sides of the quasi-circular depressions that are so prevalent there. There is a
suggestion that the layers have different spectral properties (Ferrari et al. 2018). It
has also been suggested that the layering is indicative of a, possibly primordial,
internal layering that passes through the body and that the two lobes exhibit different
orientations of this layering (Massironi et al. 2015). This would be quite remarkable
in that it would contend that the object formed in such a way that planes could arise
(e.g. through large scale sedimentary processes). It is probably fair to say that this is
not universally accepted as it would require a force field (e.g. a gravitational field) to
produce defined layers over 2 km distances. This would suggest that the (sub-)nuclei
were formed within a far larger body that disrupted. On the other hand, there are
alternatives. Thermal processing on diurnal and annual scales may provide a means
of processing the surface layer to a certain depth (see e.g. Sunshine et al. 2016).
Changes in the orbit modify those depths and thus you have a mechanism for
modifying the surface down to a fixed depth that changes occasionally after a
close encounter with Jupiter. The sedimentation/airfall mechanism may also play a
role by generating layers that are given years to consolidate by a mechanism such as
sintering. This would support layering being a local phenomenon rather than a global
one.
2.10.9 Activity in Dust-Covered Areas
One might expect that the airfall deposit should contribute to the slowing and
eventual choking of activity on the northern hemisphere of 67P. Modelling of the
outgassing from the nucleus, however, suggests that, while there were inhomogeneities, the outgassing from the northern hemisphere was broadly speaking insolationdriven (i.e. proportional to the solar insolation; Bieler et al. 2015a, b). However,
evidence of surface changes in the smooth terrain was clearly evident as seen in
Fig. 2.84. This figure shows an area of Ma’at close to some of the pits and compares
observations made more than 7 months apart. In the later image (acquired in March
2015), the surface has taken on a mottled texture in several (marked) places when
compared to the data acquired in August 2014 (panel a). Panels c and d show the
bottom of a pit from the March 2015 observation in detail. Here, too, surface changes
are visible (panel c) and, when saturating the image to exposure low intensity levels,
it can be seen that the surface was locally active (panel d).
Dust coverage has not extinguished or prevented activity and if this material is
part of the original surface (it is hard to imagine otherwise) then the depth of the
airfall deposit must be very small. Indeed, stretching Fig. 2.84 panel a, the surface
changes seen later seem to be at a place that we observe to be slightly brighter in
August 2014. This argues that the airfall deposit is really very thin which would
seem to agree with what we have seen in Fig. 2.60.
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2 The Nucleus
suggestion that the layers have different spectral properties (Ferrari et al. 2018). It
has also been suggested that the layering is indicative of a, possibly primordial,
internal layering that passes through the body and that the two lobes exhibit different
orientations of this layering (Massironi et al. 2015). This would be quite remarkable
in that it would contend that the object formed in such a way that planes could arise
(e.g. through large scale sedimentary processes). It is probably fair to say that this is
not universally accepted as it would require a force field (e.g. a gravitational field) to
produce defined layers over 2 km distances. This would suggest that the (sub-)nuclei
were formed within a far larger body that disrupted. On the other hand, there are
alternatives. Thermal processing on diurnal and annual scales may provide a means
of processing the surface layer to a certain depth (see e.g. Sunshine et al. 2016).
Changes in the orbit modify those depths and thus you have a mechanism for
modifying the surface down to a fixed depth that changes occasionally after a
close encounter with Jupiter. The sedimentation/airfall mechanism may also play a
role by generating layers that are given years to consolidate by a mechanism such as
sintering. This would support layering being a local phenomenon rather than a global
one.
2.10.9 Activity in Dust-Covered Areas
One might expect that the airfall deposit should contribute to the slowing and
eventual choking of activity on the northern hemisphere of 67P. Modelling of the
outgassing from the nucleus, however, suggests that, while there were inhomogeneities, the outgassing from the northern hemisphere was broadly speaking insolationdriven (i.e. proportional to the solar insolation; Bieler et al. 2015a, b). However,
evidence of surface changes in the smooth terrain was clearly evident as seen in
Fig. 2.84. This figure shows an area of Ma’at close to some of the pits and compares
observations made more than 7 months apart. In the later image (acquired in March
2015), the surface has taken on a mottled texture in several (marked) places when
compared to the data acquired in August 2014 (panel a). Panels c and d show the
bottom of a pit from the March 2015 observation in detail. Here, too, surface changes
are visible (panel c) and, when saturating the image to exposure low intensity levels,
it can be seen that the surface was locally active (panel d).
Dust coverage has not extinguished or prevented activity and if this material is
part of the original surface (it is hard to imagine otherwise) then the depth of the
airfall deposit must be very small. Indeed, stretching Fig. 2.84 panel a, the surface
changes seen later seem to be at a place that we observe to be slightly brighter in
August 2014. This argues that the airfall deposit is really very thin which would
seem to agree with what we have seen in Fig. 2.60.
150
2 The Nucleus