Light Scattering by Feldspar Particles: Modeling Laboratory Measurements

1 Department of Physics, University of Helsinki, Finland2 Institute of Astronomy, Kharkov National University, Ukraine3 Finnish Geodetic Institute, Finland4 Instituto de Astrofísica de Andalucía, CSIC, Spain5 Science Systems and Applications, Inc., USA6 US Army Research Laboratory, USA 7 Space Science Institute, USA

Evgenij Zubko1,2, Karri Muinonen1,3, Olga Muñoz4, Timo Nousiainen1, Yuriy Shkuratov2, Wenbo Sun5, and Gorden

Videen6,7

Laboratory measurements of single-scattering feldspar particles appear to be a huge challenge for modeling

Data adapted from Volten et al. 2001: JGR 106, pp. 17375–17401

Data adapted from Dubovik et al. 2006: JGR 111, D11208

Results of fitting feldspar at 0.442 m

Data adapted from Dubovik et al. 2006: JGR 111, D11208

Results of fitting feldspar at 0.633 m

from Dubovik et al. 2006: JGR 111, D11208

Unfortunately, the parameters were different for the different fits, and

“Simultaneous inversions of scattering matrices measured at two wavelengths (0.442 m and 0.633 m) were not successful in that a reasonably good fit was not achieved. The root-mean-square (over all elements) fit for a single wavelength was about 7–10%, while for two wavelengths the root-mean-square fit did not drop below 20%.”

Little difficulties in modeling

from Dubovik et al. 2006: JGR 111, D11208

Besides, the best fits are obtained with a mixture of highly oblate and prolate spheroids. However, the feldspar particles look highly irregular with aspect ratio being somewhat about 1.

Little difficulties in modeling

Method: Discrete Dipole Approximation (DDA)

Concept: Modeling target with set of small sub-volumes

Advantage: Arbitrary shape and internal structure

Modeling feldspar with agglomerated debris particles

More details in, e.g., Zubko et al. 2009: JQSRT 110, pp. 1741–1749

Modeling feldspar with agglomerated debris particles

Features of agglomerated debris particles:

(1) Highly irregular

(2) Equi-dimensional

(3) Fluffy (packing density =0.236)

Modeling feldspar with agglomerated debris particles

Laboratory measurements of single-scattering feldspar particles

Refractive index m is estimated to be in range m = 1.5–1.6 + 0.001–0.00001i

Data adapted from Volten et al., 2001

SEM image of feldspar

Size distribution is retrieved with the laser diffraction method

SEM image of feldspar

Modeling laboratory measurements of feldspar

Refractive index: m = 1.5 + 0i

We consider the range of particle radii r from 0.21 m through 2.25 m

at =0.442 m: x=3–32

at =0.633 m: x=2.1–22.3

Size distribution: r–2.9

For each size parameter x, we consider a minimum of 500 samples of agglomerated debris particles in random orientations.

This makes our analysis being statistically reliable!

Size parameter: x = 2r/

Modeling laboratory measurements of feldspar at =0.442 m

Modeling laboratory measurements of feldspar at =0.633 m

We model light-scattering response measured in feldspar particles at two wavelengths 0.442 and 0.633 m. We utilize model of agglomerated debris particles and compute light scattering with the discrete dipole approximation (DDA).

Measurements can be satisfactorily reproduced under realistic assumptions on size distribution and refractive index of feldspar particles. Unlike spheroidal model, agglomerated debris particles can fit measurements at both wavelengths.

Summary