materials. These can have non-negligible compressive strength with 35% porosity
sandstones have a compressive strength in excess of 14 MPa (Palchik 1999).
Nonetheless, investigations of impacts into low compressive strength material
have also been performed with particular emphasis on ejectaless craters (Housen
and Holsapple 2012).
The term, ejecta blanket, describes the material from the crater launched by the
impact in such a way that it is deposited outside the periphery of the crater
depression. For cometary-sized bodies, the weak gravity field is insufficient to
prevent the ejecta escaping the body so that the mass retained by the object in any
ejecta blanket is small. However, Housen and Holsapple (2012) showed that impacts
into highly porous materials primarily drive material downward and outward into the
floor of the expanding transient crater causing permanent compaction of pore spaces.
As a percentage of crater volume, much less material is ejected upward than for
impacts into materials of lower porosity. Penetration into the interior is therefore
more significant. If the material is highly compressible, impactors will penetrate,
boring an impactor diameter-sized hole into the target. This effect has also been
illustrated by de Niem et al. (2018).
In the event that the target has a harder layer (e.g. a “crust”) above a lower
strength material (a possible model for comets), the harder layer reduces the amount
of ejected material (e.g. Jutzi et al. 2013) and the surface manifestation of the impact
is reduced. Such a scenario would be consistent with Deep Impact observations as
discussed in Sect. 2.8.2.
Impacts also generate heat and the presence of volatiles in comets including what
are sometimes referred to as super-volatiles (e.g. CO) might lead to interesting
consequences. However, the temperature rise resulting from an impact, while
non-negligible, is not very large. Even if all the kinetic energy of the impactor
(excluding any other losses) is made available to sublime water ice, a 2 km s
À1
impactor only sublimes roughly a mass of water equivalent to the mass of the
original impactor. The more significant influence on the comet of the impact is the
resulting compaction and local damage. Figure 2.55 shows the density change and
temperature rise seen in an impact simulation (2 km s
À1 impact velocity at 45
with
respect to the surface normal). The images show the values 4s after first contact.
The density increase within the crater is around a factor of 4 while the temperature
rise is only of the order of 10 K locally. In the case of water ice, restructuring of the
material might lead to sintering with time and a harder, icy, layer (or layers) within
the crater.
The best studied examples of possible impact craters on 67P can be seen in
Fig. 2.56. The structure on the left is in the Ash region and is a circular depression
around 350 m in diameter with no obvious ejecta blanket. The rim to the lower left is
layered with local slopes approaching 90
, indicative of some structural strength.
The structure shown in Fig. 2.56 (right) is in the Imhotep region. The central
depression is roughly circular and rimmed with the southern rim being more
pronounced. Layering is evident and gives the impression that the surroundings
have been eroded leaving a structure similar to a pedestal crater.
2.10 Surface Appearance and Cometary “Geology”
123
sandstones have a compressive strength in excess of 14 MPa (Palchik 1999).
Nonetheless, investigations of impacts into low compressive strength material
have also been performed with particular emphasis on ejectaless craters (Housen
and Holsapple 2012).
The term, ejecta blanket, describes the material from the crater launched by the
impact in such a way that it is deposited outside the periphery of the crater
depression. For cometary-sized bodies, the weak gravity field is insufficient to
prevent the ejecta escaping the body so that the mass retained by the object in any
ejecta blanket is small. However, Housen and Holsapple (2012) showed that impacts
into highly porous materials primarily drive material downward and outward into the
floor of the expanding transient crater causing permanent compaction of pore spaces.
As a percentage of crater volume, much less material is ejected upward than for
impacts into materials of lower porosity. Penetration into the interior is therefore
more significant. If the material is highly compressible, impactors will penetrate,
boring an impactor diameter-sized hole into the target. This effect has also been
illustrated by de Niem et al. (2018).
In the event that the target has a harder layer (e.g. a “crust”) above a lower
strength material (a possible model for comets), the harder layer reduces the amount
of ejected material (e.g. Jutzi et al. 2013) and the surface manifestation of the impact
is reduced. Such a scenario would be consistent with Deep Impact observations as
discussed in Sect. 2.8.2.
Impacts also generate heat and the presence of volatiles in comets including what
are sometimes referred to as super-volatiles (e.g. CO) might lead to interesting
consequences. However, the temperature rise resulting from an impact, while
non-negligible, is not very large. Even if all the kinetic energy of the impactor
(excluding any other losses) is made available to sublime water ice, a 2 km s
À1
impactor only sublimes roughly a mass of water equivalent to the mass of the
original impactor. The more significant influence on the comet of the impact is the
resulting compaction and local damage. Figure 2.55 shows the density change and
temperature rise seen in an impact simulation (2 km s
À1 impact velocity at 45
with
respect to the surface normal). The images show the values 4s after first contact.
The density increase within the crater is around a factor of 4 while the temperature
rise is only of the order of 10 K locally. In the case of water ice, restructuring of the
material might lead to sintering with time and a harder, icy, layer (or layers) within
the crater.
The best studied examples of possible impact craters on 67P can be seen in
Fig. 2.56. The structure on the left is in the Ash region and is a circular depression
around 350 m in diameter with no obvious ejecta blanket. The rim to the lower left is
layered with local slopes approaching 90
, indicative of some structural strength.
The structure shown in Fig. 2.56 (right) is in the Imhotep region. The central
depression is roughly circular and rimmed with the southern rim being more
pronounced. Layering is evident and gives the impression that the surroundings
have been eroded leaving a structure similar to a pedestal crater.
2.10 Surface Appearance and Cometary “Geology”
123