Techniques d'observation spectroscopique d'astéroïdes

52 CHAPTER 2. WHY SPECTROSCOPY?
2005].
2.4.2 Spectral features
Because minerals are characterized by unique compositions in specific crystallographic structures,
each mineral has a characteristic reflectance spectrum with different spectral features.
These features come from electronic and vibrational transitions within crystals or molecules.
The wavelengths at which the features are located in the spectrum depends upon the ionic (e.g,
Fe 2+ , Fe 3+ etc. ) or molecular (e.g, H 2 O, OH, CO 3 ) species involved and the mineral structure
[Gaffey et al., 1993a]. As a general rule, crystal field theory is used to explain absorption bands
in asteroid spectra.
1.2
1.3
tel-00785991, version 1 - 7 Feb 2013
Spectral Reflectance
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
(a)
1.3
1.2
Spectral Reflectance
1.1
1
0.9
0.8
Spinel
0.5 1 1.5 2 2.5
Wavelength [um]
Spectral Reflectance
1.2
1.1
1
0.9
0.8
0.7
0.6
(b)
1.2
1.15
Spectral Reflectance
1.1
1.05
1
0.95
Pyroxene
0.5 1 1.5 2 2.5
Wavelength [um]
0.7
0.6
Olivine
0.5 1 1.5 2 2.5
Wavelength [um]
0.9
Iron − Nickel
0.5 1 1.5 2 2.5
Wavelength [um]
(c)
(d)
Figure 2.6: Reflectance spectra of several meteoritic important minerals: a) Spinel, b) Pyroxene, c) Olivine, d)
Iron-Nickel alloy.
Considering the meteoritic minerals spectra as a starting point for analyzing asteroid spectra
it can be found that several features are detectable in the range of (0.35 - 2.50) µm. The
most abundant mineral species composing the meteorites are: olivine (Fig 2.6c), pyroxene