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The planetary Ice Laboratory has been developed at the University of Bern to measure various reflectance properties of analogues for small bodies and planetary surfaces with a special focus on icy surfaces. Its core facilities are a set of devices to produce well-characterized and reproducible analogue samples and a set of instruments designed to measure the reflectance properties of the samples over different spectral ranges, different geometrical configurations, either in total light or sorted by polarisation state. We use the laboratory to work on a range of scientific questions, mostly related to current Solar System missions and the interpretation of remote-sensing data but also occasionally on telescopic observations of Solar System objects as well as distant exoplanets and planet-forming circum-stellar discs. The experimental datasets collected are then provided to public databases for use by the community. Some of our most recent investigations include: - Visible and near-infrared reflectance spectra of analogues for the icy moons of the outer Solar System. We have collected new data with salty ice samples (Fig. 1), before and after irradiation by electrons, which can be compared with reflectance data measured by previous missions. The dataset will also help with the preparation of the JUICE and Europa Clipper missions. - Simulations of cometary activity, in the framework of the CoPhyLab project led by TU Braunschweig, with IWF Graz, MPS and DLR as partners. We develop quantitative methods to derive the ice-to-dust ratio from reflectance data in order to study the sublimation of dust ice mixtures prepared as cometary analogues. The results help interpreting the data from previous missions such as Rosetta and preparing future missions such as Comet Interceptor. - Spectral reflectance properties of different types of icy and dry Martian analogues, most notably in relation with the quantitative analysis of images returned by the CaSSIS imager of Exomars TGO. These measurements are performed in a simulation chamber equipped to simulate the Martian conditions of composition, temperature and pressure and we look at the samples with an imager equipped with the spare bandpass filters from CaSSIS (Fig. 2). - The linear polarisation properties of pure and mixed minerals as analogues for the surfaces of asteroids with implications for the interpretation of measured phase curves in terms of composition. (see also Spadaccia et al., this conference). - Assessment of the potential of circular spectropolarimetry as a biosignature in reflected light for Solar System objects and exoplanets. - Measurements of artificial samples for comparison with numerical simulations. With this presentation, we will provide an overview of the main objectives of the overall project, detail the methodology used, and present some of the results from the most recent studies mentioned here. Fig. 1: Blueish salty (NaCl) ice particles (~67µm) produced after the freezing in LN2 of salty solutions. Particles freeze from the exterior to the interior, forming a thin mantle of pure crystalline ice around a core of amorphous hyper saline ice. After the LN2 has evaporated, ice and salt in the core slowly crystallize as the temperature increases, generating Rayleigh scattering. Fig. 2: The SCITEAS-2 simulation chamber configured to simulate the cold surface and atmosphere at the Martian poles and equipped with a microscope and a multispectral imager with the spare bandpass filters from CaSSIS.

