A.-T. Auger et al.: Geomorphology of the Imhotep region on comet 67P
/Churyumov-Gerasimenko from OSIRIS observations
Fig. 2.
Upper panel: mosaic of six NAC images covering the Imhotep region (see Table
1
for the list of images). Letters A to F are reference names
for the accumulations basins (see text for details).
Lower panel: geomorphological mapping of the Imhotep region.
Smooth terrains cover about one third of the Imhotep re-
gion. Most of the smooth terrains are located in the local grav-
itational lows (Fig.
4
), and all of them are located in flat areas
where the gravitational slope is lower than 15
◦
(Fig.
5
). The
largest smooth area, located at the center of the region, covers a
surface of 0.8 km
2
. It is remarkably flat, with gravitational slopes
lower than 5
◦
and very close to the gravitational lowest point of
Imhotep (Fig.
4
). Although less extended, some smooth terrains
are also visible in the area of roundish features (Figs.
4
and
9
)
that are located in the gravitational low of the region.
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A&A 583, A35 (2015)
Table 1. NAC images used for the general mapping of the Imhotep region.
Image
Date
Distance to comet center (km)
Pixel scale (m)
NAC_2014-08-03T17.19.34.549
3 Aug. 2014
273.3
5.1
NAC_2014-08-25T23.12.54.550
25 Aug. 2014
52.1
1.0
NAC_2014-09-05T06.31.16.575
5 Sept. 2014
43.2
0.8
NAC_2014-09-05T06.35.55.557
5 Sept. 2014
43.2
0.8
NAC_2014-09-05T06.40.55.560
5 Sept. 2014
43.1
0.8
NAC_2014-09-05T06.45.55.557
5 Sept. 2014
43.1
0.8
Fig. 3.
Smooth terrains on Imhotep and zooms on the fine material
for two di
fferent areas. This image was acquired with the NAC cam-
era on 5 Oct. 2014 from a distance of 18 km. The spatial resolution
is 34 cm
/pix. (NAC_2014-10-05T16.09.11.)
Fig. 4.
Gravitational heights on Imhotep, shown in transparency above a
mosaic image of the region. Accumulation basins, boulders, and smooth
terrain boundaries are also represented. The yellow cross indicates the
gravitationally lowest point of the region.
The thickness of smooth terrains seems to vary across the
region, although we cannot quantify it. Upslope on the borders of
the basins, the thickness of the fine material gradually decreases,
revealing the underlying rocky terrains. Finally, smooth terrains
are cut by terrace margins (Figs.
2
and
3
) that can be gentle or
very steep.
3.2. Rocky terrains
Rocky terrains are so called to distinguish them from smooth
terrains. Rocky terrains consist of exposed, consolidated mate-
rials. As they are covered by a fine material in flat areas, most
Fig. 5.
Gravitational slopes on Imhotep, shown in transparency above
a mosaic image of the region. Smooth terrain boundaries and boulders
are also represented.
Fig. 6.
Rocky terrains with fractures. This image was acquired with the
NAC camera on 18 Sept. 2014 from a distance of 28 km. The spatial
resolution is 54 cm
/pix. (NAC_2014-09-18T05.22.28.)
of them are only visible at the peripheries of the Imhotep region
(Fig.
2
), on high gravitational slopes, up to 60
◦
(Fig.
5
). Rocky
terrains on Imhotep are made of a highly fractured material. The
fractures present di
fferent orientations (Fig.
6
), which are some-
times locally associated with a feature (Fig.
18
, Sect. 4.3). These
fractures are quite common on the nucleus of 67P and have been
observed in other regions (
Thomas et al. 2015b
).
The notable exception to these statements is the area of
roundish features located in the gravitational low of the region
(Fig.
4
),
where rocky terrains are still visible in a more chaotic
arrangement and do not show fractures at the meter scale (Figs.
2
and
9
).
A35, page 4 of
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A.-T. Auger et al.: Geomorphology of the Imhotep region on comet 67P
/Churyumov-Gerasimenko from OSIRIS observations
Fig. 7.
Color-composite images with zoom on the bright, bluer, ar-
eas. We used three filters: blue (480 nm), green (536 nm), and or-
ange (649 nm). The images were acquired with the NAC camera
on 5 Sept. 2014 from a distance of 43 km. The spatial resolution
is 81 cm
/pix. (NAC_2014-09-05T06.45 and NAC_2014-09-05T08.00.)
3.3. Accumulation basins
Morphologically, we define the accumulation basins as areas
where a fine material and boulders seem to accumulate preferen-
tially (Fig.
2
). These basins correspond to the gravitational lows
of the region (Fig.
4
). They are all shaped like an alcove and
have steep sides with gravitational slopes exceeding 40
◦
. Except
for basin F in the south, each basin opens toward a lower basin.
We detected four minor basins of 0.1–0.2 km
2
(basins A, B, C,
and F) and two larger ones of 0.8 km
2
each (basins D and E).
Basin D corresponds to the largest smooth terrain and basin E to
the area of roundish features. Accumulation basins cover two-
thirds of the Imhotep region and thus dominate the overall geo-
morphology of the region.
3.4. Bright areas
A few bright patches are visible in the images (Fig.
2
). They are
located at the basin edges. They are associated with boulders that
have recently been illuminated (they were in the dark before due
to large localized projected shadows from surrounding terrains),
but also with a scarp east of the region that has been illuminated
daily for several months (Fig.
7
). These patches are bluer than
the average color of the region, with a contrast of 7.5 at 550 nm
(V band) between the dark and bright terrains (see
Pommerol
et al. 2015
). This may be indicative of ice and water ice in par-
ticular (
Sunshine et al. 2006
;
Pommerol et al. 2015
). Water ice is
also supported by the lack of changes of these bright areas over
a period of several weeks, which excludes more volatile species
such as CO
2
ice (
Pommerol et al. 2015
). In the Imhotep region,
bright areas represent less than 1% of the total surface area.
3.5. Linear features
The Imhotep region presents linear features as slope breaks
(Fig.
2
). These linear features are long compared to the size of
the nucleus, typically hundreds of meters and up to 1 km for
Fig. 8.
Top and middle: linear features and terraces in the center of
the Imhotep region. This image was acquired with the NAC cam-
era on 5 Sept. 2014, from a distance of 43.5 km. The spatial reso-
lution is 80 cm
/pix. (NAC_2014-09-05T05.21.16). Bottom: zoom on
terraces cut by fractures. This image was acquired with the NAC cam-
era on 16 Sept. 2014 from a distance of 28 km. The spatial resolution
is 50 cm
/pix. (NAC_2014-09-16T01.17.56.)
the longest ones. All linear features are located near the gravi-
tational low of the region, with the exception of one linear fea-
ture in basin C. Several linear features cross the interface of the
rocky to the smooth terrain without discontinuities, suggesting
a continuity of the rocky terrain topography below the smooth
material (Figs.
2
and
8
). In the vicinity of basin F, linear features
correspond to roughly concentric scarps that can be described as
terraces (Fig.
8
). We identified about ten terraces, with a height
of 5
±2 m derived from the DTM (
Jorda et al. 2014
).
3.6. Roundish features
The roundish features identified in Fig.
2
(see also Fig.
9
)
are characterized by a circular, elliptical, or irregular roundish
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