Space and ground based observations of the small body populations in the Solar System are continuously reshaping our understanding of how these objects were formed and evolved. New data and theoretical advances, as well as the discovery of interstellar objects and extrasolar comets give us new insights on the physical and dynamical properties of small bodies. The goal of this session is to highlight recent results from outer planetary system objects (comets, KBOs, Centaurs, interstellar, …) that provide fundamental clues about the early stages of planetary systems. We aim to explore the continuum of small bodies and the overlap between different populations through a balanced set of contributions from ground based observers and space missions (e.g. Rosetta, New Horizons)
Mon, 20 Sep, 10:40–11:25
Chairpersons: Jean-Baptiste Vincent, Michael Küppers, Alessandra Migliorini
Understanding cometary activity is key to exploring their materials and surface physics, with implications for comet formation and subsequent evolution. Outgassing produces a reaction force on nuclei that can alter their trajectory and spin, allowing constraints to be placed on comet models by measuring the observed orbit and rotation changes. For comet 67P/Churyumov-Gerasimenko, detailed measurements have been made by the Rosetta spacecraft and various attempts have been made to model the activity (see, e.g. [1,2]).
Here we will present ongoing work, testing various activity distributions in an effort to fit Rosetta outgassing, trajectory, and rotation data using the activity model of . We test a number of different activity distributions over the surface of the comet by varying the Effective Active Fraction (EAF), relative to pure water ice, of facets on a shape model. We investigate different spatial patterns in EAF, and attempt to correlate them to physical features on the cometary surface. Here we are able to achieve a good fit to the Rosetta data by parameterizing EAF in terms of the different geological unit types on 67P (Fig. 1). This will have important implications for understanding how activity works on the different types of surface observed on cometary nuclei, including ‘rough’, ‘smooth’, ‘dusty’ and ‘rocky’ surface morphologies.. The objective here is to constrain the shape of the activity curve on these various surfaces that a more detailed thermal model (see, for example [3,4]) must produce in order to fit the data.
In addition to the changes in rotation period examined in , we also compute changes in the rotation axis, using a method based on  in order to compare with the observations. This provides an additional constraint on the spatial distribution of activity. Finally, we also compare the trajectory information, in the form of Earth-to-comet range, with the new analysis of Rosetta radio-tracking data performed in .
Fig. 1. Peak Effective Active Fraction over the surface of 67P for a model fit to geological unit types.
- Nongravitational Effects of Cometary Activity. S. Mottola, N. Attree, L. Jorda, H.U. Keller, R. Kokotanekova, D. Marshall. Space Science Reviews 216 (1), 1-20
- Constraining models of activity on comet 67P/Churyumov-Gerasimenko with Rosetta trajectory, rotation, and water production measurements. N. Attree, L. Jorda, O. Groussin, S. Mottola, N. Thomas, Y. Brouet, E. Kührt. Astronomy & Astrophysics 630, A18
- On the activity of comets: understanding the gas and dust emission from comet 67/Churyumov-Gerasimenko’s south-pole region during perihelion. B. Gundlach, M. Fulle, J. Blum. Monthly Notices of the Royal Astronomical Society, Volume 493, Issue 3, April 2020, Pages 3690–3715
- Near-perihelion activity of comet 67P/Churyumov–Gerasimenko. A first attempt of non-static analysis. Yu. Skorov, H. U. Keller, S. Mottola and P. Hartogh. Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 3, May 2020, Pages 3310–3316
- Kramer, T., Laeuter, M., Hviid, S., et al. 2019. A&A, Volume 630, id.A3, 11 pp.
- Lasagni Manghi R., Zannoni M., Tortora P., Kueppers M., O'Rourke L., Martin P., Mottola S., et al., 2020, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19173
How to cite: Attree, N., Jorda, L., Groussin, O., Marschall, R., Lasagni Manghi, R., Tortora, P., and Zannoni, M.: Activity on different terrain types on comet 67P/Churyumov-Gerasimenko, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-17, https://doi.org/10.5194/epsc2021-17, 2021.
The Rosetta mission provided detailed data of the surface of the nucleus of comet 67P/Churyumov-Gerasimenko. The analysis of these data, and especially the images of the Narrow Angle Camera (NAC) from the Optical Spectroscopic and Infrared Remote Imaging System (OSIRIS instrument; Keller et al., 2007), revealed the morphological diversity of the nucleus surface (El-Maarry et al., 2019). Among these morphologies, depressions have been observed in several regions (Fig.1).
