It is a long-standing open question whether electrification of wind-blown sand due to tribocharging—the generation of electric charges on the surface of sand grains by particle–particle collisions—could affect rates of sand transport occurrence on Mars substantially. While previous wind tunnel experiments and numerical simulations addressed how particle trajectories may be affected by external electric fields, the effect of sand electrification remains uncertain. Here we show, by means of wind tunnel simulations under air pressure of 20 mbar, that the presence of electric charges on the particle surface can reduce the minimal threshold wind shear velocity for the initiation of sand transport, u *ft, significantly. In our experiments, we considered different samples, a model system of glass beads as well as a Martian soil analog, and different scenarios of triboelectrification. Furthermore, we present a model to explain the values of u *ft obtained in the wind tunnel that is based on inhomogeneously distributed surface charges. Our results imply that particle transport that subsides, once the wind shear velocity has fallen below the threshold for sustained transport, can more easily be restarted on Mars than previously thought.

Planetesimals are born fragile and are subject to destruction by wind erosion as they move through the gas of a protoplanetary disk. In microgravity experiments, we determined the shear stress necessary for erosion of a surface consisting of 1 mm dust pebbles down to 1 Pa ambient pressure. This is directly applicable to protoplanetary disks. Even pebble pile planetesimals with low eccentricities of 0.1 cannot survive inside of 1 au in a minimum-mass solar nebula, and safe zones for planetesimals with higher eccentricities are located even farther out.

<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>Rubble pile asteroids such as Itokawa, Ryugu, and Bennu are covered by regolith of various sizes. In some instances like on Itokawa the regolith is not distributed evenly but large boulders congregate in some areas whereas other are dominated by finer material [1].<span class="Apple-converted-space">  </span>Several mechanisms have been proposed to explain this size sorting, among them the so called ballistic sorting effect (BSE) [2]. The BSE depends on impacting particles rebounding more elastically from large targets (boulders) than when hitting a bed of fine grains. This mechanism of course not only applies to the primary impactor hitting an asteroid surface but also to secondary impacts from material ejected by the primary impact.<span class="Apple-converted-space"> </span>In order to fully understand the BSE on asteroids, it is therefore important to quantify the mass- and velocity distribution of ejecta generated by impacts into asteroid surfaces. In particular from impacts with low velocities that will then generate ejecta that is slower than the escape velocity of the asteroid.</p> <p>To conduct realistic experiments under asteroid conditions we use the ERICA (Experiments on Rebounding Impacts and Charging on Asteroids) platform [3] under the microgravity provided by the ZARM Bremen drop tower and the new GTB-Pro, also at ZARM. To provide a low but directed gravity level, i.e. asteroid gravity, ERICA consists of a vacuum chamber that contains the sample material which is mounted to a linear stage that - once the whole setup is in microgravity - provides a linear acceleration to simulate asteroid-level gravity. Using the linear stage eliminates any Coriolis forces a centrifuge would create. Due to the low g-jitter and the long microgravity time of 9.2 s in the drop tower and 2.5 s in the GTB-Pro these experiments are able to focus on ultra low velocity impacts in the cm/s rage, complementing earlier experiments by Brisset et al. [4].</p> <p>In the sample chamber we place granular beds (regolith analog) of various sizes and a launcher mechanism that impacts a basaltic projectile at the simulated asteroid surface. Using a stereo pair of cameras, as well as a high speed camera we then record the ejecta plume created by the impact. From the resulting image data we extract ejecta velocities using particle tracking (for larger ejecta particles) and digital image correlation (for smaller ejecta).<span class="Apple-converted-space"> </span></p> <p><span class="Apple-converted-space"><img src="" alt="" width="503" height="452" /></span></p> <p><em>Fig.1. Ejecta Plume generated by impacting a basaltic projectile (v = 65 cm/s)<span class="Apple-converted-space">  </span>in a bed of 0-300 μm sized particles at a gravity level of 2 • 10<sup>-4</sup> m/s^2</em></p> <p> </p> <p>[1] A. Fujiwara, J. Kawaguchi, D.K. Yeomans, M. Abe, T. Mukai, T. Okada, J. Saito, H. Yano, M. Yoshikawa, D.J. Scheeres et al., Science 312,1330 (2006)<span class="Apple-converted-space"> </span></p> <p>[2] T. Shinbrot, T. Sabuwala, T. Siu, M.V. Lazo,P. Chakraborty, Phys. Rev. Lett.118 (2017)<span class="Apple-converted-space"> </span></p> <p>[3] K. Joeris, L. Schönau, L. Schmidt, M. Keulen, V. De-sai, P. Born, J. Kollmer, EPJ Web of Conferences 249, 13003 (2021)</p> <p>[4] J. Brisset, C. Cox, S. Anderson, J. Hatchitt, A. Madison, M. Mendonca, A. Partida, D. Remie, Astron. Astrophys (2020)</p> <p> </p>

The ballistic sorting effect has been proposed to be a driver behind the observed size sorting on the rubble pile asteroid Itokawa. This effect depends on the inelasticity of slow collisions with granular materials. The inelasticity of a collision with a granular material, in turn, depends on grain size. Here we argue that determining the inelasticity of such collisions in an asteroid-like environment is a nontrivial task. We show non-monotonic dependency of the coefficient of restitution (COR) on target particle size using experiments in microgravity. Employing numerical simulations, we explain these results with the growing influence of adhesion for smaller-sized particles. We conclude that there exists an optimum impactor to target particle size ratio for ballistic sorting.