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The atmosphere of Mars is driven by different cycles – seasonal cycles of temperature, dust, ice, water vapour, CO2, etc. – which are coupled through transport: dust is blown from the surface by winds, carried aloft where it absorbs solar radiation and heats the atmosphere, impacting temperature, composition and winds. Formation of water ice crystals scavenges dust back from the surface. Water, being involved in the creation processes of clouds, is one of the most important species in the atmosphere. It controls the stability of the atmosphere, dominates its chemistry, and has a radiative impact through the formation of clouds. Dust is present everywhere on Mars, yet its abundance, physical properties, size distribution as well as impact on the composition, structure and dynamics of the atmosphere has today only barely been addressed and understood. However, knowledge of the characteristics of aerosols (such as dust, ices, clouds and haze) is crucial for the interpretation of the IR and UV spectra because they absorb in these spectral regions, and their absorption signature overlaps the absorption features of other species (such as O3 in the UV), and they regulate the travelling path of light in the atmosphere through absorption and (multiple) scattering. Their impact is difficult to accurately estimate because their spectral characteristics are not known with enough accuracy. The goal of the ROADMAP project (ROle and impAct of Dust and clouds in the Martian AtmosPhere) is to better characterize the Martian dust and clouds in terms of size, shape and loading within the atmosphere, but also to provide laboratory reference data which will be used to improve our knowledge on the genesis, transport and impact of dust and clouds on the Martian atmosphere. The team behind the RoadMap project brings together the laboratory community, scientists involved in space missions and numerical modellers to promote synergies through their different perspectives and experiences: laboratory scientists understand the reference data and know-how to extract the most value from their experiments; mission scientists know the intricacies and potential of the instruments and the details of their calibration; numerical modellers know what data, information and parameters are most pertinent to their simulations and how best to interpret the results. We will describe the methodology behind the project and show how we intend to improve space exploration (observation and modelling of the atmosphere) using new laboratory results (dust resuspension induced by saltation, scattering properties, etc). The RoadMap (ROle and impAct of Dust and clouds in the Martian AtmosPhere) project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004052.

Attempting to use dust analogue from lab to study Martian atmosphere Flimon1, J. Erwin1, A.C. Vandaele1, L. Neary1, A. Piccialli1, L. Trompet1, Y. Willame1, F. Vanhellemont, S. Bauduin2, M.Wolff4, Olga Munoz3, Julia Martikainen3, Juan Carlos Gomez Martin3, Fernando Moreno3 1 Royal Belgian Institute for Space Aeronomy, BIRA-IASB 2 Université libre de Bruxelles (ULB), Spectroscopy, Quantum Chemistry and Atmospheric Remote Sensing (SQUARES), Brussels, Belgium 3 Instituto de Astrofìsica de Andalucía, Consejo Superior de Investigaciones Científicas (CSIC), Granada, Spain 4Space Science Institute, Boulder Colorado, USA Introduction: Aerosols are present in the atmosphere of Mars and have a major effect on it. They are mainly composed of dust, water ice and CO2 ice. Dust is confined to lower altitudes during the aphelion season and can reach higher altitudes during the perihelion, especially during dust storms that frequently arise on Mars. These storms can sometimes grow up to cover the entire planet and are then called global dust storms. They have important consequences on the atmosphere as dust will heat the atmosphere and change its composition. The dust can go higher in altitude and bring, for example, water vapor to places where we do not see it outside the dust storm. Previous studies worked on dust climatology but the composition is still not well constrained. To understand the mechanisms behind the dust lifting and the impact of dust in the mesosphere, it is important to have a good understanding of the Martian dust. Knowing the size of the grain, the vertical profile, or the composition are key elements to understanding its radiative property. The goal of the RoadMap (ROle and impAct of Dust and clouds in the Martian AtmosPhere) project is to use Martian dust analogues, measure their radiative and optical properties and compare them to space measurements of the Martian atmosphere. The space measurements considered here are made by the NOMAD instrument. The NOMAD (“Nadir and Occultation for MArs Discovery”) spectrometer suite onboard the ExoMars Trace Gas Orbiter (TGO) is composed of three spectrometers. In this work, we will use the UVIS channel in occultation mode. Method: The extinction efficiency, Qext is computed using a Mie code (Bohren, et al., 1998) considering a log-normal size distribution and refractive indeces for Martian dust from (Wolff et al., 2009) and the new values obtained within the RoadMap project. To compute the extinction due to aerosols from the transmittances of UVIS, we first use the ASIMUT code (Vandaele, et al., 2006.) to make a fit of the ozone abundance. The ozone absorption (as well as the Rayleigh extinction) is then subtracted from the original transmittances. In the result should remain only the contribution due to the aerosol extinction. Local extinctions are then computed from these reduced transmittances using a simple onion peeling methodology. Preliminary results: We made comparisons of the extinction cross section from Wolff and RoadMap refractive indices. The first results obtained are a net difference in value between the two dataset. Wolff values are higher by several order of magnitude. More work will be done to understand this and show the agreement or disagreement between both datasets with real data. Acknowledgements: The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency. The ROADMAP project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004052. References: Bohren, Craig F. and Donald R. Huffman,Absorption and scattering of light by small particles, New York : Wiley, 1998, 530 p., ISBN 0-471-29340-7, ISBN 978-0-471-29340-8 (second edition) Vandaele, A.C., Kruglanski, M., Mazière, M.D., n.d. MODELING AND RETRIEVAL OF ATMOSPHERIC SPECTRA USING ASIMUT 6. Wilquet, V., Drummond, R., Mahieux, A., Robert, S., Vandaele, A.C., Bertaux, J.-L., 2012. Optical extinction due to aerosols in the upper haze of Venus: Four years of SOIR/VEX observations from 2006 to 2010. Icarus 217, 875–881. https://doi.org/10.1016/j.icarus.2011.11.002 Wolff, M.J., Smith, M.D., Clancy, R.T., Arvidson, R., Seelos, F., Murchie, S., Savijärvi, H., 2009. Wavelength dependence of dust aerosol single scattering albedo as observed by the Compact Reconnaissance Imaging Spectrometer. J. Geophys. Res. 114, E00D04. https://doi.org/10.1029/2009JE003350

