Physical modeling for the Vis-SWIR spectrometry of the Chelyabinsk meteorite

Understanding light scattering on meteorite surfaces is difficult. Multiple factors affect the reflectance spectra of meteorites, such as space weathering, terrestrial weathering, and shocks. The main focus of this thesis was to investigate how shock induced iron changes meteorite spectra. The reflectance spectra of 30 meteorite pieces were measured with the University of Helsinki spectrometer in the wavelength range of 300 to 2500 nm. A principal component analysis (PCA) was performed on the spectra and the results were compared with previous studies carried out by Pentikäinen et al. (JQSRT, 146, 2014) and Gaffey (NASA PDS, 2001). The analyses show that HED meteorites can be separated from chondrites. However, more HED measurements are needed to verify the validity of the results. The effects of shock induced iron on meteorite spectra were modeled with the SIRIS3 (Muinonen et al., 2009) light-scattering program. Three different spectra of the Chelyabinsk meteorite were modeled, each of them having experienced a different degree of shock, and thus representing a different lithology: light-colored lithology was modeled as 10% air particles in olivine, dark-colored lithology as 10% iron particles in olivine, and impact-melt lithology as 5% air particles and 5% iron particles in olivine. The modeled spectra were then compared with the spectral measurements of the three lithologies of the Chelyabinsk meteorite. The compability of the measured and the modeled spectra was fair. In general, a higher iron occurrence makes the spectra darker and more flat. The differences between the modeled and the measured spectral shapes of the impact-melt and light-colored lithologies are caused by the absence of pyroxene in the simulations, whereas the modeled and the measured spectral shapes of the dark-colored lithology are different because the occurrence of iron in the measured spectrum is probably higher

Scattering of light by dense particulate media in the geometric optics regime

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.Peer reviewe

Asteroid and meteorite compositional studies by modeling light scattering

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

Laboratory spectroscopy of meteorite samples at UV-Vis-NIR wavelengths : Analysis and discrimination by principal components analysis

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.Peer reviewe

Spectral modeling of meteorites at UV-vis-NIR wavelengths

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.Peer reviewe

Ray optics for absorbing particles with application to ice crystals at near-infrared wavelengths

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.Peer reviewe

Absolute spectral modelling of asteroid (4) Vesta

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 particlesPeer reviewe

Rigorous light-scattering simulations of nanophase iron space-weathering effects on reflectance spectra of olivine grains

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.Peer reviewe

Hedelmällisyys ja siihen vaikuttavat tekijät Suomessa lähivuosikymmeninä

Peer reviewe

Ejected Particles after Impact Splash on Mars: Aggregates and Aerodynamics

Our earlier laboratory measurements showed that low-velocity sand impacts release fine <5 {\mu}m dust from a Martian simulant soil. This dust will become airborne in the Martian atmosphere. Here, we extend this study by measuring aerodynamic properties of ejecta and characterizing deviations from the behavior of spherical, monolithic grains. We observe the settling of particles emitted as part of an impact splash. The sizes (20 to 280 {\mu}m) and sedimentation velocities (0.1 to 0.8 ms^{-1} ) of the particles are deduced from high-speed videos while the particles sediment under low ambient pressure of about 1 mbar. The particles regularly settle slower than expected, down to a factor of about 0.3. Using optical microscopy, the shape of the captured particles is characterized by simple axis ratios (longest/smallest), which show that the vast majority of particles are irregular but typically not too elongated, with axis ratios below 2 on average. Electron microscopy further reveals that the particles are typically porous aggregates, which is the most likely reason for the reduction of the sedimentation velocity. Due to the reduced bulk density, aggregates up to 10 {\mu}m in diameter should regularly be a part of the dust in the Martian atmosphere.Comment: 8 pages, 7 figure