Structures similar in shape and form to those seen in Fig. 2.92 were also evident in
more consolidated regions associated with activity. An example is shown in an
image of the Bes region (Fig. 2.93).
Circular, expanding depressions often seem to be related to sublimation processes. Malin et al. (2001), for example, have shown that pits on the south residual
cap of Mars are expanding at rates that are measurable with repeated high resolution
imaging. These pits are almost perfectly circular but in this case the subliming
material is almost pure CO 2 while on 67P we appear to have a complex mixture of
H 2 O, CO 2 and refractory materials.
2.10.12 Other Circular Structures
In the Imhotep region, we can see a series of circular structures in positive relief with
rims (Fig. 2.94). Deep Impact observations of 9P/Tempel 1 show pits with rims
(Thomas et al. 2013a) that look very similar to these structures. Thomas et al.
(2013a) concluded that volatile loss and transport from depth with on-going
Fig. 2.91 Oblique view of the edge of the smooth surface and the quasi-circular crater-like feature
in Imhotep. Note the cliff from the smooth surface down to the region with small circular structures
(Image number: N20160130T141149683ID10F22)
2.10 Surface Appearance and Cometary “Geology”
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