The transition from soluble to insoluble organic matter in interstellar ice analogs and meteorites

Context. Carbonaceous chondrites are sources of information on the origin of the Solar System. Their organic content is conventionally classiﬁed as soluble (SOM) and insoluble organic matter (IOM), where the latter represents the majority. Aims. In this work, our objectives are to identify possible relations between soluble and insoluble organic matter generated in laboratory experiments and to extrapolate the laboratory analog ﬁndings to soluble and insoluble organic matter of meteorites to test their connection. Methods. Using laboratory experiments, processes possibly linking IOM analog (IOMA) to SOM analog (SOMA) precursors are investigated by assuming that dense molecular ices are one of the sources of organic matter in the Solar System. Each organic fraction is analyzed by laser desorption coupled to a Fourier transform ion cyclotron resonance mass spectrometer on a comprehensive basis. Results. SOMA and IOMA signiﬁcantly differ in their chemical ﬁngerprints, and particularly in their aromaticity, O/C, and N/C elemental ratios. Using an innovative molecular network, the SOMA–IOMA transition was tested, revealing connection between both classes. This new network suggests that IOMA is formed in two steps: a ﬁrst generation IOMA based on precursors from SOMA, while a second IOMA generation is formed by altering the ﬁrst IOMA generation. Finally, using the same analytical technique, the molecular content of IOMA and that of the Paris IOM are compared, showing their molecular similarities for the ﬁrst time. The molecular network application to the Paris SOM and IOM demonstrates that a possible connection related to photochemical ice processing is present, but that the overall history of IOM formation in meteorites is much more complex and might have been affected by additional factors (e.g., aqueous alteration). Conclusions. Our approach provides a new way to analyze the organic fraction of extraterrestrial material, giving new insights into the evolution of organic matter in the Solar System.


Introduction
Organic matter is diversely present in many astrophysical environments, from the interstellar medium to the interplanetary bodies and planets of the Solar System (Ruf et al. 2018).While observations give information on the organic composition of some astrophysical objects, the actual molecular diversity has only been observed on Earth for fragments of asteroids and comets (carbonaceous chondritic meteorites; Schmitt-Kopplin et al. 2010, 2012;Ruf et al. 2019b).Carbonaceous matter represents up to 6% of the weight of these meteorites (Pearson et al. 2006).Depending on extraction protocols, two fractions can be recovered.A soluble fraction, called soluble organic matter (SOM), which presents a high molecular diversity as identified by high resolution mass spectrometry analyses (HRMS; Schmitt-Kopplin et al. 2010).Targeted analyses have identified numerous molecular families, ranging from amino acids, sugar derivatives, and nucleobases, to various polyaromatic hydrocarbon (PAH) compounds composing this SOM (Pizzarello 2007).Along with SOM, an insoluble organic fraction can be isolated after successive demineralization steps, and this is called insoluble organic matter (IOM).This IOM is generally seen as an agglomerate of hydrophobic macromolecules formed by small PAH units cross-linked with aliphatic bridges including a small amount of heteroatoms such as O, N, or S (Cody et al. 2002;Gardinier et al. 2000;Remusat et al. 2005a,b;Yabuta et al. 2005).This high molecular diversity was also observed when applying laser desorption ionization HRMS (LDI-HRMS) A120, page 1 of 15 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.This article is published in open access under the Subscribe-to-Open model.Subscribe to A&A to support open access publication.
to IOM, showing thousands of organic molecules with masses ranging from 150 to 700 unified atomic mass units (u), (Danger et al. 2020).These molecules are mainly formed by aromatic or condensed aromatic structures, including small amounts of heteroatoms (O, N, and/or S).A possible origin of this type of organic matters could be related to ices observed in dense molecular clouds of the interstellar medium (Danger et al. 2021).Their isotopic measurements showed an enrichment in heavier elements ( 13 C, 15 N and/or D), implying reactivity occurring at low temperatures (Remusat et al. 2010).
To complement spectral observations and meteorite analyses, experimental simulations were developed in the laboratory to verify different hypotheses and steps that could lead to the organic matter observed in meteorites.For this purpose, the evolution of dense molecular ices during a planetary system formation is simulated.These experiments show that ice irradiation by energetic particles (UV photons at Lyman α, electrons or ions) followed by thermal processes results in diverse molecular fingerprints comparable those of meteorites (Danger et al. 2016).However, significant differences remain, such as the nitrogen content, which is higher in analogs compared to meteorites.This maybe be a result of the alteration of meteoritic parent bodies, as observed in the mineralogy of carbonaceous meteorites (Brearley 2006;Vinogradoff et al. 2017).Particularly, for some objects, aqueous alteration is observed as predominant (Le Guillou et al. 2014;Le Guillou & Brearley 2014).We thus consider here that such an organic residue formed in the laboratory could be considered a pre-accretional organic analog (Danger et al. 2021).The UV irradiation at Lyman α of this pre-accetional organic analog, initially soluble in polar solvents, shows the formation of a crust at its surface, which turned out to be an insoluble fraction.Infrared spectroscopy analyses suggest some similarities with meteorite IOM (de Marcellus et al. 2017), but this technique lacked the molecular resolution, specificity, and sensitivity necessary to obtain unambiguous characterization and meteorite comparison.
In this work, experiments were performed to produce both soluble and insoluble organic matter analogs (SOMA and IOMA, respectively) and probe their transition.These samples were analyzed by laser desorption ionization using a Fourier transform resonance cyclotron mass spectrometer (LDI-FTICR-MS; Maillard et al. 2018).This technique has shown its ability to analyze a fraction of meteorite IOM (Danger et al. 2020) underlying the molecular diversity present in IOM.Using specific data representation, the SOMA/IOMA transition was investigated, showing a two-step process for the IOMA formation.Finally, IOMA and the IOM of Paris meteorite were compared, and the similarities observed may highlight that dense molecular ices have to be considered as an essential source of organics for the Solar System.for the same ice, on average the number of N and O were equivalent (Danger et al. 2013(Danger et al. , 2016;;Fresneau et al. 2017).The average number of C is 19, with an H average of 28.

