We use a dynamical model to characterize the monthly and yearly variations of the lunar meteoroid environment for meteoroids originating from short and long-period comets and the main-belt asteroids. Our results show that if we assume the meteoroid mass flux of 43.3tons per day at Earth, inferred from previous works, the mass flux of meteoroids impacting the Moon is 30 times smaller, approximately 1.4tons per day, and shows variations of the order of 10% over a year. The mass flux difference is due to the combined effect of the smaller cross- of the Moon (factor of 13.46) and Earth's larger gravitational focusing (factor of 2-2.5). The lunar surface is vaporized by these impactors at an average impact vaporization flux of 11.6x10(-16)g.cm(-2).s(-1), providing a significant source for the rarefied lunar exosphere. Our model predicts acceptable vaporization rates and reproduces the local time dependence of observations of the dust ejecta cloud, measured by the Lunar Dust Experiment on board NASA's Lunar Atmosphere and Dust Environment (LADEE) satellite. However, the predicted density of the lunar ejecta cloud is four orders of magnitude larger than reported values by LADEE. This discrepancy might be attributed to a much lower yield from meteoroid impacts on fluffy lunar regolith and/or a lower detection efficiency of the LADEE dust detector. We suggest an upper limit of 30cm per million years for the soil gardening rate from small meteoroids. Plain Language Summary The lunar surface is continuously bombarded by small but fast and abundant particles at rates that amount to 1.4tons per day. These particles originate from asteroids and comets and after striking the surface produce a variety of observable phenomena such as a thin atmosphere and a dust cloud engulfing our satellite. Our novel model describes for the first time in detail the directions and velocities of particles impacting the Moon, including their variability in time and space. This approach correctly reproduces the shape of the dust cloud that was measured by the Lunar Atmosphere and Dust Environment Explorer mission. However, the density of the dust cloud predicted by this model is thousands of times higher than what was inferred from the measurement. On the other hand, our model provides realistic numbers for the mass of particles delivered daily to the Moon and for the exosphere density, resulting in a disagreement that is yet to be understood.

The neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft collected a trove of exospheric data, including a set of high-quality measurements of radiogenic Ar-40 over a period of 142days. Data synthesis studies, using well-established exosphere simulation tools, show that the LADEE argon data are consistent with an exosphere-regolith interaction that is dominated by adsorption and that the desorption process generates the Armand distribution of exit velocities. The synthesis work has uncovered an apparent semiannual oscillation of argon that is consistent with temporal sequestration in the seasonal cold traps created at the poles by the obliquity of the Moon. In addition, the LADEE data provide new insight into the pristine nature of lunar regolith, its spatially varying sorption properties, and the influence of sorption processes on the synodic oscillation of the argon exosphere.

The Lunar Dust Experiment (LDEX) is an in situ dust detector onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission. It is designed to characterize the variability of the dust in the lunar exosphere by mapping the size and spatial distributions of dust grains in the lunar environment as a function of local time and the position of the Moon with respect to the magnetosphere of the Earth. LDEX gauged the relative contributions of the two competing dust sources: (a) ejecta production due to the continual bombardment of the Moon by interplanetary micrometeoroids, and (b) lofting of small grains from the lunar surface due to plasma-induced near-surface electric fields.

The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft will orbit the Moon at an altitude of 50 km with a payload that includes the Ultraviolet Spectrometer (UVS) instrument, which will obtain high spectral resolution measurements at near-ultraviolet and visible wavelengths (approximate to 231-826 nm). When LADEE/UVS observes the lunar limb from within the shadow of the Moon it is anticipated that it will detect a lunar horizon glow (LHG) due to sunlight scattered from submicron exospheric dust, as well as emission lines from exospheric gases (particularly sodium), in the presence of the bright coronal and zodiacal light (CZL) background. A modularized code has been developed at NMSU for simulations of scattered light sources as observed by orbiting instruments in lunar shadow. Predictions for the LADEE UVS and star tracker cameras indicate that LHG, sodium (Na) emission lines, and CZL can be distinguished based on spatial morphology and spectral characteristics, with LHG dominant at blue wavelengths (similar to 250-450 nm) and small tangent heights. If present, LHG should be readily detected by LADEE/UVS and distinguishable from other sources of optical scattering. Observations from UVS and the other instruments aboard LADEE will significantly advance our understanding of how the Moon interacts with the surrounding space environment: these new insights will be applicable to the many other airless bodies in the solar system. (c) 2010 Elsevier Ltd. All rights reserved.