Techniques d'observation spectroscopique d'astéroïdes

122 CHAPTER 7. SPECTRAL PROPERTIES OF MAIN BELT ASTEROIDS
tel-00785991, version 1 - 7 Feb 2013
homogeneous bodies with ellipsoidal shapes. This model allows the simultaneous fit of grain
density and the bulk porosity. The author calculated a grain density of (2,790±380)kg/m 3 in
agreement to the one of Vesta-like asteroids, correlated to a bulk porosity of 63% (≈55% of
macroscopic porosity +≈8% of microporosity).
For the laboratory spectra proposed by M4AST to match this asteroid spectrum, the majority
corresponds to achondrite meteorites, subtype Basaltic HED -Eucrite. This fully agrees
with the classification as a V-type asteroid. The first solution given by all four methods for
comparison with laboratory spectra shows that the spectrum of (854) Frostia is almost identical
with the spectrum of ”ALHA76005,85” meteorite. Other relevant solutions of this application
are the spectra of samples from Basaltic HED -Eucrite meteorites: ”Y − 793591,90”,
”ALH− 78132,61” (Fig. 7.2c, 7.2d, Table 7.4). Another significant result of the spectral comparison
was the fact that the first matches correspond to particulate samples with sizes less than
25 µm. This suggests that (854) Frostia is covered by fine grains of regolith.
The meteorite number 5 discovered in 1976 in Allan Hills -Antartica (”ALHA76005,85”)
was study in many papers [Olsen et al., 1978, Simon & Papike, 1983, Miyamoto et al., 1979].
Olsen et al. [1978] noted about this meteorite that is a pale gray in color and consists of a
finely divided mycrocrystalline pyroxene rich matrix that contains clastic fragments: white
Plagioclase rich rocks, grey clasts of glass, monominerallic fragments of pyroxenes, silica,
Fe
oxide minerals, sulfides and metal. On a plot of CaO against
Fe+Mg
, is placed in the middle of
the eucrite field.
The mineralogy of (854) Frostia could be refined by taking into account the precise position
of the band minima, band centers at 1 and 2 µm and the band area ratio (BAR). To estimate
these parameters the mineralogical models can be applied [Cloutis et al., 1986a, 1990]
using M4AST routines on the composed V+NIR spectrum.
The first minimum (BI minimum)
is found at 0.9309±0.0015 µm, while the BII minimum is located at 2.0049±0.0046
µm, implying a band separation of 1.0740 µm. These parameters fit in the empirical formula
y = 0.801∗x−0.536, where y is the band separation and x is the BII minimum. They
correspond to an iron content of around 55 wt%, according to the calibrations shown by
de Sanctis et al. [2011b]
After removing the continuum by considering a linear function for each band it can be found
the band centers at 0.9355±0.0012 µm for the first band, respectively 1.9972±0.0038 µm for
the second band. In the case of BII, the thermal correction can be computed using the formulas
(2) and (4) from Burbine et al. [2009]. The value found 0.002 µm is closely to the value of the
error-bar for BI center, thus its influence can be neglected. The positions of BI and BII centers
are relatively similar to those obtained for the asteroid (1459) Magnya [Hardersen et al., 2004].
If these values obtained for Frostia are placed in the context of the pyroxene studies of Adams
[1974] and Cloutis & Gaffey [1991] it can be concluded a dominant presence of orthopyroxene
on the asteroid surface. The position of the bands place the asteroid near the Eucrite region (see