Geomorphology of the Imhotep region on comet 67P/Churyumov-Gerasimenko from osiris observations
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- Fig. 19.
- 5. Conclusions
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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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 13 Document Outline
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