Evolution of circular depressions at the surface of JFCs
Authors: Benseguane Selma , Guilbert-Lepoutre Aurélie , Lasue Jérémie , Besse Sébastien , Beth Arnaud , Grieger Björn , Teresa Capria Maria 
Published: September 2022

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&lt;p&gt;&lt;strong&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Context&lt;/span&gt;&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Circular depressions and alcoves were observed on the surface of some JFCs visited by spacecrafts:&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;81P/Wild 2&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;(Brownlee et al., 2004), 9P/Tempel 1 (Belton et al., 2013), 103P/Hartley 2 (Syal et al., 2013), and 67P/Churyumov-&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Gerasimenko (Vincent et al., 2015). These features are characterized by different shapes and sizes ranging from few&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;tens to few hundreds of meters (Ip et al., 2016). Several studies investigated the thermal processing in relation to their&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;formation and evolution (Guilbert-Lepoutre et al., 2016), and found that recent thermal activity in the inner solar system&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;orbits is not sufficient to carve them. Ip et al. (2016) found that the size frequency distribution of the depressions on&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;67P, 81P and 9P has the same power law distribution, implying that they might have the same origin and formation&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;mechanism. Dynamical simulations show that the thermal history of 81P and 9P is likely shorter than 67P&amp;#8217;s and 103P&amp;#8217;s,&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;suggesting a younger surface. In this work, we investigate the thermally-induced evolution of depressions at the surface&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;of 81P, 9P, 103P, and 67P under each of their current illumination conditions.&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;strong&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;Methods&lt;/span&gt;&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;For these four nuclei, we select more than 10 surface features (i.e. depressions or alcoves). From their shape models,&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;we select multiple facets on different sides of each feature (plateaux, bottom and walls) and consider the complete&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;thermal environment for each facet, including self-heating and shadowing, either by neighboring facets or due to the&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;complex global morphology of the nucleus. We compute the energy input for each facet during a full recent orbit, with&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;a time step of&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&amp;#8764;&lt;/span&gt; &lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;8 min. The total energy received at the surface is used as an input of a 1D thermal evolution model,&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;which accounts for heat diffusion, phase transitions (sublimation of various ices and crystallization of amorphous water&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;ice), gas diffusion, erosion, and dust mantling (Lasue et al., 2008). The thermal behaviour of each surface feature is&amp;#160;&lt;/span&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;investigated in detail.&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&lt;strong&gt;Results&lt;/strong&gt;&lt;/span&gt;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;We find that self-heating can be important in deep pits and steep cliffs of 67P and 81P (~65% and ~30% of the&amp;#160;total energy input, respectively). In comparison, it is very low for 9P and 103P&amp;#8217;s (&lt;10%), where surface features&amp;#160;are shallower.&lt;br role=&quot;presentation&quot; /&gt;&amp;#8226; Plateaux tend to erode more than the shadowed bottoms of sharp features, found on 67P and 81P: i.e. circular&amp;#160;depressions become shallower with time. On 9P and 103P, erosion is more uniform since depressions are already&amp;#160;shallower (as in the southern hemisphere of 67P). Overall, sharp depressions are likely erased by cometary&amp;#160;activity.&lt;br role=&quot;presentation&quot; /&gt;&amp;#8226; Erosion sustained after the multiple perihelion passages is not able to carve depressions with the observed size&amp;#160;and shape. It is therefore very unlikely that current illumination conditions were able to carve them.&lt;br role=&quot;presentation&quot; /&gt;&amp;#8226; We have, however, performed our simulations with a uniform set of thermo-physical parameters for all facets.&amp;#160;Therefore, we cannot exclude that local or regional heterogeneities may yield different erosion rates.&lt;br role=&quot;presentation&quot; /&gt;&amp;#8226; A comparison between simulation outcomes for all nuclei allows to consider 103P as having the most altered&amp;#160;surface. 9P could be an intermediate state. 81P would thus represent the least altered, or best preserved surface&amp;#160;of these nuclei. Finally, 67P display a variety of surface ages, with areas as preserved as 81P, and a southern&amp;#160;hemisphere as altered as 9P.&lt;/span&gt;&lt;/li&gt;
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&lt;p&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;&lt;strong&gt;Acknowledgements&lt;/strong&gt;&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;span dir=&quot;ltr&quot; role=&quot;presentation&quot;&gt;This study is part of a project that has received funding from the European Research Council (ERC) under the European&amp;#160;Union&amp;#8217;s Horizon 2020 research and innovation programme (Grant agreement No. 802699). We gratefully acknowledge support from the PSMN (P&amp;#244;le Scientifique de Mod&amp;#233;lisation Num&amp;#233;rique) of the ENS de Lyon for the computing&amp;#160;resources. We thank the European Space Research and Technology Centre (ESAC) and the European Space Astronomy&amp;#160;Centre faculty council for supporting this research.&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;Belton, M. J., Thomas, P., Carcich, B., et al. 2013, Icarus, 222, 477&lt;br /&gt;Brownlee, D. E., Horz, F., Newburn, R. L., et al. 2004, Science, 304, 1764&lt;br /&gt;Guilbert-Lepoutre, A., Rosenberg, E. D., Prialnik, D., &amp; Besse, S. 2016, Monthly Notices of the Royal Astronomical&amp;#160;Society, 462, S146&lt;br /&gt;Ip, W.-H., Lai, I.-L., Lee, J.-C., et al. 2016, Astronomy &amp; Astrophysics, 591, A132&lt;br /&gt;Lasue, J., De Sanctis, M. C., Coradini, A., et al. 2008, Planetary and Space Science, 56, 1977&lt;br /&gt;Syal, M. B., Schultz, P. H., Sunshine, J. M., et al. 2013, Icarus, 222, 610&lt;br /&gt;Vincent, J.-B., Bodewits, D., Besse, S., et al. 2015, Nature, 523, 63&lt;/p&gt;
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