Clathrate hydrates FTIR spectroscopy to understand cometary ices

<p>Comet nuclei in the transneptunian region are submitted to  heating at temperatures from 30 to 50 K over the age of the solar system [1]. The timescale for sublimated volatiles to escape the objects at these temperatures is long though [1]. Once these nuclei enter the inner solar system and become active, subsurface sublimation puts a gas phase in contact of the porous and tortuous ice structure of cometary material. In this context, the formation of clathrate hydrates may be considered as a plausible trapping mechanism of these gases, occurring in subsurface layers, and allowing some of the most volatile species to subsequently survive in cometary material at temperatures higher than the sublimation temperature of the corresponding pure solid [2]. </p> <p>Hydrates are ice-like crystalline compounds, resulting from the tridimensional stacking of cages of H-bonded water molecules. Clathrates are gas hydrates, meaning that the guests are gas molecules encased in a host framework of water molecules. Gas hydrates only form and remain stable in specific temperature and pressure regimes that depend on the nature of the guest molecules [3]. Theoretical phase diagram of clathrate hydrates show that it would be possible to form clathrates at very low pressure (10<sup>-10</sup> bar) and temperature (< 80 K), but there is a critical lack of experimental data using these preparation methods [4]. Could clathrate hydrates be formed under conditions relevant to the interior of comet nuclei?  The formation and characterization of these ice-like structures under such conditions could provide valuable experimental evidence for understanding the preservation of some volatile species during the thermally-induced evolution of comets. </p> <p>In an effort to assess whether hydrates may play a role in maintaining volatile species in cometary material, FTIR spectroscopic identification of several species have been performed. We present results related to carbon dioxide and methane hydrates, in conditions relevant to cometary nuclei, i.e. at low temperature (10 K) and pressure (base pressure 10<sup>-7</sup> mbar) regimes. To understand the nature of the gas hydrates formed under these conditions, vibrational spectra of distinct gas/ice interactions (clathrate hydrate, gas in/on water ice) were compared. The behaviour of the water crystalline skeleton interactions with the trapped molecules at different temperatures, as well as the influence of the gas mixture and the deposition method, will be presented.</p> <p> </p> <p><strong>Acknowledgements</strong></p> <p>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).<span class="Apple-converted-space"> </span></p> <p> </p> <p>[1] Prialnik et al. (2004) in Comets II, Festou, Keller and Weaver (Eds.), 359-387</p> <p>[2] Mandt et al. (2017) in Comets as Tracers of Solar System Formation and Evolution, Mandt, Mousis, Bockel{\'e}e-Morvan and Russel (Eds.)</p> <p>[3] Sloan (2003) Nature, 426, 353-359</p> <p>[4] Choukroun et al. (2003) in The Science of Solar System Ices, Gudipati and Castillo-Roguez (Eds.), 409-454 </p>

On the role of collisions with meteoroids in splitting and acceleration of heliocentric velocity of comets

The hypothesis on the role of the meteoroid impacts in the comet nuclei splitting as well as acceleration of their heliocentric velocity are considered. Inclinations of the orbits of split comets relative to the movement planes of 100 known meteoroid streams are calculated. The analysis is carried out for the cases: when the cometary nodes are located from the meteoroids orbit < 0.1 AU; MOID-values less than 0.1 AU. In the case of split long-period comets irregularity (maximum near 180°) of the distribution of the inclinations has been found. Comets, constituting this maximum, could have head-on collisions with meteoroids. A similar analysis is carried out relatively to the hyperbolic comets (HCs). Analysis is based on the assumption that the acceleration of the heliocentric velocities of the comet also is caused by collisions with meteoroids. The inclinations of the orbits of 300 HCs relative to 100 known meteoroid streams have the significant maxima in the interval of 90°− 101.5°. Acceleration of comet velocity might be the result of “slanting” collisions with meteoroids.

Size scaling of crater size, ejecta mass, and momentum enhancement due to hypervelocity impacts into 2024-T4 and 2024-T351 aluminum

Momentum enhancement occurs when impactors strike objects at hypervelocities due to the formation of crater ejecta whose departure from the impact body impart more momentum to the impacted body. In previous work the momentum enhancement caused when metals, rock, and pumice were impacted have been examined [1-7]. Momentum enhancement is quantified by β, which is the ratio of the resulting target momentum by the impactor momentum. By quantifying momentum enhancement it is possible to make informed decisions about the use of hypervelocity impactors to deflect celestial bodies such as asteroids or comet nuclei.

Numerical simulation of the long-term thermal evolution of the nuclei of short-period comets:using the nucleus of comet 67p/Churyumov–Gerasimenko as an example

Using numerical models, we have studied what depth of the outer layer the comet nuclei are degassed to when they are in orbits whose perihelion is close to the Sun for tens of years. The problem is topical, because it helps to understand how much the experimentally obtained results on the composition of comet comas depend on how long the comet is in its present-day orbit and how adequately the data obtained reflect the composition of comet nuclei as a whole. The proposed approach, which is demonstrated using comet 67P/Churyumov–Gerasimenko as an example, is based on a 3D comet nucleus surface relief model and takes into account not only its orbital motion, but also its diurnal rotation. The propagation of heat in the nucleus subsurface layers is described by a 1D heat conduction equation for a porous rock-ice composition of matter. Based on this approach, we have derived the temperature distributions in the subsurface layers for several surface patches located in the Ma’at region in 20 revolutions around the Sun, ~130 years.

Physical activity of the selected nearly isotropic comets with perihelia at large heliocentric distance

Context. The systematic investigation of comets in a wide range of heliocentric distances can contribute to a better understanding of the physical mechanisms that trigger activity at large distances from the Sun and reveals possible differences in the composition of outer solar system bodies belonging to various dynamical groups. Aims. We seek to analyze the dust environment of the selected nearly isotropic comets with a perihelion distance between 4.5 and 9.1 au, where sublimation of water ice is considered to be negligible. Methods. We present results of multicolor broadband photometric observations for 14 distant active objects conducted between 2008 and 2015 with various telescopes. Images obtained with broadband filters were used to investigate optical colors of the cometary comae and to quantify physical activity of the comet nuclei. Results. The activity level was estimated with Afρ parameters ranging between 95 ± 10 cm and 9600 ± 300 cm. Three returning comets were less active than the dynamically new comets. Dust production rates of the comet nuclei were estimated between 1 and 100 kg s−1 based on some assumptions about the physical properties of dust particles populating comae. The measured colors point out reddening of the continuum for all the comets. The mean values of a normalized reflectivity gradient within the group of the comets amount to 14 ± 2% per 1000 Å and 3 ± 2% per 1000 Å in the BV and VR spectral domains, respectively. The comae of the dynamically new comets, which were observed on their inbound legs, may be slightly redder in the blue spectral interval than comae of the comets observed after the perihelion passages. The dynamically new comets observed both pre- and post-perihelion, seem to have higher production rates post-perihelion than pre-perihelion for similar heliocentric distances.