Searching for Lunar Horizon Glow With the Lunar Orbiter Laser Altimeter

Abstract

AbstractWe present the results of a 2‐year‐long systematic campaign to monitor the lunar dust exosphere for enhancements in dust concentration at altitudes <20 km both during and outside of major meteor stream periods. We utilize the radiometric capabilities of the Lunar Orbiter Laser Altimeter Laser Ranging telescope onboard the Lunar Reconnaissance Orbiter to search for forward‐scattered sunlight from exospheric dust grains, called lunar horizon glow (LHG). Specifically, we test the hypothesis that major meteor streams can produce LHG similar to what was measured in Apollo 15 coronal photography. Assuming a one‐dimensional exponential vertical dust profile and a dust grain radius r∼0.1μm, we derive an upper limit of ∼10−11 g/cm2 on the overlying column dust mass density near the terminator during stream periods, a limit roughly 10 times lower than inferred from Apollo 15. Recent studies at altitudes ∼1–20 km outside of major meteor streams and at higher altitudes during streams placed limits on the density of similarly sized grains ∼100–1,000 times lower than Apollo 15. Our results show that Apollo 15‐like LHG, if real, is also a rare occurrence at altitudes <20 km during major meteor stream periods. This study is the first to quantitatively constrain the role of meteor streams in producing Apollo 15‐like LHG at altitudes <20 km and small forward‐scattering angles and further narrows the properties of any similar high‐altitude LHG and the conditions under which it can occur.