Radiative transfer modelling of dust devils

Abstract

Dust devils are particle laden convective vortices that form at the base of convective plumes. They are typically observed in dry desert climates on Earth and have been observed to form frequently on Mars. Meteorological measurements have also indicated that martian surface spacecraft have experienced numerous dust devil transits. To date, the characterisation of dust devils through the interpretation of spectral measurements of sunlight taken during a transit has yet to be investigated. Such measurements would provide valuable information of the physical size, dust load and internal structure of dust devils. A Monte Carlo Radiative Transfer (MCRT) model was developed to simulate the attenuation of sunlight through a dust devil and to investigate the observed spectral variation during such an event. The predicted transit signature resulting from a dust devil transit is highly dependent on the method of observation. The scattered light flux increases during the transit with the magnitude dependent on the dust concentration, making it sensitive to the internal dust distribution. This dependence is not observed for the total light flux which experiences a decrease and is strongly dependent on the total extinction through the vortex and insensitive to how the dust is distributed. The implication of this work is that separate in situ measurement of both the total and scattered flux is crucial for characterising dust devils and such measurements provide a powerful tool that could be exploited by future Mars missions.