Radiolysis of H2O:CO2 ices by heavy energetic cosmic ray analogs

An experimental study on the interaction of heavy, highly charged, and energetic ions (52 MeV Ni^13+) with pure H2O, pure CO2 and mixed H2O:CO2 astrophysical ice analogs is presented. This analysis aims to simulate the chemical and the physicochemical interactions induced by heavy cosmic rays inside dense and cold astrophysical environments such as molecular clouds or protostellar clouds. The measurements were performed at the heavy ion accelerator GANIL (Grand Accelerateur National d’Ions Lourds in Caen, France). The gas samples were deposited onto a CsI substrate at 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometer at different fluences. Radiolysis yields of the produced species were quantified. The dissociation cross sections of pure H2O and CO2 ices are 1.1 and 1.9E-13 cm^2, respectively. In the case of mixed H2O:CO2 (10:1) the dissociation cross sections of both species are about 1E-13 cm^2. The measured sputtering yield of pure CO2 ice is 2.2E4 molec/ion. After a fluence of 2-3E12 ions/cm^2 the CO2/CO ratio becomes roughly constant (~0.1), independent of the of initial CO2/H2O ratio. A similar behavior is observed for the H2O2/H2O ratio which stabilizes at 0.01, independent of the initial H2O column density or relative abundance.

💡 Research Summary

This paper presents a systematic laboratory investigation of the radiolysis of astrophysical ice analogs—pure H₂O, pure CO₂, and a mixed H₂O : CO₂ (10 : 1) ice—by means of heavy, highly charged, and energetic ions. The authors employed 52 MeV Ni¹³⁺ ions from the GANIL heavy‑ion accelerator to simulate the impact of massive cosmic‑ray particles that penetrate dense, cold regions such as molecular clouds and protostellar envelopes. Ice films were deposited on a CsI substrate at 13 K, and in‑situ Fourier‑transform infrared (FTIR) spectroscopy was used to monitor the evolution of molecular vibrational bands as a function of ion fluence.

Key experimental findings include: (1) Dissociation cross sections of 1.1 × 10⁻¹³ cm² for H₂O and 1.9 × 10⁻¹³ cm² for CO₂, indicating that CO₂ is roughly 1.7 times more susceptible to ion‑induced breakup than water. (2) In the mixed ice, both constituents exhibit a convergent dissociation cross section of ≈1 × 10⁻¹³ cm², suggesting that matrix effects average out the individual susceptibilities. (3) The sputtering yield for pure CO₂ ice is measured at 2.2 × 10⁴ molecules per incident ion, a value markedly higher than that for H₂O, reflecting the more efficient momentum transfer and surface disruption for CO₂. (4) After a fluence of 2–3 × 10¹² ions cm⁻², the CO₂ : CO column‑density ratio stabilizes at ~0.1, and the H₂O₂ : H₂O ratio at ~0.01, regardless of the initial CO₂/H₂O proportion. This saturation behavior implies that the production of CO and H₂O₂ reaches a steady state where further ion impacts primarily recycle existing products rather than generate new ones.

The authors interpret these results in the context of astrochemical processes. The rapid conversion of CO₂ into CO and the formation of H₂O₂ from water are consistent with radiolytic pathways involving electronic excitation, ionization, and subsequent radical recombination. The measured cross sections and sputtering yields provide quantitative inputs for models that aim to predict the chemical evolution of icy mantles under cosmic‑ray bombardment. In dense, shielded regions where UV photons are attenuated, heavy cosmic rays become the dominant driver of ice chemistry; thus, the experimental data help to constrain the timescales over which key species such as CO, CO₂, and H₂O₂ can be altered.

Furthermore, the study highlights that the final abundances of radiolysis products are largely independent of the initial ice composition once a critical fluence is exceeded. This finding suggests that observed variations in CO₂/CO or H₂O₂/H₂O ratios across different astrophysical environments may reflect differences in exposure history rather than intrinsic compositional differences. The work also underscores the importance of considering heavy ion effects alongside more commonly studied light ions (e.g., H⁺, He⁺) because the former deposit substantially more energy per track, leading to higher sputtering yields and more efficient molecular fragmentation.

In summary, the paper delivers a comprehensive set of experimentally derived parameters—dissociation cross sections, sputtering yields, and steady‑state product ratios—that are directly applicable to astrochemical modeling of cold, dense regions. By reproducing the conditions of heavy cosmic‑ray irradiation, the authors advance our understanding of how water and carbon‑dioxide ices are chemically processed, how volatile inventories evolve, and how such processing may influence the composition of nascent planetary systems.