Formation of complex organosulfur compounds by sulfur implantation in astrophysical ice analogs – implications for the chemical evolution of the surface of icy objects

Irradiation of ices is a ubiquitous cause of chemical evolution of the surface of icy bodies of the solar system, due to solar UVs, solar wind particles, and magnetospheric particles. Sulfur is present in the solar wind and, in large quantities, in the jovian magnetosphere; in addition of acting as a projectile and inducing radiation chemistry, it is reactive and may be incorporated into the compounds produced. This may be a factor in increasing the chemical complexity of the surface of KBOs, TNOs, and jovian moons.

We have performed implantation of 105 keV sulfur ions into a water-methanol-ammonia ice at the Grand Accélérateur National d’Ions Lourds (GANIL) in Caen, France. Similar samples were also irradiated with argon (non-reactive projectiles). The samples were monitored in the infrared during the implantation process. The organic residues left after heating and sublimating the volatiles were then analyzed with Very High Resolution Mass Spectrometry (VHRMS). The infrared spectra of the argon-irradiated and sulfur-irradiated samples are qualitatively the same, but VHRMS shows the residue of the sulfur-irradiated sample contains more than a thousand of CHNOS formulas that are not present in the argon-irradiated sample. This indicates an active and rich sulfur chemistry induced by the implantation. The compounds formed are mostly aliphatic and can reach masses up to 700 amus. We discuss the implications for icy objects of the solar system and other ongoing experiments to explore the chemistry induced by sulfur implantation on the surface of the jovian moons.