1,721,026 research outputs found
ZPEC - The Zodiacal Dust Polarization Evaluation Code
No full textThe habitability of exoplanets is determined by their surface properties and atmospheres, spectropolarimetry is a powerful tool to characterize these. Numerical models can be used to simulate the polarimetric signals of these exoplanets, but these computed signals have to be compared with observations. The best test-case for such a comparison is the Earth, as the atmospheric and surface properties are well-known and the signal can be computed to a high accuracy. The Lunar Observatory for Unresolved Polarimetry of Earth (LOUPE) shall take measurements of the unresolved Earth being based on the Moon. When observing exoplanets or the Earth, dust in planetary systems acts as a noise source and a source of confusion. This so-called zodiacal dust adds to the polarimetric signal of the planet and in some cases a dust clump might be confused with a planet. The aim of the thesis is to develop a computational tool that allows to analyze the zodiacal dust in order to estimate the noise level and to determine if dust clumps can be distinguished from exoplanets using their polarization signals. The developed computational tool is called ZPEC: the Zodiacal Dust Polarization Evaluation Code. It uses a Monte-Carlo based algorithm in order to track individual photon paths. When photons are scattered their polarization properties are calculated, which are verified using the Adding-Doubling code for flat layers. ZPEC allows to analyze how photon properties change when they are scattered multiple times by a dust particle, whereas classically it is assumed that zodiacal dust disks are optically thin enough to assume photons are scattered only once. The dust model of the COBE/DIRBE mission is used to model the dust in the Solar System, a NASA collisional model was used to model the dust disk surrounding star ? Pictoris. The ZPEC photon property databases are used to generate hypothetical detector views. Using ZPEC it is concluded that the Earth sometimes seems to disappear against the polarization signal of the background dust, while the maximum brightness of the dust to the Earth signal is 0.14%. It is also concluded for the dust disk orbiting ? Pic that ignoring multiple scattering leads to errors in the calculated polarization signal that are estimated to be 4-9% (relative). In addition it is concluded that dust clumps show the same polarization behavior as the surrounding dust, while planets do not. Hence using brightness only the two can be confused, but they can be distinguished based on their polarization behavior.Astrodynamics & Space MissionsSpaceflightAerospace Engineerin
Accurate high-contrast imaging polarimetry of exoplanets with SPHERE/IRDIS
No full textThe search for and characterization of exoplanets and other sub-stellar companions are hot topics in contemporary astronomy. Currently, the characterization of the atmospheres of exoplanets through direct imaging leverages on the analysis of only the intensity of their light as a function of wavelength and time. Additional information on the composition and structure of planetary atmospheres - that cannot be obtained with spectroscopy - can be deduced with polarimetry, i.e. measuring the direction of oscillation of the electric fields of light. Not only the starlight that an exoplanet reflects is expected to be polarized, but also the thermal emission of a planet, as this radiation from inside the atmosphere will be scattered by cloud and haze particles on its way up. Indeed, the polarized thermal radiation of several field brown dwarfs has already been measured and is attributed to the scattering of the radiation by patchy clouds in their atmospheres. The degree of linear polarization of hot exoplanets at near-infrared wavelengths is expected to generally be larger than 0.1% and could be up to several percent in some cases. Measurements of the polarized thermal emission of exoplanets can provide information on the presence and patchiness of atmospheric clouds and hazes, the cloud top pressure, spatial structure such as rotational flattening and cloud bands, the atmospheric rotation rate, and the surface gravity and mass of the companion. By determining the angle of linear polarization, the planet's projected spin axis could be constrained. The recently commissioned VLT instrument SPHERE is a high spatial resolution, high-contrast, direct imaging instrument that is specifically designed to detect and characterize giant exoplanets orbiting nearby stars. SPHERE's near-infrared instrument arm IRDIS has a dual-beam polarimetric mode that is primarily used for high-contrast imaging of circumstellar disks, as it is expected to be too insensitive to directly measure the polarized thermal emission of exoplanets. However, IRDIS has already detected exoplanets with huge signal-to-noise ratio using angular differential imaging (ADI) of thermal fluxes. The aim of this thesis is therefore to investigate the feasibility of combining ADI and accurately calibrated polarimetry with SPHERE/IRDIS to for the first time detect and possibly characterize exoplanetary atmospheres through direct imaging polarimetry at near-infrared wavelengths. To assess whether IRDIS can detect the expected polarization signal of exoplanets, and using HR 8799's planetary system as a study case, IRDIS' polarimetric sensitivity is estimated by extrapolating the results from VLT/NaCo observations to IRDIS, and by simulating single-beam polarimetric measurements with real