the pit floor and is therefore a natural throttling mechanism and challenging to the
concept (although Mousis et al. (2015) have argued otherwise).
Third, the compressive strength of cometary material might be very low—at least
locally. If the material really does have a strength in the range of 10s of Pascal, then
the overburden pressure on sub-surface material might be sufficient to induce
collapse or promote collapse following a trigger. Using the density and gravitational
acceleration of 67P as an example, 20 Pa pressure would be generated at a depth of
~230 m and comparable to the tensile strength (Attree et al. 2018). The deepest pits
are of this order. However, the difference between compressive strength and tensile
strength is of particular importance here and, as we noted above, the compressive
strength is extremely ill-defined. It is conceivable that the Seth region is highly
porous with low compressive strength and could not support the overburden.
Elsewhere it is either stronger or it has already collapsed to a level at which the
internal strength can support it.
Mechanisms for initiating collapse were explored by Vincent et al. (2015)
including the possibility of having large scale voids in the interior. Three concepts
for producing voids were proposed namely
– Voids might be primordial inherited from formation,
– Voids may result from evolutionary processes through direct sublimation of super
volatiles,
– Voids may come from sublimation at depth triggered by a secondary source of
energy such as the amorphous to crystalline transition.
However, the sounding experiment on Rosetta found no evidence for voids larger
than metre-sized near the Philae landing site (Hérique et al. 2019) and the Radio
Science Investigation (RSI) has found no evidence for large-scale inhomogeneities
in the interior (Pätzold et al. 2016). This is in direct contradiction to the morphological evidence that shows these pits are concentrated in specific regions.
Vincent et al. also speculated that there may be an age-depth relationship with
deep pits being younger. The unpitted surface gradually erodes with time because of
normal sublimation processes to reach the pit-bottom level when the structure is fully
evolved. This would suggest pits in Ma’at and parts of Seth as being young, the
circular pit-like structures in Seth and Ash as being older and the circular structures
in regions such as Maftet and parts of Hatmehit (Fig. 2.59) being very old. The pits
seen on other comets would fit in this chronology.
The material under the floor of the depressions has been shown to be ice-rich
(Pajola et al. 2017) and characterized by an albedo >0.4. An outburst event on 67P
observed by the Rosetta spacecraft navigation camera shortly before perihelion was
subsequently shown to be related to the loss of material from the edge of a cliff in the
Seth region (Fig. 2.60). The cliff seems to have arisen from the removal of part of
one side of a circular depression roughly 650 m in diameter known as “Aswan”. The
floor was a candidate landing site for the Philae lander in preliminary discussions.
The structure gave the appearance of being cut in half creating a steep cliff around
150 m high from the base of the pit down to an intermediate level before descending
2.10 Surface Appearance and Cometary “Geology”
127
concept (although Mousis et al. (2015) have argued otherwise).
Third, the compressive strength of cometary material might be very low—at least
locally. If the material really does have a strength in the range of 10s of Pascal, then
the overburden pressure on sub-surface material might be sufficient to induce
collapse or promote collapse following a trigger. Using the density and gravitational
acceleration of 67P as an example, 20 Pa pressure would be generated at a depth of
~230 m and comparable to the tensile strength (Attree et al. 2018). The deepest pits
are of this order. However, the difference between compressive strength and tensile
strength is of particular importance here and, as we noted above, the compressive
strength is extremely ill-defined. It is conceivable that the Seth region is highly
porous with low compressive strength and could not support the overburden.
Elsewhere it is either stronger or it has already collapsed to a level at which the
internal strength can support it.
Mechanisms for initiating collapse were explored by Vincent et al. (2015)
including the possibility of having large scale voids in the interior. Three concepts
for producing voids were proposed namely
– Voids might be primordial inherited from formation,
– Voids may result from evolutionary processes through direct sublimation of super
volatiles,
– Voids may come from sublimation at depth triggered by a secondary source of
energy such as the amorphous to crystalline transition.
However, the sounding experiment on Rosetta found no evidence for voids larger
than metre-sized near the Philae landing site (Hérique et al. 2019) and the Radio
Science Investigation (RSI) has found no evidence for large-scale inhomogeneities
in the interior (Pätzold et al. 2016). This is in direct contradiction to the morphological evidence that shows these pits are concentrated in specific regions.
Vincent et al. also speculated that there may be an age-depth relationship with
deep pits being younger. The unpitted surface gradually erodes with time because of
normal sublimation processes to reach the pit-bottom level when the structure is fully
evolved. This would suggest pits in Ma’at and parts of Seth as being young, the
circular pit-like structures in Seth and Ash as being older and the circular structures
in regions such as Maftet and parts of Hatmehit (Fig. 2.59) being very old. The pits
seen on other comets would fit in this chronology.
The material under the floor of the depressions has been shown to be ice-rich
(Pajola et al. 2017) and characterized by an albedo >0.4. An outburst event on 67P
observed by the Rosetta spacecraft navigation camera shortly before perihelion was
subsequently shown to be related to the loss of material from the edge of a cliff in the
Seth region (Fig. 2.60). The cliff seems to have arisen from the removal of part of
one side of a circular depression roughly 650 m in diameter known as “Aswan”. The
floor was a candidate landing site for the Philae lander in preliminary discussions.
The structure gave the appearance of being cut in half creating a steep cliff around
150 m high from the base of the pit down to an intermediate level before descending
2.10 Surface Appearance and Cometary “Geology”
127