Characterisation of dust events on Earth and Mars: the ExoMars/DREAMS experiment and the field campaigns in the Sahara desert

Atmospheric dust plays an important role on the terrestrial climate, regulating the amount of solar radiation coming to the surface, affecting the development and the life time of the clouds and providing fundamental nutrients to the growth of the terrestrial and oceanic biomes. On Mars, the global effect of dust is even stronger due to the widespread presence of sources and the lack of vegetation and oceans able to mitigate its contribution. The DREAMS station and the Dust Complex, on board of the ExoMars 2016 and 2020 mission respectively, have been specifically developed for the study of the Martian airborne dust. During my Phd I joined the team that lead the DREAMS experiment and the MicroMed sensor of the Dust Complex. As a part of the instruments developing and the acquisition of martian analogous data, our team has carried out various campaigns in the Sahara desert, to study the environment and the lifting phenomena that are expected on Mars. We monitored the dust lifting events by acquiring, for the first time in literature, synchronous measurement of meteorological data, atmospheric electric field, saltation activity and suspended dust concentration. Currently, this is the most complete data set available for the study of the dust lifting processes. We worked on the development of proper detection algorithms to individuate the dust events acquired in the surveys, applicable also to the future martian missions. We studied the characteristic of the observed dust storm and dust devils activity, focusing on their electric proprieties. In particular, we obtained the first experimental indications of how the induced electric field is related to the amount of suspended grains and meteorological characteristics of the events. We compared the terrestrial results with the currently available martian data, in order to prepare the analysis of the next ExoMars measurements.Comment: Phd Thesi

Characterisation of dust events on Earth and Mars the ExoMars/DREAMS experiment and the field campaign in the Sahara desert

Atmospheric dust plays an important role on the terrestrial climate, regulating the amount of solar radiation coming to the surface, affecting the development and the life time of the clouds and providing fundamental nutrients to the growth of the terrestrial and oceanic biomes. On Mars, the global effect of dust is even stronger due to the widespread presence of sources and the lack of vegetation and oceans able to mitigate its contribution. Hence, in the frame of the Martian exploration, the study of the dust proprieties and of the dust lifting phenomena covers a key role, representing one of the goals of the present and future missions. The ExoMars 2016-2020 programme aims to search for signs of past or present life on Mars, and investigates the Martian atmosphere and the long-term climate changes. The DREAMS station and the Dust Complex, on board of the ExoMars 2016 and 2020 mission respectively, have been specifically developed for the study of the airborne dust. During my Phd I joined the team that lead the DREAMS experiment and the MicroMed sensor of the Dust Complex. As a part of the instruments developing and the acquisition of martian analogous data, our team has carried out various campaigns in the Sahara desert, to study the environment and the lifting phenomena that are expected on Mars. Indeed, there are still many open questions regarding the dust processes physics, largely due to the lack of proper field surveys. Among the other, many uncertainties are still related to the electric proprieties of the dust grains, that are able to acquire charge by triboelectricity. Our station was able to monitor the dust lifting events by acquiring, for the first time in literature, synchronous measurement of meteorological data (vertical and horizontal wind speed, pressure, air and soil humidity and temperature) and the atmospheric electric field, coupling also the observations of the saltation activity and suspended dust concentration. Currently, we have acquired the most complete data set available for the study of the dust lifting processes. One of the aim of this PhD work has been the development of proper detection algorithms to individuate the dust events acquired in the surveys. The methods could be applied also to future martian missions. We focused also on the development of a technique to evaluate the morphological proprieties of the dust devils using the tracks leaved in the meteorological data. We studied the characteristic of the observed dust storm and dust devils activity, focusing on their electric proprieties. We performed the first statistical study of the electric proprieties of the dust devils observing how the induced E-field is linearly related to the vortex pressure drop, rotatory speed and the vertical air flow speed. We found that during both dust storms and dust devils the induced E-field is linearly related to the numeric concentration of lifted grains with a compatible slope for the two dust processes. We observed also how the induced electric field is heavily influenced by the air and soil humidity and how the grains ability to acquire and hold the electric charge is probably affected by the deliquescence of the evaporites. In addition, we obtained the first experimental indication of how the induced electric field has a positive feedback on the lifting process, enhancing the amount of suspended grains when a critical value is overcome. We compared the terrestrial results with the martian data available in literature, founding a lot of points in common between the dust processes populations of the two planets. This allow to understand how the results here presented are fundamental not only for the study of the terrestrial dust proprieties but also for the analysis of the future ExoMars data

Characterisation of dust events on Earth and Mars the ExoMars/DREAMS experiment and the field campaign in the Sahara desert

