Periodic Reporting for period 4 - THEMISS (Thermal Evolution Modeling of Icy objects in the Solar System)
Reporting period: 2023-08-01 to 2025-01-31
Where, when and how did comets form? What are they made of? If our goal for studying comets is to constrain their role in the formation of planets and planetary systems, and to use them as tracers of the conditions and processes that prevailed during the early phases of the solar system, we need to constrain first which of the physical, chemical or orbital properties of comets are «primordial», thus relevant to characterise the early solar system. THEMISS aims at providing a framework to interpret the current ground-based and in situ dataset in a statistically significant manner, i.e. by studying comet populations in addition to individual objects. Three topics have been selected to provide advances beyond the current state of the art:
A — Comet nuclei display a variety of shapes and sizes. At the surface, the energy balance can be extremely varied when accounting for self-heating and shadowing effects induced by the irregular shape of the nucleus, and this could produce long-lived internal heterogeneities. We studied whether shape-induced effects are linked to the origin of surface features observed on all Jupiter Family Comets (JFCs) imaged by spacecrafts.
B — Every comet may have a unique history due to their chaotic orbital evolution and the diversity of underlying causes for activity. We aimed at assessing the degree of alteration each comet population sustains, and constrain the relevant long-lasting effects of thermally-induced evolution within a comet population.
C — Accounting for the complete thermal history of comet nuclei, through their orbital evolution from outer solar system reservoirs to the inner solar system, shows that it is difficult to maintain highly hyper-volatile species as pure ices. Yet, these species are observed at present. THEMISS has thus enabled the development of an experimental setup designed to understand whether hyper-volatile species can be trapped inside comet nuclei during their evolution.
A — Comet nuclei display a variety of shapes and sizes. At the surface, the energy balance can be extremely varied when accounting for self-heating and shadowing effects induced by the irregular shape of the nucleus, and this could produce long-lived internal heterogeneities. We studied whether shape-induced effects are linked to the origin of surface features observed on all Jupiter Family Comets (JFCs) imaged by spacecrafts.
B — Every comet may have a unique history due to their chaotic orbital evolution and the diversity of underlying causes for activity. We aimed at assessing the degree of alteration each comet population sustains, and constrain the relevant long-lasting effects of thermally-induced evolution within a comet population.
C — Accounting for the complete thermal history of comet nuclei, through their orbital evolution from outer solar system reservoirs to the inner solar system, shows that it is difficult to maintain highly hyper-volatile species as pure ices. Yet, these species are observed at present. THEMISS has thus enabled the development of an experimental setup designed to understand whether hyper-volatile species can be trapped inside comet nuclei during their evolution.
First, the project aimed at studying shape-induced effets such as self-heating and shadowing. A shape-dependent surface energy model has been developed and benchmarked, using high resolution shape models of several comet nuclei imaged by spacecrafts, and combined to an internal evolution model. We studied the origin and evolution of circular depressions, seen on all comets visited by spacecrafts. In the initial study of 67P by Benseguane et al. (2022), we find that the sublimation of water, and therefore erosion, is the main process modifying the surface, with direct illumination being the main driver for gas production and erosion. We show that cometary activity tends to erase surface features: water-driven erosion cannot carve circular depressions with the observed depths and diameters. This also applies to comets 9P, 81P and 103P: in Guilbert-Lepoutre et al (2023), we point to the Centaur phase that each comet experienced in the past as a possible region with adequate conditions to form pits. Alternatively, these could be a primordial feature reflective of the formation process of comets. This can be tested by the upcoming Comet Interceptor mission, which targets a comet with a very distinct orbital history. Finally, we have established a theoretical framework for understanding the aging of cometary surfaces that can further be tested with ground-based observations.
The second part of the project was dedicated to the long-term evolution of various comet populations. By coupling a thermal evolution model to a sample of dynamical clones, Gkotsinas et al. (2022) observe long-lasting passages close enough to the Sun for comet nuclei to be significantly altered. Several stages of numerical development were thus performed to achieve a new dedicated thermal and ice evolution model coupled to N-body dynamical simulations (e.g. Gkotsinas et al. 2023). In Gkotsinas et al. (2024), we investigate the early thermal processing of ice-rich planetesimals during and after the giant planet instability in the early Solar System. While previous models assumed static orbits, this study is the first to integrate the changing orbits of planetesimals into a thermal evolution model, leading to more realistic predictions of volatile retention. We provide a natural explanation for observed differences between comet populations, suggest distinct drivers for cometary activity, and predict a difference between new and returning long period comets, that is supported by observations including new JWST spectra of comet active at large heliocentric distances. Specific cases were also studied, like precursors of Jupiter Family Comets in Guilbert-Lepoutre et al. (2023). We have also studied the thermal processing of planetesimals for various stellar environments (Cabral et al. 2023, Cabral et al. in preparation). This is the part of the project that has had the most impact on the current state of the art.