Aerosols are key components of the Martian radiative transfer. For instance, airborne dust is ubiquitous on the planet and influences the climate by absorbing shortwave radiation, resulting in a local warming of the atmosphere. While ice clouds are related to the water cycle, forming due to adiabatic cooling of upward flows where the water vapor condenses on dust particles as the temperature is low enough. Nadir UV measurements are commonly used to monitor the aerosol loading and the ozone abundance in the Martian atmosphere and study their seasonal cycles [e.g. Perrier et al., 2006; Mateshvili et al., 2009; Clancy et al., 2016; Willame et al., 2017; Wolff et al., 2019]. The analysis of UV nadir measurements, from MEX/SPICAM in a previous work [Willame et al., 2017] and currently from the NOMAD/UVIS data, were performed using the canonical dust scattering properties obtained in [Wolff et al., 2010]. In the frame of the RoadMap project, we will assess the impact on the nadir retrievals of using new sets of dust properties derived from laboratory measurements (see related presentation/poster of Martikainen et al. about lab measurements). In the present work, we will present preliminary results of the retrieval analysis. Acknowledgements The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency. The ROADMAP project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004052. References Clancy, R.T., Wolff, M.J., Lefvre, F., Cantor, B.A., Malin, M.C., Smith, M.D., 2016. Daily global mapping of Mars ozone column abundances with MARCI UV band imaging. Icarus 266, 112–133. Mateshvili, N., Fussen, D., Vanhellemont, F., Bingen, C., Dekemper, E., Loodts, N., Tetard, C., 2009. Water ice clouds in the Martian atmosphere: two Martian years of SPICAM nadir UV measurements. Planet. Space Sci. 57, 1022–1031. Perrier, S., Bertaux, J.L., Lefèvre, F., Lebonnois, S., Korablev, O., Fedorova, A., Montmessin, F., 2006. Global distribution of total ozone on Mars from SPICAM/MEX UV measurements. J. Geophys. Res. (Planets) 111, 9. Willame Y., A.C. Vandaele, C. Depiesse, F. Lefèvre, V. Letocart, D. Gillotay, F. Montmessin. Retrieving cloud, dust and ozone abundances in the Martian atmosphere using SPICAM/UV nadir spectra. Planetary and Space Science, 2017, Volume 142, Pages 9-25. Wolff, M.J., Clancy, R.T., Goguen, J.D., Malin, M.C., Cantor, B.A., 2010. Ultraviolet dust aerosol properties as observed by MARCI. Icarus 208, 143–155. Wolff M.J., R. Todd Clancy, Melinda A. Kahre, Robert M. Haberle, François Forget, Bruce A. Cantor, Michael C. Malin, 2019. Mapping water ice clouds on Mars with MRO/MARCI, Icarus, Volume 332.

Remote observations of the light scattered by cometary comae make it possible to constrain the physical properties of the dust particles lifted into the comet atmosphere. Ground-based photometric and polarimetric observations of the light scattered by comet 67P/Churyumov-Gerasimenko (67P) present a unique opportunity to directly compare remote measurements to those performed in-situ by the ESA/Rosetta mission. The polarisation of the light scattered by cometary dust varies as a function of phase-angle (the Sun-comet-Earth angle), as well as properties of the dust particles: their size, structure and composition. Therefore, our polarimetric maps can be used to monitor the spatial and temporal evolution of the coma dust properties up to fifty thousand kilometres, while the phase-angle dependence of linear polarisation can be compared to that of other comets, and measurements from light scattering experiments and cometary models. Here, we present photometric and polarimetric observations of 67P from 2021 performed between 1-6 months pre-perihelion with FORS2 @ VLT, as well as 6 months post-perihelion observations from 2015-2016 with FORS2 @ VLT and ISIS @ WHT. This data, of unprecedentedly high S/N ratio, covers a phase-angle range of ~4-50° and heliocentric distances from 1.2-2.5 AU. Complementing previous measurements with these data points we can see that the polarimetric phase curve of 67P resembles that of a high-polarisation dusty comet. Various jet-like structures in the inner coma are visible in processed intensity images. Polarimetric maps, however, show a homogenous distribution of polarisation throughout the coma and tail, and show no evidence of dust particle evolution over the entire observing run.

