Gorden Videen

Comets are often characterized by their production of H ₂ O molecules, whose quantity is further used to constrain the fraction of active area of their nuclei. The latter retrieval is based on a theory developed half century ago. We evaluate it, suggesting improvements in light of more recent models developed to evaluate lunar temperature. We take into account the difference in temperature of the outermost surface layer and the internal volume of the nucleus and the energy exchange between these two parts via thermal emission. The resulting equation takes on a simple form for dynamically new and interstellar comets due to the extremely low temperature of their interior volume. We apply this new approach to the interstellar Comet 3I/ATLAS. The result is a significant increase in the active area of the nucleus needed to reproduce the production of H ₂ O molecules observed in Comet 3I/ATLAS.

Carbonaceous aerosols represent a major component of atmospheric particulate matter. However, current characterization methods have limitations in sensing these particles because they either measure only one type of aerosol property or rely on offline measurement from collected samples. In-situ sensing is crucial for characterizing the physical and chemical properties of carbonaceous aerosols while they remain airborne. In this work, optical trapping–Raman spectroscopy (OT–RS) was applied for in-situ sensing of single carbonaceous aerosol particles in freshly generated smoke. Nylon rope and plastic fiber were ignited inside an open chamber using the trapping laser itself. The resulting particles were captured directly in air by the optical trap and immediately analyzed using Raman spectroscopy and real-time imaging within seconds. The optically trapped particles had spatial dimensions of several micrometers and estimated masses on the order of ~10 pg. We demonstrate that this minute amount of material is sufficient to obtain detailed Raman spectral fingerprints. Furthermore, analysis of the Raman bands allowed two types of particles to be clearly distinguished based on their distinct Raman features. Spatially resolved Raman spectra further resolved that individual trapped particles are chemically heterogeneous, with organic carbon and soot located in different regions within the same particle. These results demonstrate that OT–RS provides an effective platform for in-situ characterization of single carbonaceous aerosol particles and possesses strong potential for future on-site atmospheric sensing and monitoring.

Infrared spectroscopy is based on the emission from a heated body, oftentimes dust particles. This body is generally assumed to have a spherical shape. We explore the temperature of blackbodies with non-spherical shapes. Five non-spherical shapes are considered: (1) a prolate cylinder and (2) a prolate spheroid, both with aspect ratio 2:1, (3) agglomerated debris particles, equidimensional in size, (4) ballistic particle–cluster aggregates (BPCAs), and (5) ballistic cluster–cluster aggregates (BCCAs). Such particles have been used to model the light-scattering response of particles in cometary comae and debris disks. The first three types of particles yield remarkably similar blackbody temperatures that are only ~9 K lower than a spherical blackbody. Blackbodies with BPCA and BCCA morphologies are much colder than the spherical blackbody due to the large surface area of the fractal aggregates.

Scattering and absorption of light in macroscopic discrete random media of densely packed particles (e.g., planetary regoliths) constitutes a computational challenge in electromagnetics described by the Maxwell equations. There are differing physics cases that must be tackled separately. Exact solutions are typically available only for systems with maximum sizes of only a few wavelengths. We consider a radiative-transfer coherent-backscattering (RT-CB) approximation for embedded discrete random media of particles. The first case (Case I) concerns wavelength-scale particles within a half space of optically isotropic and homogeneous material described by a complex refractive index (host medium). In comparison to a semi-infinite discrete random medium in free space, Case I introduces an interface between free space and host medium and particle scattering and absorption properties must be computed in relation to the host medium. In RT-CB, interaction paths always include refractions into and out of the host medium, as well as any internal reflections that take place at the interface. Numerical Monte Carlo computation largely resembles that for semi-infinite random media in free space. In the UV-Vis-NIR part of the spectrum (wavelengths of 0.1-2.5 microns), Case I is relevant for planetary-regolith particles large compared to the wavelength with inhomogeneous internal composition. The internal inhomogeneities can derive from agglomerated minerals or voids. In the microwave part of the spectrum relevant for radars (wavelengths of 1-100 mm), Case I is relevant by the very definition of the radar scattering problem. Case II concerns a finite layer of wavelength-scale particles in free space on a half space of optically isotropic and homogeneous material (host medium). In comparison to a semi-infinite discrete random medium in free space, Case II thus introduces an interface in the bottom of the finite layer of particles. In RT-CB, interaction paths including a refraction into the host medium do not contribute to the scattering by the medium: refracted radiation is considered to be entirely absorbed. The interaction paths can include reflections from the interface and, from the point of view of RT-CB, the paths fall into different categories. The categories include the following cases: (i) the interaction path is a pure reflection process at the interface;(ii) the interaction path includes particle scattering processes but no reflection processes;(iii) the interaction path includes reflection processes but, in the RT path, the first and final interactions are particle scattering processes;(iv) the first interaction is a reflection process and the last interaction is a particle scattering process;(v) the first interaction is a particle scattering process and the last interaction is a reflection process;(vi) the first and last interactions are reflection processes. We highlight a number of applications in scattering and absorption by airless Solar System objects in the UV-Vis-NIR and microwave spectral ranges (see Leppälä et al. and Pentikäinen et al., present conference). For UV-Vis-NIR, we apply the RT-CB computations to the case of a layer of wavelength-scale amorphous ice particles on the surface of crystalline ice particles large compared to the wavelength. For radar, we carry out a computation for absorbing particles within pure, crystalline water ice.

