Organic chemistry on the surface of jovian icy satellites: formation of complex refractory organic matter by implantation of sulfur ions into water-alkanes ices

We present the results of the implantation of sulfur ions in ice samples (H₂O:C₃H₈) in conditions relevant to Europa and other jovian satellites. The samples were prepared and irradiated at 80 K, and were thick enough to ensure the projectiles were implanted in the ice, allowing the sulfur projectiles to become part of the ensuing chemistry. The evolution of the samples was followed with Fourier Transform Infra-Red (FT-IR) spectroscopy, and the organic residues were analyzed off-site through Ultra High Resolution Mass Spectrometry (UHRMS). The UHRMS shows formation of a complex, diverse refractory organic matter, however no evidence of organosulfur formation has been found.

 

Introduction:

Energetic particles from Jupiter’s magnetosphere (electrons, protons, oxygen and sulfur ions) are likely to be a key driver of the chemical evolution of the icy satellite’s surface, especially Europa; they can alter organic matter deposited on the surface from the internal ocean. In the case of impinging ions, in addition to the stopping power and total energy of the projectile [1], their reactivity is a possible factor in the chemistry. Sulfur is abundant in Jupiter’s magnetosphere [3] due to Io’s volcanic activity, and highly reactive. Experiments with sulfur projectiles have been performed and have shown the efficient creation of sulfuric acid in water ice [4, 5]; previous experiments by our group with interstellar analogs showed it can become part of the organic chemistry triggered by irradiation and form organosulfurs [6]. Here, in temperature conditions relevant to Europa (80K), we investigate the composition of the organic residue generated by the irradiation of a water:propane ice by sulfur ions.

Experiments

We performed the experiments at the ARIBE low-energy line at the Grand Accélérateur National d’Ions Lourds (GANIL) in Caen, France. The projectiles were 105 keV  S⁶⁺ and 105 keV Ar⁷⁺ (the latter to produce a reference sample with a non-reactive projectile). The samples were prepared at 80K on a ZnSe window from a 2:1 H2O:C3H8 gas mixture and irradiated at a fluence of approximately 10¹⁴ ions. To ensure the products would be abundant enough to be detected, cycles of deposition-irradiation were repeated up to 15 times. The finally obtained samples were slowly warmed up to 300K to sublimate the volatiles and leave the refractory organic residue. This residue was analyzed off-site using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). This FT-ICR-MS technique represents the highest mass resolving power (R > 10⁶) and highest mass accuracy (<200 ppb) among all mass spectrometric instruments [7]. This technique allows for detection and identification of species well beyond what is accessible with FT-IR. Two ionization techniques were used: Laser Desorption Ionization and Electro-Spray Ionization.

Results

Infrared spectroscopy shows little difference to previous work on irradiation with electrons of similar ices [8], except for the early production of CO₂ and modest abundance of CH₄. No qualitative difference was seen between Ar-irradiated and S-irradiated samples in the IR.  

The results from the ESI or LDI FT-ICR-MS analysis show a very diverse, (3000 + annotations) refractory (average Double Bond Equivalent=14) organic matter (Figure 1). The residue features an insoluble phase. Heavy molecules (m/z>200) are common, showing that a fairly low dose is enough to form large molecules. No organosulfur could be identified, in stark contrast with our previous experiments [6]. Analysis of the soluble phase by ESI-FT-ICR shows that the oxygen-bearing products tend to be lighter. We discuss how our results compare to experiments using electrons ([8], and our own work).

 

Figure 1: Annotations (spot size is a function of intensity) obtained with the LDI-FT-ICR analysis, showing number of carbon atoms vs double bond equivalent (DBE) and number of oxygen atoms. A large amount of the residue is oxygen poor and highly aromatic.

Acknowledgements

A.B. and G.D. acknowledge the CNRS program "Programme National de Planétologie" (P.N.P, INSU), G.D. acknowledges the “Programme de Physique et Chimie du Milieu Interstellaire” (P.C.M.I, INSU) and the “Centre National d’Etudes Spatiales” (C.N.E.S) (exobiology program). H.R., P.B. and C.P.dC. acknowledge funding from Région Normandie through RIN EMERGENT SCHINOBI.

References

[1] Teolis et al. JGR: Planets 122.10 (2017): 1996-2012.
[2] von Steiger, R., Schwadron, N., Fisk, L., et al. 2000, JGR: Space Physics, 105, 27217
[3] Paranicas, C., Cooper, J., Garrett, H., Johnson, R., & Sturner, S. 2009, Europa. University of Arizona Press, Tucson, 529
[4] Ding et al. Icarus 226.1 (2013): 860-864.
[5] Strazzulla, et al. Icarus 192.2 (2007): 623-628.
[6] Ruf, et al. ApJL, 885(2), L40 (2019).
[7] Ruf et al.(2017) PNAS 114(11), 2819-2824.
[8] Hand, K. P., & Carlson, R. W. JGR: Planets, 117(E3). (2012)