The isotopes of krypton and the abundance of rare gases in comet comet 67P / Churyumov-Gerasimenko

Constraints on the origin of cometary ice from the isotopic composition of noble gases

Isotopic composition of comon argon of 67P / CG [ ³⁶ Ar / ³⁸ Ar = 5.4 ± 1.4; ( 10 )] is not precise enough to distinguish between most solar system reservoirs, including meteoritic [lecomposantQ ³⁶ Ar / ³⁸ Ar = 5.34; see ( 11 ) and his references] and solar [ ³⁶ Ar / ³⁸ Ar = 5.37 ( 11 )]. Given the error on the 67P / CG measurement, the isotopic composition of argon can not be distinguished from the identified nucleosynthetic components in pre-cerebral meteoritic grains, such as P3-Ar (5.26), N-Ar (5.87) or HL-Ar (4.41) [see( 11 ) and its references]. The isotopic composition of 67P / CG-Xe, however, is not compatible with a sun-like origin, being depleted in heavy isotopes of xenon ( ¹³⁴ Xe and ¹³⁶ Xe) and being rich in ¹²⁹ Xe compared to solar. This composition was attributed to a nucleosynthetic mixture different from that of the bulk solar system rather than being the result of an isotopic mbad fractionation, suggesting that cometary ice contains interstellar material ( 12 ). Thus, the isotopic composition of krypton can still constrain the origin of cometary material.

Figure 1 represents the isotopic composition of krypton recorded in comet comet 67P / CG in May 2016, normalized to ⁸⁴ Kr and composition SW ( 18 ) . Within the errors, the composition of 67P / CG-Kr is close to that of SW, with the possible exception of a deficit in ⁸³ Kr. This similarity contrasts with the case of the xenon in 67P / CG, whose isotopic composition is clearly different from that of the sun ( 12 ). The great excess of ¹²⁹ Xe was tentatively attributed to the disintegration of ¹²⁹ I ( T _1/2 = 16 Ma), provided that the initial iodine abundance in the parent xenon reservoir was greater by an order of magnitude than that present in meteorites at the onset of solar system formation, implying a presolar origin for the parent xenon reservoir. The deficits of ¹³⁴ Xe and ¹³⁶ Xe could not be derived from nuclear processes other than nucleosynthetic, and it was suggested that these deficits resulted from a mixture of different presolar components that of the bulk solar system ( 12 ). Adoption of the nucleosynthetic members of correlations in presolar materials ( 19 ), Marty et al . argued ( 12 ) that the 67P / CG xenon composition could be reproduced by mixing a s-type xenon component with two previously identified xenon process components r ( 19 ).

] In principle, a similar mixture should be able to reproduce the isotopic composition of krypton of the comet. Unfortunately, such an approach is not applicable to krypton because there are no process isotopes r only for krypton ( 20 ) and, therefore, the identification of the pure composition (s) can not be obtained. Thus, the mixing approach performed for xenon can not be applied directly to krypton. The absence of process krypton isotopes alone can also explain why the isotopic composition of krypton is less variable than that of xenon among solar system reservoirs and objects. Another complication is that production rates of krypton s-process isotopes in asymptotic giant stars (AGBs) may vary depending on the stellar regime. This is because ⁸⁶ Kr (and ⁸⁰ Kr) is affected by branching in the process s, unlike ^82,83,84 Kr isotopes. The abundances of ⁸⁶ Kr and ⁸⁰ Kr were found to be variable among the SiC grains ( 20 ), in proportions that are consistent with the theoretical models [forexample 21 ).

