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Objectives

Lead / Participants

AU ( BIRA-IASB, UDE, IAA)

The proposed research program will utilize the unique and internationally recognised research facility housed at Aarhus University (AU). Complementary, dedicated experiments will be set up in Duisburg-Essen University (UDE) largely using the available instrumentation for dust grain tracking and analysis. Finally, scattering properties of the Martian dust analogues will be measured at the CSIC-IAA Cosmic Dust Laboratory (hereafter CODULAB).

Science

Synthesis of Martian dust analogues

Novel preparation methods and processing routines imported from the field of functional, nano- and micro-ceramics research will be used for the synthesis of the dust analogues with tailored properties. Basaltic analogue materials will be first synthesized in the form of powders with narrow size distributions and a fully specified geometry and composition. In a second stage of the synthesis process, the obtained powder analogues will be shaped into compact bodies of varying density and porosity. These pellet samples are conceived to mimic the formation of particle aggregates during the occurrence of Martian storms.

Recreating the Martian dust and clouds in the laboratory

Systems have been developed at AU in order to generate mineral dust aerosols within the Mars simulation chamber. These involve injecting gas dust mixtures which create an aerosolizing jet thus dispersing the particles. Laser Doppler Velocimeter (LDV) is typically used in order to detect the velocity of individual suspended dust grains and thereby monitor the concentration and flow of the aerosol. It is also possible to reduce air temperatures within the facility down to at least -50 °C allowing studies of dust aerosols at low temperature.

Dust Resuspension/Aerosolization (direct and saltation generated flux)

Indirect resuspension (aerosolization) of dust can occur through saltation impact of sand particles. This process is considered by many geologists to be the dominant process of dust resuspension on Earth, though details of the process are lacking. Quantifying and parametrizing these processes experimentally is a first step towards developing new predictive models of dust resuspension. Specifically, the measured parameters will be the threshold wind speed for particle resuspension, quantifying removal rates (vertical flux), high speed imaging and microscopy to determine the detachment dynamics, in-situ measurement of the particle size and for the first time the electrification state of the aerosolized dust.

Aerosol dynamics and electrification

Of central importance to the transport dynamics of micro-particle aerosols are the processes of surface adhesion, particle cohesion (aggregation) and dispersion (breakup of aggregated particles). Here particle electrification (electrostatics) is often a dominant factor in these processes.

Aggregation in electric fields

A recent analysis suggests that the aggregation of hollow micro-spheres in electrostatic fields is mostly due to the dielectrophoretic motion of induced dipoles. As this changes the size distribution and morphology of ejected grains or already influences the ejection process itself, this is of high importance in order to characterize airborne dust. The electric fields at the surface of Mars are unknown and the low-density atmosphere and lower saltating particle fluxes might lead to significant differences from Earth.

Aerosol deposition (settling)

For all types of aerosols it is physically important to have an understanding of the aerodynamic drag properties of particles. Conventionally for low pressure environments researchers apply the General or Universal law of fall developed by Millikan in the 1920’s. This is essentially a semi-empirical model based upon a series of experiments made at that time. The few experimental tests which have been carried out in more recent times have led to broad criticism of this model and urged further experimental study. Confidence in settling rates for specific aerosols should therefore be supported by experimental measurement. A novel technique has been developed at AU to determine the settling velocity of individual aerosol particles based upon Laser Doppler Velocimetry.

Observations at the microscopic scale

Sedimentation speeds or, more generally, gas-grain coupling of dust grains are important to be known for transport within the atmosphere. The gas-grain interaction is determined by the particle size, shape and inner structure of a particle, which itself is then again influencing the optical properties and the retrieval of particle properties within the Martian atmosphere. Therefore, one fundamental measurement is the grain size directly at the time the particles are resuspended from the soil. While wind is a necessity to determine threshold conditions for particle lift and erosion rates, particle impacts in a static (no wind) environment readily allow ejecta to be traced while settling.

Microgravity experiments

In the laboratory one of the dominating forces acting on the soil and opposing lift is gravity. Under reduced Martian gravity, smaller forces are needed to eject grains. Lower gravity might generally also result in a more porous soil, as dust is settling. Therefore, scaling of laboratory results to Martian conditions is needed. To make scaling more precise we will aim at carrying out some benchmark studies under reduced gravity during parabolic flights.

Scattering properties of dust

This experimental study is motivated by the lack of satisfactory modelling approaches traditionally used to mimic the role of dust particles in the Martian radiative budget. Airborne dust particles scatter and absorb solar radiation thereby playing a key role in determining the thermal structure of Martian atmosphere. The net radiative impact of mineral dust particles in the atmosphere constitutes one of the major uncertainties in Martian atmospheric studies. At the root of this problem lies a lack of understanding of how solar radiation is scattered in all directions after interacting with a cloud of particles, i.e. lack of accurate dust/cloud scattering properties. Modelling scattering properties is straightforward for homogeneous spheres but is extremely challenging for irregular dust particles.Martian dust particles are known to have a wide variety of shapes and sizes. However, the scattering function of Martian dust particles is often derived by assuming that these particles are spherical. Experimental studies of light scattering by ensembles of dust particles covering different size ranges, and compositions are a key tool to interpret space observations.

 

Expected results and Impact