A.-T. Auger et al.: Geomorphology of the Imhotep region on comet 67P
/Churyumov-Gerasimenko from OSIRIS observations
– depleted in fine-material deposits in contrast to all other
gravitational lows on Imhotep and on the nucleus, and it is
– located close to terraces. Processes that can form round fea-
tures on the surface are a) impact cratering; b) collapsing; or
c) cometary activity.
Impact cratering can explain the rim observed for the roundish
features, with the compression induced by this process, but it
cannot explain the cylindrical shape or the bulging top of some
roundish features. In a similar way, the collapse of a primordial
cavity cannot explain the cylindrical rimmed shape. These two
processes, impact cratering and collapsing, seem excluded. We
remain with cometary activity as the most plausible process that
formed the roundish features.
Brownlee et al.
(
2004
) proposed that the cylindrical shape of
the roundish features might be an ancient gas conduit consoli-
dated by the deposit of harder particles like water ice on its walls.
Walls have been exposed to the surface following the erosion of
the surrounding terrains.
Belton & Melosh
(
2009
) developed this
idea of gaseous conduit with their concept of a “spouting flow”
of CO and CO
2
gas rising up from the comet interior to the sur-
face and leading to outbursts.
Roundish features are located close to terraces, suggesting
that they are closely linked. Following the idea of
Brownlee et al.
(
2004
), we propose that roundish features are indeed ancient de-
gassing conduits that have been revealed by the di
fferential ero-
sion of pre-existing layers (Fig.
19
). Their consolidated walls al-
low their shape to be preserved and also prevent the retreat of the
layers, which explains why some roundish features are located
at the margins of terraces and why some are only barely visi-
ble (type D in Fig. 9). The erosion explains the depletion in fine
material and boulders in this region. Northern roundish features
are more degraded, which indicates that they may have been ex-
posed for a longer time to solar illumination. Moreover, terraces
are orientated toward the north. These two observations suggest
that the erosion front proceeds from north to south. The source
of gas at the origin of the conduit formation and the reason why
the erosion is – or was – more e
fficient in basin E are still un-
clear, but the passage of the comet at perihelion may bring us
answers.
Finally, we also mention the strong similarity of the roundish
features on Imhotep with those observed on the nucleus of comet
9P
/Tempel 1 (Fig.
20
). They share similar morphological proper-
ties, are all located in a gravitational low and have similar sizes,
from some tens of meters to 350 m for the largest ones (
Thomas
et al. 2007
).
4.6. Bright areas
If bright patches, which are bluer than their surroundings, do
indeed reveal the presence of water ice (
Pommerol et al. 2015
;
Capaccioni et al. 2014
), bright areas should be the youngest part
of the region, as they are the most unstable. The presence of
bright patches further reinforces the idea that the basins have
been enlarged by the progressive retreat of their borders; this re-
treat is more e
fficient at the perihelion passage since it is driven
by sublimation. Figure
7
shows that the bright areas are indeed
associated with mass wasting, but also with collapsing pieces of
the surrounding rocky terrain. From these observations, and in
particular because of the icy scarp visible in the east-southeast
of the region (Fig.
7
, bottom), it seems likely that most if not
all rocky terrains do contain a large fraction of water ice, hidden
under a layer depleted in volatiles. It is not possible, however,
to estimate the amount of water ice and the thickness of the de-
pleted layer from our observations.
Fig. 19.
Scenario for the formation and evolution of roundish features as
ancient degassing conduits. A) Initial nucleus surface, covered by fine
material. B) Formation of active degassing conduits, resulting in the
erosion of the fine material. C) Activity of degassing conduits stops and
fine material progressively covers them. D) Di
fferential erosion starts
on the surrounding terrains, revealing ancient degassing conduits (no
longer active) from north to south. E) Current state: the ancient conduits
are more or less degraded depending on how long they were exposed to
solar illumination. Fine material still covers or hides the less exposed
conduits.
4.7. Boulders
Boulders on Imhotep are mostly related to mass wasting. The
mass wasting of the northern and western part of Imhotep are
clearly associated with the surrounding scarps, and more partic-
ularly so on the southward slopes (Fig.
