Temporal and spatial variability of lunar hydration as observed by the Deep Impact spacecraft.

The Moon is generally anhydrous, yet the Deep Impact spacecraft found the entire surface to be hydrated during some portions of the day. Hydroxyl (OH) and water (H2O) absorptions in the near infrared were strongest near the North Pole and are consistent with <0.5 weight percent H2O. Hydration varied with temperature, rather than cumulative solar radiation, but no inherent absorptivity differences with composition were observed. However, comparisons between data collected 1 week (a quarter lunar day) apart show a dynamic process with diurnal changes in hydration that were greater for mare basalts (approximately 70%) than for highlands (approximately 50%). This hydration loss and return to a steady state occurred entirely between local morning and evening, requiring a ready daytime source of water-group ions, which is consistent with a solar wind origin.