or with different cations might have occurred at some point. Curiously, although no
large carbonate grains have been identified in Stardust samples from 81P/Wild 2,
numerous submicron carbonate grains, frequently intermixed with other mineral
grains, have been found (Westphal et al. 2017). The presence of sulphides in
81P/Wild 2 particles (Westphal et al. 2009) in the form of pyrrhotite and troilite
would seem to agree with interpretations of the Spitzer data during the Deep Impact
experiment (see below).
Carbonaceous chondrite meteorites are among the most pristine material known
in the Solar System and hence there may be some relationship to comets. Their
classification has been described by Sears and Dodd (1988). From the organic
composition point of view, CI and CM are the most interesting classes, in particular
because they contain up to 2 wt% of organic carbon. In laboratory studies, more than
80 different amino acids have been identified in the Murchison meteorite alone
including some that are rare in terrestrial environments and unquestionably of
extraterrestrial origin (Botta et al. 2007).
Remote-sensing observations of the composition of the resolved nucleus rely on
infrared spectroscopy. VIRTIS-M measurements of the surface of 67P suggest that it
consists of an assemblage of various organic components, minerals and the water ice
that could be deduced from the outgassing. Filacchione et al. (2019) suggested that
the organic compounds contain COOH and OH-groups and that a refractory macromolecular material bearing aliphatic (CH 2 and CH 3 ) and polycyclic aromatic hydrocarbons would fit the infrared reflectance spectra of 67P. They also suggested that
this material is mixed with minerals, including silicates, Fe-sulphides (pyrrhotite
and/or troilite) and possibly salts with ammonium cations (Altwegg et al. 2020; Poch
et al. 2020). However, both the resolution of the instrument and the intimate
mixtures of many compounds containing similar chemical groups inevitably leads
to ambiguity. The infrared reflectance spectra of 9P/Tempel 1 acquired by Deep
Impact show very little structure and ratios between icy and non-icy regions were
required to identify surface water ice with very little evidence of other absorptions in
the spectra.
The Deep Impact mission to 9P/Tempel 1 was complemented by an extensive
Earth-based observational campaign. Of specific interest here are the results from the
Spitzer Space Telescope observations at thermal infrared wavelengths (Lisse et al.
2006). A spectrum acquired 45 min after the impact is shown in Fig. 4.71. There are
numerous features evident that might be attributable to different minerals.
A factor of ten increase in the flux density at 8–12 μm is indicative of silicates and
specifically pyroxenes and olivines. Carbonates show emission features typically
appear at 6.5 to 7.2 μm, while the broad emission around 27 μm might be attributable
to sulphides. However, the features that one sees in the spectrum have limited
contrast and this can lead to ambiguity. Lisse et al. (2006) performed spectral fitting
to the data and provided a best fit composition but the result almost certainly has
significant degeneracy (e.g. Gicquel et al. 2012).
Harker et al. (2007) performed ground-based mid-infrared spectroscopy of the
9P/Tempel 1 Deep Impact ejecta and concluded that small dust grains ($0.2 μm) of
diverse mineralogy (amorphous olivine, amorphous pyroxene, amorphous carbon,
4.14 The Non-volatile Composition of Dust and the Nucleus
387
large carbonate grains have been identified in Stardust samples from 81P/Wild 2,
numerous submicron carbonate grains, frequently intermixed with other mineral
grains, have been found (Westphal et al. 2017). The presence of sulphides in
81P/Wild 2 particles (Westphal et al. 2009) in the form of pyrrhotite and troilite
would seem to agree with interpretations of the Spitzer data during the Deep Impact
experiment (see below).
Carbonaceous chondrite meteorites are among the most pristine material known
in the Solar System and hence there may be some relationship to comets. Their
classification has been described by Sears and Dodd (1988). From the organic
composition point of view, CI and CM are the most interesting classes, in particular
because they contain up to 2 wt% of organic carbon. In laboratory studies, more than
80 different amino acids have been identified in the Murchison meteorite alone
including some that are rare in terrestrial environments and unquestionably of
extraterrestrial origin (Botta et al. 2007).
Remote-sensing observations of the composition of the resolved nucleus rely on
infrared spectroscopy. VIRTIS-M measurements of the surface of 67P suggest that it
consists of an assemblage of various organic components, minerals and the water ice
that could be deduced from the outgassing. Filacchione et al. (2019) suggested that
the organic compounds contain COOH and OH-groups and that a refractory macromolecular material bearing aliphatic (CH 2 and CH 3 ) and polycyclic aromatic hydrocarbons would fit the infrared reflectance spectra of 67P. They also suggested that
this material is mixed with minerals, including silicates, Fe-sulphides (pyrrhotite
and/or troilite) and possibly salts with ammonium cations (Altwegg et al. 2020; Poch
et al. 2020). However, both the resolution of the instrument and the intimate
mixtures of many compounds containing similar chemical groups inevitably leads
to ambiguity. The infrared reflectance spectra of 9P/Tempel 1 acquired by Deep
Impact show very little structure and ratios between icy and non-icy regions were
required to identify surface water ice with very little evidence of other absorptions in
the spectra.
The Deep Impact mission to 9P/Tempel 1 was complemented by an extensive
Earth-based observational campaign. Of specific interest here are the results from the
Spitzer Space Telescope observations at thermal infrared wavelengths (Lisse et al.
2006). A spectrum acquired 45 min after the impact is shown in Fig. 4.71. There are
numerous features evident that might be attributable to different minerals.
A factor of ten increase in the flux density at 8–12 μm is indicative of silicates and
specifically pyroxenes and olivines. Carbonates show emission features typically
appear at 6.5 to 7.2 μm, while the broad emission around 27 μm might be attributable
to sulphides. However, the features that one sees in the spectrum have limited
contrast and this can lead to ambiguity. Lisse et al. (2006) performed spectral fitting
to the data and provided a best fit composition but the result almost certainly has
significant degeneracy (e.g. Gicquel et al. 2012).
Harker et al. (2007) performed ground-based mid-infrared spectroscopy of the
9P/Tempel 1 Deep Impact ejecta and concluded that small dust grains ($0.2 μm) of
diverse mineralogy (amorphous olivine, amorphous pyroxene, amorphous carbon,
4.14 The Non-volatile Composition of Dust and the Nucleus
387