Numerical study of multiple scattering intensities obtained from dust grains of different sizes and compositions in an impact-induced ejecta curtain

Impact cratering of celestial bodies produces an ejecta plume containing underground material, which helps us understand the properties of subsurface grains and the surface evolution. In most cases, the ejecta plume forms an inverted conical structure so-called ejecta curtain. In-situ measurements of light scattering of ejecta curtain, such as the images taken at the Hayabusa2’s impact experiment help to constrain the size and composition of these less-processed grains using Monte-Carlo radiative transfer models. Here, we analyse the variation in the predicted scattered intensities of an ejecta curtain set up after that of Hayabusa2’s case and test its sensitivity to different compositions, sizes, and phase function of grains comprising the ejecta curtain. Initially, the density/optical depth of the ejecta curtain is assumed to be constant throughout. The intensities are seen to deviate from single scattering expectations in some cases, which we attribute mostly to the effect of varying scattering angles and optical depths of the photons. Further, we show the importance of using the phase function reproducing actual grain materials in place of the often-used Henyey–Greenstein (HG) phase function in impact plume modelling. The results are compared with different values of optical depth in the ejecta cone and the study is also extended to incorporate a grain-size distribution. It is observed that there is an increase in the magnitude of the model median scattered intensity with the decrease of the power index of the grain-size distribution. Nevertheless, the overall variation in the profile between the different compositions remained the same as that of the monodisperse case with uniform density across the ejecta cone. Thus, we envisage this study to serve as a useful reference for investigating the subsurface grain properties of planetary bodies via impact plume observations, and to aid in improving available ejecta scattering models and also in the selection of prospective targets for future impact missions.