The retreat of cliffs through progressive erosion on 67P seems to have resulted in
multiple landslide deposits suggesting that cliff collapse is an important process in
reshaping cometary surfaces (Lucchetti et al. 2020, in press). It has also been
proposed that rotational instabilities can induce avalanche-like behaviour. Steckloff
et al. (2016) have argued that such avalanches on 103P/Hartley 2’s were sufficient to
excavate down to CO 2 -rich material and activate the small lobe of its nucleus
producing the observed CO 2 outgassing.
2.10.8 Sedimentary Processes
One of the most important findings of the Rosetta mission was that re-accumulation
of non-escaping particles is a major process influencing the properties of the surface
layer (Thomas et al. 2015a). The process had been predicted in the 1990s by
Moehlmann (1994) where it was described as a “dust hail” phenomenon. This was
somewhat of a misnomer as hail arises from condensation of volatiles (water) in an
atmosphere. Here, dust particles are emitted from the surface, returning on quasiballistic trajectories to impact the surface at positions often remote from their origin.
The process has been described as “airfall” in analogy with volcanic processes on
Earth but the mechanism itself was accurately described by Moehlmann.
The process arises from there being a wide range of dust emission velocities with
some of these being lower than the escape velocity. Prior to Rosetta, it was accepted
that return of non-escaping particles to the surface would occur and may even act to
choke activity in the immediate proximity of an active region. However, the magnitude of the effect was totally underestimated.
There were several relevant observations made by Rosetta at 67P. First, the northfacing surfaces were usually smooth with an almost conformal coating of material
with particle sizes close to and below the resolution limits of the cameras. These
surfaces (most notably in the Ash and Ma’at regions) were bounded by rougher
material and the change in texture at the interface was abrupt. A good example of this
is shown in Fig. 2.78 where the smooth surface to the top left of the image almost
certainly arises from airfall deposits. The smooth appearance of the putative impact
crater in Ma’at (Fig. 2.56 centre) is a further indication of dust deposition and was
used to estimate the depth of the deposit to be around 5 m (although there are many
caveats associated with this number).
There have been high resolution observations of these surfaces (e.g. Fig. 2.79)
that show that the apparently smooth surface is made up of smaller particles close to
the resolution limit. Individual particles can be seen and a range of brightnesses is
evident. This implies that many of the particles are in the decimetre size range—a
size that has interesting consequences for dust ejection models as will be discussed
below.
Observations post-equinox showed that surfaces with faces directed towards the
south were mostly devoid of these smooth deposits. Keller et al. (2017) suggested
that the stronger activity from south-facing surfaces around perihelion, produced by
144
2 The Nucleus
multiple landslide deposits suggesting that cliff collapse is an important process in
reshaping cometary surfaces (Lucchetti et al. 2020, in press). It has also been
proposed that rotational instabilities can induce avalanche-like behaviour. Steckloff
et al. (2016) have argued that such avalanches on 103P/Hartley 2’s were sufficient to
excavate down to CO 2 -rich material and activate the small lobe of its nucleus
producing the observed CO 2 outgassing.
2.10.8 Sedimentary Processes
One of the most important findings of the Rosetta mission was that re-accumulation
of non-escaping particles is a major process influencing the properties of the surface
layer (Thomas et al. 2015a). The process had been predicted in the 1990s by
Moehlmann (1994) where it was described as a “dust hail” phenomenon. This was
somewhat of a misnomer as hail arises from condensation of volatiles (water) in an
atmosphere. Here, dust particles are emitted from the surface, returning on quasiballistic trajectories to impact the surface at positions often remote from their origin.
The process has been described as “airfall” in analogy with volcanic processes on
Earth but the mechanism itself was accurately described by Moehlmann.
The process arises from there being a wide range of dust emission velocities with
some of these being lower than the escape velocity. Prior to Rosetta, it was accepted
that return of non-escaping particles to the surface would occur and may even act to
choke activity in the immediate proximity of an active region. However, the magnitude of the effect was totally underestimated.
There were several relevant observations made by Rosetta at 67P. First, the northfacing surfaces were usually smooth with an almost conformal coating of material
with particle sizes close to and below the resolution limits of the cameras. These
surfaces (most notably in the Ash and Ma’at regions) were bounded by rougher
material and the change in texture at the interface was abrupt. A good example of this
is shown in Fig. 2.78 where the smooth surface to the top left of the image almost
certainly arises from airfall deposits. The smooth appearance of the putative impact
crater in Ma’at (Fig. 2.56 centre) is a further indication of dust deposition and was
used to estimate the depth of the deposit to be around 5 m (although there are many
caveats associated with this number).
There have been high resolution observations of these surfaces (e.g. Fig. 2.79)
that show that the apparently smooth surface is made up of smaller particles close to
the resolution limit. Individual particles can be seen and a range of brightnesses is
evident. This implies that many of the particles are in the decimetre size range—a
size that has interesting consequences for dust ejection models as will be discussed
below.
Observations post-equinox showed that surfaces with faces directed towards the
south were mostly devoid of these smooth deposits. Keller et al. (2017) suggested
that the stronger activity from south-facing surfaces around perihelion, produced by
144
2 The Nucleus