A search for transient water frost at the lunar poles using LOLA

Abstract:

The possibility of lunar polar ice has been considered since suggested by Harold Urey in the 1950's, and has likely been directly detected at the north pole of Mercury by MESSENGER. That detection was based on the presence of reflectance anomalies seen by the Mercury Laser Altimeter that occurred only where models of the surface temperature allow long-duration preservation of water ice against sublimation (Paige et al., 2013; Neumann et al., 2013). Similar characteristics are seen at the poles of the Moon, though the higher lunar albedo complicates the detection. In this study we seek evidence for transient water frost on polar surfaces using data from the Lunar Orbiter Laser Altimeter.

The Lunar Orbiter Laser Altimeter (LOLA) measures the backscattered energy of the returning altimetric laser pulse at its wavelength of 1064 nm, and these data are used to map the reflectivity of the Moon at zero-phase angle with a photometrically uniform data set. Global maps have been produced at 4 pixels per degree (about 8 km at the equator) and 2 km resolution within 20° latitude of each pole. The zero-phase geometry is insensitive to lunar topography and enables the characterization of subtle variations in lunar albedo, even at high latitudes where such measurements are not possible with the Sun as the illumination source. We are currently searching the data set for evidence of transient surface frost by looking for changes in reflectance as a function of temperature based on the Diviner radiometer measurements and models. Thus far one candidate region has been identified, and we are refining the calibration to ensure that this and other detections are reliable.