4.6.3 Advanced Dust Ejection Concepts
The difficulty in overcoming cohesive forces leads one to consider if there are other
mechanisms at work. There are several ideas that have not really been explored. I
give three examples. Firstly, there is a widespread belief that pressure cannot build
up underneath particles because the material is porous. This is probably incorrect as
illustrated in Fig. 2.87. Even if the surface layer is highly porous, there can be a
pressure gradient across it. It is merely a question of how far below the actual surface
the subliming front has to be to produce sufficient pressure to rupture the surface. If
sub-surface pressure can build up in this way then small-scale quasi-explosive events
may eject material. Agarwal et al. (2017) have invoked this idea, without substantiating the exact mechanism, in their explanation for an outburst recorded at 67P in
July 2016. Attainable pressures have been calculated in an idealised system by
Skorov et al. (2017) using a numerical simulation. They showed that water vapour
pressures close to 20 Pa (and thus comparable to the tensile strengths estimated for
the bulk material at 67P) can be reached at 5 mm depth if the particle size of the
non-volatile material is sufficiently small.
Secondly, the importance of the “super-volatiles”, CO and CO 2 , may be underestimated. Sub-surface pressure build-up may be driven by the super-volatiles in
some way rather than relying solely on H 2 O. In this respect, 103P/Hartley 2 may be
an extreme example where the super-volatile is so dominant that the water ice has no
chance to sublime during the ejection process. Skorov et al. (2017) also demonstrated that this may be of significance by studying CO 2 and CO. They showed
(Fig. 4.26) that even higher internal pressures than those seen for water vapour may
be reached with these super-volatiles.
Finally, ejected particles may just be large enough so that coherent forces are low.
However, this requires explanation of the measured particle size distribution, which
for 1P/Halley contained particles right down to the 10–100 nm size range (Fig. 4.22),
and an understanding of whether these large particles can be accelerated sufficiently
to produce the observed coma.
What the above discussion shows is that we are rather ignorant of the whole
ejection process and that additional effort and experiment appears to be necessary
(Thomas et al. 2019). This has been recognized in the past few years and some
progress has been made although we remain far from having clear answers. Indeed,
there may be processes at work that we have not yet identified. An example of the
challenges is given in Fig. 4.27 which is from the work of Poch et al. (2016a, b) and
is a schematic diagram of an observation made in the laboratory.
Water ice and a non-volatile organic tholin were mixed in two different ways. In
the first method, liquid water was mixed with the tholin before being sprayed
through a nebuliser and immediately frozen. The tholins were contained within the
ice particles in an “intra-mixture” (Fig. 4.27 left). In the second method, the ice
particles were produced first from pure liquid water and then simply mixed with the
tholins. This was referred to as an “inter-mixture”. The two types of sample were
then allowed to evolve by illuminating their surfaces in a vacuum chamber. The
4.6 The Lifting Dust Ejection Process
327
The difficulty in overcoming cohesive forces leads one to consider if there are other
mechanisms at work. There are several ideas that have not really been explored. I
give three examples. Firstly, there is a widespread belief that pressure cannot build
up underneath particles because the material is porous. This is probably incorrect as
illustrated in Fig. 2.87. Even if the surface layer is highly porous, there can be a
pressure gradient across it. It is merely a question of how far below the actual surface
the subliming front has to be to produce sufficient pressure to rupture the surface. If
sub-surface pressure can build up in this way then small-scale quasi-explosive events
may eject material. Agarwal et al. (2017) have invoked this idea, without substantiating the exact mechanism, in their explanation for an outburst recorded at 67P in
July 2016. Attainable pressures have been calculated in an idealised system by
Skorov et al. (2017) using a numerical simulation. They showed that water vapour
pressures close to 20 Pa (and thus comparable to the tensile strengths estimated for
the bulk material at 67P) can be reached at 5 mm depth if the particle size of the
non-volatile material is sufficiently small.
Secondly, the importance of the “super-volatiles”, CO and CO 2 , may be underestimated. Sub-surface pressure build-up may be driven by the super-volatiles in
some way rather than relying solely on H 2 O. In this respect, 103P/Hartley 2 may be
an extreme example where the super-volatile is so dominant that the water ice has no
chance to sublime during the ejection process. Skorov et al. (2017) also demonstrated that this may be of significance by studying CO 2 and CO. They showed
(Fig. 4.26) that even higher internal pressures than those seen for water vapour may
be reached with these super-volatiles.
Finally, ejected particles may just be large enough so that coherent forces are low.
However, this requires explanation of the measured particle size distribution, which
for 1P/Halley contained particles right down to the 10–100 nm size range (Fig. 4.22),
and an understanding of whether these large particles can be accelerated sufficiently
to produce the observed coma.
What the above discussion shows is that we are rather ignorant of the whole
ejection process and that additional effort and experiment appears to be necessary
(Thomas et al. 2019). This has been recognized in the past few years and some
progress has been made although we remain far from having clear answers. Indeed,
there may be processes at work that we have not yet identified. An example of the
challenges is given in Fig. 4.27 which is from the work of Poch et al. (2016a, b) and
is a schematic diagram of an observation made in the laboratory.
Water ice and a non-volatile organic tholin were mixed in two different ways. In
the first method, liquid water was mixed with the tholin before being sprayed
through a nebuliser and immediately frozen. The tholins were contained within the
ice particles in an “intra-mixture” (Fig. 4.27 left). In the second method, the ice
particles were produced first from pure liquid water and then simply mixed with the
tholins. This was referred to as an “inter-mixture”. The two types of sample were
then allowed to evolve by illuminating their surfaces in a vacuum chamber. The
4.6 The Lifting Dust Ejection Process
327