Atmospheric dust plays an important role on the terrestrial climate, regulating the amount of solar radiation coming to the surface, affecting the development and the life time of the clouds and providing fundamental nutrients to the growth of the terrestrial and oceanic biomes. On Mars, the global effect of dust is even stronger due to the widespread presence of sources and the lack of vegetation and oceans able to mitigate its contribution. Hence, in the frame of the Martian exploration, the study of the dust proprieties and of the dust lifting phenomena covers a key role, representing one of the goals of the present and future missions. The ExoMars 2016-2020 programme aims to search for signs of past or present life on Mars, and investigates the Martian atmosphere and the long-term climate changes. The DREAMS station and the Dust Complex, on board of the ExoMars 2016 and 2020 mission respectively, have been specifically developed for the study of the airborne dust. During my Phd I joined the team that lead the DREAMS experiment and the MicroMed sensor of the Dust Complex. As a part of the instruments developing and the acquisition of martian analogous data, our team has carried out various campaigns in the Sahara desert, to study the environment and the lifting phenomena that are expected on Mars. Indeed, there are still many open questions regarding the dust processes physics, largely due to the lack of proper field surveys. Among the other, many uncertainties are still related to the electric proprieties of the dust grains, that are able to acquire charge by triboelectricity. Our station was able to monitor the dust lifting events by acquiring, for the first time in literature, synchronous measurement of meteorological data (vertical and horizontal wind speed, pressure, air and soil humidity and temperature) and the atmospheric electric field, coupling also the observations of the saltation activity and suspended dust concentration. Currently, we have acquired the most complete data set available for the study of the dust lifting processes. One of the aim of this PhD work has been the development of proper detection algorithms to individuate the dust events acquired in the surveys. The methods could be applied also to future martian missions. We focused also on the development of a technique to evaluate the morphological proprieties of the dust devils using the tracks leaved in the meteorological data. We studied the characteristic of the observed dust storm and dust devils activity, focusing on their electric proprieties. We performed the first statistical study of the electric proprieties of the dust devils observing how the induced E-field is linearly related to the vortex pressure drop, rotatory speed and the vertical air flow speed. We found that during both dust storms and dust devils the induced E-field is linearly related to the numeric concentration of lifted grains with a compatible slope for the two dust processes. We observed also how the induced electric field is heavily influenced by the air and soil humidity and how the grains ability to acquire and hold the electric charge is probably affected by the deliquescence of the evaporites. In addition, we obtained the first experimental indication of how the induced electric field has a positive feedback on the lifting process, enhancing the amount of suspended grains when a critical value is overcome. We compared the terrestrial results with the martian data available in literature, founding a lot of points in common between the dust processes populations of the two planets. This allow to understand how the results here presented are fundamental not only for the study of the terrestrial dust proprieties but also for the analysis of the future ExoMars data

Design and CFD Analysis of the Fluid Dynamic Sampling System of the “MicroMED” Optical Particle Counter

MicroMED is an optical particle counter that will be part of the ExoMars 2020 mission. Its goal is to provide the first ever in situ measurements of both size distribution and concentration of airborne Martian dust. The instrument samples Martian air, and it is based on an optical system that illuminates the sucked fluid by means of a collimated laser beam and detects embedded dust particles through their scattered light. By analyzing the scattered light profile, it is possible to obtain information about the dust grain size and speed. To do that, MicroMED’s fluid dynamic design should allow dust grains to cross the laser-illuminated sensing volume. The instrument’s Elegant Breadboard was previously developed and tested, and Computational Fluid Dynamic (CFD) analysis enabled determining its criticalities. The present work describes how the design criticalities were solved by means of a CFD simulation campaign. At the same time, it was possible to experimentally validate the results of the analysis. The updated design was then implemented to MicroMED’s Flight Model

Electric properties of dust devils

Dust devils are one of the most effective phenomena able to inject dust grains into the atmosphere. On Mars, they play an important role to maintain the haze and can significantly affect the global dust loading, especially outside the dust storm season. Despite dust devils having been studied for a century and a half, many open questions regarding their physics still exist. In particular, the nature of the dust lifting mechanisms inside the vortices, the development of the induced electric field and the exact contribution to the global atmospheric dust budget are still debated topics. In this paper, we analyze the dust devil activity observed in the Moroccan Sahara desert during a 2014 field campaign. We have acquired the most comprehensive field data set presently available for the dust devils: including meteorological, atmospheric electric field and lifted dust concentration measurements. We focus our attention on the electric field induced by vortices, using this as the principal detection parameter. We present, for the first time, the statistical distribution of dust devil electric field and its relationships with the pressure drop, the horizontal and vertical vortex velocity and the total dust mass lifted. We also compare the pressure drop distribution of our sample with the ones observed on the martian surface showing the similarity of the dust devils samples and the usefulness of this study for the next martian surface missions

