Correlations between cometary ice composition and their refractory and gas organic diversity: clues from laboratory experiments

Introduction

Comets are the most primitive bodies of the solar system [1]. They are composed of low density material: mainly dust and ice. A better understanding of their composition is of particular interest to improve the comprehension of the composition of the material composing the forming solar system. Nevertheless, because of detection issues, the composition of its nucleus is only poorly known even eight years after the landing of the Philae probe from the European Space Agency (ESA) Rosetta mission. Most insights on the organic composition of 67P/Churyumov-Gerasimenko are coming from indirect analysis estimating that around 45 % of a comet is composed of organic matter ([2],[3]).

Laboratory experiments show similarities with the organic matter observed in comets [4]. Astrochemical experiments are thus a promising approach to cover this lack of data. Experiments simulating the chemical reactivity occurring on icy grains initially composed with volatile molecules such as H2O, NH3, methanol (MeOH) give information into the refractory [5] and volatile [6] organic composition that are formed in such astrophysical environments. Those experiments suggest the presence of a large organic diversity [7]. Previous work showed the correlation between the initial composition of these ices and either the volatile organic compounds (VOCs) composition [8] or the refractory organic material [9]. In this presentation, we will give the first experimental description of the correlations that exist between both three phases: the primitive ice composition, the refractory and the gas phase. 

Method

All samples were synthesized using the MICMOC experiment [10]. It is a vacuum chamber (~10^-7 mbar) where different ice compositions of H₂O, NH₃ and methanol (MeOH) were irradiated at Lyman α (121 nm) at 77 K for 5,5 h at a fixed irradiation rate of ~7 photon/MeOH. Then, 92 mbar of argon were added before the sample was warmed up to ~300 K. Four ice compositions were tested : a benchmark experiment (Ref.) with ratio 3:1:2 (H₂O:MeOH:NH₃); a H2O-rich sample, 8:1:2; a NH3-rich sample, 2:1:5; and a MeOH-rich sample, 2:5:3.

VOCs are recovered using an extra cryogenic trap coupled to a valve system for GC-EI-FT-Orbitrap analysis [11]. Three replicates are made for each ice composition. The cumulation of the three residual phases from all samples is then extracted and dissolved 10 times into methanol before positive ESI-FT-ICR. More details on the apparatus characteristics can be found in [12].

To extract correlations between all three phases (initial ice, refractory and gas), principal component analysis (PCA) were performed on the dataset standardized by variable with following characteristics :

• For the gas phase (Figure 1A): individuals are the triplicates of GC-EI-FT-Orbitrap analysis with 150 chromatographic peak areas as variables;
• For the refractory phase (Figure 1B): individuals are the analysis of methanol solubilized refractory material injected using positive ESI-FT-ICR with between 4126 to 12814 extracted exact mass peak area as variables.

Preliminary results and conclusions

PCA is a mathematical tool to find the smartest plan to differentiate individuals in a multiple variable hyperspace. Figure 1 displays the plan with highest differences from those two datasets. Axes 1 and 2 are mostly enough to discriminate samples according to their initial composition for both gas phase (Figure 1A)  and residual phase (Figure 1B). As most individual ices are present in different zones, it proves the organic composition of each phase allows to find correlation between them.

More specifically, in the gas phase (Figure 1A) only three zones can be determined : the first one contains only NH3-rich ices, the second one MeOH-rich ices and the third one both the benchmark experiment (Ref.) and H2O-rich ices. Consequently, H₂O, NH₃ and MeOH seem to have different impacts on the volatile organic compounds: H₂O seems to be less impactful on the first and second principal axes while only the third axes can discriminate it from the other zones.

Finally, all three parameters can be separated using ACP on residual phase, proving significant differences extracted from the intensity and compound nature detected by positive ESI-FT-ICR. All together, this suggests a high correlation between all three phases regarding their organic compound composition.

Shortly :

• On the gas phase : NH₃ hinders all volatile productions including nitriles, H₂O has little impact on the gas composition with respect to the benchmark experiment.
• On the refractory phase : NH₃ and H₂O are changing the nature of organics produced and making global  ESI-FT-ICR intensity. H₂O is the ice that’s producing the smaller organic diversity (tree time less compounds detected).
• In both gas and residual phase : MeOH has little impact on the final organic compound type, but enhances the total amount of them.

 

A)

B)

Figure 1. Principal component analysis (PCA) of results from each sample. In A) individuals are the triplicates of GC-EI-FT-Orbitrap analysis with chromatographic peak areas as variables. In B) individuals are each four analyses of methanol solubilized refractory material injected using positive ESI-FT-ICR with exact mass peak area as variables. Samples are labeled according to the compound exhausted in the initial composition of the ice with respect to the benchmark experiment (Ref.); H₂O for the H₂O-enriched sample; NH₃ for the NH₃-enriched sample and a MeOH for the MeOH-enriched sample.

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