Predictions for the Optical Scattering at the Moon, as Observed by the Ladee

Introduction: When viewing the lunar exosphere there appear to be three major sources of optical scattering that are typically observed: atomic line emissions from the exospheric gases [1]; the relatively bright coronal and zodiacal light (CZL) background [2]; and the putative scattering of sunlight by exospheric dust, which is also referred to as “lunar horizon glow” (LHG) [3]. The lunar exospheric environment has not been well characterized, and there is still much to discover about the various physical processes taking place. This was recognized in the NRC SCEM report by “Concept 8: Processes involved with the atmosphere and dust environment of the Moon are accessible for scientific study while the environment remains in a pristine state” [4]. In response, NASA is going to fly the Lunar Atmosphere and Dust Environment Explorer (LADEE) to address these goals [5]. LADEE will carry a neutral mass spectrometer, a dust detector, and an ultraviolet/visible (UV/Vis) spectrometer. It will be placed in a near-circular retrograde equatorial orbit at an altitude of ≈50 km, and is due for launch in 2011. Here we focus on the modeling necessary for the analysis of the UV/Vis spectrometer data. We have developed an adaptable code that can simulate the optical scattering processes that can be observed at UV and visible wavelengths from an orbiter within the shadow of the Moon. The predictions presented here indicate that the LADEE UV/Vis spectrometer will be able to readily distinguish between atomic line emissions and CZL, as well as LHG. Lunar Exospheric Environment Overview: Atomic line emissions. The lunar exosphere consists of various elements (e.g., H, He, Na, K, Ar, Rn), which are continuously sourced from the lunar surface and are either lost to space or recycled back to the surface [1]. Although only trace elements in the lunar exosphere, the bright emissions from sodium (Na) and potassium (K) with scale heights H ~ 100 km have been extensively used to study the variability of the exosphere from ground-based observatories [6,7]. The Na-D lines, at wavelengths λD1 = 589.6 nm and λD2 = 589.0 nm, are particularly bright in the visible with intensities up to several kiloRayleighs. Coronal and Zodiacal Light. This refers to both sunlight scattered by the solar corona, and that scattered by interplanetary dust in the inner solar system. The Zodiacal dust population is distributed about the ecliptic plane and produced by asteroids and comets [2]. A formulation for the spatial and spectral characteristics of CZL was developed [8], and later refined by fitting to spectral broad measurements from the Clementine Star Tracker cameras [2]. Lunar Horizon Glow. Observations of a glow along the lunar horizon suggest that there is a greater abundance of lunar exospheric dust than is expected to be produced by meteoritic ejecta alone [9]. Therefore, it has been proposed that electric charging of the lunar surface by plasma and solar UV could result in the electrostatic transport of dust [e.g.,10]. The Surveyor landers observed LHG along the local western horizon after sunset, which indicated that micron-scale dust grains (radii a ~ 5 μm) were “levitating” within ≈1 m of the surface. Direct detection of highly-charged lunar dust moving at ~100 m s was likely provided by the Apollo 17 Lunar Ejecta and Meteorites (LEAM) experiment [10]. The LEAM dust counts peaked around the terminator, which strongly suggests a connection with LHG [11]. Evidence for exospheric dust “lofted” to higher altitudes came from Apollo astronaut observations and coronal photography from orbit while within lunar shadow [3,12]. These observations indicated that the high-altitude dust was submicron (a ~ 0.1 μm) with H ~ 10 km [13]. Further evidence from the astrophotometer aboard the Soviet Lunokhod-II rover revealed a brighter than expected lunar twilight sky [14], while the Apollo 16 Far-Ultraviolet Camera/Spectrograph possibly detected LHG during the lunar daytime [15].