Europa’s photometric and polarimetric behaviour in UV-Vis-NIR light provides constraints on the microphysical properties of its uppermost regolith. At small phase angles, approximately below 20 degrees, atmosphereless bodies exhibit non-linear brightening near the backscattering direction as well as negative linear polarization, in which the scattered light is polarized parallel to the Sun–object–observer scattering plane. These phenomena are sensitive to particle size, shape, and refractive index, as well as to regolith packing density, as they arise from electromagnetic wave scattering in discrete media composed of small particles, including interference between reciprocal light paths traversed in opposite directions. Photometric and polarimetric observations can therefore be used to probe the structure and composition of the near-surface scattering medium. In earlier work, we modeled the small-phase-angle polarimetric phase curves of the Galilean satellites [1 , 2] using radiative-transfer coherent-backscattering (RT-CB) calculations [3 –5] with ensemble-averaged scattering matrices. For Europa, this approach reproduced key characteristics of the observed negative polarization branch using disk-integrated polarimetric observations. A recent study [6] further demonstrated the diagnostic potential of Mueller matrix elements beyond those describing linear polarization alone. In the present work, we extend this modeling framework by replacing generic single-scattering inputs with scattering matrices derived from physically constrained particle models for Europa’s icy surface. The particle ensembles considered here include randomly shaped and randomly oriented particles with effective sizes in the approximate range 0.1–2.0 μm. Their single-scattering properties are computed using numerical electromagnetic methods, including the Discrete Dipole Approximation (ADDA) [7] (0.1–0.5 μm) and the surface-integral-equation method [8] (0.1–2.0 μm). These calculations are complemented by the scattering properties of larger Gaussian particles generated with the SIRIS-4 code [9 , 10], resulting in ensemble-averaged scattering matrices representative of a physically motivated near-surface composition. The resulting ensemble-averaged scattering matrices are used as input for the RT-CB modeling of Europa’s photometric and polarimetric phase curves. Furthermore, decomposition of the ensemble-averaged scattering matrix into polarization-conserving Mueller matrices [4] enables RT-CB computations for discrete random media composed of nonspherical particles [5]. This decomposition facilitates the interpretation of near-surface structure and composition by comparing RT-CB model results with observations [11]. We apply extended RT-CB computations to a surface model consisting of a layer of wavelength-scale amorphous ice particles overlying crystalline ice particles that are large compared with the wavelength; see Muinonen et al., present conference. At radar wavelengths, we model absorbing particles embedded in pure crystalline water ice. Using radar observations of same-circular (SC) and opposite-circular (OC) polarization [12, 13], we aim to estimate the relative abundance of pure water ice and more absorbing impurities. Previous studies have addressed this using coherent-backscatter and multiple-scattering models (e.g., [14]). Here, we apply the extended RT-CB model to investigate whether these polarization measurements can provide additional constraints on the relative abundance of pure water ice and more absorbing impurities. By simulating light scattering from near-surface composition models with specified physical properties and comparing the results with ensemble-averaged scattering matrices and observations, we obtain constraints on the compositional and structural characteristics of Europa’s surface, including particle size distribution, packing density, and impurity content. The RT-CB model, combined with photometric and polarimetric measurements, provides a valuable tool for characterizing Europa, other icy satellites, and airless bodies using both ground-based observations and spacecraft measurements. Regolith parameters retrieved through RT-CB modeling can serve as boundary conditions for simulations of particle release into the surface-bounded exospheres of Jovian moons, thereby supporting the science goals of the JUICE and Europa Clipper missions.references[1] N. Kiselev et al., ”New Polarimetric Data for the Galilean Satellites : Europa Observations and Modeling” Planet. Sci. J. 3, 134 (2022)[2] N. Kiselev et al., ”New Polarimetric Data for the Galilean Satellites: Io and Ganymede Observations and Modeling,” Planet. Sci. J. 5, 10 (2024)[3] K. Muinonen et al., ”Coherent Backscattering Verified Numerically for a Finite Volume of Spherical Particles,”ApJ 760, 118 (2012)[4] K. Muinonen, A. Penttilä, ”Scattering matrices of particle ensembles analytically decomposed into pure Mueller matrices,” JQSRT 324, (2024)[5] K. Muinonen et al., ”Coherent backscattering in discrete random media of particle ensembles,” JQSRT 330,(2025)[6] A. Leppälä, K.Muinonen, A. Penttilä, et al., JQSRT, submitted (2026)[7] M. A. Yurkin, A. G. Hoekstra, ”The discrete-dipole-approximation code ADDA: Capabilities and known limitations,” JQSRT 112, 13 (2011)[8] J. Markkanen, ”Surface-integral-equation solution for solid particles with wavelength-scale surface roughness,” JQSRT 341, (2025)[9] K. Muinonen et al., ”Light scattering by Gaussian particles with internal inclusions and roughened surfaces using ray optics,” JQSRT 110, (2012)

