the subject of a fly-by by the Rosetta spacecraft 4 years before the spacecraft
encountered 67P (Thomas et al. 2012). Even at 60 m px
À1 , the surface of the asteroid
is covered with the evidence of impacts in most areas. The only exception is the
lower surface density of impact structures in a region referred to as Baetica (and
specifically the North Polar Crater Cluster or NPCC within this region) which is to
the left in Fig. 2.54. The region is assumed to have been impacted relatively recently
(within the past 1 billion years) and hence its surface is much younger.
As discussed in Sect. 2.4, crude energy balance calculations show that the surface
of a Jupiter family comet can lose between 1 and 10 m of material locally per
apparition although the average erosion over the surface is required to be a factor of
10 less than this to match the observed mass loss. The more detailed calculation in
Fig. 2.34 also indicates values of this order for 67P while Fig. 2.13 shows that
latitude-correlated variations in the erosion rates across the nucleus are to be
expected and this should be the case for all nuclei.
Larger impactors should result in the complete destruction of the nucleus
although gravitational re-accumulation of disrupted material has been shown to be
potentially important (Fig. 2.24) if the impact energy is not too high. On the
terrestrial planets (e.g. Mercury) craters smaller than about 10 km in diameter are
bowl-shaped and referred to as “simple” (see Pike 1988). The depth/diameter ratios
are, in these cases, between 0.15 and 0.2. Hence, if there was initially a 500 m
diameter crater on a comet, one could expect it to be eroded and lost on timescales of
perhaps 10–200 apparitions. For such a large crater in relation to the nucleus
dimensions and for the comet’s lifetime inside the orbit of Jupiter, this is an
extremely short time. The almost complete absence of unambiguously identifiable
impact craters on the surfaces of cometary nuclei is therefore unsurprising. Nonetheless circular structures that resemble conventional impact craters have been found
on 9P/Tempel 1 (Thomas et al. 2007) and 67P (Thomas et al. 2015a). Flat-bottomed
depressions were seen in abundance on 81P/Wild 2 but these are probably not impact
structures (at least not conventional ones). Potentially similar structures were also
seen on 67P (Fig. 2.94).
The simulation of impacts in solid surfaces is already at a somewhat advanced
level. Codes such as iSALE (impact-Simplified Arbitrary Lagrangian Eulerian) are
now available (Amsden et al. 1980). iSALE is a multi-material, multi-rheology
shock physics code (sometimes called a hydrocode). It is now well-established and
has been used in studies of the formation of large impact craters on the Earth. This
has been extended to planetary physics and used to investigate the influence of target
property variations on crater formation, the influence of a water layer on crater
formation, as well as investigating the mobility of large rock avalanches. It has been
expanded over the past 30 years and a 3D version of the code is freely available
3 for
non-commercial use (Elbeshausen et al. 2009).
There are however some issues with applying hydrocodes to cometary nuclei.
The principal problem is that the material properties of the target are very poorly
3 https://isale-code.github.io
2.10 Surface Appearance and Cometary “Geology”
121
encountered 67P (Thomas et al. 2012). Even at 60 m px
À1 , the surface of the asteroid
is covered with the evidence of impacts in most areas. The only exception is the
lower surface density of impact structures in a region referred to as Baetica (and
specifically the North Polar Crater Cluster or NPCC within this region) which is to
the left in Fig. 2.54. The region is assumed to have been impacted relatively recently
(within the past 1 billion years) and hence its surface is much younger.
As discussed in Sect. 2.4, crude energy balance calculations show that the surface
of a Jupiter family comet can lose between 1 and 10 m of material locally per
apparition although the average erosion over the surface is required to be a factor of
10 less than this to match the observed mass loss. The more detailed calculation in
Fig. 2.34 also indicates values of this order for 67P while Fig. 2.13 shows that
latitude-correlated variations in the erosion rates across the nucleus are to be
expected and this should be the case for all nuclei.
Larger impactors should result in the complete destruction of the nucleus
although gravitational re-accumulation of disrupted material has been shown to be
potentially important (Fig. 2.24) if the impact energy is not too high. On the
terrestrial planets (e.g. Mercury) craters smaller than about 10 km in diameter are
bowl-shaped and referred to as “simple” (see Pike 1988). The depth/diameter ratios
are, in these cases, between 0.15 and 0.2. Hence, if there was initially a 500 m
diameter crater on a comet, one could expect it to be eroded and lost on timescales of
perhaps 10–200 apparitions. For such a large crater in relation to the nucleus
dimensions and for the comet’s lifetime inside the orbit of Jupiter, this is an
extremely short time. The almost complete absence of unambiguously identifiable
impact craters on the surfaces of cometary nuclei is therefore unsurprising. Nonetheless circular structures that resemble conventional impact craters have been found
on 9P/Tempel 1 (Thomas et al. 2007) and 67P (Thomas et al. 2015a). Flat-bottomed
depressions were seen in abundance on 81P/Wild 2 but these are probably not impact
structures (at least not conventional ones). Potentially similar structures were also
seen on 67P (Fig. 2.94).
The simulation of impacts in solid surfaces is already at a somewhat advanced
level. Codes such as iSALE (impact-Simplified Arbitrary Lagrangian Eulerian) are
now available (Amsden et al. 1980). iSALE is a multi-material, multi-rheology
shock physics code (sometimes called a hydrocode). It is now well-established and
has been used in studies of the formation of large impact craters on the Earth. This
has been extended to planetary physics and used to investigate the influence of target
property variations on crater formation, the influence of a water layer on crater
formation, as well as investigating the mobility of large rock avalanches. It has been
expanded over the past 30 years and a 3D version of the code is freely available
3 for
non-commercial use (Elbeshausen et al. 2009).
There are however some issues with applying hydrocodes to cometary nuclei.
The principal problem is that the material properties of the target are very poorly
3 https://isale-code.github.io
2.10 Surface Appearance and Cometary “Geology”
121