Stefano Spadaccia

The field of planetary system formation relies extensively on our understanding of the aerodynamic interaction between gas and dust in protoplanetary disks. Of particular importance are the mechanisms triggering fluid instabilities and clumping of dust particles into aggregates, and their subsequent inclusion into planetesimals. We introduce the timed Epstein multi-pressure vessel at low accelerations, which is an experimental apparatus for the study of particle dynamics and rarefied gas under micro-gravity conditions. This facility contains three experiments dedicated to studying aerodynamic processes: (i) the development of pressure gradients due to collective particle–gas interaction, (ii) the drag coefficients of dust aggregates with variable particle–gas velocity, and (iii) the effect of dust on the profile of a shear flow and resultant onset of turbulence. The approach is innovative with respect to previous experiments because we access an untouched parameter space in terms of dust particle packing fraction, and Knudsen, Stokes, and Reynolds numbers. The mechanisms investigated are also relevant for our understanding of the emission of dust from active surfaces, such as cometary nuclei, and new experimental data will help interpreting previous datasets (Rosetta) and prepare future spacecraft observations (Comet Interceptor). We report on the performance of the experiments, which has been tested over the course of multiple flight campaigns. The project is now ready to benefit from additional flight campaigns, to cover a wide parameter space. The outcome will be a comprehensive framework to test models and numerical recipes for studying collective dust particle aerodynamics under space-like conditions.

The Rosetta mission revealed the extraordinary complexity of the surface of comet 67P/Churyumov-Gerasimenko (67P) nucleus. The images and spectra from the OSIRIS and VIRTIS instruments showed a great variety of morphology structures as well as some variability of spectral features and slopes [1][2][3]. The study of water ice in comets is paramount for understanding the water content and evolution of those bodies. The surface of 67P is mainly covered by a mantle of organic-rich dust. When warmed by the solar thermal waves, the volatiles trapped in the subsurface layers start sublimating, and sometimes the upper dust mantle is disrupted, revealing the volatile-rich nature of the subsurface. These spots of exposed water ice show a flatter (“bluer”) spectral slope in the range 500-800 nm with respect to the average slope of the nucleus (11%/100 nm at 1.3° phase angle) [4][5]. The exposed ice spots can show diurnal variations or be stable over months [6], and the sublimation-triggered exposure mechanism has been demonstrated in the laboratory under comet-like conditions [5]. Observations showed that water vapor can also recondense on the shadowed regions, where the surface temperature is lower [7]. The recondensed frost becomes visible when the shadow retreats and disappears within minutes. Thermal modeling indicates that the ice thickness must be about 10 µm or lower [8], suggesting that the ice in these regions is most probably inter-mixed with dust particles. We are investigating in the laboratory the detection of frost condensation on asteroid simulants (CR-type from [9]). We use the POLarimeter for ICE Samples (POLICES) at the University of Bern, installed inside a black airtight enclosure. The sample is in the middle of the enclosure, illuminated by an optical fiber connected to a monochromator and held by a motorized arm that illuminates the sample with different incidence angles. A camera is placed on the other side of the arm looking at the sample at an angle of 45°. Finally, a full-Stokes polarimeter (Dual PEM II/FS42-47) is placed on top of the enclosure above the sample, measuring the polarization state of the light scattered by the sample. The dust sample is actively kept at a temperature of approximately 150 K through liquid nitrogen circulation. At the beginning of the experiment, the dust layer is uncovered and air with 15% relative humidity is pushed into the enclosure while keeping the sample cold. The phase angle is fixed, and the sample is alternatively illuminated every 30 seconds with blue (450 nm) and red (750 nm) light. At each wavelength, the reflectance and polarization state of the sample are measured over time, while the frost grows on top of the cold grains. In Fig. 1 we show an example of experimental results obtained following this procedure, with the sample illuminated with an incidence angle of 12°. In the first 20 minutes of condensation, the small particles of ice deposited on top of the sample are transparent and the overall reflectance increases by 0.4% in the blue and 0.2% in the red, while the linear polarization decreases from the initial value of -0.45% to a value of -1.15% in the blue and -0.67% in the red. The polarization continues to decrease until