The transient appearance of surface ices was also evident in infrared spectra.
Filacchione et al. (2016b) identified CO 2 ice on the surface of a cometary nucleus for
the first time but, as can be seen in Fig. 2.107, the CO 2 was transient and had
disappeared less than 4 weeks later. 67P had not yet reached perihelion at this time
and hence the heat input to the surface element was still increasing.
While “pore” ice (ice present within pores of a non-volatile matrices or condensed
on the matrices) has frequently been offered as an explanation for the absence of pure
ice surfaces, the bright patches are indicative that “massive” ice (volumes dominated
by ice) do exist within cometary nuclei. This observation seems to be incompatible
with a constantly eroding, compositionally homogeneous, surface layer.
2.10.16 Other Surface Changes
For 67P, losing 0.1% of the mass (Sect. 2.3) implies a 0.1% loss in volume
corresponding to around 0.6 m of loss in radius if the mass loss is equally distributed
over the nucleus. The heat input as a function of latitude (Fig. 2.13) and the irregular
shape (Sect. 2.2) imply that a homogeneous distribution of loss is not to be expected.
We have seen evidence of surface changes during the Rosetta mission in, for
example, Figs. 2.88, 2.89, 2.94, 2.95, and 2.105 (cf El-Maarry et al. 2019). However,
there has not been any clear quantitative evidence of erosion presented to this point.
Figure 2.108 (left) shows part of the Anhur region which was studied by
Fornasier et al. (2017, 2019). The colour enhancement of this image is a cruder
version of that used by them. Fornasier et al. (2019) identified the formation of new
scarps in the vicinity and showed evidence of surface erosion locally of 14 Æ 2 m
during the perihelion passage. The formation of a 14 m deep cavity has also been
reported in the Khonsu region by Hasselmann et al. (2019).
Fornasier et al.’s observations also indicated significant colour differences. This is
evident in the image (Fig. 2.108 left) and in low spectral resolution filter data from
Rosetta/OSIRIS (Fig. 2.108 right). For the latter, the data from the sub-areas B to D
have been ratioed to sub-area A and normalized at 986 nm (Fornasier et al. (2017)
normalized at 535 nm). The strong blue colour comes from a 50% change in relative
reflectance in sub-area B. The control areas, A and C shows only 2–3% variations
with respect to each other which probably illustrates the accuracy with which the
relative reflectance can be determined. Sub-area D shows around 10% relative
absorption in the visible.
It is noteworthy that the blue material is in depressions although a cluster of bluer,
more “rocky” material (marked by the arrow in Fig. 2.108 left) can also be seen.
However, this blue material is highly localised and covers a relatively small total
area of the surface. It is very tempting to interpret the bluer material as ice-rich but
caution is warranted. Fine icy material should sublime fairly quickly (e.g. Fig. 2.106)
so why is the material on the floor blue whereas the surrounding cliffs that are
assumed to be sources of this material are not? Although higher ice content remains
the most plausible explanation, other possibilities such as particle size effects in
2.10 Surface Appearance and Cometary “Geology”
173
Filacchione et al. (2016b) identified CO 2 ice on the surface of a cometary nucleus for
the first time but, as can be seen in Fig. 2.107, the CO 2 was transient and had
disappeared less than 4 weeks later. 67P had not yet reached perihelion at this time
and hence the heat input to the surface element was still increasing.
While “pore” ice (ice present within pores of a non-volatile matrices or condensed
on the matrices) has frequently been offered as an explanation for the absence of pure
ice surfaces, the bright patches are indicative that “massive” ice (volumes dominated
by ice) do exist within cometary nuclei. This observation seems to be incompatible
with a constantly eroding, compositionally homogeneous, surface layer.
2.10.16 Other Surface Changes
For 67P, losing 0.1% of the mass (Sect. 2.3) implies a 0.1% loss in volume
corresponding to around 0.6 m of loss in radius if the mass loss is equally distributed
over the nucleus. The heat input as a function of latitude (Fig. 2.13) and the irregular
shape (Sect. 2.2) imply that a homogeneous distribution of loss is not to be expected.
We have seen evidence of surface changes during the Rosetta mission in, for
example, Figs. 2.88, 2.89, 2.94, 2.95, and 2.105 (cf El-Maarry et al. 2019). However,
there has not been any clear quantitative evidence of erosion presented to this point.
Figure 2.108 (left) shows part of the Anhur region which was studied by
Fornasier et al. (2017, 2019). The colour enhancement of this image is a cruder
version of that used by them. Fornasier et al. (2019) identified the formation of new
scarps in the vicinity and showed evidence of surface erosion locally of 14 Æ 2 m
during the perihelion passage. The formation of a 14 m deep cavity has also been
reported in the Khonsu region by Hasselmann et al. (2019).
Fornasier et al.’s observations also indicated significant colour differences. This is
evident in the image (Fig. 2.108 left) and in low spectral resolution filter data from
Rosetta/OSIRIS (Fig. 2.108 right). For the latter, the data from the sub-areas B to D
have been ratioed to sub-area A and normalized at 986 nm (Fornasier et al. (2017)
normalized at 535 nm). The strong blue colour comes from a 50% change in relative
reflectance in sub-area B. The control areas, A and C shows only 2–3% variations
with respect to each other which probably illustrates the accuracy with which the
relative reflectance can be determined. Sub-area D shows around 10% relative
absorption in the visible.
It is noteworthy that the blue material is in depressions although a cluster of bluer,
more “rocky” material (marked by the arrow in Fig. 2.108 left) can also be seen.
However, this blue material is highly localised and covers a relatively small total
area of the surface. It is very tempting to interpret the bluer material as ice-rich but
caution is warranted. Fine icy material should sublime fairly quickly (e.g. Fig. 2.106)
so why is the material on the floor blue whereas the surrounding cliffs that are
assumed to be sources of this material are not? Although higher ice content remains
the most plausible explanation, other possibilities such as particle size effects in
2.10 Surface Appearance and Cometary “Geology”
173