Figure 1: Example of studied depression located on Ma'at region (NAC image, 1 m/pixel). The white arrows indicate the depressions
The origin of these structures remains unclear and several hypotheses have been proposed: (1) the depressions could be indicative of scarp retreat (Vincent et al., 2016; El-Maary et al., 2017), (2) they mark the location of future cliff collapses (Pajola et al., 2016d), and (3) they are seasonal structures shaped by the changes induced by perihelion approach (Groussin et al., 2015). In a previous study, we studied two of these depressions, located in the Ash region, by a comparative morphometrical analysis (Bouquety et al., 2021). We observed that the two depressions grew by several meters during the last perihelion passage, and that this growth is not necessarily linked with cliff collapses. Thus, in that case, the sublimation of ices certainly played a key role in shaping these depressions.
On Earth and Mars, there are similar depressions with the same shape and geometry that are controlled by thaw processes. These depressions are called thermokarstic lakes on Earth and scallops depressions on Mars (Fig.2). On both planets, these periglacial structures result from the degradation of an ice rich permafrost (Costard and Kargel, 1995; Morgenstern et al., 2007; Séjourné et al., 2011).
Figure 2: Example of thaw depressions. (a) Thermokarstic lakes in Alaska on Earth (Digital Orthophoto Quadrangle DOQ, 5 m/pixel). (b) Scalloped terrain in Utopia planitia on Mars (HiRISE image, 50 cm/pixel).
Due to their processes and morphological similarities, we decided to compare the depressions observed on 67P’s surface with thermokarstic lakes on Earth and scallops depressions on Mars to constrain their origin.
Data and Method
We used the same method as in Bouquety et al., (2021). This comparative morphometrical analysis (CMA) allows to study surface features via a morphological and geometrical approach, with a great level of detail, in order to build an interplanetary database which can be used for comparison.
In order to perform the comparison, a list of parameters and criteria that can be applied on Earth, Mars and 67P. For each depression we measured 10 parameters: the length, width, area, perimeter, depth, slope (max, min, mean), elongation and the circularity index (Ulrich et al., 2010; Séjourné et al., 2011; Morgenstern et al., 2011; Niu et al., 2014). Based on different dataset and their associated DTM (Earth: DOQ/3DEP; Mars: HiRISE/HiRISE DTM; 67P: NAC/SPC method (Jorda et al.,2016)), we measured a total of 432 depressions, namely 200 on Mars (Utopia planitia), 101 on Earth (Arctic coastal plain) and 131 on the whole 67P’s surface.
Figure 3: Example of measurement. Gravitational: (a) slopes and (b) height draped on NAC images. (c) Topographic profile extracted from GH.
Results and interpretations
The depressions are exclusively located in terrains covered by fine deposit particules (FDP), and seems to be present in all topographical contexts. The highest depression densities are located on the body, where FPD covers the majority of the region (Thomas et al., (2018); Fig.4).
Figure 4: Density map of the measured depression according to the region.
The analysis also revealed that the set of measured parameters is consistent with the references known in the literature. Remarkably, all the measured parameters on 67P depressions are included in the range that characterized scallops terrains on Mars and thermokarstic lakes on Earth (Ulrich et al., 2010; Séjourné et al., 2011; Morgenstern et al., 2011; Niu et al., 2014). Moreover, depressions from 67P follow the same area/perimeter trend as scallops on Mars and thermokarstic lakes on Earth (Fig.5). Finally, more than 90% of 67P depressions topographic profiles show a slope asymmetry (Fig.1c). This slope asymmetry has been observed on thermokarstic lakes on Earth and scallops on Mars and interpreted to be characteristic of depressions shaped by the obliquity-driven insolation (Plescia, 2003; Morgenstern et al., 2007; Séjournée et al., 2011).
Figure 5: Area versus perimeter for all the measured depressions on 67P, Mars, and Earth.