a minimum is reached in both wavelengths (after 45 and 65 minutes from the beginning of the experiment). At this point, the ice thickness makes it opaque and the reflectance increases more steeply, while the linear polarization decreases because of the increase in multiple scattering. The 450 – 750 nm spectral slope decreases by 1%/(100 nm) in the first 20 minutes. We would like to stress that a change in linear polarization of 1% at 450 nm is very significant, changing the shape of the whole polarization phase curve drastically. The polarization phase curve at small phase angles reflects important properties of the surface materials, and its sensitivity can be used to distinguish different classes of asteroids. We simultaneously observed the ice deposition with a long-distance microscope (~10 µm resolution) and in the first 30 minutes of the experiment, no visible changes affected the sample, confirming that at the beginning the frost layer is smaller than 10 µm and mainly transparent. We demonstrated that variations of linear polarization over time (especially at the shortest visible wavelengths) are a viable way to detect thin layers of frost depositing on top of dark dust layers when the spectral slope is still not affected significantly. Measuring the frost deposition with additional wavelengths and phase angles is a future extension of this work, and will allow us to compile the full spectropolarimetric properties of this process. [1] Sierks, Holger, et al. "On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko." Science 347.6220 (2015): aaa1044. [2] Thomas, N., et al. "The morphological diversity of comet 67P/Churyumov-Gerasimenko. Science 347: aaa0440." (2015). [3] Capaccioni, Fabrizio, et al. "The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta." Science 347.6220 (2015): aaa0628. [4] Fornasier, Sonia, et al. "Spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from the OSIRIS instrument onboard the ROSETTA spacecraft." Astronomy & Astrophysics 583 (2015): A30. [5] Pommerol, Antoine, et al. "OSIRIS observations of meter-sized exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments." Astronomy & Astrophysics 583 (2015): A25. [6] Filacchione, Gianrico, et al. "An orbital water-ice cycle on comet 67P from colour changes." Nature 578.7793 (2020): 49-52. [7] De Sanctis, Maria Cristina, et al. "The diurnal cycle of water ice on comet 67P/Churyumov–Gerasimenko." Nature 525.7570 (2015): 500-503. [8] Fornasier, Sonia, et al. "Rosetta’s comet...

Polarization phase curves of asteroids and other small airless bodies are influenced by the compositional and physical properties of their regolith. The mixing of minerals composing the regolith influences the negative polarization at small phase angles as it changes the multiple scattering properties of the medium. In recent years, two new asteroid classes have been identified by their polarization phase curve at small phase angles [1][2]. The F-class shows an inversion angle (i.e. the phase angle at which the polarization becomes positive) relatively small to other asteroid families (about 14-16°), and the L-class (the so-called “Barbarians” after the prototype of this class (234) Barbara) show very high inversion angles, about 25-30°. The interpretation of these phase curves is challenging, and it has been proposed that the polarization properties of Barbarians are directly correlated to their surface mineralogy, in particular to the presence of spinel-bearing calcium-aluminum inclusions in a dark matrix [3][4]. The F-class peculiar negative polarization has been interpreted as the result of a homogeneous composition of the surface [1]. We know that the surface mineralogy is directly influencing the polarization phase curve of asteroids. In fact, asteroids tend to group in their respective spectral classes when their minimum of polarization is plotted versus  [5]. It has been experimentally demonstrated that a mixture of minerals with different albedos can deepen the negative polarization with respect to the polarization phase curves of the single minerals [6][7], and we call this effect the “mixing effect”. A systematic study on the “mixing effect”, however, is lacking in the literature. The mixing of bright ad dark minerals has been studied in polarization only for a few minerals, and it has been used to explain the polarization behavior of F-type asteroids and trans-Neptunian objects [8]. Our work aims to demonstrate experimentally how the “mixing effect” influences the polarization phase curve at small phase angles for different mineralogies relevant for asteroids and to determine how different aggregate sizes affect the negative polarization. We prepared a set of binary and ternary mixtures with different common minerals on asteroids and one set of the same mixture with different aggregate sizes. We measured their reflected light at 530 nm with full-Stokes polarimetry at phase angles ranging from 0.8° to 30°. The mixing effect of those mixtures