Complex ice chemistry: A comparative study of electron irradiated ices and the formation of volatile irradiation products

Abstract

Ices are ubiquitous in the Solar System: They are found on the surfaces of planets and moons, on Kuiper belt objects, and are important constituents of cometary nuclei. The pristine ices consist mainly of simple molecules, such as H2O, N2, and CH4 [1, 2]. Laboratory experiments on the energetic processing of ice have shown that complex molecules form, including prebiotic molecules [3,4]. Furthermore, energetic processing of ices can sputter and desorb molecules from the ice, therefore populating the faint atmosphere of planetary objects. We simulate ice chemistry with laboratory experiments using the in-house designed and constructed ICEBEAR setup. Briefly, a copper sample holder is placed in a high vacuum chamber (P<10–8 mbar) and cooled down to temperatures as low as 4 K using a cryostat. Gas mixtures are prepared in a separate gas mixing line and injected into the chamber via a leak valve, where the gas mixtures condense on the sample holder due to the low temperatures and form a layer of ice. The setup is equipped with an electron gun to process the ice. After irradiation, a temperature-programmed desorption – mass spectrometry (TPD-MS) measurement is performed, during which the sample holder is heated from 20 to 300 K with a heating rate of 2 K/min and the desorption products are monitored using a Quadrupole Mass Spectrometer (QMS). We present the results of TPD-MS measurements of electron-irradiated methane-containing CH4:H2O, CH4:N2, and CH4:H2O:N2 ices and the influence of the different components on the formation of volatile irradiation products is discussed. Our studies show the formation of C2 and C3 hydrocarbons for all irradiated ices. However, their abundance and desorption temperature strongly depend on the starting composition and irradiation duration, with an increase in irradiation time for the ices containing H2O generally leading to a decrease in the abundance of the observed volatile products. The addition of molecular nitrogen to an H2O:CH4 mixture also reduces the abundance of volatile desorption products recorded with the QMS. Several organic molecules are detected, such as methanol, as well as mass spectrometric signatures pointing to the formation of larger hydrocarbons and alcohols. These results underline how the molecular composition and irradiation durations of ice affect the resulting products and their desorption behavior. The experimental data helps understand which kind of products are expected to be found on the surfaces of methane-rich planetary objects or contribute to their atmospheres upon their release to the gas-phase.