Background: Spectral observations in visual and near-infrared wavelengths are used to infer their surface material composition. Similar laboratory measurements of meteorites and various minerals and are used to link the known composition in laboratory measurements to asteroid composition. However, many parameters in observations and measurements can introduce effects on the resulting spectra. With regolith-type powdered grains packed to a slab, the observing angles, particle size distribution, particle shapes, and space-weathering state of the material can all alter the spectra. Here, we try to model the particle size effects so that at least this effect could be understood when comparing different spectra. Size effects in spectra: One can find spectral measurements of the same material but with varying size fractions in spectral databases such as RELAB (https://pds-speclib.rsl.wustl.edu/) and SSHADE (https://www.sshade.eu/). The typical spectral effects of different particle size are that the smaller particles are generally brighter, have redder slope, and deeper (absolute) absorption band depths. However, it is also possible to find counterexamples of different behavior. Trend from light-scattering theory: If we limit to optically large particles (several times larger than the wavelength) and assume that different particle sizes are simply scaled versions of one particle, the particle size effects in spectra can be understood as interplay between surface reflections and absorption in the volume. If we assume that the surfaces are flat in the wavelength-scale, the surface reflections follow Fresnel reflections. For light that is refracted from the surface into the homogeneous material, the Beer-Lambert absorption gives the attenuation in the volume. The Fresnel-type reflections do not absorb and are also only weakly dependent on the imaginary part k of the complex refractive index of the material, m=n+ik. The Beer-Lambert absorption, on the other hand, is only dependent on k and the distance traveled in the material, d. If the same volume is divided into smaller particles, typical d in the material between air-surface or surface-air interfaces and reflections or refractions from these is decreasing. Since absorption is taking place only in the volume, the increased surface reflections are increasing the brightness. This is the simple mechanism behind increasing brightness with decreasing particle size. At the same time, the exponent-term in the Beer-Lambert absorption affects so that similar change in d in the particle for a bright material has larger effect than the same d in darker material. This means that the absorption band depths are also increasing, in absolute units, when particle size is decreasing. The possible increase of red slope with decreasing particle size in spectra cannot be explained with optically large particles. The longer wavelengths are ‘seeing’ the distances inside the particles shorter, but the effect is quite small for particles that are several times larger than the wavelength. However, if the sample also includes wavelength-scale particles, these are relatively more abundant in smaller size fractions and can introduce a red slope. This is because the scattering efficiency of particles increase when they reach wavelength-scale whereas for optically large particles it is constant. Numerical modeling: We will verify and quantify the abovementioned theoretical trends with numerical simulations. We use a two-fold scheme where we first simulate the single-scattering properties (single-scattering albedo and the phase function) of individual random particle shapes, and then simulate the multiple-scattering effect in a packed slab of these particles. We model the particles with a Voronoi cell particle model, see Fig. 1. In this model, a large volume is first filled with random seed points. Then, a 3d Voronoi division is done on the points, resulting the volume to be divided into Voronoi cells having flat surfaces and sharp corners. Finally, single cells are extracted from the volume, except the cells close to the edge of the volume to avoid edge-effects. The light-scattering properties of single Voronoi particles are computed using a geometric-optics ray-tracing code SIRIS (https://bitbucket.org/planetarysystemresearch/siris4-framework/). These computations are run for 200 realizations of the particle model and changing the size parameter (physical size divided by the wavelength) and the real and imaginary parts of the refractive index. The results are averaged over the particle shape realizations. The single-particle properties are fed into the Monte Carlo radiative transfer code RT-CB (https://bitbucket.org/planetarysystemresearch/rtcb_public/) where the amount of reflected light from a large slab containing the single particles is computed. This gives us the brightness as a function of particle size, wavelength, and real and imaginary parts of the refractive index. Preliminary results: We have a grid of size parameters and refractive indices computed, and the results are consistent with what we expected from the simple theory of the changing ratio of surface reflections to volume absorption. We can, for example, plug in a typical behavior of the imaginary part of olivine as a function of wavelength, keep the real part of the refractive index constant for simplicity (it does not change much in visual/near-infrared wavelengths), and plot the spectra of different-sized olivine particles, see Fig. 2. Conclusions and future work: Currently, we are increasing the parameter grid to include both smaller size parameters and smaller imaginary k values. From the results we have, it seems that we can fit an analytical function to the results with quite small prediction errors. If successful, this analytical model can be used to model the size effect on the reflectance spectra of particulate materials if the particles are clearly larger than the wavelength. If we can model, we can also remove the effect if we know the particle sizes and receive size-standardized spectra that can be better compared between the samples. As a side product, we can use this data and the model to invert the imaginary part of the refractive index for materials from which we have reflectance spectra measurements and the particle size.

The scattering matrices of three well-characterized Martian dust simulants (JSC Mars-1, MGS-1 and MMS-2) have been measured at the Cosmic Dust Laboratory in Granada as part of the experimental work package of the ROle and impAct of Dust and clouds in the Martian AtmosPhere (ROADMAP) project. These simulants have been chosen from a range of representative dust analogues discussed in the recent literature (Allen, Jager, et al., 1998; Allen, Morris, et al., 1998; Cannon et al., 2019; Morris et al., 2001; Peters et al., 2008). The bulk chemical composition and mineralogy have been retrieved from the sample manufacturers and the relevant literature and double checked in our facilities. The samples have been size-segregated using state-of-the-art Materials Science techniques. The particle sizes have been measured for the size fractions obtained, showing narrow size-distributions unprecedented in the context of dust light scattering studies. Information about the morphology of the particles has been obtained from electron microscopy images. Reflectance spectra have been measured as well. The three Martian analogue samples with size distributions in the 6 μm < r < 30 μm range have similar scattering matrices at 488 nm and 640 nm. The simulant that deviates most from the rest is JSC Mars-1, with a shallower phase function and generally higher values of the other non-zero elements of the scattering matrix. The differences are interpreted in terms of the absorption coefficients and the morphology of the dust particles. ReferencesAllen, C. C., Morris, R. V., Jager, K. M., Golden, D. C., Lindstrom, D. J., Lindstrom, M. M., & Lockwood, J. P. (1998). Martian Regolith Simulant JSC Mars-1. In Lunar and Planetary Science Conference (p. 1690).Allen, C. C., Jager, K. M., Morris, R. V., Lindstrom, D. J., Lindtsrom, M. M., & Lockwood, J. P. (1998). Martian soil simulant available for scientific, educational study. Eos, Transactions American Geophysical Union, 79(34), 405–409. https://doi.org/10.1029/98EO00309Cannon, K. M., Britt, D. T., Smith, T. M., Fritsche, R. F., & Batcheldor, D. (2019). Mars global simulant MGS-1: A Rocknest-based open standard for basaltic martian regolith simulants. Icarus, 317, 470–478. https://doi.org/10.1016/J.ICARUS.2018.08.019Morris, R. V., Golden, D. C., Ming, D. W., Shelfer, T. D., Jørgensen, L. C., Bell, J. F., et al. (2001). Phyllosilicate-poor palagonitic dust from Mauna Kea Volcano (Hawaii): A mineralogical analogue for magnetic Martian dust? Journal of Geophysical Research: Planets, 106(E3), 5057–5083. https://doi.org/10.1029/2000JE001328Peters, G. H., Abbey, W., Bearman, G. H., Mungas, G. S., Smith, J. A., Anderson, R. C., et al. (2008). Mojave Mars simulant—Characterization of a new geologic Mars analog. Icarus, 197(2), 470–479. https://doi.org/10.1016/J.ICARUS.2008.05.004

