Casey I. Honniball
NASA Goddard Space Flight Center
Lunar Surface Hydration: a View from Earth
Hydration on the lunar surface was first reported in 2009 by three spacecraft and manifested as a strong absorption at 3 µm. The hydration at 3 µm is produced by hydroxyl (OH) attached to metal cations and/or molecular water (H2O). Measurements of the 3 µm band were revolutionary but the returned spacecraft data have limitations in wavelength coverage, spatial resolution, global coverage, and lunar time of day. To bridge the gap we use two Earth based observatories to characterize the true nature of the 3 µm band and to unambiguously detect molecular water. Using the SpeX infrared cross-dispersed spectrograph at the NASA InfraRed Telescope Facility (IRTF) at Maunakea Observatory we address diurnal variability of the 3 µm band and with the NASA/DLR Stratospheric Observatory For Infrared Astronomy (SOFIA) we observe the Moon at 6 µm where a purely H2O feature is exhibited.
Observations with the IRTF reveal total water (OH + H2O) abundances ranging from 0 to ~500 ppm H2O. From this new data set with improved thermal removal, we find diurnal variations of the 3 µm band along with variations with latitude and composition. We observe a decrease in abundance with increasing lunar local time, an asymmetric trend about the equator that favors the South, and higher abundances in highland regions. Data from SOFIA of the Clavius crater and surrounding region reveal abundances of ~100 to 412 ppm H2O from the 6 µm emission band that we attribute to molecular water on the Moon. All spectra acquired at the Clavius region exhibit a 6 µm emission band. We are unaware or any other lunar material that may exhibit an isolated 6 µm band. This is the first direct, unambiguous detection of H2O on the Moon outside the permanent shadows at the lunar poles.