Lunar Hydration: UV Measurements of Diurnally-Varying Thermal Desorption from LRO LAMP

The Lyman Alpha Mapping Project (LAMP), a far-ultraviolet (FUV) spectrograph on the Lunar Reconnaissance Orbiter (LRO), has been observing the Moon since September 2009. In addition to using stellar sources and interplanetary hydrogen emissions to study water in permanently shadowed regions of the lunar poles, LAMP measures reflected sunlight from the dayside of the Moon. The presence of a strong water absorption near 165 nm allows small amounts of hydration to be sensed in the LAMP reflectance spectra, and indeed LAMP measures spectral variations across the surface attributable to thermal desorption of diurnally-varying levels of hydration. In this report, we combine LAMP data with surface temperatures measured by LRO-Diviner to study diurnally-varying hydration — and estimate activation energies — across the entire lunar surface. The activation energy of desorption is the energy required for a water molecule to be released from the surface. We find that derived apparent activation energies vary with latitude and terrain type, which could be consistent with the idea that activation energy, related to the molecular trapping energy that is a result of defects and impurities in the grains, is expected to be higher for rougher, more mature grains that are present at lower latitudes if a gradient in space weathering effects is present with latitude.