The origin of Insoluble Organic Matter: Formation of macromolecules from heavily irradiated simple ices

Comets and asteroids are among the most pristine objects in the Solar System, being formed out of the materials available in the proto-Solar Nebula [3]. Especially interesting are the complex organic molecules in these bodies, as they likely contribute towards the elemental composition of forming planets, as well as potentially being delivered after accretion of planetary bodies via impacts and thus contribute to their molecular inventory [4, 5].

Many primitive solar system objects, such as carbonaceous chondrites or interplanetary dust particles (IDPs), contain organic matter, which itself can be divided into a solvent-soluble (soluble organic matter, SOM) and insoluble fraction (Insoluble Organic Matter, IOM) [1, 6]. The formation environment of the IOM, which mainly consist out of macromolecules, is an area of ongoing research and IOM could have formed either in the interstellar medium or in the proto-Solar Nebula [5].

Even though it makes up the majority of the organic carbon in solar system objects, IOM has not been extensively studied in the laboratory. However, the formation pathways of IOM are crucial to understand the complex – insoluble – organic molecules available for the formation of planetary bodies. In this study, irradiation experiments on ice are performed with the ICEBEAR setup [2]. The setup consists of a stainless-steel vacuum chamber with base pressures of mbar. Vacuum-grade aluminium foil is fixed onto a copper sample holder, which is mounted on the cold head of a closed cycle helium cryostat, allowing for cooling of the sample holder to ~5 K.

A H₂O:CH₃OH:N₂ gas mixture is leaked into the chamber, where it adsorbs onto the cold (~10 K) aluminium foil, forming an ice film on the aluminum foil. Next, the ice is irradiated with 5 keV electrons, resulting in the formation of soluble organic matter. For several samples, a second irradiation is performed, which has been observed to lead to the formation of a darker residue, presumed to be insoluble organic matter.

The produced residues are analysed using micro-Raman spectroscopy at ETH Zürich with a laser operating at 532 nm. Raman spectroscopy is a powerful tool to investigate the structure of carbonaceous material. Especially interesting are the D (disordered) and G (graphite) bands of carbon. The peak widths and positions of the two bands, as well as their ratio, give valuable information about the structural order of the material. The results are compared to IDPs, which are thought to contain some of the most primitive organic matter in the solar system [6].

The initial analysis of the residue of a double irradiated ice sample with micro-Raman spectroscopy hints towards the formation of amorphous carbon that resembles the IOM extracted from IDPs.

 

[1] Garcia et al., ACS Earth and Space Chemistry, 2024. doi: 10.1021/acsearthspacechem.3c00366

[2] Kipfer et al., Icarus, 2024, doi: 10.1016/j.icarus.2023.115742

[3] Caselli & Ceccarelli, Astron Astrophys Rev 20, 2012, doi : 10.1007/s00159-012-0056-x

[4] Chyba & Sagan , Nature, 1992, doi: 10.1038/355125a0

[5] Alexander et al. Geochemistry, 2017, doi: 10.1016/j.chemer.2017.01.007

[6] Riebe et al., Earth and Planetary Science Letters, 2020, doi: 10.1016/j.epsl.2020.116266