Ozone in Planetary Ices: Solid-State Detection under Enceladus-like conditions

Introduction:

Once considered enigmatic, Enceladus has attracted steadily growing scientific interest over the past 20 years since the first visual detection of cryovolcanic plumes venting from the tiger stripes in 2005 [1]. A combination of magnetospheric and gravity data revealed that under a kilometer's thick icy crust, Enceladus harbours a warm subsurface ocean maintained by tidal dissipation and hydrothermal vents on the oceanic floor [2, 3, 4]. This ocean escapes the icy crust through plumes that expel a mixture of vapour and icy grains composed mainly of water, salts and traces of organic material [5]. Material from these plumes feeds the E-ring or falls back to the surface depending on the particle’s velocity. While the surface of Enceladus is mostly rich in water, patches of CO₂ ice have been observed in-between the tiger stripes, possibly due to gas exsolution from the ocean and diffusion through small fissures in the icy crust [6]. Despite the wealth of these new discoveries, many of the moon’s endogenic and surface processes remain unresolved to date. For instance, how material is exchanged between the ocean and the surface, or how surface ice is affected by the external environment are key questions of interest. In this work, we studied the UV-photolysis of H₂O and CO₂-rich ices under Enceladus environmental conditions to determine the products of photo-induced reactions as well as the lifetime of these ices [7, under review]. 

Methods:  

To this end, we use a cryogenic ultra-high vacuum (UHV) setup housed at Leiden University’s Laboratory for Astrophysics that is employed to investigate the formation of complex organic molecules through energetic processing of simulated Enceladus ices. Thin CO₂-rich ice films are grown at a base pressure of 10^-11 mbar on a substrate that is cryogenically cooled and thermally controlled.  These ices are photo-irradiated using a specialized microwave discharge hydrogen-flow lamp, producing vacuum ultraviolet (VUV) light with an SED including Ly-α and H₂-emissions. The UHV system is equipped with two diagnostic tools for the spectroscopic analysis of thin ices across a range of compositions and temperatures, allowing for detailed investigation of the physico-chemical processes within the ice. Changes in the solid phase are tracked via Fourier-transform infrared (FTIR) spectroscopy, while a quadrupole mass spectrometer provides complementary detection of gas-phase species that desorb during linear warming of the substrate from 70 to 200 K.   

Results and Discussion: 

By systematically probing VUV-photolysed pure (H₂O, CO₂, and NH₃) and mixed ices (H₂O:CO₂:NH₃), the detection of photo-induced ozone in an Enceladus-like ice at 70 K is verified spectroscopically, besides other O and N-bearing chemical products forming in the ice. We found that ozone entrapment in CO₂-rich ice occurs at temperatures as high as 88 K. Most likely, ozone is produced from the photodissociation of segregated CO₂into CO + O, followed by consecutive O-atom addition reactions that form molecular oxygen (O₂) and, finally, ozone (O₃). Further, the survival of the VUV-irradiated parent molecules, CO₂ and NH₃, is quantified based on the fitted UV photodestruction cross-sections and half-lives, assuming first-order kinetics. The molecular half-lives are found to range from a few to several weeks on Enceladus. Our experimental work highlights that such short geological timescales possibly suggest an ongoing replenishing process that supplies surface CO₂ as a key precursor for the formation of ozone. Future in-situ or remote-sensing detections of this molecule may serve as an indicator of geological activity involving surface renewal processes. We showed that ozone remains trapped in the solid-state in CO₂-rich ices at temperatures up to 88 K and may therefore be found on Enceladus - or other sufficiently cold Solar System bodies that are exposed to comparable or higher levels of UV-radiation.  

References:
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