Planet. Sci. J. 2 238     DOI 10.3847/PSJ/ac38a4 10.3847/PSJ/ac38a4

Kruss M., Salzmann T., Parteli E., Jungmann F., Teiser J., Schönau L., Wurm G.

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.

Schematical diagram of the main relevant forces acting on a particle that is protruding from the granular bed considered in our experiments.

Astrophysical Journal Supplement Series (2023); http://dx.doi.org/10.3847/1538-4365/acf0be

J. Martikainen, O. Muñoz, T. Jardiel, J. C. Gómez Martín, M. Peiteado, Y. Willame, A. Penttilä, K. Muinonen, G. Wurm, and T. Becker

We present an advanced light-scattering model to retrieve the optical constants of three Martian dust analogs: Johnson Space Center regolith simulant, Enhanced Mojave Mars Simulant, and Mars Global Simulant. The samples are prepared to have narrow particle-size distributions within the geometric-optics domain. We carry out laboratory measurements to obtain the particle-size distributions, shapes, and diffuse reflectance spectra of the Martian analogs deposited on a surface. Our model framework includes a ray-optics code to compute scattering properties for individual particles, and a radiative-transfer treatment to simulate the surface. The irregular shapes of the dust particles are taken into account in the model. We compare our derived imaginary parts of the refractive indices with those in the literature and find that they are much smaller than the ones that are commonly used for Martian dust. A sensitivity study shows that the retrieved optical constants are sensitive to the particle shape, which needs to be accounted for in applications that use different shapes. Finally, the derived values are validated by using them to reproduce the reflectance spectrum of the Martian surface regolith as observed by the Nadir and Occultation for Mars Discovery instrument on board the ExoMars mission.

GRS1 particles used in the model (right) compared to the MMS-2 FESEM image (left).

Atmosphere (2023); https://doi.org/10.3390/atmos14071065

F. C. Onyeagusi, C. Meyer, J. Teiser, T. Becker, and G. Wurm

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.

Total charge on two solid basalt spheres before and after a collision. The solid line is a linear approximation of the data. The dashed line visualizes charge conservation. The basalt bead systems tend to lose charge after a collision.

2023 (PSJ) https://doi.org/10.3847/PSJ/acd32f 

S. Aoki, K. Shiobara, N. Yoshida, L. Trompet, T. Yoshida, N. Terada, H. Nakagawa, G. Liuzzi, A. C. Vandaele, I. R. Thomas, G. L. Villanueva, M. A. Lopez-Valverde, A. Brines, M. R. Patel, S. Faggi, F. Daerden, J. T. Erwin, B. Ristic, G. Bellucci, J. J. Lopez-Moreno, H. Kurokawa and Y. Ueno

The atmosphere of Mars is mainly composed by carbon dioxide (CO₂). It has been predicted that photodissociation of CO₂ depletes ¹³C in carbon monoxide (CO). We present the carbon ¹³C/¹²C isotopic ratio in CO at 30–50 km altitude from the analysis of the solar occultation measurements taken by the instrument Nadir and Occultation for Mars Discovery on board the ExoMars Trace Gas Orbiter (ExoMars-TGO). We retrieve ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O volume mixing ratios from the spectra taken at 4112–4213 cm⁻¹, where multiple CO isotope lines with similar intensities are available. The intensities of the ¹²C¹⁶O lines in this spectral range are particularly sensitive to temperature, thus we derive the atmospheric temperature by retrieving CO₂ density with simultaneously measured spectra at 2966–2990 cm⁻¹. The mean δ¹³C value obtained from the ¹³C¹⁶O/¹²C¹⁶O ratios is −263‰, and the standard deviation and standard error of the mean are 132‰ and 4‰, respectively. The relatively large standard deviation is due to the strong temperature dependences in the ¹²C¹⁶O lines. We also examine the ¹³C¹⁶O/¹²C¹⁸O ratio, whose lines are less sensitive to temperature. The mean δ value obtained with ¹²C¹⁸O instead of ¹²C¹⁶O is −82‰ with smaller standard deviation, 60‰. These results suggest that CO is depleted in ¹³C when compared to CO₂ in the Martian atmosphere as measured by the Curiosity rover. This depletion of ¹³C in CO is consistent with the CO₂ photolysis-induced fractionation, which might support a CO-based photochemical origin of organics in Martian sediments.

Synthetic spectra of the NOMAD measurements taken with diffraction order 183 (a), order 184 (b), order 185 (c), and order 186 (b) around 30 km tangent height. The assumed vertical profile of total CO volume mixing ratio is 1000 ppm (uniform) and along the line of sight. The isotopic ratios defined in the HITRAN2020 databases are assumed. The red, blue, and green curves illustrate contributions due to ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O absorption lines. The origin of the Y-axis for the red, blue, and green curves have been offset to improve visibility.