Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021


Comets, Trojans, Centaurs, TNOs, & Interstellar Objects 

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)

Convener: Jean-Baptiste Vincent | Co-conveners: Aurelie Guilbert-Lepoutre, Michael Küppers, Alessandra Migliorini

Session assets

Discussion on Slack

Oral and Poster presentations and abstracts

Chairpersons: Jean-Baptiste Vincent, Michael Küppers, Alessandra Migliorini
Comet 67P and Rosetta
Nicholas Attree, Laurent Jorda, Olivier Groussin, Raphael Marschall, Riccardo Lasagni Manghi, Paulo Tortora, and Marco Zannoni

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 [2]. 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 [2], we also compute changes in the rotation axis, using a method based on [5] 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 [6].

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,, 2021.

Axel Bouquety, Olivier Groussin, Laurent Jorda, Antoine Séjourné, François Costard, and Sylvain Bouley


          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.