A&A 583, A35 (2015)
Fig. 9.
Roundish features in the center of the Imhotep region. Upper
panel: examples of roundish features: (A) with a rim and a depres-
sion on top, (B) with a rim and a fine-material mesa on top, some-
times bulging and (C) multiple roundish features that are nested in or
are vertically stacked on top of each other, and (D) roundish feature
covered by fine material. 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.) Lower panel: anaglyph of
the roundish feature area from two NAC images acquired on 22 Nov.
2014 from a distance of 31 km. The spatial resolution is 56 cm
/pix.
(NAC_2014-11-22T06.52.)
shape, elevated relative to the surroundings. They are located in
the gravitationally lowest area of the Imhotep region and have
not yet been observed in any other region of 67P (
Thomas et al.
2015b
). They present a rim and at their top either a depression
(Fig.
9
, type A) or a mesa of a fine material that sometimes forms
a bulge (Fig.
9
, type B). Many of these roundish features appear
in groups that are nested in or are vertically stacked on top of
each other (Fig.
9
, type C). A few roundish features appear to
Fig. 10.
Size distribution of roundish features. We counted more
than 70, with sizes between 2 m (lower limit with our images) and 59 m.
Fig. 11.
Regions in the south of Imhotep that begin to be illuminated,
revealing additional roundish features. This image was acquired with
the NAC camera on 31 Oct. 2014 from a distance of 33 km. The spatial
resolution is 63 cm
/pix. (NAC_2014-10-31T14.19.35.)
be covered by a fine material (Fig.
9
, type D). Some roundish
features are also located on the terraces and more particularly on
the margins of the terraces close to basin F. From south to north,
roundish features seem to be more degraded and less filled with
a fine material.
With a spatial resolution of 50 cm
/pix (Fig.
9
), we identi-
fied more than 70 roundish features, with sizes (diameter) be-
tween 2 m (lower limit with our images) and 59 m (Fig.
10
).
Their size distribution is neither flat nor Gaussian or logarith-
mic, so that there does not seem to be a characteristic size for
these features. The general trend is a decrease of their number
for larger sizes, but this is subject to caution because of the low
statistic.
Finally, as 67P approaches perihelion, the southern part of
Imhotep starts to be illuminated, and additional roundish fea-
tures become visible (Fig.
11
).
3.7. Boulders
Similar to many other regions on 67P (
El-Maarry et al. 2015
;
Pajola et al. 2015
), Imhotep shows many boulders, with sizes
A35, page 6 of
13
A.-T. Auger et al.: Geomorphology of the Imhotep region on comet 67P
/Churyumov-Gerasimenko from OSIRIS observations
Fig. 12.
Cumulative size distribution of 2207 boulders, which follows a
power law with an exponent of 2.8
± 0.1.
(diameter) from 2 m (lower limit with our images) to 90 m. We
counted 2207 boulders in Fig.
2
. Their cumulative size distribu-
tion is shown in Fig.
12
and follows a power law with an expo-
nent of
−2.8 ± 0.1. It is complete down to 6 m in size.
Boulders are mainly located on the slopes surrounding the
regional gravitational low, and more particularly on southward
slopes in the western half of the region (Figs.
4
and
5
). Their
slope distribution peaks around 10
◦
, and most boulders are lo-
cated on intermediate slopes between 5
◦
and 25
◦
. There are
no boulders on slopes steeper than 50
◦
, and 98% of them are
located on gravitational slopes lower than 35
◦
(Fig.
13
, upper
panel). There are very few boulders on smooth and flat terrains
(slope < 3
◦
). Finally, there is no obvious correlation between the
size of the boulders and the gravitational slopes of the terrain on
which they stand (Fig.
13
, lower panel).
The boulders di
ffer not only in size, but also in texture. They
could either be conglomerate or highly fractured (Fig.
15
). If
they are fractured, fractures cross each other and do not have a
specific and unique orientation.
4. Discussion: geomorphology and processes
4.1. Smooth terrains
Smooth terrains can be considered as relatively undisturbed ar-
eas that evolve slowly and where material has time to settle and
accumulate. No fracture cuts through the smooth terrains as it
does in rocky terrains, which suggests that the smooth terrains
are more recent than the fractures or that they consist of loose
material that is unable to retain fractures.
The erosion of rocky terrains and boulders may form the fine
material constituting the smooth terrains. This eroded material
may originate from di
fferent regions on the nucleus: a) it can be
formed in situ, at the current location of the smooth terrain; b) it
can be transported by gravity from the borders of the Imhotep
basins; c) or it can be air-fall deposits from the coma following
Fig. 13.
Upper panel: histogram of the number of boulders as a func-
tion of the gravitational slope of the terrain on which they stand. Lower
panel: gravitational slope of the terrain on which boulders stand as a
function of their size.
an ejection processes in any other part of the nucleus. We explore
here several processes related to these di
fferent origins.
Thermal fatigue – smooth terrain on small bodies is not un-
common and has already been subject to several interpretations.
An explanation for the ponds on Eros is the in situ erosion of
boulders by thermal fatigue (
Dombard et al. 2010
). The ponds
and boulders in question are located at or near the equator of
Eros, similar to Imhotep on 67P, where the diurnal thermal cy-
cles are the strongest. However, smooth terrains around boulders
on Eros do not extend very far, typically a few boulder radii. It
therefore seems unlikely that the large amount of fine material
on Imhotep is entirely due to thermal fatigue.
Dust levitation –
Colwell et al.
(
2005
) adopted the idea of
dust levitation reported by
Lee
(
1996
) to provide another expla-
nation for the formation of dust ponds on Eros. Dust is trans-
ported in photoelectron layers and redeposited in shadow areas
or trapped in gravitational lows. If applicable to 67P, this process
only applies to particles smaller than
∼1 µm and thus only con-
cerns a small fraction (in mass) of the observed smooth terrain,
which includes larger particles up to the decimeter scale.
Air-fall deposits – on 67P, the sublimation process leads to
the ejection of particles that fall back onto the surface if they do
not reach the escape velocity. Their fall is guided by the gravi-
tational attraction and is then oriented toward the nearest grav-
itational low. However, this transport mechanism is limited to
boulders smaller than the meter scale because larger boulders are
A35, page 7 of
13