IRDIS data. Based on these estimates, SPHERE/IRDIS is expected to reach sub-percent polarimetric sensitivity when combining extreme adaptive optics, coronagraphy, ADI with advanced data reduction techniques and dual-beam polarimetry. A model describing the modification of the polarization signal induced by the telescope and instrument is established and validated with available internal calibration measurements and observations of a standard star. It appears that for some filters and particular combinations of the parallactic and altitude angle, only a very small part (~10%) of the incident linearly polarized signal is actually measured by the instrument. This loss of signal is accompanied by large offsets in the angle of linear polarization. It is also found that the instrumental polarization of the telescope and SPHERE's first mirror varies with telescope altitude angle and can reach values of a few percent. To limit the instrumental polarization and the loss of signal, an observation strategy is presented for IRDIS' polarimetric mode. In addition, a data reduction method is developed that uses the instrument model to derive an exoplanet's true degree and angle of linear polarization from the measured polarization signal. It is estimated that, after correcting for the modification of the polarization signal induced by the complete optical system, a polarimetric accuracy of ≤0.1% is reached. Given that an exoplanet will generally be between a few tenths of a percent and a percent polarized, it is concluded that, if SPHERE/IRDIS can indeed attain the predicted sub-percent polarimetric sensitivity when combining angular differential imaging and polarimetry, exoplanetary atmospheres can be characterized for the first time through direct imaging polarimetry.Aerospace EngineeringSpace EngineeringAstrodynamics and Space Mission
Mapping and detection of the mesospheric CO2 clouds on Mars using MEx/OMEGA instrument
No full textIn 1997, the descent of the lander Pathfinder into the Martian atmosphere revealed temperatures in the meso- sphere below CO2 condensation point. As CO2 is the main constituent of the martian atmosphere, the pres- ence of mesospheric CO2 ice clouds has been suggested, and then confirmed by spectro-imaging observa- tions coming from satellite data. These clouds fascinate scientists because none of the current atmospheric models have been able to predict them. They also raise important questions about the martian mesosphere dynamics and properties. Since the early 2000s, a lot of instruments study these clouds in order to detect and map them. These observations are very important in order to be able to model better the atmospheric processes that take place on Mars as the CO2 cycle has a significant impact on its climate. Several studies already developed methods to detect them but now these have to be improved and analysed better to fully characterise the clouds. What I am going to do will be then to analyse the three last years of observation from the OMEGA instrument onboard Mars Express, map the CO2 clouds and study their distribution. The aim of this thesis will be to study the inter-annual variations and the properties of the clouds and try to corre- late them with atmospheric parameters (amount of dust in the atmosphere, temperature, altitude) coming from other observations. The main objective here is to determine how these clouds form and what the source of the condensation nuclei is. With the results of our analysis, the climatology of Mars and its atmosphere dynamics will be refined.Aerospace EngineeringSpace EngineeringMSc Space Flight, track Space Exploratio
Silicate cloud formation in the atmospheres of close-in super-Earths and gas giants
No full textContext: Clouds form in the atmospheres of brown dwarfs and extrasolar planets. Recent observations of planets orbiting extremely close (<0.1 AU) to their stars indicate possible atmospheres with silicate compositions resulting due to vaporization of silicate magma from their surface. Such atmospheres are heavily dependent on compositions of the planetary crust which in turn might influence the kind of dust particles that form in such atmospheres. Aims: We identify five types of silicate compositions commonly found on Earth and derive atmospheric chemistry with Earth silicates as starting compositions using an equilibrium chemistry atmospheric model. Following the mineral cloud modelling approach for hot atmospheres of brown dwarfs and giant gas planets, we model the dust cloud formations resulting due to varying Earth silicate compositions and apply that to investigate the possibility of clouds on sample atmospheres of a giant gas planet, 55 Cnc e, HD149 026b and CoRoT-7b. Methods: Atmospheric compositions for the planets have been derived using a previously validated Equilibrium chemistry code. We derive our atmospheric chemistry using element abundances from previously studied Earth surface compositions which is provided as an input to the 1D kinetic cloud formation model, DRIFT. We perform cloud modelling on each of the atmospheres with varying silicate compositions and study the resulting cloud properties such as particle growth, particle sizes and their composition at various stages. Results: We present the cloud structures resulting due to varying Earth silicate compositions on four different types of planets. The clouds show variations in the dust properties due to different starting compositions with differing average particle sizes but the formation conditions such as average particle size, cloud thickness and condensation altitude largely remain dependent on the local gas density and temperature. The cloud layers on 55 Cnc e, HD149 026b are found to be greatly varying in terms of their geometrical thickness, particle sizes and number densities and are primarily composed of silicates of elements such as Mg, Si, Fe and Al.Aerospace EngineeringSpace Engineerin
