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| Titel |
A Monte Carlo model for the reflection of polarized light on surfaces |
| VerfasserIn |
D. Guirado, D. M. Stam, M. Smit |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250066237
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| Zusammenfassung |
| An accurate modeling of the reflection of polarized light on rough surfaces is crucial for
the interpretation of polarimetric observations of planets and moons, as well as of
remote sensing measurements on Earth. We have developed an accurate Monte Carlo
code to predict polarization signatures of sunlight reflected from the surface of a
planetary body. Using this code for predictions of polarization signatures of the Jovian
Moon Ganymede, we find that characteristic features near scattering angles around
90 deg. can mark small differences in refractive index, such as may result from
ice-stress.
As the phenomenon of reflection of light by a surface occurs locally, we assumed a
plane-parallel geometry for our problem. We first considered a surface as a layer of
compressed dust grains. A Monte Carlo (MC hereafter) technique was then applied in order
to account for multiple scattering and absorption of the individual photons entering the
medium. In the MC approach, packets of photons with an initial weight W = 1 are launched.
Each packet is split into two parts. One goes to the bottom of the surface or to infinity and is
subsequently lost from the packet. The other part undergoes scattering into the
surface. The optical thickness to the point of scattering is calculated according
to a probability density function depending on the distribution of particles. The
weight of the scattered part is reduced according to the single scattering albedo of
the scatterer. We follow the path of a photon until W < Wmin, and then another
one is launched. This MC scattering model has a limitation on the packing of the
particles because it is based on the hypothesis that each scatterer is in the far field
zone of the others. As a consequence, it is valid only for fluffy media. In order
to make a more realistic model, we included a specular (Fresnel) or Lambertian
reflection for light reaching the bottom of the layer of compressed dust. So the whole
model consists of a plane-parallel solid block that reflects light in a specular or
Lambertian way covered by a thin layer of compressed dust with a low packing
density.
The main purpose of this modeling is the interpretation of data collected by SPEX, the
Spectrometer for Planetary Exploration, that will be proposed to fly to Jupiter and Ganymede
on ESA’s JUICE mission. Due to mission restrictions, SPEX would orbit Ganymede in a
plane that is mostly perpendicular to the direction of the solar incident light. Due to this
constraint, observations will be performed only for scattering angles near 90 deg., but will
nevertheless allow us to retrieve information on the stress in the surface ice from the
observations by SPEX. If ice stress exists on a surface, different zones of the material are
under different pressures, which leads to slightly different refractive indices. We found out
that very important variations of the maximum of the curve of linear polarization
occur at scattering angles around 90 deg. for small changes of the refractive index. |
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