2.7 Visible Colour
The reflectance factor of most (solid) planetary surfaces is wavelength dependent
and absorptions characteristic of minerals can be found in spectra. Indeed, reflectance spectroscopy in the wavelength range 0.2–5 μm has proven to be the most
powerful technique for determining surface mineralogical composition by remote
sensing although thermal emission spectroscopy in the range 5–50 μm is also now an
established technique. For comets, however, spectra in what is loosely referred to as
the visible wavelength range (from 0.2 to 1 μm) appear to be fairly featureless. We
shall defer discussing composition and compositional changes using the full wavelength range to a later sub-section. But for the optical reflectance here it is necessary
to appreciate that the reflectances of cometary nuclei at optical wavelengths also
show wavelength dependence and that this dependence seems to vary between
comets.
In Fig. 2.21, we can see an image of the surface of 67P acquired by the Rosetta/
OSIRIS instrument. Three areas have been isolated corresponding to morphologically different types of material. The image sequence contained images over the full
wavelength range of OSIRIS. The reflectances for the three areas are shown in
Fig. 2.22 and a linear fit has been made to the data for the dusty material.
Fig. 2.20 The reflectance factor (“I/F”) of 1P/Halley as derived from Giotto/HMC data in the HMC
clear filter (λ ¼ 652.9 nm, Δλ ¼ 372.6 nm; Thomas and Keller 1990a). The values of reflectance
have been multiplied by 1000 to produce the contour labels. The observations were acquired at a
phase angle of 107
. Left: HMC image 3457. Right: HMC image 3491
66
2 The Nucleus
The reflectance factor of most (solid) planetary surfaces is wavelength dependent
and absorptions characteristic of minerals can be found in spectra. Indeed, reflectance spectroscopy in the wavelength range 0.2–5 μm has proven to be the most
powerful technique for determining surface mineralogical composition by remote
sensing although thermal emission spectroscopy in the range 5–50 μm is also now an
established technique. For comets, however, spectra in what is loosely referred to as
the visible wavelength range (from 0.2 to 1 μm) appear to be fairly featureless. We
shall defer discussing composition and compositional changes using the full wavelength range to a later sub-section. But for the optical reflectance here it is necessary
to appreciate that the reflectances of cometary nuclei at optical wavelengths also
show wavelength dependence and that this dependence seems to vary between
comets.
In Fig. 2.21, we can see an image of the surface of 67P acquired by the Rosetta/
OSIRIS instrument. Three areas have been isolated corresponding to morphologically different types of material. The image sequence contained images over the full
wavelength range of OSIRIS. The reflectances for the three areas are shown in
Fig. 2.22 and a linear fit has been made to the data for the dusty material.
Fig. 2.20 The reflectance factor (“I/F”) of 1P/Halley as derived from Giotto/HMC data in the HMC
clear filter (λ ¼ 652.9 nm, Δλ ¼ 372.6 nm; Thomas and Keller 1990a). The values of reflectance
have been multiplied by 1000 to produce the contour labels. The observations were acquired at a
phase angle of 107
. Left: HMC image 3457. Right: HMC image 3491
66
2 The Nucleus