CFD Analysis of the unpredicted bimodal size distribution histograms for the ExoMars MicroMED sensor

The sensor "MicroMED", which is scheduled to reach Mars in the framework of the ExoMars mission, is an Optical Particle Counter (OPC) that aims at performing the first ever direct measurement at the surface of airborne Martian dust. MicroMED has been mostly developed and integrated at the Astronomical Observatory of Capodimonte (INAF-OAC) premises and will provide size distribution and concentration of airborne dust on Mars. During performance and calibration tests with monodispersed grain samples, some unexpected bimodal size distributions have been obtained, highlighting the need for a verification of MicroMED's precision in size retrieving. This paper provides an overview of the analysis performed to individuate the causes of this behavior, and the verification of the impact on performances. Such analysis could be of interest to whoever is implied in the design of an OPC.