Evolution of circular depressions at the surface of JFCs

 

Context

Circular depressions and alcoves were observed on the surface of some JFCs visited by spacecrafts: 81P/Wild 2 (Brownlee et al., 2004), 9P/Tempel 1 (Belton et al., 2013), 103P/Hartley 2 (Syal et al., 2013), and 67P/Churyumov- Gerasimenko (Vincent et al., 2015). These features are characterized by different shapes and sizes ranging from few tens to few hundreds of meters (Ip et al., 2016). Several studies investigated the thermal processing in relation to their formation and evolution (Guilbert-Lepoutre et al., 2016), and found that recent thermal activity in the inner solar system orbits is not sufficient to carve them. Ip et al. (2016) found that the size frequency distribution of the depressions on 67P, 81P and 9P has the same power law distribution, implying that they might have the same origin and formation mechanism. Dynamical simulations show that the thermal history of 81P and 9P is likely shorter than 67P’s and 103P’s, suggesting a younger surface. In this work, we investigate the thermally-induced evolution of depressions at the surface of 81P, 9P, 103P, and 67P under each of their current illumination conditions.

 

Methods

For these four nuclei, we select more than 10 surface features (i.e. depressions or alcoves). From their shape models, we select multiple facets on different sides of each feature (plateaux, bottom and walls) and consider the complete thermal environment for each facet, including self-heating and shadowing, either by neighboring facets or due to the complex global morphology of the nucleus. We compute the energy input for each facet during a full recent orbit, with a time step of ∼ 8 min. The total energy received at the surface is used as an input of a 1D thermal evolution model, which accounts for heat diffusion, phase transitions (sublimation of various ices and crystallization of amorphous water ice), gas diffusion, erosion, and dust mantling (Lasue et al., 2008). The thermal behaviour of each surface feature is investigated in detail.

 

Results

• 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.
  • 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.
  • 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.
  • 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.
  • 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.

 

Acknowledgements

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.

 

References

Belton, M. J., Thomas, P., Carcich, B., et al. 2013, Icarus, 222, 477
Brownlee, D. E., Horz, F., Newburn, R. L., et al. 2004, Science, 304, 1764
Guilbert-Lepoutre, A., Rosenberg, E. D., Prialnik, D., & Besse, S. 2016, Monthly Notices of the Royal Astronomical Society, 462, S146
Ip, W.-H., Lai, I.-L., Lee, J.-C., et al. 2016, Astronomy & Astrophysics, 591, A132
Lasue, J., De Sanctis, M. C., Coradini, A., et al. 2008, Planetary and Space Science, 56, 1977
Syal, M. B., Schultz, P. H., Sunshine, J. M., et al. 2013, Icarus, 222, 610
Vincent, J.-B., Bodewits, D., Besse, S., et al. 2015, Nature, 523, 63