with both bright and dark minerals significantly changes the behavior of the phase curves in terms of minimum polarization, phase angle of the minimum, and inversion angle with respect to the mineral components that are mixed together (Fig. 1). Some of the binary mixtures show a deepening of the negative polarization when a low reflectance mineral is mixed with a high reflectance mineral. The inversion angle is affected similarly, increasing respect to the inversion angle of the single minerals. Interestingly, some binary mixtures with high and low reflectance minerals show no deepening of the negative polarization (e.g. spinel and graphite). In general, the polarization phase curve changes significantly every time that two or more minerals are mixed together. Furthermore, we find that the presence of aggregates up to cm-size does not affect the negative polarization. Our binary mineral mixtures can explore large areas of the space (Fig. 2). Our results show that F-class asteroids could be not as homogeneous as previously thought, since similar reflectance,  and can be obtained by a mixture with 1:3 mass ratio of bright and dark minerals (forsterite and graphite). Barbarians lay in a region that is not explored by our binary mixtures. Nonetheless, we observe that our binary mixtures have a higher inversion angle with increasing contrast of the two end-members (pure Mg-spinel excluded). Other studies found similar results for mixing bright and dark materials: [6] found that the inversion angle increases by 3° compared to pure fine silicates when adding 10% of 10 nm soot, and [7] found that a 1:1 mixture of sub-micron MgO and Fe2O3 shows a 9° higher inversion angle with respect to the highest inversion angle of the end-members (MgO). In our sample, the largest increase in inversion angle is given by a 1:1 mixture of silica and magnetite, with an excess of 1° compared to the inversion angle of pure silica. We expect that mixtures of very fine dark and bright particles could increase even more substantially the inversion angle compared with the single end-members. From this perspective, we propose that the polarimetric behavior of Barbarians is not merely related to the presence of a single mineral, but to a combination of very fine regolith having both bright  (CAIs) and dark components that are mixed finely together. A good synergy between modeling, observations, and laboratory experiments has the potential to strongly aid in interpreting the surface properties of regolith when observing reflected polarized light. Both future sample-return missions and in-situ high sensitive polarimetric observations will greatly improve ourunderstanding of asteroid regolith properties, helping in the interpretation of astronomical measurements and constraining laboratory simulations to more realistic mineralogies. [1] Belskaya, I. N., et al. "The F-type asteroids with small inversion angles of polarization." Icarus 178.1 (2005): 213-221 [2] Cellino, A., et al. "The strange polarimetric behavior of Asteroid (234) Barbara." Icarus 180.2 (2006): 565-567. [3] Sunshine, J. M., et al. "Ancient asteroids enriched in refractory inclusions." Science 320.5875 (2008): 514-517. [4] Devogèle, Maxime, et al. "New polarimetric and spectroscopic evidence of anomalous enrichment in spinel-bearing calcium-aluminium-rich inclusions among...

Context.Homochirality is a generic and unique property of life on Earth and is considered a universal and agnostic biosignature. Homochirality induces fractional circular polarization in the incident light that it reflects. Because this circularly polarized light can be sensed remotely, it can be one of the most compelling candidate biosignatures in life detection missions. While there are also other sources of circular polarization, these result in spectrally flat signals with lower magnitude. Additionally, circular polarization can be a valuable tool in Earth remote sensing because the circular polarization signal directly relates to vegetation physiology.Aims.While high-quality circular polarization measurements can be obtained in the laboratory and under semi-static conditions in the field, there has been a significant gap to more realistic remote sensing conditions.Methods.In this study, we present sensitive circular spectropolarimetric measurements of various landscape elements taken from a fast-moving helicopter.Results.We demonstrate that during flight, within mere seconds of measurements, we can differentiate (S∕N> 5) between grass fields, forests, and abiotic urban areas. Importantly, we show that with only nonzero circular polarization as a discriminant, photosynthetic organisms can even be measured in lakes.Conclusions.Circular spectropolarimetry can be a powerful technique to detect life beyond Earth, and we emphasize the potential of utilizing circular spectropolarimetry as a remote sensing tool to characterize and monitor in detail the vegetation physiology and terrain features of Earth itself.