Issue |
A&A
Volume 410, Number 3, November II 2003
|
|
---|---|---|
Page(s) | 1055 - 1062 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361:20031317 | |
Published online | 17 November 2003 |
Optical constants of powdered limestone obtained by taking into account the grain shapes: Applicability to Martian studies
1
Space Research Centre, Polish Academy of Sciences, Bartycka 18a, 00-716 Warsaw, Poland e-mail: aaj@cbk.waw.pl
2
Department of Physics, University of Lecce, Via Arnesano, CP 193, 73100 Lecce, Italy e-mail: anna.cinzia.marra@le.infn.it; blanco@le.infn.it
Corresponding author: V. Orofino, orofino@le.infn.it
Received:
4
February
2003
Accepted:
14
August
2003
The modelling and the interpretation of infrared
spectra exhibited by astronomical dusty objects require fair acquaintance
with complex refractive indices, the
so-called “optical constants", of cosmic analog materials. It turns out
that the spectra of the latter, in case of a crystalline granular
material, depend on the size and the shape of the grains and may differ
from the spectra of the same material but in bulk form. This phenomenon
can be very elegantly accounted for by considering optical lattice
excitations specific to small particles, the so-called “surface modes".
We present a study of the spectral behaviour, in the 1.5–62.5 μm
range, of the optical constants of a particulate sample of limestone,
a typical carbonate material mainly composed of calcite
(CaCO).
Shape effects have been accounted for by considering a collection of
randomly oriented ellipsoids with various continuous distributions of
shape parameters.
It is shown that in the spectral region around the bands at
32 μm
and 44 μm, whose assignment to surface modes raises no doubt, the
optical constants derived for various shape distributions are markedly
different from each other. We find that the best agreement between laboratory
and theoretical spectra is obtained for spheres while for two continuous
distributions of ellipsoids the fits are slightly worse. In other words the
optical constants, that describe best the interaction between electromagnetic
radiation and our limestone sample, are those derived by using Mie theory
(valid for spheres); this is in agreement with SEM analysis which indicates a
spheroidal shape of the particles.
Such conclusions,
valid for limestone particles, cannot be extrapolated directly to other
particles and/or materials, since every case has to be treated
independently. They should nevertheless be helpful in avoiding the
possible error of interpreting absorption spectra of particulate
crystalline stuffs without taking into account the effects of particle
size and shape.
Key words: methods: numerical / planets and satellites: individual: Mars
© ESO, 2003
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