A reflectron time-of-flight mass spectrometric study on the degradation pathways of glycine on mars in the presence of perchlorates and ionizing radiation

The absence of abundant organics on the Martian surface is a much discussed observation. So far, no explanation is completely satisfactory. In this study we aim for a deeper understanding of the degradation processes of organics in the presence of perchlorates that can take place on the Martian surface. Our primary goal is to study the radiation-induced decomposition process of glycine (H₂NCH₂COOH) in the absence and presence of an oxidizer relevant to the Martian surface—perchlorate anions $\left({{ClO}_4}^{-}\right)$. Glycine and various samples of glycine-1-¹³C (⁺H₃NC${H_2}^{13}$COO⁻)–magnesium perchlorate hexahydrate (Mg(ClO₄)₂ · 6H₂O) were exposed to energetic electrons mimicking secondary electrons originating from the interaction of galactic cosmic rays (GCRs) with the Martian regolith. Using isotope-labeled and deuterated pure glycine samples such as glycine-1-¹³C, glycine-d₅ (⁺D₃NCD₂COO⁻), glycine-N,N,N-d₃ (⁺D₃NCH₂COO⁻), and glycine-2,2-d₂ (⁺H₃NCD₂COO⁻), we can conclude that decarboxylation (carbon dioxide loss) of the glycine molecule is exclusively the first decay step during irradiation regardless of whether perchlorate anions are present or not. In pure glycine samples, the decarboxylation co-product methylamine (CH₃NH₂) and its radiolytic decay product ammonia could both be detected explicitly for the first time. In the presence of perchlorates, (partial) oxidation of the glycine decarboxylation product CH₃NH₂ may occur. Because the decarboxylation is an equilibrium reaction and the CH₃NH₂ is effectively removed from the system by this oxidation, glycine cannot be recycled. Therefore the depletion of the CH₃NH₂ facilitates the process, resulting in an overall 10-fold increase in the formation rate of carbon dioxide and its elevated concentrations in the perchlorate-containing irradiated samples.