Introduction Solar System small bodies are presumed relics from the eve of the Solar System, as they were the first objects to accrete inside the protoplanetary disk. The P-/D-type asteroids are particularly interesting because of the similarity of their spectra, in the visible (Vis) and near infrared (NIR) wavelengths, with cometary nuclei, suggesting that they are the most primitive types of small bodies [1-3]. In the mid-infrared (MIR), emission spectra of both P-/D-type asteroids surfaces and cometary comae display a signature around 10 µm due to the fundamental mode of vibrations of Si-O in silicates [2, 4-5]. P-/D-type asteroids are among the low-albedo class, and are characterized by linear polarimetric phase curves in V-band with a minimum of polarization around 1.2 ± 0.4 % and an inversion angle around 8.5 ± 2.5° [6]. There are various indications that the surface of these primitive small bodies are covered by so-called “hyperfine” grains, with individual grain sizes smaller than the wavelength at which they are observed (< 1 µm) [7]. The spectral characteristics of these objects in the Vis-NIR and MIR have also been attributed to their peculiar micro-texture and grain size [4, 5]. Here, we investigate how the Vis-NIR spectra, MIR spectra and Vis polarimetric phase curves of surfaces made of hyperfine grains are influenced by the relative abundance of materials having strongly different optical indexes. Methods We used olivine and iron sulphide (a mixture of pyrrhotite and troilite; labelled as “FeS”) having contrasted optical indexes. Following a dedicated grinding protocol, we have produced grains of average diameter ranging from 0.3 to 0.6 µm, as imaged by electron microscopy [8]. Mixtures were produced by mixing the two powders in a mortar manually with the pestle for about 10 min. Reflectance spectra in the Vis-NIR range were obtained at IPAG with the SHADOWS instrument [9] (emergence e=30º, incidence i=0º). MIR reflectance spectra were obtained using a Brucker Vertex 70V FT-IR spectrometer equipped with a reflectance kit A513/QA. Because small bodies MIR observations are emissivity spectra, we show on Figure 1b the experimental spectra as “1 - reflectance” to approximate their emissivity spectra according to the Kirchhoff’s law. Polarimetric phase curves were measured at the University of Bern with the POLICES instrument at 530 nm [10]. Results Figure 1a presents the evolution of normalized reflectance spectra of olivine-FeS mixtures with decreasing volume concentration of olivine. We observe a general decrease of reflectance, associated to a modification of the spectral slope, as the concentration of FeS (opaque in the Vis) increases, with a blueing followed by a reddening of the spectra (Fig. 1a). The measurements reveal that mixtures of hyperfine grains made of two components with contrasted optical indexes have spectral and polarimetric properties which varies in strongly nonlinear way in the Vis-NIR (Fig. 1a,b, Fig. 2). In the MIR, while the spectra of the endmembers are relatively flat, the spectra of mixtures containing high concentrations of FeS exhibit the absorption bands of Si-O in the olivine around 10 µm. Discussion Spectra of mixtures of olivine and iron sulphide (or anthracite, not shown here) exhibit an emissivity feature in the MIR and various degree of bluing or reddening in the Vis-NIR, as observed on several small bodies (Fig. 1). Moreover, the same mixtures exhibiting the 10-µm feature also have a polarimetric phase curve similar to P-/D-type asteroids (Fig. 2). Spectra of pure olivine, or mixtures with high concentration of olivine exhibit spectral features mainly due to reflectivity effects. As the concentration of FeS grains increases, these features are progressively replaced by the absorption bands of olivine material, maximum around 10 to 11 µm. In these cases, the olivine grains are well dispersed in a matrix of FeS grains which diffuse the light and enable photons to escape the sample after some absorption by olivine grains. A similar mechanism was pointed out with mixtures in KBr [5], but while mixtures with KBr are very reflective, mixtures with FeS have a much lower reflectance, compatible with the low reflectance and high emissivity values observed on small bodies. The resemblance between mid-IR spectra of our hyperfine mixtures and P-/D-type asteroids emission feature (Fig. 1d) implies that elevated porosity is not a requirement for the presence of a silicate signatures at 10 µm. We show here that a relatively compact surface (porosity of the order of 50 %) exhibits similar mid-IR feature as cometary dust tails. An interpretation that can be proposed is that in both cases an optical separation of olivine grains occurred, whether by vacuum in the case of comae, or by optically featureless grains in the case of P-/D-type asteroids. Finally, we note that some mixtures of hyperfine grains (such as olivine-FeS 10:90 vol%) exhibit altogether a red spectral slope in the Vis-NIR, a 10-µm feature in the MIR, and a V-band polarimetric phase curve similar to P-/D-type asteroids, reinforcing the hypothesize that these bodies are made of powdery mixtures of hyperfine grains. Acknowledgments: We acknowledge funding from the European Research Council (ERC) (SOLARYS ERC-CoG2017_771691) References: [1] Capaccioni et al. (2015) Science 347, 6620. [2] Vernazza and Beck (2017) in Planetesimals, Cambridge Univ. Press. [3] Poch et al. (2020) Science 367, 6483. [4] Emery et al. (2006) Icarus 182, 496-512. [5] Vernazza et al. (2012) Icarus 221, 1162–1172. [6] Belskaya et al. (2017) Icarus 284, 30–42. [7] Levasseur-Regourd et al. (2018) Space Sci Rev 214. [8] Sultana et al. (2021) Icarus 35, 11412. [7] Mustard and Hays (1997) Icarus 125, 145-163. [9] Potin et al. (2018) App. Optics 57, 28. [10] Poch, (2018) JGR Planets, 123.