Martian environmental chamber: Dust system injection

NessunaThe aim of this work is to describe the development and implementation of an experimental setup able to reproduce some characteristics of the Martian atmosphere. The development of such setup fits into the context of MicroMED project, that foresees the development of an optical particle counter to be accommodated on the ExoMars 2020 Surface Platform, as part of the suite of sensors named Dust Complex. MicroMED will perform the first direct measurement of the size distribution of the powder close to Martian surface. The experimental setup is able to reproduce the characteristics of the Martian atmosphere: pressure, atmospheric composition, the actual temperature in which MicroMED will operate (from 20 C to 40 C) and the most important thing: the presence of suspended dust. The main result obtained in this work was the right configuration of an experimental setup in which to test sensors or instruments that work in Martian conditions. In particular, a dust injection system has been developed in order to obtain a dust distribution that was localized and without the formation of particles aggregates, for a correct calibration of the instrument

Signal-adapted tomography as a tool for dust devil detection

Dust devils are important phenomena to take into account to understand the global dust circulation of a planet. On Earth, their contribution to the injection of dust into the atmosphere seems to be secondary. Elsewhere, there are many indications that the dust devil’s role on other planets, in particular on Mars, could be fundamental, impacting the global climate. The ability to identify and study these vortices from the acquired meteorological measurements assumes a great importance for planetary science. Here we present a new methodology to identify dust devils from the pressure time series testing the method on the data acquired during a 2013 field campaign performed in the Tafilalt region (Morocco) of the North- Western Sahara Desert. Although the analysis of pressure is usually studied in the time domain, we prefer here to follow a different approach and perform the analysis in a time signal-adapted domain, the relation between the two being a bilinear transformation, i.e. a tomogram. The tomographic technique has already been successfully applied in other research fields like those of plasma reflectometry or the neuronal signatures. Here we show its effectiveness also in the dust devils detection. To test our results, we compare the tomography with a phase picker time domain analysis. We show the level of agreement between the two methodologies and the advantages and disadvantages of the tomographic approach

CFD analysis and optimization of the sensor “MicroMED” for the ExoMars 2020 mission

Characterization of dust is a key aspect in recent space missions to Mars. Dust has a huge influence on the planet's global climate and it is always present in its atmosphere. MicroMED is an optical particle counter that will be part of the "Dust Complex" suite led by IKI in the ExoMars 2020 mission and it will determine size distribution and concentration of mineral grains suspended in martian atmosphere. A Computational Fluid Dynamic (CFD) analysis was performed aimed at the optimization of the instrument's sampling efficiency in the 0.4-20 μm diameter range of the dust particles. The analysis allowed to understand which conditions are optimum for operations on Mars and to consequently optimize the instrument's fluid dynamic design

The role of the atmospheric electric field in the dust-lifting process

Mineral dust particles represent the most abundant component of atmospheric aerosol in terms of dry mass. They play a key role in climate and climate change, so the study of their emission processes is of utmost importance. Measurements of dust emission into the atmosphere are scarce, so that the dust load is generally estimated using models. It is known that the emission process can generate strong atmospheric electric fields. Starting from the data we acquired in the Sahara desert, here, we show for the first time that depending on the relative humidity conditions, electric fields contribute to increase up to a factor of 10 the amount of particles emitted into the atmosphere. This means that electrical forces and humidity are critical quantities in the dust emission process and should be taken into account in climate and circulation models to obtain more realistic estimations of the dust load in the atmosphere. <P /

Structural Optimization of MicroMED Dust Analyzer

This research work describes the structural optimization of the MicroMED Dust Analyzer, an Optical Particle Counter developed for the ESA ExoMars 2022 mission. Topology Optimization, a non-conventional design technique was adopted to obtain a lighter component, a valuable achievement for aerospace and space scientific instruments design. In particular, two solutions for the instrument optical bench were proposed and assessed relying on a classical finite element approach, comparing the improved performance with the current design. The optimization outcome proved the adopted design workflow robustness and provided promising results in view of a possible mechanical design enhancement of the MicroMED Dust Analyzer instrument. Indeed, a mass budget saving of about 55% of the considered design domain was achieved, and the dynamic behaviour of the optical bench was improved by up to 50% of the first natural frequency value. Finally, a mockup of the lightened optical bench was manufactured, and the redesign effectiveness was proven by comparing the numerical mechanical resonances with the ones obtained experimentally. An error smaller than 5% was found on the first natural frequency, validating the performed optimization approach