Experimental results and analysis
The two distributions that differ by the number of N are also identified by the represented double bond equivalent (DBE) versus the number of carbon plots (Figs.1B and B.3). Zone A presents the highest DBE for a similar C count (Fig. 1B).Zone A is enriched in N and depleted in O, and it includes CHNO molecules and a large part of the CHN group (Fig. B.3).In contrast, Zone B is depleted in N and enriched in O, with a lower DBE.These two distributions can be related to the ones observed with ESI, where the nitrogen-enriched distribution detected in positive ESI mode can be related to Zone A, and the carbon-rich distribution can be related to Zone B (Fresneau et al. 2017).
The X C pie charts (Fig. 1B), with m = 0.5 for SOMA and m = 0.75 for IOMA, confirm the higher aromaticity of Zone A, since 97% can be related to condensed aromatic structures, 2% to aromatics and only 0.2% to aliphatics.For Zone B, aliphatics represent 15%, while condensed aromatics decrease to 50%, and aromatics represent 35% of all molecular formulas.With LDI, the SOMA presents a higher aromaticity than observed in previous works with ESI, due to the different ionization selectivity between these two ionization techniques.LDI in positive mode tends to favor the ionization of unsaturated structures and molecules bearing nitrogen (Cho et al. 2013).This is also corroborated by van Krevelen (VK) representations that present lower H/C and N/C (Fig. 1C) compared to ESI mode, even if H/C is not lower to due to the high level of N in the molecular formula.However, similar distributions of compounds are observed in the VK space, implying similar distributions of compounds such as unsaturated hydrocarbons or aromatic structures with low amounts of O and various nitrogen chemical functions.For high DBE, we can assume that N is incorporated in the backbone of molecules, especially for molecules included in Zone A (Fresneau et al. 2017).
To evolve toward IOMA, the SOMA was further irradiated with UV.Only the first 0.1 µm of the SOMA were affected (de Marcellus et al. 2017).Therefore, a thin layer of IOMA is formed at the top of the SOMA (de Marcellus et al. 2017).The IOMA mass spectrum is different to SOMA (Fig. 1A).The number of attributions is 9738 for IOMA, three times more than for SOMA.The molecular structures of the IOMA are more easily ionized by LDI, indicating a potential higher aromaticity (Cho et al. 2013).This is supported by the DBE versus #C and X C representations (Fig. 1B), where the detected compounds show PAH characteristics, and structures were mainly related to condensed aromatic ones (93%).This is also visible in the VK representation, which is very different from the SOMA one (Figs.1C and B.4).In IOMA, attributions extend to H/C = 0.5 implying higher aromaticity.A decrease of N/C was also observed (Fig. 1C).These structures are related to Next to this IOMA, the remaining soluble fraction of the irradiated SOMA has the mark of this irradiation process.Indeed, its analysis with electrospray ionization showed that some molecules bearing high O/H and O/C disappeared once the SOMA was processed, suggesting a decarboxylation process (Gautier et al. 2020).This trend was confirmed by infrared spectra comparison that indeed present a decrease of vibrators corresponding to COOR chemical functions (Gautier et al. 2020).A gradation in the process of aromatization may occur across the 0.1 µm of irradiated SOMA.To identify these possible SOMA-IOMA transitions, a new representation was developed based on the present LDI analyses using a molecular network illustrating the SOMA-IOMA transition (Fig. 2A).The molecular network was generated by mass difference matching of molecular formulas (nodes in the network) with a set of in-silico transformations that represent potential chemical reactions of H 2 , O, CO 2 , CO, and NH 3 (Ruf & Danger 2022), induced by the UV processing (edges in the network; Gautier et al. 2020).This data processing should allow us to identify a possible molecular connection between SOMA and IOMA.These connections could then highlight possible precursors transformed during the SOMA irradiation and leading to the first IOMA molecules.One component of the obtained network, representing 70% of all detected SOMA-IOMA molecular formulas, connects SOMA to IOMA attributions.SOMA molecules are grouped on the outer part and aligned by #H, #C, and DBE gradients (Figs. 2 and B.5). SOMA-IOMA transitions are displayed in three zones.Two are related to Zone B of SOMA (enriched in O, Fig. 1B).These two SOMA zones are different in their transformations' fingerprints and element maps (e.g., #H, #C, #N, #O, and m/z).Next, a third zone is related to Zone A (enriched in N).Connections (Fig. 2B) are driven equally by H 2 edges (balanced in addition/subtraction), followed by similar amounts of CO, O, and NH 3 edges.CO, O, and NH 3 are preferentially subtracted (IOMA -> SOMA, see histograms in Fig. 2B) confirming previous observations.SOMA transition nodes (Fig. 2C, molecules that are directly connected to IOMA molecules) show a wide range in #C, from 5 to 27, low #O counts (maximum of #O = 3), and high #N (maximum of #N = 11).
In this network, it is interesting to note that SOMA/IOMA connections are only located in two lobes.Regarding elementary maps (Fig. B.5), from SOMA-IOMA lobes to IOMA alone (gray, Fig. 2A), an increase in #DBE, #C, and #N is observed, while #O decreases.This gradient seems to follow an evolutionary process related to UV irradiation of the material.It would start from SOMA molecules that evolve to form the first IOMA molecules, decreasing the #O counts while starting to increase the DBE as well as #C and #N counts.This process occurred on the first 0.1 µm of UV penetration depth in SOMA.While the first IOMA molecules directly formed from SOMA, they can be further irradiated leading to an evolution of the IOMA itself (gradient observed in elementary maps).Therefore, two IOMA generations can be formed, one of which is directly connected to the SOMA, as observed in the two lobes, and another one that is linked to the degradation of the first IOMA generation itself under the UV processing (Fig. 3).