We applied advanced light-scattering models on laboratory measurements to retrieve the scattering properties of Martian dust analogues JSC Mars-1, MMS-2, and MGS-1. The retrieval was performed in two parts: first, we computed the optical constants of the three samples. Then, the obtained values were used together with two different scattering databases to model the measured scattering matrices of the samples. Optical constants are needed from modelling single-scattering within the atmospheric dust to simulating the global Martian climate. Reliable complex refractive indices of Martian dust are difficult to find in the literature. Previous studies by Wolff et al. (2009) & (2010) have obtained optical constants from analysing the observed dust storm spectra at 258-2900 nm region by using cylindrical particle shapes. However, these values could not be supported with direct laboratory measurements. In this work, we retrieved the complex refractive indices of the three Martian dust analogues at UV-vis-NIR wavelengths by using the measured size distributions, diffuse reflectance spectra, and an advanced light-scattering model with realistic particle shapes. The scattering properties of the analogues were retrieved by utilizing the derived refractive indices, measured scattering matrices, and two databases of single scattering properties of triaxial ellipsoids (Meng et al. 2010) and hexahedral shapes (Saito et al. 2021) The databases have been developed by a combination of three computational methods: the T-matrix method, the discrete dipole approximation (DDA), and an improved geometric optics method. Each database consists of pre-calculated scattering properties over a broad range of complex refractive indices, particle sizes, particle shapes, and wavelengths. The modelled scattering matrices were then compared with those obtained using spherical shapes. Finally, the best-fit model gives us physical properties of the particles such as cross sections, single scattering albedos, extinction efficiencies, asymmetry factors, and Legendre polynomials used to mathematically calculate the phase functions. References: Wolff et al. (2009), JGR Planets, 114, E00D04 ; Wolff et al. (2010), Icarus, 208, 143-155 ; Meng et al. (2010), J. Atmos. Sci., 41.5: 501-512 ; Saito et al. (2021), J. Atmos. Sci., 78, 2089-2111.

We present a multi-scale light-scattering model that is capable of simulating the reflectance spectra of a regolith layer. In particular, the model can be applied to a case where the regolith grains have varying amounts of nanophase inclusions due to space weathering of the material. As different simulation tools are employed for different size scales of the target geometry (roughly, nano-, micro-, and millimeter scales), the particle size effects, the surface reflections, and the volume scattering can all be properly accounted for. Our results with olivine grains and nanophase iron inclusions verify the role of the nanoinclusions in the reflectance spectra of space-weathered materials. Together with the simulation results, we give simplified explanations for the space-weathering effects based on light scattering, namely the decrease of albedo, the general increase of the red spectral slope, and the dampening of the spectral bands. We also consider the so-called ultraviolet bluing effect and show how the change in the spectral slope over the ultraviolet-visual wavelengths is due to the decrease of reflectance in the visual wavelengths rather than the increase of reflectance in the ultraviolet part.

Aims. We perform light curve inversion for 491 asteroids to retrieve phase curve parameters, rotation periods, pole longitudes and latitudes, and convex and triaxial ellipsoid shapes by using the sparse photometric observations from Gaia Data Release 2 and the dense ground-based observations from the DAMIT database. We develop a method for the derivation of reference absolute magnitudes and phase curves from the Gaia data, allowing for comparative studies involving hundreds of asteroids. Methods. For both general convex shapes and ellipsoid shapes, we computed least-squares solutions using either the Levenberg-Marquardt optimization algorithm or the Nelder-Mead downhill simplex method. Virtual observations were generated by adding Gaussian random errors to the observations, and, later on, a Markov chain Monte Carlo method was applied to sample the spin, shape, and scattering parameters. Absolute magnitude and phase curve retrieval was developed for the reference geometry of equatorial illumination and observations based on model magnitudes averaged over rotational phase. Results. The derived photometric slope values showed wide variations within each assumed Tholen class. The computed Gaia G-band absolute magnitudes matched notably well with the V-band absolute magnitudes retrieved from the Jet Propulsion Laboratory Small-Body Database. Finally, the reference phase curves were well fitted with the H, G1, G2 phase function. The resulting G1, G2 distribution differed, in an intriguing way, from the G1, G2 distribution that is based on the phase curves corresponding to light curve brightness maxima.

