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 ± 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 , CO 2 , and 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 cm − 1 or 2.5 - 14.3 μ 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 CO 3 in an H 2 O : CO 2 : NH 3 ice matrix due to the clustering of CO 2 at elevated temperatures. Hereby, dissociation of segregated CO 2 provides the necessary oxygen atoms to form O 3 via the enhanced mobility and addition reaction of O-atoms. At 15 K, CO , OCN − , H 2 CO , CH 3 OH , HCOOH and possibly NH 2 OH were induced by VUV-photons. Similarly, these species were detected at 70 K with a tentative assignment for HCOOH and NH 2 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 CO 2 -rich or mixed 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. • Photon Energy Effects: VUV photons (MDHL) induce significant chemical changes, leading to the formation of various organic and inorganic compounds. SRL photons, with lower energies, did not induce chemical evolution above the detection threshold, suggesting that VUV photons (6.9-10.8 eV in this work) are most responsible for ice chemistry. • Temperature Effects: At 15 K, photoproducts such as carbon monoxide (CO), cyanate ion (OCN {-} ), formaldehyde (H 2 CO), methanol (CH 3 OH) and formic acid (HCOOH) were observed. At 70K, additional products were detected, including ozone (O 3 ) and carbon trioxide (CO 3 ), due to a higher mobility of oxygen atoms and CO 2 segregation. • Ozone is likely formed through O-atom addition reactions in CO 2 -rich ice regions and is potentially trapped on Enceladus’surface below 88 K. • The estimated half-life of solid-state CO 2 and NH 3 on the surface of Enceladus is ∼ 11 and ∼ 5 days, respectively, suggesting an on-going replenishing process.