Techniques d'observation spectroscopique d'astéroïdes

CHAPTER 2. WHY SPECTROSCOPY? 51
−9.5
−10
−10.5
log10(Flux)
−11
−11.5
−12
−12.5
Thermal emission
Reflected solar
−13
0 1 2 3 4 5 6 7
Wavelength [um]
tel-00785991, version 1 - 7 Feb 2013
Figure 2.5: The components of the radiation received from 1km square lunar mare area (dark basaltic plain
on Moon formed by ancient volcanic eruptions) having an albedo of 006, considering the average Earth-Moon
distance, phase angle 0, T = 395K. The flux is measured in Watts per square meter per micron. Source
McCord & Adams [1977]
The following relation can be written to describe the spectrum recorded by a detector on a
ground base observatory:
Y(λ)=[X(λ)·HA(λ)+T(λ)]·HT(λ) (2.9)
where Y(λ) is the radiation flux recorded by the spectrometer, X(λ) is the radiation flux from
the Sun, HA(λ) is the transfer function of the asteroid, HT(λ) include the transfer function of
the Earth atmosphere and of optical instrument and T(λ) is the thermal infrared emission of
the asteroid
In the VNIR spectral region (0.40, 2.50) µm the thermal emission of the asteroids can be
neglected compared to the reflected radiation for the majority of asteroids. Thus in this spectral
region, the reflection spectra are studied.
In some particular cases, some asteroids become warm enough such that the thermal flux
can not be ignored. These are low-albedo NEAs that become warm enough to emit detectable
thermal flux at 2.5 µm when they are located near perihelion. In this case the thermal radiation
can account for 33% of the total flux for an object with an albedo 0.04 at 1.0 AU. Rivkin et al.
[2005] defined a quantity called "thermal excess" to describe this phenomenon:
γ = R 2.5+ T 2.5
R 2.5
− 1 (2.10)
where R 2.5 is the reflected flux at 2.5 µm and T 2.5 is the thermal flux at 2.5 µm. Usually R 2.5 is
determined by extrapolating a linear continuum from shorter wavelengths up to 2.5 µm. It was
shown that the lower albedo give larger values of γ as also do smaller solar distances. Beyond
1.9 AU, the expected thermal excess is close to zero for all modeled albedo [Rivkin et al.,