Modelling the Circular Polarisation of Earth-like Exoplanets
No full textCircular polarisation is commonly induced through the multiple scattering of light by aerosols in a planetary atmosphere and the multiple reflection of light by a rough surface. More interestingly, light reflected by organisms displays unique spectral circular polarisation behaviour, presenting the possibility of circular polarimetry for the detection of life. It has previously been neglected due its relatively small signal and the need for advanced instrumentation in its measurement. The purpose of this thesis is to investigate the circular polarisation that can be induced by the atmosphere of an exoplanet and the impact it would have on the measurement of surface features. This was done through the modelling of the multiple scattering of light by aerosols for varying aerosol and atmospheric conditions. The influence of partial cloud cover was also modelled. A degree of circular polarisation between 10-3 or 10-5 was typically observed. The maximum values were found at phase angles of 30° to 70°, with an additional peak at 110° to 140° and near-zero values around 90°. The particle size, real refractive index and cloud and gas optical thickness values were the most significant parameters. The introduction of partial cloud cover increased the variability of circular polarisation values and high cloud symmetry with respect to the equator led to a decrease in the circular polarisation. The modelled circular polarisation may be measurable for some cases but not all. The best phase angles for observation are around 40° and 140°, where the circular polarisation is near peak value and the linear polarisation is relatively small. The symmetry of the clouds is advantageous for the observation of surface features, as circular polarisation values cancel out due to having opposite sign. However, a fully cloudy planet would make surface observations very challenging. This work can be built upon with the modelling of surface and biological circular polarisation. Most importantly, observations of organism-dense areas are required to determine the potential of circular polarimetry for the detection of life.Aerospace EngineeringPlanetary Science
Polarimetric modeling of non-transiting and transiting exorings
No full textPlanetary rings seem to be a common appearance around large planets because every large planet in the solar system has them. They have, however, not yet definitively been detected around exoplanets. Because the formation of rings might be closely related to planetary formation, as well as the formation of moons, much could be learned from discovering rings around exoplanets. To aid in this search, we developed a radiative transfer model that includes all orders of scattering and polarization and can calculate the reflected and transmitted light for any planetary orbit and ring geometry. Previous studies have analyzed the effect that rings have on a transit as well as on the reflected light but those studies have not included the polarization of light. Using the developed model, the effect that rings have on the reflected flux, polarized flux, and the degree of polarization was characterized by varying the orbit orientation, ring orientation, ring size, the optical thickness of a ring, and the ring particle properties. Our study showed that rings introduce unique features in both the flux and degree of polarization curves. Especially the existence of ring-plane crossings, when the ring is illuminated from the side, causes distinct features in the light curves. Adding polarimetric capabilities to the next generation of telescopes could help with determining the presence of rings due to the different scattering behavior of the ring and planet.The developed model allows for a fast and easy generation of accurate light curves which allowed for two additional studies to be done. These were a bit smaller in scope and the first involved fitting the transit of a planet with a potential ring. The result of the fit and subsequent analysis is that there is likely no ring. However, because the model was not initially designed to deal with the close proximity of the planet to its star no definitive conclusion can be drawn. In the second study, the reflected flux and polarization of the transiting planet J1407b, which is suspected to have a large circumplanetary disk, were computed to assess the detectability. These generated light curves and values were then compared to unpublished observations made with the SPHERE/ZIMPOL instrument that detected no signal. The computed flux and degree of polarization lie well below the detection limit and are thus in agreement with the SPHERE/ZIMPOL observations.Aerospace Engineerin
ARM: Asteroid Reflection Model: The implementation of an asteroid polarimetry model and its application to interpret asteroid (3200) Phaethon observations