We present a new physics-based approach to model the absolute reflectance spectra of asteroid (4) Vesta. The spectral models are derived by utilizing a ray-optics code that simulates light scattering by particles large compared to the wavelength of the incident light. In the light of the spectral data obtained by the Dawn spacecraft, we use howardite powder to model Vesta's surface regolith and its particle size distribution for 10-200 μm sized particles. Our results show that the modelled spectrum mimics well the observations. The best match was found using a power-law particle size distribution with an index 3.2. This suggests that Vesta's regolith is dominated by howardite particles

We present optical, radio, and X-ray observations of SN2020bvc (=ASASSN20bs; ZTF20aalxlis), a nearby ($z=0.0252$; $d$=114 Mpc) broad-lined (BL) Type Ic supernova (SN). Our observations show that SN2020bvc shares several properties in common with the Ic-BL SN2006aj, which was associated with the low-luminosity gamma-ray burst (LLGRB) 060218. First, the 10 GHz radio light curve is on the faint end of LLGRB-SNe ($L_\mathrm{radio} \approx 10^{37}$erg/s): we model our VLA observations (spanning 13-43 d) as synchrotron emission from a mildly relativistic ($v \gtrsim 0.3c$) forward shock. Second, with Swift and Chandra we detect X-ray emission ($L_X \approx 10^{41}$erg/s) that is not naturally explained as inverse Compton emission or as part of the same synchrotron spectrum as the radio emission. Third, high-cadence ($6\times$/night) data from the Zwicky Transient Facility (ZTF) shows a double-peaked optical light curve, the first peak from shock-cooling emission from extended low-mass material (mass $M10^{12}$cm) and the second peak from the radioactive decay of Ni-56. SN2020bvc is the first confirmed double-peaked Ic-BL SN discovered without a GRB trigger, and shows X-ray and radio emission similar to LLGRB-SNe: this is consistent with models in which the same mechanism produces both the LLGRB and the shock-cooling emission. For four of the five other nearby ($z\lesssim0.05$) Ic-BL SNe with ZTF high-cadence data, we rule out a first peak like that seen in SN2006aj and SN2020bvc, i.e. that lasts $\approx 1$d and reaches a peak luminosity $M \approx -18$. X-ray and radio follow-up observations of future such events will establish whether double-peaked optical light curves are indeed predictive of LLGRB-like X-ray and radio emission., Accepted to ApJ. 39 pages, 17 figures. Comments welcome

The optical constant of the material, meaning the complex refractive index m=n+i k, is an essential parameter when considering the reflection and absorption properties of that material. The refractive index is a function of wavelength of the light, and usually the imaginary part k is what governs the reflection or transmission spectral behavior of the material. The knowledge of the complex refractive index as a function of wavelength, m(λ), is needed for light scattering simulations. On the other hand, rigorous scattering simulations can be used to invert the refractive index from measured or observed reflection spectra. We will show how the combination of geometric optics and radiative transfer codes can be used in this task. In this work, the possible application is with the future visual-near infrared observations of Mercury by the ESA BepiColombo mission. That application in mind, we have used four particulate igneous glassy materials with varying overall albedo and in several size fractions in reflectance spectra measurements (hawaiitic basalt, two gabbronorites, anorthosite, see details from Carli et al, Icarus 266, 2016). The grounded material consist of particle with clear edges and quite flat facets, and we choose to model the particle shapes by geometries resulting from Voronoi division of random seed points in 3D space (see Fig. 1). Figure 1. On the left, a SEM image of anorthosite particles, size fraction 224–250 µm (from Carli et al, Icarus 266, 2016). On the right, eight example particles from Voronoi division algorithm. The refractive index inversion is done here using first a geometric optics code SIRIS (Muinonen et al, JQSRT 110, 2009) to simulate the average Mueller matrix, albedo, and scattering efficiency for a single Voronoi particle. Then, these properties are fed into radiative transfer code RT-CB (Muinonen, Waves in Random Media 14, 2004) to produce the reflective properties of a semi-infinite slab of these particles. This procedure is repeated for a 2D grid of particle size parameters x=2πr/λ, where r is the radius of particle, and imaginary part k of refractive index. In Vis-NIR wavelengths, the real part n is quite constant and is estimated to be about 1.58 for all the four glasses. From the simulated slab reflectance data with the 2D x, k parameter grid, we can first interpolate, and then invert the k parameter for any reflectance value with given wavelength and particle size. The resulting spectral behavior of k for the four glasses and for all the size fractions was seems very realistic. Carli et al. (Icarus 266, 2016) inverted the k spectral behavior for these same samples using Hapke modeling, and the results are quite similar. An example of inverted k for one sample (anorthosite) is shown in Fig. 2. Figure 2. Inverted imaginary part of refractive index, k, as a function of wavelength for different size fractions of anorthosite. We show that rigorous light scattering simulations can be used to invert the optical constant of a material from observed reflectance spectra, at least if some assumptions regarding particle sizes can be done. These methods can be valuable when analyzing various illumination and observing geometries, intimate mixtures of multiple materials, and other cases where Hapke modeling might not be enough for accurate inversion. We foresee employing these methods to analyze the observations of Mercury from ESA BepiColombo mission. Acknowledgements: Research supported, in part, by the Academy of Finland (project 325805).

Asteroids have remained mostly the same for the past 4.5 billion years, and provide us information on the origin, evolution and current state of the Solar System. For the physical characterization of asteroids, one of the best data sources is photometry: the measurement of the disk-integrated brightness of the asteroid. An asteroid's lightcurve (i.e. the dependency of the brightness as a function of time) is indicative of its surface scattering properties, as well as its shape and the state of spin. In this work, we apply novel Bayesian inverse methods (Muinonen et al., A&A 2020, in revision) to derive phase curve parameters from photometric lightcurve observations. Our aim is to validate and expand the existing analyses by applying the methods to of the order of tens to hundreds of asteroids using photometric data from the Gaia Data Release 2 (Gaia Collaboration, Spoto et al., A&A 616, A13, 2018) and ground-based photometry (Durech et al., A&A 513, A46, 2010) from Database of Asteroid Models from Inversion Techniques (DAMIT). We derive phase curve linear slopes by using four techniques: 1) convex inversion on all data, 2) convex inversion on all data with fixed rotation parameters from DAMIT, 3) ellipsoid inversion on Gaia DR2 data only, and 4) ellipsoid inversion on Gaia DR2 data only with fixed rotation parameters from DAMIT. Finally, we compare the obtained slope parameters with the presumed Bus-DeMeo (DeMeo et al., Icarus 202, 160, 2009) taxonomic classes of the asteroids, and study possible correlations with the geometric albedos. The Gaia photometry has milli-magnitude precision and is thus extremely valuable when used to carry out asteroid taxonomic classification.

