A&A 583, A35 (2015)
Fig. 1.
Overview of the Imhotep region, located on the largest lobe of
the nucleus, whose largest dimension is 4.1 km. This image was ac-
quired with the NAC camera on 3 Aug. 2014 from a distance of 282 km.
The spatial resolution is 5.3 m
/pix. (NAC_2014-08-03T16.19.34.)
In this paper, we focus on the Imhotep region (Fig.
1
), as
defined in
Thomas et al.
(
2015b
) and
El-Maarry et al.
(
2015
),
which is very interesting for various reasons. This region is lo-
cated close to the equator and is relatively flat compared to the
overall shape of the nucleus, so that it is illuminated daily from
aphelion (5.7 AU) to perihelion (1.2 AU) and never enters polar
night during the comet revolution around the Sun. It is a complex
region with a broad diversity of geomorphological features, the
most striking of which are a large smooth area of 0.8 km
2
and
several round features located in the gravitational lows. These
particular illumination conditions and overall geomorphology
render Imhotep a good candidate for being an active region when
approaching perihelion (
Keller et al. 2015
). This point is fur-
ther reinforced by the observations of
Vincent et al.
(
2013
), who
suggested the equatorial region as the source of the strongest
jet observed on 67P close to perihelion. Finally, Imhotep may
be representative of the regions of the southern hemisphere that
have not yet been illuminated and imaged, but will exit polar
night in March 2015 and be fully illuminated at perihelion in
August 2015.
The aim of this paper is to describe the geomorphology of
the Imhotep region using the images of the OSIRIS cameras on-
board Rosetta (Sects. 2 and 3) and to propose di
fferent scenarios
and processes for the formation and evolution of the observed
structures (Sects. 4 and 5).
2. Data and tools
2.1. Images
The OSIRIS instrument is composed of two cameras: a Narrow
Angle Camera (hereafter NAC) and a Wide Angle Camera
(Keller et al. 2007). This work is based on the analysis of the
NAC images of the Imhotep region acquired from 3 August 2014
to 22 November 2014. For the regional mapping, we used im-
ages with a spatial resolution between 5.0 m
/pix and 0.8 m/pix
(Table
1
), while for the local analysis we used images with a
better spatial resolution, up to 34 cm
/pix.
2.2. Gravitational heights and slopes
On Earth, the upslope and downslope directions are intuitive
as gravity points towards the Earth’s center to a very good
approximation. On small bodies of a few kilometers such as 67P,
the very irregular shape and the rotation strongly influence the
shape of the equipotential lines, and the direction of slopes is
no longer intuitive (
Thomas 1993
). On 67P, we calculated the
gravity vector of a surface point by taking into account the grav-
ity of the body itself, derived from its tridimensional shape
(
Gaskell et al. 2008
;
Jorda et al. 2014
) assuming a homoge-
neous nucleus with a density of 470 kg
/m
3
(
Sierks et al. 2015
),
and the centrifugal force resulting from its rotation (
Werner &
Scheeres 1996
). To emphasize the role of the gravitational forces
in the formation and evolution of the Imhotep morphologies, we
used gravitational heights or “dynamic heights” as defined by
Thomas
(
1993
). The gravitational heights are calculated as fol-
lows (
Vanicek & Krakiwsky 1986
;
Thomas 1993
):
H
=
W
1
− W
0
g
0
,
(1)
where
W
1
and W
0
are the gravitational potential energies per unit
mass of the measured point (W
1
) and reference point (W
0
) and g
0
is the local gravity at the reference point.
2.3. Tools
The images and digital terrain models (DTM) were imported
into the ArcGis 10.2 software. We used ArcGis for mapping
and projections (e.g., Fig.
2
), area and size measurements (e.g.,
Fig.
13
) and downslope direction estimate (e.g., Fig.
14
).
3. Geomorphology of the Imhotep region
The Imhotep region is located close to the equator and exhibits a
wide variety of features that are di
fferent in texture, morphology,
and photometric properties (Fig.
2
). These variations hold clues
to understanding the processes that shaped the surface and the
underlying structure of the object. The first step to unravel these
processes is to map the surface, grouping terrains with similar
properties (landforms, texture, albedo) into morphological units
that may have experienced a similar formation and evolution.
The nature, location, and stratigraphic arrangement of the mor-
phological units provide insights into their history and ultimately
constrain the formation and evolution scenario of the comet.
We used six NAC images to map the Imhotep region. They
were assembled to cover the entire region with a spatial resolu-
tion from 0.8 m
/pix to 5.1 m/pix (Fig.
2
a and Table
1
). From this
geomorphological mapping (Fig.
2
b), we identified
– two types of terrains: smooth (Sect. 3.1) and rocky
(Sect. 3.2),
– two specific areas: accumulation basins (Sect. 3.3) and bright
spots (Sect. 3.4), and
– three remarkable morphological features: linear features
(Sect. 3.5), roundish features (Sect. 3.6), and boulders
(Sect. 3.7).
3.1. Smooth terrains
Smooth terrains are striking on Imhotep and are characterized
by a material that is spatially unresolved in images with a pixel
scale of 1 m (Fig.
2
). However, images with a better spatial res-
olution of 34 cm
/pix show a textured surface in some areas or
directly allow detecting individual grains in other areas (Fig.
3
).
They show that the material in the smooth terrains consistes of
relatively fine grains, with a size of up to a few tens of centime-
ters for the largest ones. There are spatial inhomogeneities in the
size distribution of grains in the smooth areas.
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