15
). The profusion of
boulders in the east may be mass wasting from the eastern high-
lands, but the top of the mass wasting is flat, which makes this
A35, page 11 of
13
A&A 583, A35 (2015)
Fig. 20.
Roundish features on comet 9P
/Tempel 1, from tens of meters
to 350 m for the largest ones.
hypothesis doubtful. An in situ conglomerate that is eroding and
then reveals its constitutive boulders seems more likely.
As discussed in previous paragraphs, boulders likely are the
erosion products of rocky terrains. But rocky terrains can be a
homogeneous material or a conglomerate, so that boulders might
also be remobilized boulders. We do not intend to answer the
question of the primordial or evolutionary nature of boulders
here, but it is clear that many boulders tend to be cut and recut
by fractures.
The origin of the large isolated boulders in the middle of
some smooth terrains is a key point (Fig.
2
). With a size of tens
of meters, they cannot be air-fall deposits (Sect. 4.1), unless a
large, highly speculative outburst were assumed. We prefer the
scenario where these boulders are the remnants of a previous
mass wasting at the foot of the previous scarp location, when
basins were less wide. They have slightly sunk since, as material
accumulated around them. They do not show a dust deposit on
their top, which further reinforces the idea that air-fall deposits
only account for a small fraction of the smooth terrains.
5. Conclusions
We have shown that the Imhotep region is of double interest: its
location near the equator, which makes it representative of a re-
gion illuminated from aphelion to perihelion that is even poten-
tially active at perihelion, and its wide variety of morphologies,
which allows examining a broad panel of processes. Although it
is very low, gravity plays a significant role in the formation and
evolution of the regional geomorphology, implying mass wast-
ing and transport of materials. Cometary processes are respon-
sible for the formation of roundish cylindrical features by gas
pressure and contribute to the erosion of the cli
ffs by sublima-
tion, with the highest intensity expected at perihelion.
This overview of the geomorphology of Imhotep and of the
processes responsible for its landscape allow us to propose a sce-
nario for the formation and evolution of this region:
Cyclic and on-going processes:
1. Formation of basins – the collapse of large cavities, tens of
meters or more, leads to the formation of basins. These cav-
ities are primordial voids, resulting from the nucleus forma-
tion process. Some first-generation basins may no longer be
visible today.
2. Enlargement and infill of basins – the sublimation of ices
leads to the erosion of basins and to their progressive infill
with fine material and boulders by mass wasting.
3. Formation of smooth terrains – the degradation of boulders
and fine material from mass wasting, plus air-fall deposits,
leads to the accumulation of smooth material in gravita-
tional lows. Fracturing probably exacerbates this degradation
process.
Transient events:
1. Formation of basin F – the formation of this large structure
and associated fractures was triggered by either an impact or
the rising up of a gas bubble from the interior of the nucleus.
2. Formation of roundish features – these cylindrical features
are probably ancient degassing conduits revealed by the dif-
ferential erosion of surrounding less compacted materials.
The layers suggested by the terraces around basin F probably
predate the formation of this basin. They might either be pri-
mordial, resulting from the nucleus formation process, or result
from an ancient evolutionary process.
This scenario implies a general flattening of the region to-
ward smooth terrains, a process similar to that observed on
Earth, where old geological regions tend to be flatter than young
regions. The next step to further constrain this scenario is to de-
tect and monitor changes on the Imhotep region with Rosetta,
as 67P approaches perihelion. A fundamental question is where
erosion occurs today on Imhotep, if it does at all.
Acknowledgements. OSIRIS was built by a consortium of the Max-Planck-
Institut für Sonnensystemforschung, Göttingen, Germany, CISAS-University
of Padova, Italy, the Laboratoire d’Astrophysique de Marseille, France, the
Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the Research and
Scientific Support Department of the European Space Agency, Noordwijk, The
Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain,
the Universidad Politéchnica de Madrid, Spain, the Department of Physics and
Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik und
Kommunikationsnetze der Technischen Universität Braunschweig, Germany.