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 L-type asteroids." Icarus 304 (2018): 31-57. [5] Belskaya, I. N., et al. "Refining the asteroid taxonomy by polarimetric observations." Icarus 284 (2017): 30-42. [6] Zellner, B., T. Lebertre, and K. Day. " II-Laboratory polarimetry of dark carbon-bearing silicates." Lunar and Planetary Science Conference Proceedings. Vol. 8. 1977. [7] Shkuratov, Yu G. "Negative Polarization of Sunlight Scattered from Celestial Bodies-Interpretation of the Wavelength Dependence." Soviet Astronomy Letters 13 (1987): 182. [8] Bagnulo, Stefano, et al. "Exploring the surface properties of transneptunian objects and Centaurs with polarimetric FORS1/VLT observations." Astronomy & Astrophysics 450.3 (2006): 1239-1248.

Icy pebbles may play an important role in planet formation close to the water ice line of protoplanetary discs. There, dust coagulation is more efficient and re-condensation of vapor on pebbles may enhance their growth outside the ice line. Previous theoretical studies showed that disruption of icy pebbles due to sublimation increases the growth rate of pebbles inside and outside the ice line, by freeing small silicate particles back in the dust reservoir of the disc. However, since planet accretion is dependent on the Stokes number of the accreting pebbles, the growth of planetesimals could be enhanced downstream of the ice line if pebbles are not disrupting upon sublimation. We developed two experimental models of icy pebbles using different silicate dusts, and we exposed them to low-temperature and low-pressure conditions in a vacuum chamber. Increasing the temperature inside the chamber, we studied the conditions for which pebbles are preserved through sublimation without disrupting. We find that small silicate particles (

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., 7 pages, 6 figures

1. INTRODUCTION Planet formation in protoplanetary discs is a process whereby the primitive solids that are initially of microscopic scale, must be converted into larger objects such as pebbles (mm-cm size), planetesimals, and eventually planets. It has been acknowledged that drifting pebbles play an important role in the core accretion scenario by triggering streaming instabilities or by aiding the growth of planetary cores. Moreover, ice lines of volatile species such as water seem to be promising sites for this process. At the water ice line, the higher surface energy of ice promotes coagulation and the sublimated vapor can diffuse outward in the disk and deposit onto pebbles, allowing fast growth [1][2]. These processes can then trigger streaming instabilities or core formation through gravitational collapse of small bodies. However, very little is known about the evolution of these objects’ cohesive properties and volatile content as they drift and encounter various conditions throughout the disc. Investigating the morphology, chemistry, and physical processes of pebbles close to the ice line is essential to have an insight into the overall evolution of small bodies in the early phases of protoplanetary discs. We are interested in studying the different outcomes of the sublimation of an icy pebble, to understand how its optical properties are changing over time and if the dust aggregates survive to the sublimation process. In the Laboratory for Outflow Studies of Sublimating Materials (LOSSy) at the University of Bern [3], we are researching optical and physical properties of ice-dust mixtures with relevance for protoplanetary discs and planet formation, with focus on the role of ice sublimation in changing these properties. 