Browsing by Subject "scattering"

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  • Martikainen, Julia; Penttilä, Antti; Gritsevich, M.; Videen, Gorden; Muinonen, Karri Olavi (2019)
    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 mu m 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
  • Muinonen, K.; Torppa, J.; Wang, X-B; Cellino, A.; Penttilä, A. (2020)
    Context. We assess statistical inversion of asteroid rotation periods, pole orientations, shapes, and phase curve parameters from photometric lightcurve observations, here sparse data from the ESA Gaia space mission (Data Release 2) or dense and sparse data from ground-based observing programs.Aims. Assuming general convex shapes, we develop inverse methods for characterizing the Bayesian a posteriori probability density of the parameters (unknowns). We consider both random and systematic uncertainties (errors) in the observations, and assign weights to the observations with the help of Bayesian a priori probability densities.Methods. For general convex shapes comprising large numbers of parameters, we developed a Markov-chain Monte Carlo sampler (MCMC) with a novel proposal probability density function based on the simulation of virtual observations giving rise to virtual least-squares solutions. We utilized these least-squares solutions to construct a proposal probability density for MCMC sampling. For inverse methods involving triaxial ellipsoids, we update the uncertainty model for the observations.Results. We demonstrate the utilization of the inverse methods for three asteroids with Gaia photometry from Data Release 2: (21) Lutetia, (26) Proserpina, and (585) Bilkis. First, we validated the convex inverse methods using the combined ground-based and Gaia data for Lutetia, arriving at rotation and shape models in agreement with those derived with the help of Rosetta space mission data. Second, we applied the convex inverse methods to Proserpina and Bilkis, illustrating the potential of the Gaia photometry for setting constraints on asteroid light scattering as a function of the phase angle (the Sun-object-observer angle). Third, with the help of triaxial ellipsoid inversion as applied to Gaia photometry only, we provide additional proof that the absolute Gaia photometry alone can yield meaningful photometric slope parameters. Fourth, for (585) Bilkis, we report, with 1-sigma uncertainties, a refined rotation period of (8.5750559 0.0000026) h, pole longitude of 320.6 degrees +/- 1.2 degrees, pole latitude of - 25.6 degrees +/- 1.7 degrees, and the first shape model and its uncertainties from convex inversion.Conclusions. We conclude that the inverse methods provide realistic uncertainty estimators for the lightcurve inversion problem and that the Gaia photometry can provide an asteroid taxonomy based on the phase curves.
  • Caro, Pedro; Helin, Tapio; Kujanpää, Antti; Lassas, Matti (2019)
    Scattering from a non-smooth random field on the time domain is studied for plane waves that propagate simultaneously through the potential in variable angles. We first derive sufficient conditions for stochastic moments of the field to be recovered from empirical correlations between amplitude measurements of the leading singularities, detected in the exterior of a region where the potential is almost surely supported. The result is then applied to show that if two sufficiently regular random fields yield the same correlations, they have identical laws as function-valued random variables.
  • Markkanen, Johannes; Agarwal, Jessica; Väisänen, Timo; Penttilä, Antti; Muinonen, Karri (2018)
    We show that the scattering phase functions of the coma and the nucleus of the comet 67P/Churyumov-Gerasimenko measured by the Rosetta/Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument can be reproduced by a particle model involving clustered, densely packed submicrometer-sized grains composed of organic material and larger micrometer-sized silicate grains. The simulated and measured coma phase functions suggest that near the nucleus scattering is dominated by large particles, and the size distribution of dust particles varies with time and/or local coma environment. Further, we show that the measured nucleus phase function is consistent with the coma phase function by modeling a nucleus-sized object consisting of the same particles that explain the coma phase functions.
  • Saajasto, Mika; Juvela, Mika; Lefevre, Charlene; Pagani, Laurent; Ysard, Nathalie (2021)
    Context. Light scattering at near-infrared (NIR) wavelengths has been used to study the optical properties of the interstellar dust grains, but these studies are limited by the assumptions on the strength of the radiation field. On the other hand, thermal dust emission can be used to constrain the properties of the radiation field, although this is hampered by uncertainty about the dust emissivity.Aims. Combining light scattering and emission studies allows us to probe the properties of the dust grains in detail. We wish to study if current dust models allow us to model a molecular cloud simultaneously in the NIR and far-infrared (FIR) wavelengths and compare the results with observations. Our aim is to place constraints on the properties of the dust grains and the strength of the radiation field.Methods. We present computations of dust emission and scattered light of a quiescent molecular cloud LDN1512. We use NIR observations covering the J, H, and K-S bands, and FIR observations between 250 and 500 mu m from the Herschel space telescope. We constructed radiative transfer models for LDN1512 that include an anisotropic radiation field and a three-dimensional cloud model.Results. We are able to reproduce the observed FIR observations, with a radiation field derived from the DIRBE observations, with all of the tested dust models. However, with the same density distribution and the assumed radiation field, the models fail to reproduce the observed NIR scattering in all cases except for models that take into account dust evolution via coagulation and mantle formation. The intensity from the diffuse interstellar medium like, dust models can be increased to match the observed one by reducing the derived density, increasing the intensity of the background sky and the strength of the radiation field between factors from two to three. We find that the column densities derived from our radiative transfer modelling can differ by a factor of up to two, compared to the column densities derived from the observations with modified blackbody fits. The discrepancy in the column densities is likely caused because of a temperature difference between a modified blackbody fit and the real spectra. The difference between the fitted temperature and the true temperature could be as high as Delta T = +1.5 K.Conclusions. We show that the observed dust emission can be reproduced with several different assumptions about the properties of the dust grains. However, in order to reproduce the observed scattered surface brightness, dust evolution must be taken into account.