These three results indicate that: (1) the depressions from 67P follow the same growth ratio than the scallops and the thermokarstic lakes while keeping their characteristic circularity, and (2) the sublimation induced by perihelion passages is the main erosion process that shaped these depressions on the comet (Bouquety et al., 2021). Our morphometrical analysis allowed to conclude that depressions on 67P are analogues to scalloped terrain on Mars and thermokarstic lakes on Earth.
Bouquety, A., Jorda, L., Groussin, O., et al. 2021, Astronomy & Astrophysics
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How to cite: Bouquety, A., Groussin, O., Jorda, L., Séjourné, A., Costard, F., and Bouley, S.: Evidence for scalloped terrains on 67P, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-25, https://doi.org/10.5194/epsc2021-25, 2021.
Introduction: The data from the Rosetta mission enabled the reconstruction of the shape of comet 67P/Churyumov-Gerasimenko (hereafter 67P) and the identification of the terrains and features forming its surface. The highly irregular shape of the comet poses a challenge for the depiction of these geological features on two-dimensional maps.
Techniques: Standard global map projections cannot display the complete surface of 67P because different points on the surface can have the same longitude and latitude. As a consequence, the geological maps published to date are created on top of comet images, making them dependent on the viewing angle and image coverage and resolution.
Here, we make use of the recently published Quincuncial Adaptive Closed Kohonen (QuACK) map . It projects the complete surface of 67P unambiguously onto a square. The QuACK map is topologically equivalent to the Peirce quincuncial projection of the world, which makes it possible to define generalized longitudes and latitudes. These can be used within any global map projection in order to obtain an unambiguous QuACK version.
Results: The mapping of geological features is carried out in three dimensions employing the Small Body Mapping Tool (SBMT), Fig. 1. We use images from the OSIRIS Narrow Angle Camera aboard Rosetta which have been projected onto the shape model of the SBMT, Fig. 2. The three-dimensional coordinates are then projected onto two-dimensional maps, either in the QuACK map projection or in the QuACK version of the equidistant cylindrical projection, Fig. 3. We present individual maps for 17 of the 26 regions of 67P, mostly located in the northern hemisphere. The new maps combine features published in previous studies with newly identified features.
We discuss the distribution of geological features and the characteristics of the regions. In order to align region boundaries with geological features, we propose two modifications of region definitions. Products are fully available to the community through the PSA Guest Storage Facility .
Fig. 1: OSIRIS image projected onto the shape model using the SBMT.
Fig 2: Identification of features in the Hatmehit region from images projected on the SHAP4S shape model and annotated on the SBMT.
Fig 3: Combined geological map of regions in North centered QuACK projection.
Acknowledgments: The authors thank the European Space Research and Technology Centre and the European Space Astronomy Center faculty councils for supporting this research. M. Leon-Dasi was supported by an internship from the European Space Agency (ESA) at the European Space Astronomy Center, Madrid, in the period March to December 2020.
References:  Grieger (2019) A&A, 630, doi:10.1051/0004-6361/201834841.  Besse, S. et al. (2017) Planetary and Space Science, 10.1016/j.pss.2017.07.013.
How to cite: Besse, S., Leon-Dasi, M., Grieger, B., and Kueppers, M.: Mapping a Duck: Geological Features and Region Definitions on Comet 67P/Churyumov-Gerasimenko, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-43, https://doi.org/10.5194/epsc2021-43, 2021.
We have analyzed high-resolution images of the Wosret region acquired at different wavelength in the 400-1000 nm range with the NAC camera of the OSIRIS imaging system on board the Rosetta mission. This region is located on the small lobe of the 67P/Churyumov-Gerasimenko comet and it is subject to strong heating during the perihelion passage. It includes the two last landing sites of the Philae lander, and notably Abydos, the final one where the lander performed most of its measurements.
Wosret has unique geomorphological features among the 26 regions of comet 67P showing a high erosion level, a pervasive fracturing, and an overall lack of dust deposits compared to other regions. It is also one of the most active regions, showing the highest estimated water production rate  and originating about 40 activity events, including one of the brightest outburst caught by Rosetta observations.