It has been observed ( ) that krypton isotope ratios of SiC grains define good mixing correlations between two nucleosynthetic ends identified as a G-Kr labeled processing component and a component. "normal" labeled N-Kr. Normal composition has integrated the contribution of several nucleosynthetic sources and could represent the nucleosynthetic ancestor of the krypton solar system. Later, a qualitatively similar scenario was proposed ( 22 ) to derive isotopic compositions of xenon and krypton from meteorites. A material rich in exotic and weak processes was added before the formation of the solar system to an ancestral presolar component (labeled "P3") resembling the solar composition. We badume that the isotopic composition of 67P / CG krypton is the result of the contribution of an exotic component, the process s (G) krypton to normal krypton (N) ( 20 ), as observed in the grains of SiC [givenonthefinallimbssynthesizedin 11 )]. On the basis of this hypothesis, the composition of 67P / CG krypton, including the slight deficit of ⁸³ Kr, may be misleading by adding ~ 5% of the G component (s-process) to the N component (Fig. 1). Taking SW krypton instead of N-Kr would give comparable results because N-Kr is isotopically close to SW-Kr. The data are better adjusted by baduming a low s-process component with a low Kr ^Kr Kr / ⁸⁴ ratio corresponding to a low neutron flux in the AGB envelopes (Figure 1). [19659007TheScenarioiscorrectthereforetheattributionoftheprocessmaterialbeforetheformationoftheolairsystemsmustalsoaffecttheionotropicpositionoftheunitbyincreasingtheprocess'scontributiontotheprocessbecauseitisnecessaryfor67P/CGxenon( 12 ). In additional materials, we modeled krypton and xenon compositions baduming a similar mixing scenario for the two noble gases. For a normal composition, represented by N-Kr and N-Xe [( 11 ) for the final member compositions] an exotic component (X-Kr and X-Xe, rich in process isotopes) is added. For X-Kr, it is not possible to define the r-process contribution for the reason given above, and we use G-Kr ( 20 22 ). For xenon, we use a similar mixture of process components and processes that reproduce the cometary composition of xenon ( 12 ). The proportions of the mixture that best match the xenon data are 35% s-process xenon and 65% r-process xenon (the latter being the average of both ends of the process r ( 19 ). the s-process xenon component, taking the composition identified by ( 19 ) or the composition G-Xe gives similar results. For reasons of coherence with krypton, we select G-Xe. 67P / CG is obtained for X-Xe = 80% and N-Xe = 20%

The much higher proportion of the excess component added to xenon (80%) compared to that added to krypton (5%) implies that the ratio (Xe / Kr) _X of the exotic component must be greater than the ratio (Xe / Kr) _N of the normal component (N). illustration, we badume below that the ratio N- (Xe / Kr) is similar to the solar relation.In accordance with this requirement, we no that the Xe / Kr ratio of the comet is 4.7 ± 1.8 times the data in Table 1 and ( 13 )]. We calculate that the ratio (Xe / Kr) _X of the exotic component is ~ 80 times the solar ratio for a proportion of 5% G-Kr (the mixing equations and the calculations are given in the Additional Materials ). We can also calculate what would be the corresponding fraction of exotic xenon in cometary xenon, which gives 80%, the remaining 20% ​​being N-Xe. This fraction is consistent with an exotic 80% exotic xenon fraction required to best fit the xenon isotopic data (Figure S5)

The mixture model proposed above is capable of providing a solution satisfactory to explain the contrasting isotopic compositions of krypton and xenon. However, this scenario is not without its problems because it requires that the exotic, process-rich component of the comet be significantly enriched in xenon compared to krypton in relation to solar abundance. Studies of presolar materials have shown that xenon is commonly enriched relative to krypton in the ends of the process, often referred to as chemical fractionation ( ). These enhancements may be related to differences in xenon and krypton behaviors during stellar envelope expulsion, such as the preferential ionization of xenon versus krypton, and / or selective implantation. xenon in the dust. It is not known whether such a chemical fractionation could have played a role in the enrichment of ice grains with noble gases derived from AGB. Selective trapping or xenon retention during ice-related processes may also have played a role, and experiments involving the trapping and desorption of rare gases in ice under conditions relevant to the outer solar system are highly needed.

In FIG. 4, the relative abundances of the noble gases 67P / CG (Table 2) are represented in a ⁸⁴ Kr / ³⁶ Ar against ¹³² Xe / ³⁶ Ar format, as well as solar, terrestrial, Martian ( 11 ) and chondritic gas compositions [Centre de Recherches Pétrographiques et Geochimiques (CRPG), Nancy compilation of Carbonaceous Orgueil (CI) and Carbonaceous Murchison (CM) data] including upper bounds for Titan ( 23 ). The 67P / CG is clearly depleted in argon compared to solar, which can be explained by an ice-trapping temperature that would not allow the complete retention of argon or additional xenon additions ( and Krypton). The trapping of noble gas in amorphous ice as a function of temperature is represented by the thin blue line and the blue bars given by ( 24 ) according to ( 25 ). Taken at their nominal value, the relative abundances of 67P / C-G noble gases would require a relatively high entrapment temperature (≥70 K), which is not compatible with other 67P / C-G coma measures. For example, the ratio CO / N ₂ requires temperatures ≤50 K ( 26 ).

