The challenges of this project consist in simulating through laboratory experiments, the chemical evolution of interstellar ices, and grains to understand, which could be the different states of the organic matter during the life cycle of interstellar grains. Currently, our team is developing two complementary approaches to study this evolution. First, we are investigating the chemical reactivity that can occur within analogs of primary or cometary ices, by working on small size systems (one to three reactants). Based on molecules that have already been detected in the corresponding astrophysical environments, we investigate specific chemical reactions (e.g. formic acid reactivity with water or ammonia), or highlight pathways for the formation of specific molecules that might explain their detection in these environments (e.g. formation of glycine or aminoacetonitrile). Our ice analogs are submitted to photochemical irradiation and/or thermal process, analysis being performed by mid- and far- infrared spectroscopy, and low resolution mass spectrometry. In parallel of these studies, we develop another approach in collaboration with Roland Thissen and Véronique Vuitton from the The “Institut de Planétologie et d’Astrophysique de Grenoble” (IPAG), which consists in studying refractory residues coming from more complex ice analogs, as those observed in astrophysical environments. These ice analogs are submitted to VUV irradiation or hydrogenation, and then warmed up to give, after sublimation of volatile species, these refractory residues. These residues are then recovered and characterized without any degradation to understand their composition, and the reaction pathways that can lead to their formation. These studies are based on ultra high resolution mass spectrometry (using an Orbitrap set-up), as well as on the development of statistical analysis tools for processing data coming from these analyses. From these two complementary projects (reactivity in ices and refractory residue analyses), we can already have a first approach of the evolution of organic matter within these interstellar ices. However, for a better understanding on chemical reactions that can lead to the formation of refractory residues, facilitating their characterization, and also providing preliminary information on species that would sublimate during the warming of cometary nucleus, we propose to develop a new approach, which consists in implementing an analytical system for the Volatile Analyses coming from the Heating of Interstellar Ice Analogs, the VAHIIA project. This project will bridge the two existing approaches. Thereby, we want to acquire a unit of gas chromatography with a mass spectrometer (GC-MS), which will be directly coupled to a cryogenic system. The GC-MS is a technique already used and developed for space mission like the Rosetta mission, where it provides the analysis of gas coming from cometary nucleus or from the heating of nucleus samples [1-5]. Therefore, the development of such a device is relevant with VAHIIA’s objectives, since it will provide chemical data on precursors of refractory residues and that it will simulate in the laboratory, the in situ experiments of Rosetta, giving important information for the understanding of Rosetta analyses. Furthermore, beyond the VAHIIA project, this GC-MS apparatus will provide a new analytical tool for the analysis of samples coming from the two others project. To our knowledge, such a comprehensive experimental approach, as shown in Figure 2, has never been proposed. This comprehensive approach will provide us a better understanding of the chemistry, which occurs in the interstellar or cometary medium, and the form in which the organic matter can be found in different astrophysical environments.

Scientific Objectives

Our objective is to understand the constitution and the chemical reactivity that occurs in interstellar ices during the life cycle of interstellar grains and that leads to the formation of refractory residues, suspected to be present into interstellar and/or cometary grains. In this context, the first step consists in simulating the formation of interstellar "primitive" ice as they are identified through astronomical observations. This step is already being developed in our team. Once formed at low temperature (10 K to 77 K) and low pressure (10^-9 mbar), these analogs are subjected to various processes such as irradiation by ultraviolet photons (Lyman a) or atomic hydrogen bombardment. They may also be subjected to thermal processes to temperatures up to 800 K. This first step can be approximated to an activation step of molecules constituting these primitive ices, which is monitored by in situ infrared spectroscopy. Infrared spectroscopy has the advantage of offering a direct connection with astronomical observations, making it a preferred method for analyzing the chemical composition of the solids in the interstellar medium. Once the volatile species are sublimated, the sample moves towards the formation of refractory residues (in our experiments for temperatures around 300 K). As we said earlier, the understanding of the formation of refractory residues is an important step to establish what types of organic matter are available in interplanetary objects such as in comets or asteroids. What we want through this present application, is to develop a new method for analyzing volatile species sublimating from the ice analogs during their warming, what will thus complete the two approaches described above, and leads to a global approach concerning the study of the evolutionary cycle of ice and interstellar grains.

During the warming of simple ice mixture (one to three reactants), low resolution mass spectroscopy (quadrupole) can be used for the analyses of sublimating-species in order to confirm infrared spectroscopy data. However, low resolution mass spectrometry is not efficient to identify the sublimating species coming from complex primitive ices, because for a same m/z several ions can be detected, and due to the saturation of the detection system caused by a large amount of material sublimating. When the sample is heated, the quantity of species sublimating below 200 K may already be significant and may not allow the use of a low resolution mass spectrometer for their identification. But this becomes critical at 200 K, temperature, which corresponds to the sublimation of water. When the water sublimes, which is the major constituent of interstellar ices; it carries away a large part of the remaining molecules constituting the ice, which are potential precursors of refractory residues subsequently formed on the grain surface. The characterization of these species is therefore an essential step in understanding the chemical processes that can occur during evaporation of cometary ice mantle, as well as during the formation of refractory residues, which represent the bulk of organic matter available in planetary systems. Therefore, to analyze and characterize these species, it is necessary to develop an original analytical system allowing us to improve the resolution and capacity of the analysis. That is why we want to complete the approaches already available in our laboratory by developing an in situ analysis of gas species, which sublimate during the temperature increase of complex primitive ices (H₂O, CH₃OH, CO₂, NH₃). To our knowledge, no such analyses have been performed. This analytical system will consist in a gas chromatography-mass spectrometry (GC-MS) apparatus, which will be directly connected to our cryogenic system in which ice is formed and warmed. Therefore, during the warming of the ice, sublimating species will be directly analyzed by this chromatographic system. The chromatographic column will provide the separation of analytes, and the ion trap mass spectrometer will allow the identification of the molecular species (chemical ionization) and, if necessary, their fragmentation pattern by collision in the ion trap, or by electron impact to perform a structural analysis in order to confirm their identification. However, because desorption kinetics of sublimated species can be slow, the direct injection of these species in the chromatographic system is not feasible because it would lead to low efficiencies. It is therefore necessary to have a device providing the pre-concentration of these species prior to their injection for analysis. Therefore, the implementation of this system requires the use of a specific gas chromatography using an injector cooled with liquid nitrogen, on which species will condense and will be concentrated before their injection. It will allow a flash desorption of the condensed species, that maximizes the efficiency of the signals. In addition, the compounds to be analyzed present a variety of physico-chemical properties (polar, non polar, with presence of more or less water depending on the temperature of sublimation). It is therefore necessary to have a multi-column gas chromatography system, which will, after proper calibration, provides the separation of these species on an appropriate column (polar or non polar columns). This type of GC-MS system is already used for the analysis of atmospheric gases, and of molecular species sublimated from heated ice cores. It was already developed for space missions like the Rosetta mission, where it provides the analysis of volatiles coming from cometary nucleus, or from the heating of nucleus samples [1-4]. The only adjustment required concerns the junction between the injector of the chromatographic system (atmospheric pressure) and our cryostat (low pressure). To play the role of airlock for transferring the sample from the cryostat to the GC, the injector or a finger cooled with liquid nitrogen will be placed in the center of a system bounded by four valves. For the development of this system, we are in contact with several companies such as Thermo Fisher Scientific instruments/Interscience.