The support of the national funding agencies of Germany (DLR), France
(CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical
Directorate is gratefully acknowledged. We thank David Romeuf from the
University Claude Bernard Lyon 1 (France) for creating the red and blue
anaglyph in Fig.
9
.
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1
Aix-Marseille
Université,
CNRS,
LAM
(Laboratoire
d’Astrophysique de Marseille) UMR 7326, 13388 Marseille,
France
e-mail: anne-therese.auger@lam.fr
2
Laboratoire GEOPS (Géosciences Paris Sud), Bât. 509, Université
Paris Sud, 91405 Orsay Cedex, France
3
Institut de Mécanique Céleste et de Calcul des Éphémérides,
UMR 8028, 77 avenue Denfert Rochereau, 75014 Paris, France
4
Planetary Science Institute, Tucson, AZ 85719, USA
5
Physikalisches Institut, Sidlerstr. 5, University of Bern, 3012 Bern,
Switzerland
6
Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen,
Germany
7
Department of Physics and Astronomy, Padova University, Vicolo
dell’Osservatorio 3, 35122 Padova, Italy
8
Centro de Astrobiologia (INTA-CSIC), 28691 Villanueva de la
Canada, Madrid, Spain
9
International Space Science Institute, Hallerstrasse 6, 3012 Bern,
Switzerland
10
Scientific
Support
O
ffice, European Space Agency, 2201
Noordwijk, The Netherlands
11
Department of Physics and Astronomy, Uppsala University,
Box 516, 75120 Uppsala, Sweden
12
PAS Space Research Center, Bartycka 18A, 00716 Warszawa,
Poland
13
Institute for Geophysics and Extraterrestrial Physics, 38106
Braunschweig, Germany
14
Department of Astronomy, University of Maryland, College Park,
MD 20742-2421, USA
15
LESIA, Obs. de Paris, CNRS, Univ Paris 06, Univ. Paris-Diderot,
5 place J. Janssen, 92195 Meudon, France
16
LATMOS, CNRS
/UVSQ/IPSL, 11 boulevard d’Alembert, 78280
Guyancourt, France
17
University of Padova, CISAS, via Venezia 15, 35100 Padova, Italy
18
Department of Mech. Engineering University of Padova, via
Venezia 1, 35131 Padova, Italy
19
CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
20
UNITN, Universit di Trento, via Mesiano, 77, 38100 Trento, Italy
21
INAF–Osservatorio Astronomico, via Tiepolo 11, 34143 Trieste,
Italy
22
Instituto de Astrofisica de Andalucía (CSIC), Glorieta de la
Astronomía s
/n, 18008 Granada, Spain
23
Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489
Berlin, Germany
24
Institute for Space Science, Nat. Central Univ., 300 Chung Da Rd.,
32054 Chung-Li, Taiwan
25
Operations Department, European Space Astronomy Centre
/ESA,
PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain
26
Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302,
USA
27
INAF, Osservatorio Astronomico di Padova, 35122 Padova, Italy
28
Centro di Ateneo di Studi ed Attivitá Spaziali “Giuseppe Colombo”
(CISAS), University of Padova, 35131 Padova, Italy
29
Institut für Datentechnik und Kommunikationsnetze der TU
Braunschweig,
Hans-Sommer-Str.
66,
38106
Braunschweig,
Germany
30
University of Padova, Department of Information Engineering, Via
Gradenigo 6
/B, 35131 Padova, Italy
31
Instituto Nacional de Tecnica Aeroespacial, Carretera de Ajalvir,
p.k. 4, 28850 Torrejon de Ardoz, Madrid, Spain
A35, page 13 of
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Document Outline - Introduction
- Data and tools
- Images
- Gravitational heights and slopes
- Tools
- Geomorphology of the Imhotep region
- Smooth terrains
- Rocky terrains
- Accumulation basins
- Bright areas
- Linear features
- Roundish features
- Boulders
- Discussion: geomorphology and processes
- Smooth terrains
- Rocky terrains
- Accumulation basins
- Terraces
- Roundish features
- Bright areas
- Boulders
- Conclusions
- References
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