Fig. 4 Relative Abundances of Rare Gases Compared to Other Solar System Reservoirs

Data Sources: 67P / CG: Production Ratio Reports with 1-σ Errors (SEM and Calibration Uncertainties ) Derived from Table 1, ⁸⁴ Kr / ³⁶ Ar = 0.058 ± 0.013, and ¹³² Xe / ³⁶ Ar = 0.013 ± 0.003; solar: ( 18 ); Earth and Mars: ( 43 ); chondritic: CRPG compilation of CI and CM data; amorphous water ice: ( 24 ). The blue arrow indicates the presumed composition of the Earth's initial atmosphere before the secondary loss of xenon, and perhaps krypton, in space through the geologic periods. The two red arrows represent the upper limits of the ratios ¹³² Xe / ³⁶ Ar and ⁸⁴ Kr / ³⁶ Ar measured at Titan by the Cbadini-Huygens Probe ( 23 )

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The relative abundances of the volatile species in comet comet 67P / CG are the result of their trapping efficiency in cometsimals, as well as their release and diffusion. in all the porosity of cometary ice. Comet 67P / C-G belongs to the Jupiter Family Comet Group (JFC), which has probably spent most of its life on the scattered disk. These objects do not easily become JFCs but they are thought to have undergone a series of gravitational diffusion processes, which leave them for several million years as Centaurs at intermediate distances from the Sun ( 27 ), while the comets of the Oort Clouds (OCC) have a very different dynamic history and their trajectories differ significantly from those of the JFCs

The thermal evolution of the comet 67P / CG baduming 10 Ma in a Centaur orbit at 7 AU was simulated ( 28 ). These simulations give inhomogeneous temperatures up to 80-90 K in the core at a depth of at least one kilometer. In combination with the amorphous to crystalline transition or any kind of ice destabilization, this leads to an inhomogeneous subterranean composition as an evolutionary process, even before the comet's first appearance in the inner solar system.

At these temperatures and over a period of time of several million years, highly volatile species can partially diffuse from the comet and be lost ( 29 ). As a result, the abundance of highly volatile species can be modified in JFCs and different from OCCs, provided that mbad degbading occurs from an altered layer. The rate of CO production of 67P / C-G around perihelion is about 1% compared to water. This is down from CO production rates compared to other comets, where production rates extend up to 25% ( 30 ). Comets are depleted of nitrogen by at least a factor of 5. This is explained by not incorporating the complete complement of N ₂ or the loss of N ₂ during the life of the comet ( 31 ). The low abundance of highly volatile CO and N ₂ ( 26 ) could have the same origin, namely a selective diffusion in amorphous ice. This would also have led to the loss of argon and, if it was initially present, to neon

The erosion of 67P / CG is estimated to be a few tens of meters per pbadage of the pericenter ( 9 ). The comet has been in its current orbit since a close encounter with Jupiter in 1959, but even before that, it could have been inside 5 AU for several centuries ( 32 ), and so , several hundred meters of surface erosion could have significantly altered the size and shape of the comet. It may have lost most of the heat affected layer at the Centaur stage, but this is difficult to badess.

Another possibility of apparent discordance of our results with respect to amorphous ice laboratory measurements is the specific set of conditions (eg, gas composition and pressure) that are not representative of conditions in nature. The release temperature of the volatile species, in particular amorphous ice, can be significantly different from their trapping temperature and depends on the ice in which they are sunk ( 33 ). Therefore, Kouchi and Yamamoto ( 34 ) also investigated the entrapment and release of gases in mixtures better representing volatile species in a comet. In particular, the significant abundance of CO ₂ influences the trapping and subsequent release of volatile amorphous ice species. Parts of the CO ₂ are embedded in ice water, while CO plates ₂ are also formed. With a 65: 10: 10: 15 mixture of H ₂ O: CO ₂ : CH ₄ : CO, the highly volatile species, CO and CH ₄ trap much better and are released in a complicated model compared to mixtures with ice only pure water. The same behavior can be observed for argon and N ₂ in mixtures of H ₂ O and CO ₂ which raises the question of trapping behavior (relative) of the other noble gases neon, krypton and xenon ( 35 ). From mixtures of H ₂ O: Ne at different temperatures between 15 and 35 K, the ratio Ne / H ₂ O drops sharply already to 35 K for the value of 10 ^-4 ( 36 ). The gas is released from amorphous ice during crystal ice transformations from 130 to 160 K. The estimated upper limit of 5 × 10 ^-8 would raise the cometary ice formation temperature to 40 K. However, the release of krypton and xenon in 67P / CG was detected in the southern hemisphere, very late in the mission. At this time of the mission, the sublimation of southern water was more or less cut off, while the CO ₂ was still abundant ( 17 ). The abundance of CO ₂ in relation to water suggests that these rare gases are mainly trapped in the CO ₂ fraction of ice. This may also explain the low correlation of argon with H ₂ O ( ). The interpretation of our results requires more laboratory experiments, where trapping in more comet-like mixtures, with H ₂ O and at least CO ₂ ] is studied. ice studies, ice trapping and desorption capabilities such as clathrate hydrates or polycrystalline ice should also be investigated in depth using appropriate gas mixtures. Many laboratory experiments, including species such as argon, krypton, and xenon, have been made in the range of 100 to 273 K ( 37 ), but none of them have been found in Canada. is close to astrophysical conditions and compositions. The argon, krypton and xenon abundances in the clathrates formed in the protosolar nebula in the vicinity of the giant planets were predicted near observed values ​​( 38 ). However, the gas mixture used in this simulation is dominated by CO, which is not consistent with the observations 67P / CG where CO was much less abundant compared to H ₂ O and CO ₂ . In addition, the pressure and temperature conditions were chosen to reflect the average nebulous conditions and could differ from the actual region where the 67P / C-G was formed. They also performed the same calculation for a mixture dominated by CH ₄ which gave very different abundances of noble gases. These experiments show that the results strongly depend on the main molecule (s) and the pressure and the temperature. No trapping or noble gas release data exist for polycrystalline ice. Therefore, additional laboratory results based on appropriate mixtures are needed for comparison with ROSINA observations. This includes the study to determine whether the simultaneous release of CO ₂ and noble gases is compatible with clathrates. Another well-known problem with the growth of clathrates in low-pressure environments in the external protosolar nebula (trans-neptunian) is their very slow formation kinetics, which may last longer than the lifetime of the disc, mainly due to lack of availability. Ice "fresh" ( 38 ). One way to overcome this problem is to badume that collisions between ice grains during planesimal formation produce fresh ice on their surface and facilitate the formation process of clathrates.

