Emission of dust up to a few microns in size by impacts of sand grains during saltation is thought to be one source of dust within the Martian atmosphere. To study this dust fraction, we carried out laboratory impact experiments. Small numbers of particles of about 200 μm in diameter impacted a simulated Martian soil (bimodal Mars Global Simulant). Impacts occurred at angles of ∼18° in vacuum with an impact speed of ∼1 m s−1. Ejected dust was captured on adjacent microscope slides and the emitted particle size distribution (PSD) was found to be related to the soil PSD. We find that the ejection of clay-sized dust gets increasingly harder the smaller these grains are. However, in spite of strong cohesive forces, individual impacts emit dust of 1 μm and less, i.e., dust in the size range that can be suspended in the Martian atmosphere. More generally, the probability of ejecting dust of a given size can be characterized by a power law in the size range between 0.5 and 5 μm (diameter).
<p>Dust in the Martian atmosphere is (under regular conditions) not larger than a few micrometers in diameter.<br />Liberation through impacts of sand grains during saltation is thought to be one source of this fine dust within&#160;<br />the atmosphere, as windspeeds usually do not exceed the threshold windspeed needed to pick up the highly&#160;<br />adhesive smallest particles directly.<br />We conducted a laboratory experiment to take a closer look at these saltating impacts and the resulting PSD of&#160;<br />the Ejecta on a microscopic scale: A small number of particles of about 200&#956;m in diameter impacted a&#160;<br />simulated Martian soil (bimodal <em>Mars Global Simulant</em>). Impacts occurred at flat angles in fine vacuum&#160;<br />(10<sup>-2 </sup>mbar) with an impact speed of &#8764; 1 m/s. The ejected dust was captured on adjacent&#160;<br />microscope slides and its size distribution was analyzed.<br />We find that the probability for ejection decreases dramatically with decreasing size. However, in spite of&#160;<br />strong adhesive forces, individual impacts still emit dust of 1&#956;m and less. In fact, the probability&#160;<br />of ejecting dust of a given size can be characterized by a power law in the decade between 0.5&#956;m&#160;<br />and 5&#956;m (diameter).</p>
In recent years, collisional charging has been proposed to promote the growth of pebbles in early phases of planet formation. Ambient pressure in protoplanetary disks spans a wide range from below 10−9 mbar up to way beyond mbar. Yet, experiments on collisional charging of same material surfaces have only been conducted under Earth atmospheric pressure, Martian pressure and more generally down to 10−2 mbar thus far. This work presents first pressure dependent charge measurements of same material collisions between 10−8 and 103 mbar. Strong charging occurs down to the lowest pressure. In detail, our observations show a strong similarity to the pressure dependence of the breakdown voltage between two electrodes and we suggest that breakdown also determines the maximum charge on colliding grains in protoplanetary disks. We conclude that collisional charging can occur in all parts of protoplanetary disks relevant for planet formation.
Grain collisions in aeolian events, e.g., due to saltation, result in atmospheric aerosols. They may regularly be electrically charged, but individual charge balances in collisions including small grains are not easily obtained on the ground. We therefore approach this problem in terms of microgravity, which allows for the observation of collisions and the determination of small charges. In a drop tower experiment, ∼1 mm dust aggregates are traced before and after a collision within the electric field of a plate capacitor. The sum of the electric charge of two particles (total charge) before and after the collision often strongly deviates from charge conservation. Due to the average low collision velocities of 0.2 m/s, there is no large scale fragmentation. However, we do observe small charged particles emerging from collisions. The smallest of these particles are as small as the current resolution limit of the optical system, i.e., they are at least as small as tens of µm. In the given setting, these small fragments may carry 1 nC/m2–1 µC/m2 which is between 1% and ten times the surface charge density of the large aggregates. These first experiments indicate that collisions of charged aggregates regularly shed charged grains into the atmosphere, likely down to the suspendable aerosol size.
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