter Produces Long-Sought Evidence for Water on the Moon

By Emily Lakdawalla
September 25, 2009
Water feature on the Moon as seen by M3
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Water feature on the Moon as seen by M3
This version of the Moon Mineralogy Mapper data represents the strength of a spectral absorption feature at 3 microns, which indicates the presence of water or hydroxyl in the surface soil. The blue arrow points to Goldschmidt, a relatively fresh crater in the lunar highlands that has a relatively strong water/hydroxyl signature. Credit: ISRO / NASA / JPL / Brown University

Although India's Chandrayaan-1 lunar orbiter fell silent less than halfway through its planned mission, the data it returned included the first evidence of widespread water in the rocks across the surface of the Moon. Chandrayaan-1's Moon Mineralogy Mapper (M3), provided to the Indian mission by NASA, found a diagnostic spectral absorption feature from water that was particularly strong near the poles. Followup observations with the spectrometer on Deep Impact confirmed the observations, and a dip into decade-old data from Cassini's Visual and Infrared Mapping Spectrometer (VIMS) provided a third corroborating line of evidence.

M3 mapped 90% of the lunar surface with a spatial resolution of 140 meters per pixel and a spectral resolution of 30 nanometers, sampling 86 different slices of the electromagnetic spectrum from 430 to 3000 nanometers. Its data can be used to provide maps of mineral abundances across the surface of the Moon. But at the long-wavelength end of its detection capability, it saw a spectral absorption feature from water. M3 principal investigator Carlé Pieters said in a press briefing conducted yesterday: "There is a variety of hypotheses for what physical form this water could take on the surface. What we're seeing occurs in the uppermost surface of the lunar soil, the upper two millimeters. It could occur as one monolayer a few molecules thick; it could be mixed into the surface; it could be altered minerals; there could be various gradients."

More On Chandrayaan-1


Determining how much water is present requires assumptions, but reasonable assumptions give ranges of 10 to 1000 parts per million in the upper ~2 millimeters of the soil (the maximum depth that these infrared spectrometers can sample). The highest number is equivalent to 1 liter of water per 1 ton of lunar soil.

There may be diurnal variations in the abundance of water. If so, this likely requires the water to be replenished daily. One possible mechanism to replenish the water daily would be through the interaction of solar wind protons with oxygen-bearing surface minerals. If this process is found to generate detectable, economically useful quantities of water on the lunar surface, it bodes well for human exploration of the solar system, becuase the same process likely operates on other dry, airless bodies like Mercury and asteroids.

For more detail on the discovery of water on the Moon, visit The Planetary Society Blog.