Lifting dust and sand into the thin Martian atmosphere is a challenging problem. Atmospheric pressure excursions within dust devils have been proposed to support lifting. We verify this idea in laboratory experiments. Pressure differences up to a few Pa are applied along particle layers of 50 to 400 µm. As samples we used glass beads of ∼50 µm diameter and irregular basalt grains of ∼20 µm in size. The total ambient pressure of air was set to 600 Pa. Particles are ejected at pressure differences as low as 2.0 ± 0.8 Pa. In the case of glass beads, the ejected grains returning to the particle bed can trigger new particle ejections as they reduce cohesion and release the tension from other grains. Therefore, few impacting grains might be sufficient to sustain dust lifting in a dust devil at even lower pressure differences. Particle lift requires a very thin, ∼100 µm, low permeability particle layer on top of supporting ground with larger pore space. Assuming this, our experiments support the idea that pressure excursions in Martian dust devils release grains from the ground.

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<p>In the formation of a planetary system, the objects involved pass through a wide range of sizes starting with micrometre-sized particles and ending up as full-grown planets. An intermediate step in this evolution is represented by kilometre-sized planetesimals, which might consist of very loosely bound millimetre dust granules [1]. Their orbital velocity differs from that of the surrounding gas in the protoplanetary disk resulting in a headwind with relative velocities of the order of 50 m/s [2]. Since self gravity of such a planetesimal is very small, there is a possibility that it loses mass due to wind erosion. This raises the question at which wind speeds and ambient pressures the planetesimal is stable and at which it is not.</p> <p>To recreate wind erosion on planetesimals in protoplanetary disks as realistically as possible, low pressures and a microgravity environment are needed. The latter can be achieved by placing an experiment in an aircraft that performs parabolic flights (<em>A310 ZERO-G</em> by Novespace). In a cylindrical vacuum chamber a second smaller cylinder is located in its center and can rotate at high frequencies up to 200 Hz to create a shear flow. With this setup, a laminar wind profile can be generated over a simulated planetesimal surface placed within at ambient pressures down to 10<sup>-2</sup> mbar.</p> <p>We used this setup with millimetre dust aggregates consisting of SiO<sub>2</sub>. The aggregates were produced in an analogous way as dust aggregates at the bouncing barrier in protoplanetary disks might form, i.e. by collisions of micrometre-sized particles, sticking and growing up to the bouncing size. This experimental setup has already been used in previous parabolic flight campaigns with glass spheres as sample [3]. This time, a more realistic approach was applied with these SiO<sub>2</sub> aggregates.</p> <p>Wind erosion was observed at an ambient pressure that was an order of magnitude lower than before. Furthermore, by accurately measuring the residual gravity during the microgravity phases, it was possible to determine an angle of repose under the given conditions. Here we report the latest results of the parabolic flight campaigns.</p> <p>Applied to planet formation, our results support and expand earlier findings that wind erosion might generate forbidden zones for pebble pile planetesimals, i.e. closer to the star [3,4] and wind erosion might filter out excentric orbits [5].</p> <p> </p> <p>References</p> <p>[1] Wahlberg Jansson K., Johansen A., Bukhari Syed M., Blum J., 2017, ApJ, 835, 109</p> <p>[2] Weidenschilling S. J., 1977, MNRAS, 180, 57</p> <p>[3] Demirci T., Schneider N., Steinpilz T., Bogdan T., Teiser J., Wurm G., 2020, MNRAS, 493, 5456-5463</p> <p>[4] Rozner M., Grishin E., Perets H. B., 2020, MNRAS, 496. 4827-4835</p> <p>[5] Cedenblad L., Schaffer N., Johansen A., Mehlig B., Mitra D., 2021, ApJ, 921, 123</p> <p> </p> <p>Acknowledgements</p> <p>This project is funded by DLR space administration with funds provided by the BMWK under grant 50 WM 2140. T. B. is funded by DLR space administration with funds from the BMWK under grant 50 WM 2049. K. J. is funded by DLR space administration with funds from the BMWK under grant 50 WM 1943. F. C. O., F. J. and M. K. are funded by DLR space administration with funds from the BMWK under grant 50 WM 2142. N. S. is funded by the DFG under grant WU 321/16-1. M. F. is funded by the DFG under grant TE 890/7-1. We also thank M. Aderholz who helped bringing the idea of this experiment to real life.</p>

<p>In eolian events, where large amounts of dust are carried through an atmosphere, strong electric fields can be generated above ground. Electric charge is transmitted via tribocharging during inter-particle collisions which can have a great impact on further particle transport and sedimentation. If different grain sizes, for example, charge differently, this might lead to size dependent particle and charge separation [1]. It could also promote particle lifting [2,3]. This shows, that understanding the charging behavior of particles and aggregates in strong electric fields is important in the context of particle transport in atmospheres. Especially on Mars, any kind of support for particle lifting might be crucial.</p> <p>We investigate the charging behavior of mm-sized basaltic dust aggregates with the help of microgravity experiments at Bremen drop tower. Our setup consists of a 50 x 50 x 110 mm chamber which we operate in an air environment. The sides of the chamber are copper plates which function as electrodes. At the bottom of the chamber, the sample is placed inside a cylindric aluminum container, which is also coated with basalt dust. The dust grains making up the agglomerates are in the µm size range. The aggregates themselves range from 0.4 – 2.2 mm in diameter.</p> <p>Before the microgravity phase, we shake the aggregates for 15 minutes in order to electrically charge them. As soon as the sample is ejected into an 8 kV DC field, the aggregates are accelerated towards one of the electrodes. Through this acceleration, we can estimate the charge of the individual agglomerates. This way, we observe initial charges up to 10<sup>5</sup>­ e, both negative and positive without an obvious bias in polarity. Once the aggregates reach an electrode, they either instantly stick to it or bounce off, but eventually cling to the copper plate. Most agglomerates larger than 0.4 mm do, however, recharge while sticking on the electrode until the repelling Coulomb force outweighs the adhesive sticking force. The sticking time is on the order of 0.05 – 0.5 s. The agglomerates charge up to 10<sup>7</sup> e until they are accelerated to the opposite electrode and recharge again. This charge gained on the electrodes is up to two orders of magnitudes higher than the initial charge. When agglomerates bounce on an electrode, no significant charge is transmitted. The experiments are in agreement with a model where conductive grains on a conductor in an electric field charge, increasing the repulsive force until the different contacts can no longer oppose lifting with their adhesive forces [4].</p> <p>Our results show that the basaltic dust aggregates are moderately electrically conductive. This presumably is caused by water clinging to the surface and the inside of the agglomerates, making the impact of an electric field on particle transport dependent on the humidity of the ambient atmosphere. In any case, these measurements allow us to quantify the charges and the lifting forces within a given field. If electric fields were present on Mars, electrostatic repulsion might support reducing the threshold friction velocity for saltation.</p> <p> </p> <p>References:</p> <p>[1]          K.M. Forward, D.J. Lacks, R.M. Sankaran, 2009, Geophys. Res. Lett.,<sub> </sub>36, p. L13201<br />[2]          Renno N. O., Kok J. F., 2008, Space Sci. Rev., 137, 419 <br />[3]          von Holstein-Rathlou C., Merrison J. P., Brædstrup C. F., Nørnberg P., 2012, Icarus, 220, 1<br />[4]          F. C. Onyeagusi, F. Jungmann, J. Teiser, G. Wurm, (in prep).</p>