2021ApJ...920...73Z

The observation of complex organic molecules (COMs) in the gas phase of cold molecular clouds has coined a freeze-out paradox in astrophysics: COMs should be accreted on low-temperature interstellar grains, but not observable in cold molecular clouds. Still, validated mechanisms transporting molecules from the grains back into the gas phase are still elusive, but critical for our understanding of the chemical evolution of the molecular universe. Here we report on the first characterization of rapid radical reactions involving methyl (CH₃) and formyl (HCO) radicals in interstellar analogous ices of methane (CH₄) and carbon monoxide (CO) upon exposure to proxies of galactic cosmic rays. Rapid radical chain reactions and explosive desorption occurred once the accumulated radicals surpassed critical concentrations of about 1% in the ices at temperatures of cold molecular clouds (5-10 K). These processes may explain the ejection and observation of COMs in the gas phase of cold molecular clouds and potentially rapid outbursts of comets.