Techniques d'observation spectroscopique d'astéroïdes

118 CHAPTER 7. SPECTRAL PROPERTIES OF MAIN BELT ASTEROIDS
spectrum of 9147 Kourakuen is almost identical with the spectrum of a sample from Pavlovka
meteorite (Fig. 7.1c). This meteorite sample is of type achondrite howardite already studied
so far [Olsen et al., 1990, Labotka & Papike, 1980]. The bulk composition of the chondrules
from this meteorite contains SiO 2 (50.1%), MgO(23.7%), FeO(15%), Al 2 O 3 (6.2%),
CaO(3.8%) [Olsen et al., 1990].
Other meteorite laboratory spectra similar to the spectrum of (9147) Kourakuen are those of
the meteorites Roda (Achondrite Diogenite), Le Teilleul (Achondrite, Howardite) and Kapoeta
(Basaltic HED Howardite). The first fifty solutions that matched our spectrum are HED
(Howardite Eucrite Diogenite) meteorites. These are basaltic meteorites believed to result from
large asteroids that melted to form a metallic core and basaltic magma after the formation.
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Another solution of this application is a spectrum of a man-made mixture of Pyroxene Hypersthene
Plagioclase Bytownite Ilmenite (Fig. 7.1d). This man-made mixture reproduces quite
well the natural composition of volcanic rocks or melting rock of volcanic beds, and is consistent
to the V-type mineralogical composition of asteroids. In all laboratory spectra proposed
by M4AST to match this asteroid spectrum, the majority corresponds to HED achondrite meteorites.
While the standard deviation measures the overall matching between the two spectra, the
correlation coefficient finds those spectra for which the spectral features positions and shapes
are very close. In the case of spectrum of (9147) Kourakuen, a very high correlation coefficient
(more than 0.99) characterize the first matching solutions (Table 7.4).
Since only the NIR part of the spectrum is available, we can only compute the band minima.
The high signal to noise ratio of this spectrum ensures that there is a small error in computing
the band minima. The first minimum is at 0.9217 ± 0.0005 µm and the second minimum is at
1.9517±0.0062 µm, which imply a band separation of 1.03 µm. The band separation provides
a way of estimating the iron content. Cloutis et al. [1990] noted that the band separation is a
linear function of the BII minimum for orthopyroxenes and that both parameters increase with
the iron content. If we refer to the relation obtained by de Sanctis et al. [2011b], the parameters
that we found match their formula y = 0.801∗x−0.536, where y is the band separation and
x is the BII minimum. These parameters correspond to an iron content of around 40 wt%.
However, the laboratory calibrations suggest that the correspondence is true for a number of
low aluminum orthopyroxenes but invalid for mixtures of olivine, metal, and both ortho- and
clino-pyroxenes [de Sanctis et al., 2011b].
Concluding this section, based on an accurate near-infrared spectrum of the asteroid (9147)
Kourakuen a description of its surface composition was made. The comparisons with meteorites
spectra which revealed a spectral matching with HED type meteorites (in particular with
the spectrum of Pavlovka meteorite) agrees and is complementary to the taxonomical classification
and to the mineralogical solution found.