  • Saajasto, Mika; Juvela, Mika; Malinen, Johanna (2018)
    Context. Regarding the evolution of dust grains from diffuse regions of space to dense molecular cloud cores, many questions remain open. Scattering at near-infrared wavelengths, or "cloudshine", can provide information on cloud structure, dust properties, and the radiation field that is complementary to mid-infrared "coreshine" and observations of dust emission at longer wavelengths. Aims. We examine the possibility of using near-infrared scattering to constrain the local radiation field and the dust properties, the scattering and absorption efficiency, the size distribution of the grains, and the maximum grain size. Methods. We use radiative transfer modelling to examine the constraints provided by the J, H, and K bands in combination with mid-infrared surface brightness at 3.6 mu m. We use spherical one-dimensional and elliptical three-dimensional cloud models to study the observable effects of different grain size distributions with varying absorption and scattering properties. As an example, we analyse observations of a molecular cloud in Taurus, TMC-1N. Results. The observed surface brightness ratios of the bands change when the dust properties are changed. However, even a change of +/- 10% in the surface brightness of one band changes the estimated power-law exponent of the size distribution gamma by up to similar to 30% and the estimated strength of the radiation field K-ISRF by up to similar to 60%. The maximum grain size A(max) and gamma are always strongly anti-correlated. For example, overestimating the surface brightness by 10% changes the estimated radiation field strength by similar to 20% and the exponent of the size distribution by similar to 15%. The analysis of our synthetic observations indicates that the relative uncertainty of the parameter distributions are on average A(max), gamma similar to 25%, and the deviation between the estimated and correct values Delta Q <15%. For the TMC-1N observations, a maximum grain size A(max) > 1.5 mu m and a size distribution with gamma > 4.0 have high probability. The mass weighted average grain size is <a(m)> = 0.113 mu m. Conclusions. We show that scattered infrared light can be used to derive meaningful limits for the dust parameters. However, errors in the surface brightness data can result in considerable uncertainties on the derived parameters.
  • Escobar-Cerezo, J.; Palmer, C.; Munoz, O.; Moreno, F.; Penttilä, A.; Muinonen, K. (2017)
    The effect of internal inhomogeneities and surface roughness on the scattering behavior of large cosmic dust particles is studied by comparing model simulations with laboratory measurements. The present work shows the results of an attempt to model a dust sample measured in the laboratory with simulations performed by a ray-optics model code. We consider this dust sample as a good analogue for interplanetary and interstellar dust as it shares its refractive index with known materials in these media. Several sensitivity tests have been performed for both structural cases (internal inclusions and surface roughness). Three different samples have been selected to mimic inclusion/coating inhomogeneities: two measured scattering matrices of hematite and white clay, and a simulated matrix for water ice. These three matrices are selected to cover a wide range of imaginary refractive indices. The selection of these materials also seeks to study astrophysical environments of interest such as Mars, where hematite and clays have been detected, and comets. Based on the results of the sensitivity tests shown in this work, we perform calculations for a size distribution of a silicate-type host particle model with inclusions and surface roughness to reproduce the experimental measurements of a dust sample. The model fits the measurements quite well, proving that surface roughness and internal structure play a role in the scattering pattern of irregular cosmic dust particles.
  • Escobar-Cerezo, J.; Penttilä, Antti; Kohout, T.; Munoz, O.; Moreno, F.; Muinonen, K. (2018)
    Lunar soil spectra differ from pulverized lunar rocks spectra by reddening and darkening effects, and shallower absorption bands. These effects have been described in the past as a consequence of space weathering. In this work, we focus on the effects of nanophase iron (npFe(0)) inclusions on the experimental reflectance spectra of lunar regolith particles. The reflectance spectra are computed using SIRIS3, a code that combines ray optics with radiative-transfer modeling to simulate light scattering by different types of scatterers. The imaginary part of the refractive index as a function of wavelength of immature lunar soil is derived by comparison with the measured spectra of the corresponding material. Furthermore, the effect of adding nanophase iron inclusions on the reflectance spectra is studied. The computed spectra qualitatively reproduce the observed effects of space weathered lunar regolith.