Finally, the initial composition of the gas might not be solar. xenon isotopic measurements ( 12 ), and the comparison with the amorphous ice data, which badumes an initial solar type gas, may not be entirely relevant. The same must be considered in future experiments using also other types of ice. We believe that the problem is far from being solved and that the current measurement of cometary noble gases, which has never been done before, will motivate further experimental and theoretical studies.

The Earth and Mars data points of FIG. mixing lines that join solar, chondrite or cometary compositions. These trends were taken as evidence of a cometary contribution to the Earth and Mars ( 39 ) based on amorphous ice measurements ( ). Recently, however, the xenon depletion of the Earth's atmosphere has been attributed to the preferential loss of this gas in space during geological periods ( 40 41 ]) because the isotopic composition of xenon in the Archean Eon was intermediate between that of the primordial atmosphere and that of the modern atmosphere (the modern terrestrial atmospheric xenon being isotopically fractionated from to the primordial xenon). A smaller fraction of krypton may also have been lost (modern atmospheric krypton is also fractionated but to a lesser extent than xenon). A similar process could also have acted on Mars since the Martian atmospheric xenon is also depleted and isotopically fractionated. Thus, the initial composition of the Earth's (and possibly Martian) atmosphere was likely shifted to higher Xe / Ar and Kr / Ar values ​​("initial earth", blue arrow in Figure 4), which would place data points closer to a line of mixing between chondritic and cometary. Once corrected for atmospheric xenon loss on Earth, the noble gas composition of the "restored" Earth would fall between the average chondrite and cometary compositions of FIG. 4. This location suggests that the atmosphere has received contributions from both cosmochemical sources. This possibility is in complete agreement with the independent evidence based on xenon isotopes that comets contributed 22 ± 5% of cometary xenon to a chondritic xenon atmosphere of 78 ± 5% ( 12 ). Limits of possible ratios Kr / Ar and Xe / Ar estimated in the atmosphere of Titan from the measurements made by the gas chromatograph-mbad spectrometer (GC-MS) aboard the Cbadini-Huygens probe. Of the primordial noble gases, only ³⁶ Ar was firmly detected by the GC-MS with an atmospheric mole fraction of ~ 2.06 × 10 ^-7 ( 23 ). On the other hand, only the upper limits of 1 × 10 ^-8 were deduced for the atmospheric molar fractions of krypton and xenon (we badume that the upper limits reported correspond to the major isotopes ^{] 84} Kr and ¹³² Xe). Some of the rare gases may have been trapped in the aerosol formation in Titan's atmosphere, which deposited on the surface, thus lowering krypton and xenon below the limit of detection (. ). A comparison between the upper bounds of the ratios Xe / Ar and Kr / Ar in Titan and 67P / CG shows that the ratio ⁸⁴ Kr / ³⁶ Ar measured in the comet (5.85 ± 1.33) × 10 ^-2 overlaps in the error bars with the upper limit (4.85 × 10 ^-2 ) measured in Titan. In addition, the upper limit of the ratio ¹³² Xe / ³⁶ Ar derived from Titan does not exclude the value of 67P / C-G. Both, Titan and 67P / CG, formed beyond the snowline in the protosolar nebula, and our data do not fully exclude the common origin of the building blocks from which both bodies have agglomerated