The last part of the project was not identified in the initial work plan: the flexibility of the ERC project has allowed to be very reactive to our first results, and opened a new, unplanned line of research. To reconcile the apparent contradiction between observations and our simulation results, we decided to test the formation of hydrates in cometary conditions, i.e. at very low pressure and temperature. Theoretical phase diagrams of clathrate hydrates suggest that it would be possible to form these structures under extreme conditions, but no experimental data currently support this claim. We experimented on existing facilites and available setups in Marseille, Bordeaux and Nantes. We identified that working in such extreme conditions would require a dedicated setup. We thus designed the setup specifically to perform water-gas deposition experiments in a cryogenic cell device, able to reproduce conditions met in the Kuiper belt and the active phase of comets. This complex task took about three years to complete. With a design adapted to control the formation conditions (gas/water flow rate, temperature and pressure, substrate, etc), this device makes it possible to obtain a set of complementary data on the same deposit (FTIR, Raman, Xray). We additionally secured 120 hours of synchrotron beam time at the SOLEIL infrastructure to provide a detailed analysis of the atomic structure of various ice deposits. This setup will continue to be exploited by the collaboration in the future.
The second part of the project was dedicated to the long-term evolution of various comet populations. By coupling a thermal evolution model to a sample of dynamical clones, Gkotsinas et al. (2022) observe long-lasting passages close enough to the Sun for comet nuclei to be significantly altered. Several stages of numerical development were thus performed to achieve a new dedicated thermal and ice evolution model coupled to N-body dynamical simulations (e.g. Gkotsinas et al. 2023). In Gkotsinas et al. (2024), we investigate the early thermal processing of ice-rich planetesimals during and after the giant planet instability in the early Solar System. While previous models assumed static orbits, this study is the first to integrate the changing orbits of planetesimals into a thermal evolution model, leading to more realistic predictions of volatile retention. We provide a natural explanation for observed differences between comet populations, suggest distinct drivers for cometary activity, and predict a difference between new and returning long period comets, that is supported by observations including new JWST spectra of comet active at large heliocentric distances. Specific cases were also studied, like precursors of Jupiter Family Comets in Guilbert-Lepoutre et al. (2023). We have also studied the thermal processing of planetesimals for various stellar environments (Cabral et al. 2023, Cabral et al. in preparation). This is the part of the project that has had the most impact on the current state of the art.
The last part of the project was not identified in the initial work plan: the flexibility of the ERC project has allowed to be very reactive to our first results, and opened a new, unplanned line of research. To reconcile the apparent contradiction between observations and our simulation results, we decided to test the formation of hydrates in cometary conditions, i.e. at very low pressure and temperature. Theoretical phase diagrams of clathrate hydrates suggest that it would be possible to form these structures under extreme conditions, but no experimental data currently support this claim. We experimented on existing facilites and available setups in Marseille, Bordeaux and Nantes. We identified that working in such extreme conditions would require a dedicated setup. We thus designed the setup specifically to perform water-gas deposition experiments in a cryogenic cell device, able to reproduce conditions met in the Kuiper belt and the active phase of comets. This complex task took about three years to complete. With a design adapted to control the formation conditions (gas/water flow rate, temperature and pressure, substrate, etc), this device makes it possible to obtain a set of complementary data on the same deposit (FTIR, Raman, Xray). We additionally secured 120 hours of synchrotron beam time at the SOLEIL infrastructure to provide a detailed analysis of the atomic structure of various ice deposits. This setup will continue to be exploited by the collaboration in the future.
Can we truly consider comets as pristine remnants of the formation of the solar system? THEMISS provides a theoretical framework to assess how primitive comets really are. We show how seasons are key for understanding cometary activity, and how surfaces might be shaped more by past dynamical history than by current solar-driven activity. We have established a clear evolutionary sequence of cometary surfaces, connecting surface features to orbital evolution, composition, and thermal history. We have provided simple explanations for the observed diversity in CO abundance among comets, naturally explained by differences in their past orbital evolution, and suggest that activity may be driven by distant phase transition between comet populations, and thus provide more realistic predictions of volatile retention. Finally, we have developed an experimental setup geared toward cometary conditions that can continue to be exploited in the future to gain insights into the molecular behaviour of cometary material in extreme conditions never tested before.