Molecular Water Detected on the Moon

This illustration highlights the Moon’s Clavius Crater with an illustration depicting water trapped in the lunar soil there, along with an image of Stratospheric Observatory for Infrared Astronomy (SOFIA) that detected the signature of water on the lunar surface. Credit: NASA/Daniel Rutter.

The conventional understanding that the Moon is completely dry has been challenged multiple times in the past. Samples collected by the Apollo missions showed up to a few thousand parts per million of water bound to the minerals in the rocks and regolith. Similarly the Lyman Alpha Mapping Project (LAMP), the Lunar Exploration Neutron Detector (LEND), and Miniature Radio Frequency (Mini-RF) instruments on the Lunar Reconnaissance Orbiter have suggested the presence of water ice in permanently shadowed craters in the lunar polar regions.

Several other missions and observatories have looked broadly across the lunar surface and found evidence of hydration features at 3 microns. However, whether these features correspond to OH (hydroxyl) or H2O (molecular water) remained undetermined. A recent study led by Casey Honniball from NASA Goddard Space Flight Center utilized the Stratospheric Observatory For Infrared Astronomy (SOFIA) to detect evidence of H2O (molecular water) in Clavius Crater, one of the largest craters visible from Earth located in the Moon’s southern hemisphere, using its Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST). Water has a spectral feature at 6 microns that distinguishes it from hydroxyl and lies in one of the spectral bands of the FORCAST instrument. The SOFIA airborne observatory allows the instrument to fly above much of the telluric (water) absorption in Earth’s atmosphere that otherwise prevents observing this feature from the ground.

This detection of molecular water raises a number of questions such as where did this water originate and where is it stored? The water could be of exogenous origin, delivered by micrometeorites accreted onto the lunar surface. Alternatively, it could be a reaction product between hydrogen delivered by the solar wind and oxygen from the regolith materials. Honniball and colleagues suggest the water could be trapped within impact glasses or pore spaces in the soil in order to survive the harsh lunar environment. Water is a valuable resource on planetary bodies, therefore, this discovery is very important for future space exploration.  Follow-up flights with SOFIA will look for this signature of water in additional locations across the lunar surface. The data will contribute to creating the first water resource maps of the Moon for future human visitors. READ MORE