We report the development of a prototype that can measure the single-particle phase function of circular intensity differential scattering (CIDS), i.e., the angular scattering ratio distribution of the Mueller matrix element −S14/S11 from individual single particles drawn into the system from the atmosphere or an aerosol generator. The system is developed from a laboratory experimental setup, improved, and constructed into a portable prototype with a dimension of 24 ^{′ ′} ×18 ^{′ ′} ×12 ^{′ ′} for field measurements. The design facilitated the fast detection of bioaerosol particles or others with spiral/helical molecular structures at a size from 1 to 10 micron in diameter. The system achieved a maximum particle screening rate of 50,000 particles/sec.

This study is part of an ongoing project in which we experimentally examine the effect of composition (refractive index) and surface roughness on the scattering matrix elements of a set of well-characterized rough regolith simulants. A set of four cylindrical Mars regolith simulants with controlled degrees of porosity and surface roughness is studied. Photopolarimetric measurements, spanning scattering angles from 94° to 177°, were conducted at a wavelength of 488 nm at the Instituto de Astrofísica de Andalucía Cosmic Dust Laboratory. These results are compared with those previously obtained at 640 nm. The higher absorption of the Mars simulants at 488 nm compared to 640 nm produces a significant effect on all elements of the scattering matrix. The trends previously observed in the diagonal elements of the scattering matrix and in the albedo as a function of surface roughness remain unaffected by the differences in refractive indices at both wavelengths. The comparison with the previous study enables the investigation of sample reddening across the scattering-angle range, with a trend also observed between surface roughness and reddening. Moreover, within our measured angular range, the effect of absorption is consistent with Umov’s law, whereas the effects of surface roughness do not appear to support its validity. Nearly all samples exhibit a shallow negative polarization branch.

We present a numerical study comparing light scattering by small clusters composed of helices, capsules, and spheres. Using the discrete-dipole approximation (DDA), we compute orientation-averaged Mueller-matrix elements M₁₁, M₁₂, and M₁₄ for clusters with varying number of monomers (N = 5–45) and mean center-to-center separation (1–10 particle diameters). Our analysis isolates the influence of particle morphology on angular scattering intensity, linear polarization, and circular intensity differential scattering (CIDS), providing a direct comparison of symmetric and chiral shapes. Helices display persistent angular fine structure in M₁₁ and deep, side-scattering maxima in M₁₂, while spheres and capsules converge to smoother polarization curves with increasing separation. CIDS from symmetric monomers manifests as small oscillations around zero that decay rapidly with monomer separation and number. In contrast, helices produce a stable backward CIDS slope that is largely separation-independent but gradually flattens with increasing number of monomers. These trends confirm that morphology alone can influence key polarization characteristics and provide insights for interpreting scattering from complex-shaped particles. Such morphology-related features may help in the interpretation of polarization data in aerosol and planetary remote sensing and justify the refinement of the design of optical setups for studying irregular or chiral particles in controlled environments.

We have measured the degree of linear polarization of the second interstellar Comet 3I/ATLAS in 2025 November, while the comet was passing within the snow line of the solar system. Our results unambiguously differ from previous observations of the same comet made beyond the snow line by Z. Gray et al., suggesting a change in the microphysics of the coma particles. The model developed by E. Zubko & G. Videen to fit the polarimetric observations of Comet 3I/ATLAS beyond the snow line also can be used to fit our observations within the snow line by omitting the contribution of the water-ice particles. This suggests stability in the microphysics and relative abundances of the refractory species of the 3I/ATLAS coma between the middle of July and late November of 2025.