Ultraviolet photodesorption as a driver of water migration on the lunar surface

We have studied the photodesorption of monolayer coverage of water ice coating amorphous carbon by 193 nm photons. We found that the column density of water decreases exponentially with fluence during irradiation at 120 K and obtain a desorption cross-section of (7.4±0.5)×10–19 cm2. We estimate the lifetime for photodesorption to be ∼14 h for average solar activity at the sub-solar point on the Moon. The photodesorbed molecules, based on other experiments, are expected to be slower than the escape velocity and to be re-adsorbed on the surface, contributing to water migration. For comparison, we determine the effect of 4 keV He+ on water-coated Apollo 16 soil (65901) under similar experimental conditions. We find a sputtering cross-section of (1.2 ±0.4)×10–16 cm2. We calculate that photodesorption by solar UV is four orders of magnitude more effective than sputtering by the solar wind in causing migration of adsorbed water on the lunar surface.