We present a hybrid radiative transfer geometric optics approximation to model multiple light scattering in arbitrary finite discrete random media in the geometric optics regime. In the hybrid model, the medium is divided into a mantle composed of discrete particles and into a diffusely scattering core. In the mantle, multiple scattering is handled by using a ray-tracing algorithm with the generalized Snel’s law for inhomogeneous waves, whereas, in the core, ray tracing with diffuse scatterers is incorporated to approximate multiple scattering and absorption. The extinction distances required to compute the scattering in the core are derived numerically by tracing the distances of the scattering and absorption events instead of using the classical extinction mean free path length. We have written a new framework that can treat arbitrary meshes consisting of watertight surface meshes with multiple diffuse scatterers and refractive indices. Comparison between the “ground truth” obtained from pure geometric optics ray tracing, the solutions obtained by using radiative transfer, and the hybrid model show that the hybrid model can produce better results, particularly, if a densely-packed medium is studied. In the future, the new approximation could be used to solve light scattering from larger media, such as asteroid surfaces, that are out of reach for the pure geometric optics methods due to their computational complexity.

Theoretical, numerical, and experimental methods are presented for multiple scattering of light in macroscopic discrete random media of densely-packed microscopic particles. The theoretical and numerical methods constitute a framework of Radiative Transfer with Reciprocal Transactions (R2T2). The R2T2 framework entails Monte Carlo order-of-scattering tracing of interactions in the frequency space, assuming that the fundamental scatterers and absorbers are wavelength-scale volume elements composed of large numbers of randomly distributed particles. The discrete random media are fully packed with the volume elements. For spherical and nonspherical particles, the interactions within the volume elements are computed exactly using the Superposition T-Matrix Method (STMM) and the Volume Integral Equation Method (VIEM), respectively. For both particle types, the interactions between different volume elements are computed exactly using the STMM. As the tracing takes place within the discrete random media, incoherent electromagnetic fields are utilized, that is, the coherent field of the volume elements is removed from the interactions. The experimental methods are based on acoustic levitation of the samples for non-contact, non-destructive scattering measurements. The levitation entails full ultrasonic control of the sample position and orientation, that is, six degrees of freedom. The light source is a laser-driven white-light source with a monochromator and polarizer. The detector is a mini-photomultiplier tube on a rotating wheel, equipped with polarizers. The R2T2 is validated using measurements for a mm-scale spherical sample of densely-packed spherical silica particles. After validation, the methods are applied to interpret astronomical observations for asteroid (4) Vesta and comet 67P/Churyumov-Gerasimenko (Figure 1) recently visited by the NASA Dawn mission and the ESA Rosetta mission, respectively. © 2019 JoVE. European Research Council, ERC: 320773 The present manuscript summarizes the findings of a project funded for five years in 2013-2018 by the European Research Council (ERC): Scattering and Absorption of ElectroMagnetic waves in ParticuLate media (SAEMPL, ERC Advanced Grant). SAEMPL succeeded in meeting its three main goals: first, novel numerical Monte Carlo methods were derived for multiple scattering by discrete random media of densely-packed particles16,17,18; second, novel experimental instrumentation was developed and constructed for controlled laboratory measurements of validation samples in levitation15; third, the numerical and experimental methods were applied to interpret astronomical observations19,20. Research supported by the ERC Advanced Grant № 320773. We thank the Laboratory of Chronology of the Finnish Museum of Natural History for the help with sample characterization.

We describe a novel data-processing and analysis pipeline for optical observations of moving objects, either of natural (asteroids, meteors) or artificial origin (satellites, space debris). The monitoring of the space object populations requires reliable acquisition of observational data, to support the development and validation of population models and to build and maintain catalogues of orbital elements. The orbital catalogues are, in turn, needed for the assessment of close approaches (for asteroids, with the Earth; for satellites, with each other) and for the support of contingency situations or launches. For both types of populations, there is also increasing interest to detect fainter objects corresponding to the small end of the size distribution. The ESA-funded StreakDet (streak detection and astrometric reduction) activity has aimed at formulating and discussing suitable approaches for the detection and astrometric reduction of object trails, or streaks, in optical observations. Our two main focuses are objects in lower altitudes and space-based observations (i.e., high angular velocities), resulting in long (potentially curved) and faint streaks in the optical images. In particular, we concentrate on single-image (as compared to consecutive frames of the same field) and low-SNR detection of objects. Particular attention has been paid to the process of extraction of all necessary information from one image (segmentation), and subsequently, to efficient reduction of the extracted data (classification). We have developed an automated streak detection and processing pipeline and demonstrated its performance with an extensive database of semisynthetic images simulating streak observations both from ground-based and space-based observing platforms. The average processing time per image is about 13 s for a typical 2k-by-2k image. For long streaks (length >100 pixels), primary targets of the pipeline, the detection sensitivity (true positives) is about 90% for both scenarios for the bright streaks ( SNR > 1 ), while in the low-SNR regime, the sensitivity is still 50% at SNR = 0.5 .

