A new analysis of data from two lunar missions finds evidence that the Moon’s water is widely distributed across the surface and is not confined to a particular region or type of terrain. The water appears to be present day and night, though it’s not necessarily easily accessible.
Bandfield and colleagues came up with a new way to incorporate temperature information, creating a detailed model from measurements made by the Diviner instrument on NASA’s Lunar Reconnaissance Orbiter, or LRO. The team applied this temperature model to data gathered earlier by the Moon Mineralogy Mapper, a visible and infrared spectrometer that NASA’s Jet Propulsion Laboratory in Pasadena, California, provided for India’s Chandrayaan-1 orbiter.
The new finding of widespread and relatively immobile water suggests that it may be present primarily as OH, a more reactive relative of H2O that is made of one oxygen atom and one hydrogen atom. OH, also called hydroxyl, doesn’t stay on its own for long, preferring to attack molecules or attach itself chemically to them. Hydroxyl would therefore have to be extracted from minerals in order to be used.
The research also suggests that any H2O present on the Moon isn’t loosely attached to the surface.
“By putting some limits on how mobile the water or the OH on the surface is, we can help constrain how much water could reach the cold traps in the polar regions,” said Michael Poston of the Southwest Research Institute in San Antonio, Texas.
Sorting out what happens on the Moon could also help researchers understand the sources of water and its long-term storage on other rocky bodies throughout the solar system.
The researchers are still discussing what the findings tell them about the source of the Moon’s water. The results point toward OH and/or H2O being created by the solar wind hitting the lunar surface, though the team didn’t rule out that OH and/or H2O could come from the Moon itself, slowly released from deep inside minerals where it has been locked since the Moon was formed.
Remote-sensing data from lunar orbiters have revealed spectral features consistent with the presence of OH or H2O on the lunar surface. Analyses of data from the Moon Mineralogy Mapper spectrometer onboard the Chandrayaan-1 spacecraft have suggested that OH/H2O is recycled on diurnal timescales and persists only at high latitudes. However, the spatial distribution and temporal variability of the OH/H2O, as well as its source, remain uncertain. Here we incorporate a physics-based thermal correction into analysis of reflectance spectra from the Moon Mineralogy Mapper and find that prominent absorption features consistent with OH/H2O can be present at all latitudes, local times and surface types examined. This suggests the widespread presence of OH/H2O on the lunar surface without significant diurnal migration. We suggest that the spectra are consistent with the production of OH in space-weathered materials by the solar wind implantation of H+ and formation of OH at crystal defect sites, as opposed to H2O sourced from the lunar interior. Regardless of the specific composition or formation mechanism, we conclude that OH/H2O can be present on the Moon under thermal conditions more wide-ranging than previously recognized.
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