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

¹ School of Electronic Engineering and Computer Science, Queen Mary University of London, London E1 4NS, UK
e-mail: s.ioppolo@qmul.ac.uk
² Astronomical Institute of Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
³ School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
⁴ ISA, Department of Physics and Astronomy, Aarhus University, Aarhus 8000, Denmark
⁵ School of Physical Sciences, University of Kent, Canterbury CT2 7NH, UK
⁶ INAF – Osservatorio Astrofisico di Catania, Catania 95123, Italy

Received: 14 August 2020
Accepted: 23 December 2020

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.