A radiolytic origin of organic matter in primitive chondrites and trans-neptunian objects? New clues from ion irradiation experiments

Abstract

We question here the radiolytic origin of (i) polyaromatic insoluble organic matter (IOM) recovered from primitive chondrites, and (ii) organics at the surface of reddish Trans-Neptunian Objects (TNOs), some minor planets and icy satellites. Organic synthesis by ion irradiation was investigated through experiments on a variety of targets: Polyethylene glycol 1450, lignin, cellulose and sucrose, exposed to low (C 40 keV and Ne 170 keV) and high energy (C 12 MeV, Ni 17 MeV, 78 Kr 59 MeV) ions. These experiments show that all carbonaceous precursors evolves towards a sp 2 -rich amorphous carbon (a-C) above a critical nuclear dose of 10 −7 +10 eV.atom -1 . A thorough review of the literature shows that this value applies for a large range of carbonaceous materials, including C-rich simple ices. Below this critical dose, irradiated targets are carbonized and transformed into cross-linked polymeric disordered solids, with abundant radiolytic olefinic and acetylenic bonds, but devoid of aromatic or polyaromatic species. Ion irradiation of simple compounds, e.g. ices, is thereby not a viable process to synthesize IOM. However, in the case of aromatic-rich precursors, swift heavy ions irradiation leads to polyaromatic materials, by bridging existing aromatic or polyaromatic units. In the context of Early Solar System, i.e. Galactic Cosmic Rays (GCR) irradiation during 10-20 Myr, the formation of chondritic IOM from simple ices mixed with interstellar Polycyclic Aromatic Hydrocarbons (PAHs) appears as a plausible mechanism. This scenario, based on the recycling of existing carbonaceous interstellar grains under lowtemperature conditions, would account for the heterogeneity of the D, 15 N and 13 C isotopic fractionations at the molecular scale, and the preservation of deuterium hot spots that are highly sensitive to high-temperature conditions (> 300 °C). At the surface of TNOs, sp 2 -rich amorphous carbons are formed by the implantation of GCRs and Solar wind ions. The electronic dose is also very high for an irradiation time of several Gyr (> 100 eV.atom -1 ), leading to the formation of reddish disordered solids, provided that the surface contains a minimum abundance of carbonaceous species. Finally, sp 2 -rich amorphous carbons produced in the laboratory (e.g. the ACAR compound from Zubko et al., 1996) are fair analogues of the darkening agent produced by radiolysis.

The Interstellar Interlopers

“…An unfortunate consequence of the destruction of bonds is that irradiated materials lack the characteristic vibrational spectral features on which spectroscopic chemical identifications are based (e.g. Faure et al (2021)). Probably for this reason, the reflection spectra of most comets, like those of 'Oumuamua and 2I/Borisov, tend to be linear, featureless and difficult or impossible to compositionally diagnose.…”

The Interstellar Interlopers

“…This irradiated material can have low volatility and very low albedo, which is quite different from the initial material. An unfortunate consequence of the destruction of bonds is that irradiated materials lack the characteristic vibrational spectral features on which spectroscopic chemical identifications are based (e.g., Faure et al 2021). Probably for this reason, the reflection spectra of most comets, like those of 'Oumuamua and 2I/Borisov, tend to be linear, featureless, and difficult or impossible to compositionally diagnose.…”