H 2 O (Feldman et al. 2015). Neutral carbon (CI) emissions could also be attributed to
dissociation of CO 2 via a similar mechanism. Subsequently, Feldman et al. (2018b)
could state that distinct spectral signatures arising from multiple dissociative excitation processes were present in ALICE spectra. Electron impact processes are
sensitive to the electron temperature, T e , and, as we shall see below, T e varies
strongly with cometocentric distance so that results are very sensitive to the extrapolation of plasma properties measured at the spacecraft to the immediate vicinity of
the nucleus. This is especially true when the cometary activity is low and the contact
surface (see below) is close to the nucleus. Evidence for this has also been presented
by Bodewits et al. (2016) who used narrow-band filter observations with the OSIRIS
Wide Angle Camera. In the WAC, the filters were centred on the emission lines and
bands of various daughter species in the optical and near-UV (Keller et al. 2007).
The observations at 67P were also interpreted as indicating that the emissions
observed in the OH, OI(
1 D), CN, NH, and NH 2 filters were mostly produced by
dissociative electron impact excitation of parent species. A sudden decrease in
intensity levels was observed after equinox in March 2015, which was attributed
to decreased T e in the first few kilometers above the surface as the activity of the
nucleus increased and the size of the diamagetic cavity increased (see Chap. 5).
3.5.6 Spatial and Temporal Variations of Parent
and Daughter Species
We have seen in Sect. 3.4.9 that the temperature distribution across the surface can
lead to compositional changes in the outgassing. But this was insolation-driven. Is
there any evidence for chemical inhomogeneity in the coma driven by nucleus
compositional inhomogeneity?
A’Hearn et al. (2012) reviewed observations of the ratios of CO, CO 2 and H 2 O
production rates from various sources. Notable is that measurements of the same
comet under similar conditions can produce markedly different results. For example,
the CO 2 /H 2 O ratio of 22P/Kopff was measured five times by Ootsubo et al. (2012)
resulting in values ranging from 0.041 to 0.201 which suggests high temporal
variability.
Evidence for different spatial distributions of CO 2 and H 2 O species at the nucleus
of 9P/Tempel 1 was presented by Feaga et al. (2007) from infrared imaging
spectroscopy data acquired by Deep Impact. Finklenburg et al. (2014) attempted to
model the observations of H 2 O using a DSMC code and showed that there was some
sensitivity to the source distribution on the nucleus (Fig. 3.54). The CO 2 distribution
shown by Feaga et al. was markedly different suggesting a different source distribution. The situation at 103P/Hartley 2 (Fig. 3.2) appears to have been even more
extreme (A’Hearn et al. 2011) with CO 2 and H 2 O ice being emitted from the end of
the small lobe of the bi-lobate structure and H 2 O vapour being emitted from the
longer side.
3.5 Reaction Chemistry and the Extended Coma
269
dissociation of CO 2 via a similar mechanism. Subsequently, Feldman et al. (2018b)
could state that distinct spectral signatures arising from multiple dissociative excitation processes were present in ALICE spectra. Electron impact processes are
sensitive to the electron temperature, T e , and, as we shall see below, T e varies
strongly with cometocentric distance so that results are very sensitive to the extrapolation of plasma properties measured at the spacecraft to the immediate vicinity of
the nucleus. This is especially true when the cometary activity is low and the contact
surface (see below) is close to the nucleus. Evidence for this has also been presented
by Bodewits et al. (2016) who used narrow-band filter observations with the OSIRIS
Wide Angle Camera. In the WAC, the filters were centred on the emission lines and
bands of various daughter species in the optical and near-UV (Keller et al. 2007).
The observations at 67P were also interpreted as indicating that the emissions
observed in the OH, OI(
1 D), CN, NH, and NH 2 filters were mostly produced by
dissociative electron impact excitation of parent species. A sudden decrease in
intensity levels was observed after equinox in March 2015, which was attributed
to decreased T e in the first few kilometers above the surface as the activity of the
nucleus increased and the size of the diamagetic cavity increased (see Chap. 5).
3.5.6 Spatial and Temporal Variations of Parent
and Daughter Species
We have seen in Sect. 3.4.9 that the temperature distribution across the surface can
lead to compositional changes in the outgassing. But this was insolation-driven. Is
there any evidence for chemical inhomogeneity in the coma driven by nucleus
compositional inhomogeneity?
A’Hearn et al. (2012) reviewed observations of the ratios of CO, CO 2 and H 2 O
production rates from various sources. Notable is that measurements of the same
comet under similar conditions can produce markedly different results. For example,
the CO 2 /H 2 O ratio of 22P/Kopff was measured five times by Ootsubo et al. (2012)
resulting in values ranging from 0.041 to 0.201 which suggests high temporal
variability.
Evidence for different spatial distributions of CO 2 and H 2 O species at the nucleus
of 9P/Tempel 1 was presented by Feaga et al. (2007) from infrared imaging
spectroscopy data acquired by Deep Impact. Finklenburg et al. (2014) attempted to
model the observations of H 2 O using a DSMC code and showed that there was some
sensitivity to the source distribution on the nucleus (Fig. 3.54). The CO 2 distribution
shown by Feaga et al. was markedly different suggesting a different source distribution. The situation at 103P/Hartley 2 (Fig. 3.2) appears to have been even more
extreme (A’Hearn et al. 2011) with CO 2 and H 2 O ice being emitted from the end of
the small lobe of the bi-lobate structure and H 2 O vapour being emitted from the
longer side.
3.5 Reaction Chemistry and the Extended Coma
269