Searching for Lunar Horizon Glow with the LRO Star Tracker Cameras

Apollo-era observations of "lunar horizon glow" phenomena have been interpreted as being due to the forward scattering of sunlight by very small dust grains above the lunar surface. High altitude lunar horizon glow (LHG) seen in coronal photographs taken above orbital sunset during Apollo 15 is consistent with a population of exospheric dust grains with radii of ≈0.1 μm extending to altitudes of ≈10 km; while near-surface LHG observed by the TV cameras aboard a few of the Surveyor landers is consistent with dust grains with radii of ≈5 μm within about a meter of the surface. More recent searches have been undertaken for high altitude LHG, or the associated dust population, using the Clementine star tracker cameras (sensitive to visible and near-IR), the Lyman Alpha Mapping Project (LAMP) far-UV spectrograph on the Lunar Reconnaissance Orbiter (LRO), and the Lunar Dust Experiment (LDEX) on the Lunar Atmosphere and Dust Environment Explorer (LADEE). These searches have only produced upper limits for these exospheric dust abundances, as opposed to a clear detection. This motivated a search for LHG with the LRO star tracker cameras. Despite being designed for spacecraft navigation, the images these cameras produce are very suitable for scientific use. They also offer benefits over instruments previously used in terms of spatial resolution and ability to probe to low altitudes (both are of order a few hundred meters at the limb), as well as sensitivity to a similar wavelength range as the Apollo-era observations. Interestingly, the initial series of searches have resulted in some images that show bright patches at the limb that could be possible evidence for LHG. However, since these patches appear to typically extend only ~1000 m horizontally and just a few hundred meters vertically, this raises the possibility that they are simply due to sunlight reflected off surface topography along the limb. Initial simulations using a 64 pixel/degree digital elevation model (DEM) indicate that the bright patches could not be explained by surface-reflected sunlight; however, this is being further investigated with higher resolution DEMs to provide the best possible assessment. If these bright patches are best explained as LHG, then the exospheric dust abundances consistent with these calibrated measurements will be determined.