shows that the gas velocities over the whole nightside can be factors of several below
those on the dayside over most of the hemisphere.
Finally, we can make a computation of the gas flow field for 67P incorporating
both H 2 O and CO 2 using a more physically realistic thermal model to initialise the
source. An example of such a computation is shown in Fig. 3.42. These diagrams
show 2D cuts through a 3D domain. We can see on the left a spherical nucleus as a
test case and on the right, a model based on 67P. In both models, we can see how the
CO 2 dominates the nightside emission and reduces the dayside to nightside gas
density gradient. In the 67P model, we can also see the influence of geometry. There
are strong afternoon jets in the CO 2 distribution resulting from extended high solar
incidence. The plots also indicate that the CO 2 flow field can be extremely complex
when the shape of the nucleus is irregular.
3.5 Reaction Chemistry and the Extended Coma
3.5.1 Daughter Products and the Haser Model
Molecules emitted from the nucleus into the inner coma immediately begin to
undergo reactions through numerous pathways (Table 3.9). Photo-ionization,
photo-dissociation, charge-exchange, and electron impact ionization all influence
gas densities even very close to the nucleus and hence the density equations need
modification when the influence on parent species begins to become significant. This
was well-known at an early stage in comet research. These reactions influence the
coma composition and can provide molecules and newly created radicals with
additional energy.
The major species are influenced by photo-dissociation reactions for which the
rates are given in Table 3.10. The difference in rates between the major species spans
two orders of magnitude and consequently the relative abundances of the major
species in the coma change rapidly beyond a few thousand kilometres from the
nucleus.
Spectroscopy at visible wavelengths in the 1950s revealed that radicals such as
CN and OH were present in gas comae. A more recent example from the bright
comet C/1996 B2 Hyakutake that came relatively close to the Earth in 1996 is shown
in Fig. 3.44. The absence of strong emissions in the visible from putative parents of
these radicals led to increased emphasis on the study of these daughter products. The
Haser (1957) model was developed for computations of densities arising through
these processes and allows one to avoid discussion and modelling of reaction
kinetics by treating the reaction chemistry in terms of scalelengths arising from the
expansion of the gas.
As the daughter products are produced, the parent is lost, and in the simplest case,
an exponential decay function is used so that, the total number of parent molecules
crossing a spherical surface, q p , at a distance, r, from a point source is given by
3.5 Reaction Chemistry and the Extended Coma
251
those on the dayside over most of the hemisphere.
Finally, we can make a computation of the gas flow field for 67P incorporating
both H 2 O and CO 2 using a more physically realistic thermal model to initialise the
source. An example of such a computation is shown in Fig. 3.42. These diagrams
show 2D cuts through a 3D domain. We can see on the left a spherical nucleus as a
test case and on the right, a model based on 67P. In both models, we can see how the
CO 2 dominates the nightside emission and reduces the dayside to nightside gas
density gradient. In the 67P model, we can also see the influence of geometry. There
are strong afternoon jets in the CO 2 distribution resulting from extended high solar
incidence. The plots also indicate that the CO 2 flow field can be extremely complex
when the shape of the nucleus is irregular.
3.5 Reaction Chemistry and the Extended Coma
3.5.1 Daughter Products and the Haser Model
Molecules emitted from the nucleus into the inner coma immediately begin to
undergo reactions through numerous pathways (Table 3.9). Photo-ionization,
photo-dissociation, charge-exchange, and electron impact ionization all influence
gas densities even very close to the nucleus and hence the density equations need
modification when the influence on parent species begins to become significant. This
was well-known at an early stage in comet research. These reactions influence the
coma composition and can provide molecules and newly created radicals with
additional energy.
The major species are influenced by photo-dissociation reactions for which the
rates are given in Table 3.10. The difference in rates between the major species spans
two orders of magnitude and consequently the relative abundances of the major
species in the coma change rapidly beyond a few thousand kilometres from the
nucleus.
Spectroscopy at visible wavelengths in the 1950s revealed that radicals such as
CN and OH were present in gas comae. A more recent example from the bright
comet C/1996 B2 Hyakutake that came relatively close to the Earth in 1996 is shown
in Fig. 3.44. The absence of strong emissions in the visible from putative parents of
these radicals led to increased emphasis on the study of these daughter products. The
Haser (1957) model was developed for computations of densities arising through
these processes and allows one to avoid discussion and modelling of reaction
kinetics by treating the reaction chemistry in terms of scalelengths arising from the
expansion of the gas.
As the daughter products are produced, the parent is lost, and in the simplest case,
an exponential decay function is used so that, the total number of parent molecules
crossing a spherical surface, q p , at a distance, r, from a point source is given by
3.5 Reaction Chemistry and the Extended Coma
251