The photochemistry of irradiated Enceladus ice analogues: Implications for the formation of ozone and carbon trioxide

Detailed observations of Enceladus by the Cassini spacecraft revealed its astrobiological potential and transformed our perception of ocean worlds in the Solar System. Beneath Enceladus’ icy crust lies a warm ocean sustained by tidal heating. This ocean expels subsurface material through fissures at the south pole region into space as plumes. The particles in these plumes reaccrete on Enceladus’ surface, while some of the volatiles present in the sub-surface ocean diffuse through the ice shell to reach the surface. In this study, we irradiated thin Enceladus ice analogues in an ultra-high vacuum chamber optimised for ice chemistry at a surface temperature of 70 $\pm$ 2 K and compared the resulting composition with ices typical to the ISM (15 K). We studied the irradiation of ices composed of $H_{2}O$, ${\mathrm{CO}}_2$, and ${\mathrm{NH}}_3$ mixtures as a function of wavelength by using two different radiation sources that cover high and low photon energy ranges: The microwave-discharge hydrogen-flow lamp (MDHL) generating vacuum-ultraviolet (VUV) light, that is, between 115 - 180 nm, and the solar radiation Xe-arc lamp (SRL), simulating the solar broadband radiation from 200 nm - 1800 nm. Upon irradiation, solid-state photoproducts were identified using a Fourier-transform infrared (FTIR) spectrometer in the mid-infrared range (4000 - 700 ${\mathrm{cm}}^{-1}$ or 2.5 - 14.3 $\mu$m). Sublimating gas-phase species were tracked using a quadrupole mass spectrometer (QMS). At 70 K, energetic photons from the MDHL formed new species such as $O_3$ and ${\mathrm{CO}}_3$ in an $H_2{\mathrm{O:CO}}_2:{\mathrm{NH}}_3$ ice matrix due to the clustering of ${\mathrm{CO}}_2$ at elevated temperatures. Hereby, dissociation of segregated ${\mathrm{CO}}_2$ provides the necessary oxygen atoms to form $O_3$ via the enhanced mobility and addition reaction of O-atoms. At 15 K, $\mathrm{CO},{\mathrm{OCN}}^{-},H_2\mathrm{CO},{\mathrm{CH}}_3\mathrm{OH}$, HCOOH and possibly ${\mathrm{NH}}_2\mathrm{OH}$ were induced by VUV-photons. Similarly, these species were detected at 70 K with a tentative assignment for HCOOH and ${\mathrm{NH}}_2\mathrm{OH}$. The SRL caused no chemical evolution of the ice due to insufficient photon energies. In conclusion, we predict the formation of ozone by gardening of ${\mathrm{CO}}_2$-rich or mixed ${\mathrm{CO}}_2:H_{2}O$ ice, found, for example, in-between the tiger stripes on Enceladus or on other icy bodies in our Solar System with surface temperatures cooler than 88 K.