Analogs versus Paris meteorite
We propose that a fraction of the IOM of meteorites can be generated by the processing, under radiations (UV, electrons and/or ions), of a soluble ice residue (Danger et al. 2021) at the surface of grains before the parent body accretion.In this section, the IOM of Paris is compared to the IOMA (Fig. 4).The comparison of DBE versus #C representations show that attributions are aligned along the same slope, which is the PAH line.Furthermore, as described in Danger et al. (2020), two zones were present for the Paris IOM.Even if these two zones are less visible for IOMA, they are still present.These observations imply that the Paris IOM and IOMA share similar aromaticities.This is confirmed by the X C factor showing a same percentage of condensed aromatic, aromatics and aliphatics (Fig. 4A).Both Paris IOM and IOMA are depleted in O (Fig. 4B).Furthermore, both also have a higher number of carbon atoms per molecule than for hydrogen, confirming the high aromaticity of these materials.The comparison of VK H/C versus O/C also A120, page 4 of 15 shows interesting similarities in the attribution distribution in the VK space (Fig. 4B).Attributions are aligned to H/C = 1, with H/C ranging from 0.5 to 2 with a maximum of O/C around 0.2.Therefore, regarding aromaticity and O content, IOMA and the Paris IOM present similarities.However, a higher amount of N is present in the IOMA compared to the Paris IOM (Fig. B.4).This is coherent with the higher N content observed in SOMA compared to the meteorite SOM (Danger et al. 2016).Indeed, the SOMA enriched in N, the IOMA, which originates from it, keeps this N enrichment, even if a decrease in N is observed from SOMA to IOMA due to the UV processing.It should also be noted that in our analog formation experiment, no sulfur is yet incorporated.S is very abundant in the SOM and IOM of meteorites and certainly plays an important role in the chemistry that led to these organic materials.In particular, it may impact the redox chemistry in the parent bodies of the meteorites if aqueous alteration has occurred.Therefore, the next step is to incorporate this heteroatom into our experiments to test its impact on the chemical reactivity leading to our SOMAs and IOMAs, as we begin to experiment by bombarding the ices as well as the SOMAs with sulfur ions (Ruf et al. 2021(Ruf et al. , 2019a)).
Finally, since the molecular network presented in Fig. 2 provides an insight into the SOMA-IOMA transitions generated by UV processing, it was applied to the SOM-IOM of the Paris meteorite in order to identify if some SOM-IOM transitions related to the same process could be isolated.The Paris SOM was firstly analyzed with LDI-FTICR-MS.The SOM-IOM molecular network was then obtained (Fig. B.6) using the same edges (H 2 , CO, O, CO 2 , NH 3 ) as SOMA-IOMA.The obtained molecular network is clearly different.This implies that the putative evolution of SOM into IOM would be much more complex than only through UV photochemistry.This may be the consequence of parent-body evolution.Even if the Paris meteorite is among the less altered, secondary processings have occurred in the parent body and have induced the modification of the accreted OM (Vinogradoff et al. 2017).Furthermore, numerous pathways could have led to IOM and SOM formation in the meteorite parent bodies (Remusat et al. 2010;Vinogradoff et al. 2017).However, it is interesting to note that two components linking SOM to IOM have been identified (Figs. B.6 and B.7), implying that a possible connection could exist as observed in our experiments, since the same edges are used.The number of connections is much lower than the one observed for SOMA-IOMA, but this could suggest that a part of SOM and IOM of the Paris meteorite is related through radiation processes and that the organic content of the Paris meteorite kept this trace.