We present a multi-scale light-scattering model that is capable of simulating the reflectance spectra of a regolith layer. In particular, the model can be applied to a case where the regolith grains have varying amounts of nanophase inclusions due to space weathering of the material. As different simulation tools are employed for different size scales of the target geometry (roughly, nano-, micro-, and millimeter scales), the particle size effects, the surface reflections, and the volume scattering can all be properly accounted for. Our results with olivine grains and nanophase iron inclusions verify the role of the nanoinclusions in the reflectance spectra of space-weathered materials. Together with the simulation results, we give simplified explanations for the space-weathering effects based on light scattering, namely the decrease of albedo, the general increase of the red spectral slope, and the dampening of the spectral bands. We also consider the so-called ultraviolet bluing effect, and show how the change in the spectral slope over the ultraviolet-visual wavelengths is due to the decrease of reflectance in the visual wavelengths rather than the increase of reflectance in the ultraviolet part.

Meteorite samples are measured with the University of Helsinki integrating-sphere UV-vis-NIR spectrometer. The resulting spectra of 30 meteorites are compared with selected spectra from the NASA Planetary Data System meteorite spectra database. The spectral measurements are transformed with the principal component analysis, and it is shown that different meteorite types can be distinguished from the transformed data. The motivation is to improve the link between asteroid spectral observations and meteorite spectral measurements. (C) 2017 Elsevier Ltd. All rights reserved.

Light scattering by particles large compared to the wavelength of incident light is traditionally solved using ray optics which considers absorption inside the particle approximately, along the ray paths. To study the effects rising from this simplification, we have updated the ray-optics code SIRIS to take into account the propagation of light as inhomogeneous plane waves inside an absorbing particle. We investigate the impact of this correction on traditional ray-optics computations in the example case of light scattering by ice crystals through the extended near-infrared (NIR) wavelength regime. In this spectral range, ice changes from nearly transparent to opaque, and therefore provides an interesting test case with direct connection and applicability to atmospheric remote-sensing measurements at NIR wavelengths. We find that the correction for inhomogeneous waves systematically increases the single-scattering albedo throughout the NIR spectrum for both randomly-oriented, column-like hexagonal crystals and ice crystals shaped like Gaussian random spheres. The largest increase in the single-scattering albedo is 0.042 for hexagonal crystals and 0.044 for Gaussian random spheres, both at λ = 2.725 µm. Although the effects on the 4 × 4 scattering-matrix elements are generally small, the largest differences are seen at 2.0 µm and 3.969 µm wavelengths where the correction for inhomogeneous waves affects mostly the backscattering hemisphere of the depolarization-connected P22/P11, P33/P11, and P44/P11. We evaluated the correction for inhomogeneous waves through comparisons against the discrete exterior calculus (DEC) method. We computed scattering by hexagonal ice crystals using the DEC, a traditional ray-optics code (SIRIS3), and a ray-optics code with inhomogeneous waves (SIRIS4). Comparisons of the scattering-matrix elements from SIRIS3 and SIRIS4 against those from the DEC suggest that consideration of the inhomogeneous waves brings the ray-optics solution generally closer to the exact result and, therefore, should be taken into account in scattering by absorbing particles large compared to the wavelength of incident light.

We present a novel simulation framework for assessing the spectral properties of meteorite specimens. The framework utilizes a ray-optics code, which simulates light scattering by Gaussian-random-sphere particles large compared to the wavelength of the incident light and accounts for internal diffuse scatterers. The code uses incoherent input and computes phase matrices by utilizing incoherent scattering matrices. Reflectance spectra are modeled by introducing a combination of olivine, pyroxene, and iron, the most common materials present in meteorites that dominate their spectral features. The complex refractive indices of olivine and iron are obtained from existing databases. The refractive indices of pyroxene are derived using an optimization that utilizes our ray-optics code and the measured spectrum of the material. We demonstrate our approach by applying it on the measured meteorite reflectance spectra obtained with the University of Helsinki integrating-sphere UV-vis-NIR spectrometer. (C) 2017 Elsevier Ltd. All rights reserved.

