Evolution of circular depressions at the surface of JFCs

<p> </p> <p><strong><span dir="ltr" role="presentation">Context</span></strong></p> <p><span dir="ltr" role="presentation">Circular depressions and alcoves were observed on the surface of some JFCs visited by spacecrafts:</span> <span dir="ltr" role="presentation">81P/Wild 2 </span><span dir="ltr" role="presentation">(Brownlee et al., 2004), 9P/Tempel 1 (Belton et al., 2013), 103P/Hartley 2 (Syal et al., 2013), and 67P/Churyumov- </span><span dir="ltr" role="presentation">Gerasimenko (Vincent et al., 2015). These features are characterized by different shapes and sizes ranging from few </span><span dir="ltr" role="presentation">tens to few hundreds of meters (Ip et al., 2016). Several studies investigated the thermal processing in relation to their </span><span dir="ltr" role="presentation">formation and evolution (Guilbert-Lepoutre et al., 2016), and found that recent thermal activity in the inner solar system </span><span dir="ltr" role="presentation">orbits is not sufficient to carve them. Ip et al. (2016) found that the size frequency distribution of the depressions on </span><span dir="ltr" role="presentation">67P, 81P and 9P has the same power law distribution, implying that they might have the same origin and formation </span><span dir="ltr" role="presentation">mechanism. Dynamical simulations show that the thermal history of 81P and 9P is likely shorter than 67P’s and 103P’s, </span><span dir="ltr" role="presentation">suggesting a younger surface. In this work, we investigate the thermally-induced evolution of depressions at the surface </span><span dir="ltr" role="presentation">of 81P, 9P, 103P, and 67P under each of their current illumination conditions.</span></p> <p> </p> <p><strong><span dir="ltr" role="presentation">Methods</span></strong></p> <p><span dir="ltr" role="presentation">For these four nuclei, we select more than 10 surface features (i.e. depressions or alcoves). From their shape models, </span><span dir="ltr" role="presentation">we select multiple facets on different sides of each feature (plateaux, bottom and walls) and consider the complete </span><span dir="ltr" role="presentation">thermal environment for each facet, including self-heating and shadowing, either by neighboring facets or due to the </span><span dir="ltr" role="presentation">complex global morphology of the nucleus. We compute the energy input for each facet during a full recent orbit, with </span><span dir="ltr" role="presentation">a time step of</span> <span dir="ltr" role="presentation">∼</span> <span dir="ltr" role="presentation">8 min. The total energy received at the surface is used as an input of a 1D thermal evolution model, </span><span dir="ltr" role="presentation">which accounts for heat diffusion, phase transitions (sublimation of various ices and crystallization of amorphous water </span><span dir="ltr" role="presentation">ice), gas diffusion, erosion, and dust mantling (Lasue et al., 2008). The thermal behaviour of each surface feature is </span><span dir="ltr" role="presentation">investigated in detail.</span></p> <p> </p> <p><span dir="ltr" role="presentation"><strong>Results</strong></span></p> <ul> <li><span dir="ltr" role="presentation">We find that self-heating can be important in deep pits and steep cliffs of 67P and 81P (~65% and ~30% of the total energy input, respectively). In comparison, it is very low for 9P and 103P’s (<10%), where surface features are shallower.<br role="presentation" />• Plateaux tend to erode more than the shadowed bottoms of sharp features, found on 67P and 81P: i.e. circular depressions become shallower with time. On 9P and 103P, erosion is more uniform since depressions are already shallower (as in the southern hemisphere of 67P). Overall, sharp depressions are likely erased by cometary activity.<br role="presentation" />• Erosion sustained after the multiple perihelion passages is not able to carve depressions with the observed size and shape. It is therefore very unlikely that current illumination conditions were able to carve them.<br role="presentation" />• We have, however, performed our simulations with a uniform set of thermo-physical parameters for all facets. Therefore, we cannot exclude that local or regional heterogeneities may yield different erosion rates.<br role="presentation" />• A comparison between simulation outcomes for all nuclei allows to consider 103P as having the most altered surface. 9P could be an intermediate state. 81P would thus represent the least altered, or best preserved surface of these nuclei. Finally, 67P display a variety of surface ages, with areas as preserved as 81P, and a southern hemisphere as altered as 9P.</span></li> </ul> <p> </p> <p><span dir="ltr" role="presentation"><strong>Acknowledgements</strong></span></p> <p><span dir="ltr" role="presentation">This study is part of a project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 802699). We gratefully acknowledge support from the PSMN (Pôle Scientifique de Modélisation Numérique) of the ENS de Lyon for the computing resources. We thank the European Space Research and Technology Centre (ESAC) and the European Space Astronomy Centre faculty council for supporting this research.</span></p> <p> </p> <p><strong>References</strong></p> <p>Belton, M. J., Thomas, P., Carcich, B., et al. 2013, Icarus, 222, 477<br />Brownlee, D. E., Horz, F., Newburn, R. L., et al. 2004, Science, 304, 1764<br />Guilbert-Lepoutre, A., Rosenberg, E. D., Prialnik, D., & Besse, S. 2016, Monthly Notices of the Royal Astronomical Society, 462, S146<br />Ip, W.-H., Lai, I.-L., Lee, J.-C., et al. 2016, Astronomy & Astrophysics, 591, A132<br />Lasue, J., De Sanctis, M. C., Coradini, A., et al. 2008, Planetary and Space Science, 56, 1977<br />Syal, M. B., Schultz, P. H., Sunshine, J. M., et al. 2013, Icarus, 222, 610<br />Vincent, J.-B., Bodewits, D., Besse, S., et al. 2015, Nature, 523, 63</p> <p> </p>