We observed a few morphological changes in Wosret, related to local dust coating removal with estimated depth of about 1 m, and the formation of a cavity measuring 30 m in length corresponding to a total mass loss of 1.2x106 kg. The spectrophotometry of the region is typical of medium-red regions of comet 67P, with spectral slope values of 15-16 %/(100 nm) in pre-perihelion data acquired at phase angle 60o. Few tiny bright spots are observed, but very small in size (1-1.5 m2). One of them is very bright and we estimated a local water icy enrichment up to 60%. Morphological features like goosebumps/clods are widely present in Wosret, and they appear consolidated, highly irregular in shape and height. Their measured equivalent diameter ranges from 2 to about 12 m, with an average value of 4.7±1.5 m.
By comparing the physical ad mechanical properties of Wosret with those of Anhur and Khonsu [2,3], two southern hemisphere regions located in the big lobe, highly active, and that experienced the same high heating level than Wosret, we highlight some differences between the two lobes:
1) First of all, the water ice-enriched regions directly exposed at the surface of Wosret are less frequent and smaller in size than those observed in Anhur, where water ice rich area of several squared meters and up to 1600 m2 were observed . Also, the spectrally bluer area ice-enriched by frost, usually found close to shadows, are less frequently observed in Wosret.
2) All these three regions are highly active, but the activity results in different surface re-shaping. In fact important morphological changes were observed in Anhur and Khonsu (new scarps, dust bank removal up to 14m in depth, vanishing structures of several tenth of meters in length, boulders displacement and fragmentation, cavities formation) corresponding to a total mass loss in the order of 108 kg [2,3]. Conversely, no major morphological changes are observed in Wosret except for the formation of a new cavity, and the dust coating removal tentatively estimated in about 1 m depth, locally. We discard observation biases because these regions were observed under similar spatial resolution and illumination conditions pre- and post-perihelion.
3) Polygonal block in Wosret are, on average, two times larger than the goosebumps/clods found in different regions of the big lobe. These structures have been interpreted as being representative of the original cometesimals forming, by aggregation, cometary nuclei, or as result of fracturing processes caused by seasonal and diurnal thermal gradients.
Morphological analysis of the 67P regions alone also points to differences in the physical and mechanical properties of the material composing the two lobes of 67P nucleus . The limited morphological changes and low exposure of volatiles observed in Wosret compared to Anhur/Khonsu indicate that the surface material on Wosret could be less fragile and more consolidated than the one composing the southern regions of the big lobe, and that the small lobe could have a lower volatile content, at least on its top layers, than the big lobe.
All these evidences support the hypothesis, formulated by Massironi et al.  from the analysis of the layering of two lobes, that comet 67P is composed of two distinct bodies that merged during a low-velocity collision in the early Solar System.
References:  Marshall et al., 2017, A&A 603, A87;  Fornasier et al., 2019, A&A 630, A13;  Hasselmann et al., 2019, A&A 639, A8,  Fornasier et al., 2016, Science 354, 1566;  El-Maarry et al., 2016, A&A 593, A110;  Massironi et al., 2015, Nature 526, 402
How to cite: Fornasier, S., Bourdelle de Micas, J., Hasselmann, P. H., Hong Hoang, V., Barucci, M. A., and Sierks, H.: Differences between the small and big lobes of 67P/Churyumov-Gerasimenko comet revealed from highly eroded regions analysis, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-171, https://doi.org/10.5194/epsc2021-171, 2021.
In this work, the motion of dust particles in the coma of the comet 67P/Churyumov-Gerasimenko is investigated. This is done by analyzing sets of images taken by OSIRIS, the main imaging system on board Rosetta, in which these dust particles can be seen as bright tracks instead of points sources, as result of the combination of motions of both particles and spacecraft. A fundamental obstacle to deriving the dust size and velocity distribution from such data is that without additional information, the apparent brightness of a particle constrains only the ratio of size and distance, but neither quantity individually.
While previous works in this area focused on obtaining the distance to individual detected particles in the image, a novel approach to analyze the dynamics of the dust is introduced in this work. This new approach is based on the statistical comparison between the images obtained by the camera, and synthetic images created by modeling the dynamics of the dust in the coma. The main advantage of this approach is to bypass the mentioned distance determination to the particles, which lifts the strict requirement on the observation conditions that were imposed by the earlier methods. This allows us to analyze a much larger set of images, and then, characterize the dust distribution and dynamics at the coma in a variety of conditions.