2. EXPERIMENTAL SETUP Two new methods for the preparation of ice-dust aggregate with mm-size have been developed. The first method (Pebble-A) uses an inclined superhydrophobic surface with dust on it; a mm-size droplet of distilled water rolls on it and collects the dust, then it falls into liquid nitrogen. The second method (Pebble-B) exploits the capillary forces between water and solid grains: a droplet impinges a dust bed and penetrates it forming a wet aggregate, which is then sunk in liquid nitrogen. PAs have generally ~50%wt of ice and the dust is mainly accumulated close to the surface of the pebble, while the core has more ice. PBs have lower amounts of ice (~15%wt) and the dust grains are connected through ice films. Different types of dust with relevance for protoplanetary discs are used, such as olivine, pyroxene, corundum, serpentine, CI and CR asteroids simulants [4]. Humic acid (HA) is used as an organic compound, simulating complex organics that can be found on comets. The SCITEAS-2 (Simulation Chamber for Imaging the Temporal Evolution of Analogue Samples version 2.0) vacuum chamber provides a low-pressure and low-temperature environment for the sublimation of icy samples, and a hyperspectral measurement of the sample over time in the VIS and NIR [5]. The pressure inside the chamber is kept low (around 10-6 mbar) through a turbomolecular vacuum pump, and a He-cryocooler guarantees a temperature at the base of the sample of ~120 K. A fast sublimation of the icy pebbles is achieved by letting the temperature evolve freely up to room temperature, while the vacuum pump pumps out the vapor formed in the chamber. The overall process lasts around 20 hours. 3. NOTABLE RESULTS Ice sublimation, gravity, grain size distribution of the dust, type of dust, ice content, and presence of organics concur all together to determine the sublimation outcome of the icy pebble. Is it going to disrupt to dust or will it maintain its shape? We list here some interesting preliminary observations: - ice sublimation is detectable in the VIS and NIR reflectance spectra of the pebbles. Although the disappearance of the ice absorption bands provides information on ice content at the surface of the pebble only, the core could still contain ice; - PAs seem to disrupt more easily with respect to PBs made of the same dust, due to the higher amount of ice that is embedded in the core, which pushes the particles away when sublimating; - in PAs, the presence of HA mixed with mineral powders prevents disruption, which is observed in the absence of HA (Fig.1); - in PBs made of pyroxene, different grain sizes result in different outcomes: PBs made of dust with grain sizes bigger than 100 microns disrupt, while smaller grain sizes are more efficient in maintaining the pebble intact, and are not affected by sublimation. Furthermore, a PB made of coarse pyroxene dust (100-300 microns) mixed with fine dust ( 4. FUTURE EXPERIMENTS A campaign of experiments is ongoing on PAs and PBs. The aim is to investigate the different physical processes that concur in the final sublimation outcome, for different dust properties. More experiments will study: - The grain size distribution effects on the disruption. Different minerals are expected to have different size distributions that will avoid disruption during sublimation. - The role of different organics in binding with mineral dust. - The interaction of a gas stream at low pressure with a sublimated PB or PA, to understand their shear strength and determine their stokes number. ACKNOWLEDGMENTS The team from the University of Bern is supported by the Swiss National National Science Foundation and through the NCCR PlanetS. REFERENCES [1] Musiolik, G., & Wurm, G. (2019) The Astrophysical Journal, 873(1), 58.[2] Ros, K., Johansen, A., Riipinen, I., & Schlesinger, D. (2019) Astronomy & Astrophysics, 629, A65.[3] Pommerol, A., Jost, B., Poch, O., Yoldi, Z., Brouet, Y., Gracia-Berná, A., ... & Blum, J. (2019) Space science reviews, 215(5), 37.[4] Britt, D. T., Cannon, K. M., Donaldson Hanna, K., Hogancamp, J., Poch, O., Beck, P., ... & Metzger, P. T. (2019) Meteoritics & Planetary Science, 54(9), 2067-2082.[5] Pommerol, A., Jost, B., Poch, O., El-Maarry, M. R., Vuitel, B., & Thomas, N. (2015) Planetary and space science, 109, 106-122.