Dense molecular ices as a source of chondritic SOM and IOM in meteorites
Numerous experiments have shown that dense molecular ice analogues can generate an important molecular diversity during A120, page 5 of 15 A&A 667, A120 (2022) simulated solar nebula evolution.Organics formed are soluble in polar solvent, and their molecular diversity can be related to the SOM of meteorites that is the last step of evolution in the interplanetary objects of the Solar System.However, SOMA is significantly different in its molecular content than the Paris SOM.This can be explained because SOMA directly originates from ices and thus has not undergone parent body evolution, which opposite to the case of the organic molecules contained in chondrites.That is why SOMA has to be considered as a possible analogue of the pre-accretionnal organic matter that was then incorporated in chondrite parent bodies.This hypothesis is strengthened when a SOMA is altered in conditions simulating a secondary aqueous alteration at 150 °C and six bars, commensurable to asteroid conditions (Danger et al. 2021).It leads to the formation of molecules sharing numerous similarities with chondrite SOM.Hence, the content of nitrogen is depleted after 100 days of reaction.Consequently, dense molecular cloud ices could be considered as precursors of a fraction of the SOM observed in chondritic meteorites.One may question if it is the same for the insoluble organic fraction of chondritic meteorites.
In addition to a soluble fraction, dense molecular ices can also lead to the formation of insoluble organic material if subjected to intense UV irradiation.The material is formed by the evolution of the soluble fraction at the surface of evolved ice grains.It could then be accreted with silicate and metal grains to form parent bodies of meteorites.It must be noted that formation of IOM from soluble compounds during alteration under asteroidal conditions is hampered by the presence of clay minerals (Viennet et al. 2022;Vinogradoff et al. 2020).IOMA and IOM share several molecular characteristics: aromaticity, molecular diversity, and O content, even if some differences remain, such as a higher amount of N in IOMA.However, as shown for SOMA, IOMA may also undergo secondary alteration once incorporated in the meteorite parent bodies.Some experiments have indeed shown that the processing of organic content of meteorites released an important fraction of nitrogen (Pizzarello et al. 2011).If the same effect as the one observed on SOMA occurs, a possible evolution of IOMA could occur and bring it closer to the IOM molecular content.
Experimental simulations thus give important clues on the role that dense molecular cloud ices play in the origin of the organic matter available in the Solar System.During their evolution through various types of irradiation (VUV, X-Rays, Cosmic Rays and/or electrons), radical and thermal chemistry led to their molecular evolution and resulted in an increased molecular diversity.The obtained organic compounds could then be modified by solar radiative environments and thermal processes in the mid-plane of the protosolar nebula, toward an insoluble organic material.After accretion on the parent bodies, these organic materials can then undergo secondary processes, notably through aqueous alteration, which induces the aromatization and oxidation of the accreted organic components (Kebukawa et al. 2020;Vinogradoff et al. 2018).Consequently, next to the gas phase, the solid phase (e.g.particularly dense molecular ices) has to be taken into account as a source of organic matter of the Solar System objects.

