A light scattering grid for irregular lunar dust grains: A resource for
surface and orbital investigations
Abstract
Observations from the LADEE, Apollo and Surveyor missions have provided
evidence of an active lunar dust environment where dust is ejected from
the surface into the exosphere by natural physical mechanisms (e.g.,
meteoroid impacts and possibly electrostatic lofting). Characterizing
the spatial and temporal distribution of different exospheric dust
populations, and how they are coupled to other components of the dynamic
lunar environment, is important to understanding the surface evolution
of the Moon. Additionally, anthropogenic dust ejected from the surface
by human/robotic exploration activities can be a significant source of
exospheric dust. Experience from the Apollo missions showed that dust
has the potential to be hazardous as it can interfere with the operation
of mechanical, thermal and optical systems. Therefore, it is vitally
important to be able to monitor the dust environment to ensure mission
safety for future exploration, especially in the Artemis era. An
effective method for observing any dust populations in the lunar
exosphere is to measure the intensity of sunlight scattered by the dust.
This can reveal the dust abundance and spatial distribution, as well as
constrain the average grain size by measuring the angular width of the
forward scattering lobe. The largest uncertainties in such measurements
lies in the grain scattering properties, namely: scattering coefficient,
phase function shape and polarization. All of these become more
important at larger scattering angles, where diffraction no longer
dominates. Typically, the dust size distribution cannot be measured
uniquely, and must be constrained by a set of forward simulations at
multiple wavelengths and scattering angles. We present a precomputed
grid of light scattering properties for irregularly shaped grains
(Richard et al., 2011), which are a more realistic representation of
lunar dust than the Mie models commonly employed. The grid spans UV to
near-IR wavelengths and encompasses a wide range of grain size.
Scattering from smaller grains is computed using the Discrete Dipole
(DDA) method (DDSCAT) and at larger sizes using the Hapke-Equivalent
Slab with Allen diffraction (this will soon be replaced by ray tracing
with diffraction). Applications of this grid include observation
interpretation and modelling of sunlight scattered by meteoroid impact
plumes, lunar horizon glow, and exploration activities.