No full textAsteroid Reflection Model (ARM) is a newly developed model that simulates reflected radiation and polarization on an asteroid's surface. The working principle behind the model is radiative transfer using Fourier series expansions of reflection matrices. Input models and parameters are: a triangle polyhedron shape model of an asteroid, a surface scattering model, and the desired asteroid location and orientation and the phase angle. Output parameters are the reflected Stokes vector and the degree and direction of polarization, for each individual surface facet and disk-integrated. ARM is fully verified. Generated phase-polarization curves are validated using polarimetric data of four asteroids, including (3200) Phaethon, for which various surface scattering models are deployed. These phase-polarization curves are fit to the Phaethon data, but this did not result in a good match for any of the surface scattering models, since they fail to simulate the opposition effect. However, the effect of the shape, orientation and the rotational motion is clearly visible in the results, as is the relation between wavelength and polarization. Finally, it was concluded that the polarization is favored over the flux when determining an asteroid's surface characteristics and shape, and that both flux and polarization can be used together when determining its size.Section 3.4 is left out due to publication reasons.Aerospace Engineerin
Modelling polarization and flux surface observations of Martian mesospheric CO2 ice clouds during twilight
No full textMartian mesospheric CO2 ice clouds are formed out of the main constituent of the Martian atmosphere: CO2 gas. The nature of these clouds is poorly understood, characterization of these clouds in terms of particle morphology would give valuable insight into the Martian climate. So far, these CO2 clouds have only been detected from space. In this Master Thesis, feasibility of a surface-based observation strategy has been investigated to predict the flux and polarization signal of the CO2 clouds, using a Monte Carlo radiative transfer code. Simulation results suggest that the CO2 clouds are observable from the surface during twilight. Furthermore, it is concluded that observation from the surface allows for detection of the cloud's degree of polarization at a broad range of scattering angles, which is useful for effective characterization of cloud particle morphology.Aerospace Engineerin
Automated detection of methane emissions in TROPOMI data using Neural Networks
No full textAerospace Engineering | Space Fligh
Reflective Light-Curves of Ellipsoidally-Shaped Outer Solar-System Objects and Exoplanets
No full textFaculty EWI and TNW. BSc Industrial and Applied Mathematics and Applied Physics. In this thesis we consider reflective light-curves of planets, graphs of the intensity of the light originating from the parent star that the planet reflects versus time. In almost all instances, planets are modelled as spheres, like in [1] and [2]. However, planets are better approximated by ellipsoids as proved by Isaac Newton in the Principia [3]. In our own solar system, we can observe that planets are not spheres when we look at for example Jupiter [4] and Haumea [5]. We study the effect on the light-curve of the change from a spherical to an ellipsoidal model. For example, a spherical model of a homogeneous planet at edge-on observation would predict a constant light-intensity during one rotation around its axis due to the symmetry of the model. Note that we assumed here that the planet sits approximately still in the sky during one rotation around its axis. However, an ellipsoidal model of a homogeneous planet at edge-on observation would predict a variable light-intensity. [5] shows a difference between the maximum and the minimum value of the measured light-curve of Haumea of 0.32 magnitudes. This shows that an ellipsoidal model has a significant effect on the light-curve. Furthermore, in contrast to the spherical model, with the light-curves for the ellipsoidal model we can for example calculate the spin of the planet in certain cases, which makes them more interesting. By calculating the light-curve with the ellipsoidal model, we can determine the shape of planets and therefore gain knowledge about the internal structure of planets. Where others, like [5], have calculated the light-curve for an ellipsoidally-shaped planet numerically, we calculate them analytically. We accomplish this with the analytical equation of the light-curve postulated by [6]. We assume that planets have a homogeneous reflecting surface, we assume parallel incident light-rays, we assume Lambertian reflection and we assume that the planet is in a circular orbit around its parent star. We did not calculate the light-curves for non-circular orbits. However, the results can easily be modified to include elliptical Kepler orbits since the light-curve depends linearly on the orbital radius. We consider the applications of a solar system triaxially-shaped planet, like Haumea, a spheroidally-shaped exoplanet and a tidally-locked, triaxially-shaped exoplanet. We considered both edge-on and face-on observation. We confirmed the dimensions given by [5] for the dwarf planet Haumea. We found that for a given tilt of the planet’s rotation axis, there are enough measurable Fourier coefficients to determine the dimensions of the planet in each of our applications. The only exception we found is a spheroidally-shaped exoplanet at edge-on observation without tilt. In that case, we do not have enough information to differentiate between the flattening and the size of the exoplanet.Electrical Engineering, Mathematics and Computer ScienceBsc Industrial and Applied Mathematics and Applied Physic
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