Fig. 1 .
Fig. 1.SOMA versus IOMA.(A) Laser desorption ionization FT-ICR mass spectra of analogues of soluble (SOMA, 3756 attributions) and insoluble (IOMA, 9738 attributions) organic matter.Pie charts display the repartition of molecular groups observed (CHNO, CHN) in SOMA and IOMA.(B) Representations of double-bond equivalent (DBE) versus the number of carbon (#C) for each molecular formula identified in SOMA and IOMA, with the dotted line indicating the PAH trend.Also displayed are the two different zones of the SOMA (Zone A in green and Zone B in purple).X C pie chart of SOMA (m = 0.5) and IOMA (m = 0.75) data are also reported showing attribution repartitions between aliphatic, aromatic and condensed aromatic structures.m corresponds to the proportion of O atoms included in double bonds.(C) Van Krevelen diagrams H/C versus N/C for SOMA and IOMA.

Fig. 3 .
Fig. 3. Possible evolution process leading to the formation of IOMA from SOMA.The upper 0.1 µm layer of the SOMA are UV irradiated, leading to the formation of the first IOMA molecules (dark).Once formed, these IOMA first-generation molecules are UV irradiated, leading to an evolved generation of IOMA (brown).This evolution is visible in the gradient of #C, #N, and DBE of elementary maps (Fig. B.5).

Fig. 4 .
Fig. 4. IOMA versus IOM of Paris.(A) Representations of DBE versus the number of carbon (#C) for IOMA and IOM, with X C pie charts showing attribution repartitions between aliphatic, aromatic, and condensed aromatic structures.(B) Van Krevelen H/C versus O/C for attributions identified in IOMA and IOM of Paris meteorite.For the Paris meteorite, data were reproduced from Danger et al. (2020).

Fig. B. 2 .
Fig. B.2. Frequencies of atoms in each molecular formula for all attributions, CHNO, or CHN (groups for SOMA and IOMA).