Physical characterization of asteroid surfaces by studying the scattered light is challenging as the light-scattering processes are affected by the particle sizes, shapes, and materials, which in most cases are unknown. When interpreting remote-sensing observations, it is important to choose the correct methods for realistic analyses. In the past decades, several extensive studies have been carried out to understand asteroid surfaces, however, none of the previously used models are able to interpret spectroscopy, photometry, and polarimetry at the same time with sufficient precision. In the thesis, light-scattering methods were developed and utilized together with laboratory measurements to characterize asteroid regoliths and meteorite surfaces. The light-scattering methods presented in the thesis take into account both wavelength-scale particles and particles larger than the wavelength of the incident light, which is important as the models used for each wavelength domain are different, and the resonance region is difficult to account for. First, the reflectance spectra of three meteorite samples are simulated using a model combining olivine, pyroxene, and iron. The results are promising as we are able to match the simulated spectra fairly well with the measured spectra. Second, spectroscopic, photometric, and polarimetric modeling of asteroid (4) Vesta shows good results as the reflectance spectra can be modeled with reasonable precision, and the modeled photometry and polarimetry produce non-linear brightening and negative degree of linear polarization in the backscattering direction that is also seen in the observations. Finally, asteroid taxonomic classification is analysed by performing lightcurve inversion for 491 asteroids using convex and ellipsoid shapes. We retrieve phase curve slope parameters, rotation periods, pole orientations, shapes, reference phase curves, and absolute magnitudes in the G band of the ESA Gaia space telescope. Our analysis indicates that there can be mis-classifications in the current taxonomic systems. Asteroid photometry complements the existing classifications based on spectroscopy and provides us with a way to find the correct taxonomy. The forward methods of modeling the scattering properties of surfaces used in the thesis are vital in future studies as they can be applied to other Solar System objects, such as comets and satellite surfaces. Furthermore, retrieving the scattering properties of asteroid surfaces plays a vital role in the future space missions, including asteroid mining. Asteroid lightcurve inversion is useful especially when carrying out taxonomic classification as it provides additional information on the physical properties of the surface. The upcoming Gaia Data Release 3 will be extensive enough for rotational pole retrievals and will improve our current knowledge of the asteroids' physical properties. Asteroidien pintojen tutkiminen valonsironnan avulla on haastavaa, koska pinnan sirontaprosessit riippuvat pinnan hiukkasten koosta, muodosta sekä materiaalista, joita ei yleensä ennalta tunneta. Kaukokartoitushavaintojen tulkintaa ja realistista analysointia varten on tärkeää valita oikeat menetelmät. Viime vuosikymmenten aikana useat tutkimukset ovat keskittyneet asteroidien pintojen mallintamiseen, mutta yksikään aiemmin käytetty malli ei ole pystynyt tulkitsemaan spektroskooppisia, fotometrisia ja polarimetrisia havaintoja yhtäaikaisesti riittävällä tarkkuudella. Väitöskirjani tavoite on kehittää valonsirontamenetelmiä ja käyttää niitä yhdessä laboratoriomittausten kanssa luonnehtimaan asteroidien sekä meteoriittien pintoja. Tässä väitöskirjassa esitetyt valonsirontamenetelmät ottavat huomioon sekä aallonpituusluokan hiukkaset että aallonpituutta suuremmat hiukkaset. Tämä on tärkeää, koska eri aallonpituusalueilla käytetyt mallit ovat erilaisia, erityisesti resonanssialuetta on vaikea mallintaa. Ensimmäiseksi kolmen meteoriittinäytteen heijastusspektrejä simuloidaan käyttämällä mallia, joka yhdistää asteroideissa ja meteoriiteissa yleisimmin esiintyviä materiaaleja: oliviinia, pyroksiinia ja rautaa. Tulokset ovat lupaavia, sillä simuloidut spektrit vastaavat melko hyvin laboratoriossa mitattuja spektrejä. Toiseksi asteroidi (4) Vestan spektroskooppisia, fotometrisia ja polarimetrisia havaintoja mallinnetaan erinomaisin tuloksin. Heijastusspektrejä voidaan simuloida riittävällä tarkkuudella, fotometrinen malli tuottaa epälineaarisen kirkkauden kasvun ja polarimetriassa nähdään negatiivinen lineaarinen polarisaatioaste takaisinsirontasuunnassa, mikä nähdään myös havainnoissa. Lopuksi asteroidien taksonomista luokittelua analysoidaan valokäyräinversion avulla, jossa käytetään sekä konvekseja että epäkonvekseja muotoja. Inversiosta saadaan arvioitua asteroidien vaihekäyrien kulmakertoimet, pyörähdysajat, napojen suunnat, muodot, todelliset vaihekäyrät ja absoluuttiset magnitudit Euroopan avaruusjärjestön (ESA) Gaia-avaruusteleskoopin G-magnitudin aallonpituuskaistassa. Tarkempi analysointi osoittaa, että nykyisissä taksonomisissa järjestelmissä voi esiintyä väärää luokittelua. Fotometriset havainnot täydentävät olemassa olevia spektreihin perustuvia luokitteluja ja auttavat määrittämään asteroidien oikean taksonomian. Väitöskirjassa käytetyt suorat valonsirontamenetelmät ovat ratkaisevan tärkeitä tulevissa tutkimuksissa, sillä niitä voi soveltaa myös muihin Aurinkokunnan kohteisiin, kuten komeettoihin ja satelliitteihin. Lisäksi asteroidien pintakerroksen valonsirontaominaisuuksien laskeminen on tärkeää tulevilla avaruuslennoilla, mukaan lukien tulevaisuuden kaivostoiminta asteroideilla. Asteroidien valokäyräinversio on hyödyllinen taksonomisessa luokittelussa, sillä se antaa lisää tietoa pinnan ominaisuuksista. Gaian kolmas tiedonjulkistus (Gaia Data Release 3) tulee lisäämään ymmärrystämme asteroidien pinnoista ja on lisäksi riittävän laaja naparatkaisujen laskemiseen.

BepiColombo has a larger and in many ways more capable suite of instruments relevant for determination of the topographic, physical, chemical and mineralogical properties of Mercury’s surface than the suite carried by NASA’s MESSENGER spacecraft. Moreover, BepiColombo’s data rate is substantially higher. This equips it to confirm, elaborate upon, and go beyond many of MESSENGER’s remarkable achievements. Furthermore, the geometry of BepiColombo’s orbital science campaign, beginning in 2026, will enable it to make uniformly resolved observations of both northern and southern hemispheres. This will offer more detailed and complete imaging and topographic mapping, element mapping with better sensitivity and improved spatial resolution, and totally new mineralogical mapping.We discuss MESSENGER data in the context of preparing for BepiColombo, and describe the contributions that we expect BepiColombo to make towards increased knowledge and understanding of Mercury’s surface and its composition. Much current work, including analysis of analogue materials, is directed towards better preparing ourselves to understand what BepiColombo might reveal. Some of MESSENGER’s more remarkable observations were obtained under unique or extreme conditions. BepiColombo should be able to confirm the validity of these observations and reveal the extent to which they are representative of the planet as a whole. It will also make new observations to clarify geological processes governing and reflecting crustal origin and evolution.We anticipate that the insights gained into Mercury’s geological history and its current space weathering environment will enable us to better understand the relationships of surface chemistry, morphologies and structures with the composition of crustal types, including the nature and mobility of volatile species. This will enable estimation of the composition of the mantle from which the crust was derived, and lead to tighter constraints on models for Mercury’s origin including the nature and original heliocentric distance of the material from which it formed.