Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: formation and destruction of solid carbonic acid upon 1 keV electron irradiation

Abstract

Context. Carbonic acid (H2CO3) is a weak acid relevant to astrobiology which, to date, remains undetected in space. Experimental work has shown that the β-polymorph of H2CO3 forms under space relevant conditions through energetic (UV photon, electron, and cosmic ray) processing of CO2- and H2O-rich ices. Although its α-polymorph ice has been recently reassigned to the monomethyl ester of carbonic acid, a different form of H2CO3 ice may exist and is synthesized without irradiation through surface reactions involving CO molecules and OH radicals, that is to say γ-H2CO3. Aims. We aim to provide a systematic set of vacuum ultraviolet (VUV) photoabsorption spectroscopic data of pure carbonic acid that formed and was destroyed under conditions relevant to space in support of its future identification on the surface of icy objects in the Solar System by the upcoming Jupiter ICy moons Explorer mission and on interstellar dust by the James Webb Space Telescope spacecraft. Methods. We present VUV photoabsorption spectra of pure and mixed CO2 and H2O ices exposed to 1 keV electrons at 20 and 80 K to simulate different interstellar and Solar System environments. Ices were then annealed to obtain a layer of pure H2CO3 which was further exposed to 1 keV electrons at 20 and 80 K to monitor its destruction pathway. Fourier-transform infrared (FT-IR) spectroscopy was used as a secondary probe providing complementary information on the physicochemical changes within an ice. Results. Our laboratory work shows that the formation of solid H2CO3, CO, and O3 upon the energetic processing of CO2:H2O ice mixtures is temperature-dependent in the range between 20 and 80 K. The amorphous to crystalline phase transition of H2CO3 ice is investigated for the first time in the VUV spectral range by annealing the ice at 200 and 225 K. We have detected two photoabsorption bands at 139 and 200 nm, and we assigned them to β-H2CO3 and γ-H2CO3, respectively. We present VUV spectra of the electron irradiation of annealed H2CO3 ice at different temperatures leading to its decomposition into CO2, H2O, and CO ice. Laboratory results are compared to Cassini UltraViolet Imaging Spectrograph observations of the 70−90 K ice surface of Saturn’s satellites Enceladus, Dione, and Rhea.

Linear and Helical Carbonic Acid Clusters

“…7 Ultimately, carbonic acid has been shown to form readily through ionizing radiation and vacuum−UV light on H 2 O + CO 2 ice. 8 As noted by Peeters and co-workers, both H 2 O and CO 2 have been found in the outer Solar System on satellites orbiting Jupiter, Saturn, Uranus, and Neptune. 7 These environments have radiation which can enable the formation of carbonic acid.…”

“…Solid carbonic acid has also been produced through acid–base reactions between HBr and KHCO 3 under vacuum at low temperature . Ultimately, carbonic acid has been shown to form readily through ionizing radiation and vacuum–UV light on H 2 O + CO 2 ice . As noted by Peeters and co-workers, both H 2 O and CO 2 have been found in the outer Solar System on satellites orbiting Jupiter, Saturn, Uranus, and Neptune .…”

On the origin of molecular oxygen on the surface of Ganymede

“…The experiments described here were performed using the custom-made Portable Astrochemistry Chamber (PAC), a high vacuum (HV) system with a base pressure of 10 -9 mbar. The apparatus is described in detail in Ioppolo et al (2020Ioppolo et al ( , 2021. Here briefly, the PAC consists of a compact spherical cube chamber (Kimball Physics) connected to a turbo molecular pump, a closed cycle helium cryostat with a base temperature of 20 K, and a 1 keV electron gun (Kimball Physics).…”

Unusual Chemical Processes in Interstellar Chemistry: Past and Present

“…Cosmic rays are mainly highly ionized ions ranging in mass from protons to heavy nuclei. The laboratory study of radiolysis using high-energy protons or electrons but occasionally high-energy nuclei (Wakelam et al, 2021) is well advanced with many experiments in many laboratories (Bennett et al, 2005;Hudson and Moore, 2018;Christoffersen et al, 2020;Ioppolo et al, 2021) and some corresponding theory (see Figure 6). It was not until a few years ago that a theoretical approach to the chemistry was developed by astrochemists , tested successfully against simple cold interstellar ices in the laboratory such as O 2 and amorphous water, and applied to interstellar icy mantles Shingledecker et al, 2020).…”