MicroMED: study of the relation between signal durations and grain diameters

MicroMED is an Optical Particle Counter (OPC) on board the ExoMARS 2022 lander. It aims to perform the first direct monitoring of the martian suspended dust, measuring grain concentration and size distribution. In an OPC single dust grains are sampled in an illuminated spot and their size is retrieved based on the amplitude of the scattered light signal. In this study we analyze the possibility to determine the grain size using an alternative approach based on the durations of the signals rather than their magnitude. For this purpose, we studied the motion of the sampled grains by using a Computational Fluid Dynamic (CFD) simulation, to assure the consistency of the method. We have then performed a laboratory test campaign in martian simulated environment, to measure the signal durations relative to different monodispersed samples. The test results are here presented and compared with the numerical ones obtained using the CFD simulations. Using the acquired data, we present a preliminary duration-diameter calibration function, discussing the pros and cons of this method and how it can support the standard OPC one based on the signal magnitude.