Our method can be divided in three main parts. First, dust tracks on the OSIRIS images are detected using an algorithm based on the Hough transform method. Secondly, the trajectories of dust particles under the influence of gas drag, nucleus gravity, and solar radiation pressure are modeled. These trajectories vary depending on the dust properties, such as density and size. The combination of the trajectories and the information about spacecraft position and orientation allows us to generate synthetic images for each type of modeled dust. Lastly, the distribution of selected track properties, like orientation, length and total brightness, obtained from the real images, is fitted by combining the ones obtained from the synthetic images. This allows us to estimate the contents of each type of dust in the coma.
How to cite: Lemos, P.: Dust distribution and dynamics in the coma of 67P/Churyumov-Gerasimenko, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-256, https://doi.org/10.5194/epsc2021-256, 2021.
Some of the comets visited by spacecraft missions display some circular depressions at their surface: 81P/Wild 2 (Brownlee et al. 2004), 9P/Tempel 1 (Belton et al. 2013), 103P/Hartley 2 (Bruck Syal et al. 2013), 67P/C-G (Vincent et al. 2015). For 67P, they consist of circular holes, half holes or cliffs, with a size range of tens of meters to a few hundreds of meters (Ip et al. 2016). Owing to the high precision of the shape model obtained from the Rosetta/OSIRIS images (Preusker et al. 2015, Sierks et al. 2015), it is possible to investigate the thermal processing of 67P’s surface in relation to the formation and evolution of these features (Mousis et al. 2015, Vincent et al. 2015, Guilbert-Lepoutre et al. 2016).
We aim to investigate the formation and evolution of 67P’s circular depressions (or pits, thereafter) by thermally-induced processes (for instance sublimation and amorphous water ice crystallization) on its current orbit. In a departure from the aforementioned studies, we consider a high-resolution shape model of the nucleus, which allows to study several facets for each pit: at the bottom, and on the walls. For each facet, the complete thermal environment is considered, including self-heating and shadowing, either by neighboring facets or due to the complex global morphology of the comet. We compute the illumination, self-heating and shadowing conditions for 125k facets during a full orbit, with a time step of ~8 min, then use these conditions as an input of a 1D thermal evolution model for each facet. The model includes standard features: heat conduction, phase transitions, gas diffusion, erosion, dust mantling (De Sanctis et al. 2005, 2010, Lasue et al. 2008). Various initial setups have been considered, and many tests were conducted to assess the influence of each parameter. The behaviour of 30 circular depressions (pits, half pits and cliffs) was studied in detail (see Figure 1).
Results and discussion
- We find that the following processes do not contribute significantly to the evolution of pits: sublimation of CO and CO2, crystallization of amorphous water ice, and dust mantling. When added to the model, they induce a relatively limited effect, altering the results by less than 10%. Sublimation of water, and therefore erosion, is the main acting process.
- We find that direct illumination is the main driver for gas production and erosion. Self-heating is not negligible, and in many cases, it allows to sustain some processing for longer periods of time and enhance local erosion. This is especially true for surface features located close to the neck, where facets additionally receive the VIS+IR flux from the small lobe. The total flux received per orbit is crucial, so is the flux received at perihelion. In this regard, we find strong differences between the Northern and Southern hemispheres of the nucleus, observed in other studies (Keller et al. 2015, Tosi et al. 2019). Finally, there is a tendency for facets in the North which are directed towards the equator to sustain more erosion than other facets at similar latitudes.
- At the scale of a given pit, there is a general tendency for cliffs and walls to receive more energy than the bottoms, and thus erode more. With time, the fate of a circular depression on 67P is thus to become wider and shallower. Nevertheless, in limited instances of small deep pits (such as Seth01), self-heating can be the driver for erosion of both the walls and bottom, since direct illumination is very limited. However, local erosion rates remain relatively low compared to erosion rates sustained by pits with direct illumination by the Sun.
- In general, we find that the erosion sustained after 10 orbits cannot reach the size extent of pits as they were observed by Rosetta. It is therefore very unlikely that current illumination conditions were able to produce those features. This results joins previous studies (Besse et al. 2015, 2017, Guilbert-Lepoutre et al. 2016).
- Because we have performed this study with a uniform set of thermo-physical parameters for all facets, we cannot exclude that local heterogeneities, such as the presence of ice patches in the bottom of some pits (Lamy et al. 2018) may help accelerate the erosion at depths in those pits.