Several spacecraft have identified a potential signal of water in the Moon’s surface material. This signal, a spectral absorption feature at a wavelength of 3 micrometers, could correspond to either molecules of water (H2O) itself or water’s close cousin, hydroxyl (OH). One complication of interpreting spacecraft data is that the 3-micrometer-wavelength region features a mixture of reflected sunlight and thermal emission emitted by the Moon itself. This thermal contribution can be calibrated out of the data, but only with accurate observations of the Moon’s thermal emission at wavelengths beyond 3 micrometers.
NASA’s Deep Impact spacecraft made such measurements during a flyby of the Moon and observed that lunar surface hydration varied with the Moon’s surface temperature. However, Deep Impact’s observations were not extensive enough to confirm this discovery. NASA’s Moon Mineralogy Mapper (M3) studied the lunar surface in far more detail as part of India’s Chandrayaan-1 lunar mission. Unfortunately, M3 could not observe at wavelengths beyond 3 micrometers, making removal of lunar thermal emission extremely difficult.
Several teams have attempted to calibrate the M3 data around 3 micrometers, with widely divergent results. Now Honniball et al. present new, independent ground-based observations to resolve the discrepancy. They used NASA’s Infrared Telescope Facility (IRTF) in Hawaii to collect more than 3,000 spatially resolved spectra spanning a wavelength range from 1.67 to 4.2 micrometers. Their observations cover a range of latitudes on the Moon as well as different solar positions relative to the lunar surface that correspond to lunar times of day. These independent measurements at wavelengths longer than 3 micrometers allowed for a more accurate removal of thermal emission.
Using the thermally corrected IRTF data, the authors confirm the temperature-dependent variation of hydration on the lunar surface. The surface appears less hydrated closer to local noon, at which time the surface reaches its maximum temperature. They also observe a latitudinal dependence, with more hydration appearing at higher latitudes, particularly in the southern hemisphere. These conclusions are broadly consistent with prior work conducted at other wavelengths, although the authors note that many smaller discrepancies remain and say that additional observations are required to ultimately characterize lunar surface hydration. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2020JE006484, 2020)
—Morgan Rehnberg, Science Writer