source region is fundamentally different in terms of physical properties—either the
dust to gas production rate ratio is larger or the emitted sizes are different when
compared to adjacent areas. Vincent et al. (2016a) traced back the structures from the
coma back to the nucleus to try to determine their footpoints and, by comparing the
distribution to the surface appearance, suggested that active cliffs on the nucleus may
be responsible for these structures. It was subsequently shown by Marschall et al.
(2017) by fitting of measured gas densities in the inner coma that Rosetta data could
not distinguish between a model with insolation-driven sublimation and a comparable model with only cliffs being active. Consequently, the idea is consistent with
available data. The only challenge to this hypothesis is that cliffs are emitting
roughly tangentially to the main body of the nucleus. In order to become (roughly)
radial as observed the emitted material must be forced into that direction presumably
by gas flow.
A third possibility is that the fine structure represents the result of local minima in
the production rate. We saw in Figs. 3.26 and 3.27 that if the Kn p is small (<1) then
interaction between adjacent active areas can occur. The interaction region can then
have locally higher densities. If the particles are well coupled to the gas, then an
enhancement in the dust density along the line of the interaction region can result.
This, rather counter-intuitive, idea was studied by Knollenberg (1994) (see also
Rauer 2010). A simple case was that of an inactive cap surrounded by uniformly
active regions. The flow of gas into the region above the inactive surface led to
enhancements in the local dust density above the inactive part of the nucleus. This is
instructive in that it illustrates that density enhancements in the dust coma do not
necessarily require local increases in production and indeed the opposite can be the
case. A similar concept was also investigated in 2-D by Finklenburg (2009) using
Bird’s DSMC code. Figure 4.39 shows how interaction between gas emitted from
separate sources can lead to maxima in the gas density above the inactive area
between them (see also Fig. 3.27). This concept requires that the dust is well coupled
to the gas and that the Knudsen penetration number is less than 1.
Another possibility is that we are seeing effects of topography. This was also
investigated by Knollenberg (1994) (see also Keller et al. 1994) using an Euler
equation solver and trace particles. The result of a similar calculation, using DSMC
for the gas in this case, is shown in Fig. 4.40. The calculation is based on a spherical
nucleus with a concave depression (or cavity) directly facing the Sun. The gas
production rate is set at 100 kg s
À1 with an insolation-driven distribution. The key
point is that the concave cavity focusses the outflow and fine structure is produced.
This illustrates that topography can strongly control the appearance of the innermost
coma.
It should be apparent that cliffs and larger scale roughness can generate these
types of structures. This raises the possibility that by tracing coma structures back to
cliffs, Vincent et al. (2016a) may have been identifying focussing by topography
rather than locally increased dust production.
Although this has not yet been investigated in detail, the dust brightness distribution along the jet axis should allow one to distinguish between the jet production
4.9 Observation of Non-uniform Dust Emission
345
dust to gas production rate ratio is larger or the emitted sizes are different when
compared to adjacent areas. Vincent et al. (2016a) traced back the structures from the
coma back to the nucleus to try to determine their footpoints and, by comparing the
distribution to the surface appearance, suggested that active cliffs on the nucleus may
be responsible for these structures. It was subsequently shown by Marschall et al.
(2017) by fitting of measured gas densities in the inner coma that Rosetta data could
not distinguish between a model with insolation-driven sublimation and a comparable model with only cliffs being active. Consequently, the idea is consistent with
available data. The only challenge to this hypothesis is that cliffs are emitting
roughly tangentially to the main body of the nucleus. In order to become (roughly)
radial as observed the emitted material must be forced into that direction presumably
by gas flow.
A third possibility is that the fine structure represents the result of local minima in
the production rate. We saw in Figs. 3.26 and 3.27 that if the Kn p is small (<1) then
interaction between adjacent active areas can occur. The interaction region can then
have locally higher densities. If the particles are well coupled to the gas, then an
enhancement in the dust density along the line of the interaction region can result.
This, rather counter-intuitive, idea was studied by Knollenberg (1994) (see also
Rauer 2010). A simple case was that of an inactive cap surrounded by uniformly
active regions. The flow of gas into the region above the inactive surface led to
enhancements in the local dust density above the inactive part of the nucleus. This is
instructive in that it illustrates that density enhancements in the dust coma do not
necessarily require local increases in production and indeed the opposite can be the
case. A similar concept was also investigated in 2-D by Finklenburg (2009) using
Bird’s DSMC code. Figure 4.39 shows how interaction between gas emitted from
separate sources can lead to maxima in the gas density above the inactive area
between them (see also Fig. 3.27). This concept requires that the dust is well coupled
to the gas and that the Knudsen penetration number is less than 1.
Another possibility is that we are seeing effects of topography. This was also
investigated by Knollenberg (1994) (see also Keller et al. 1994) using an Euler
equation solver and trace particles. The result of a similar calculation, using DSMC
for the gas in this case, is shown in Fig. 4.40. The calculation is based on a spherical
nucleus with a concave depression (or cavity) directly facing the Sun. The gas
production rate is set at 100 kg s
À1 with an insolation-driven distribution. The key
point is that the concave cavity focusses the outflow and fine structure is produced.
This illustrates that topography can strongly control the appearance of the innermost
coma.
It should be apparent that cliffs and larger scale roughness can generate these
types of structures. This raises the possibility that by tracing coma structures back to
cliffs, Vincent et al. (2016a) may have been identifying focussing by topography
rather than locally increased dust production.
Although this has not yet been investigated in detail, the dust brightness distribution along the jet axis should allow one to distinguish between the jet production
4.9 Observation of Non-uniform Dust Emission
345