Effect of Perchlorates on Electron Radiolysis of Glycine With Application to Mars

Abstract

This work explores the radiolytic decomposition of glycine (H 2 NCH 2 COOH) under simulated Martian conditions in the presence of perchlorates (-ClO 4), which are abundant oxidizers on the surface of Mars, by energetic electrons at 10, 160, 210, and 260 K, mimicking the radiation exposure of the Martian regolith in the first 5-10 cm depths over about 250 million years. Our experiments present quantitative evidence that the rate constants of the glycine decomposition in the presence of magnesium perchlorate hexahydrate (Mg(ClO 4) 2 • 6H 2 O) were a factor of about two higher than that of the pure glycine, suggesting that energetic oxygen atoms (O) released from the-ClO 4 have a significant effect on the decomposition rates and accelerate them by providing a unique oxidizing environment in the radiolyzed samples. Hence, two decay mechanisms exist: radiolysis by the electrons and oxidation by the O atoms. Within the Mars-relevant temperature range covering 160-260 K, the destruction rates are nearly temperature invariant with rates varying as little as 5%. Further, the formation rates of carbon dioxide (CO 2) and carbon monoxide (CO) are both accelerated in the presence of-ClO 4 by a factor of three to five, supporting our conclusion of an active oxygen-initiated chemistry. In addition, the degradation rates are significantly higher than the formation rates of CO 2 and CO. This suggests that, besides the decarboxylation, alternative degradation pathways such as a polymerization of glycine must exist. Finally, besides CO 2 and CO, three alternative products were identified tentatively: methylamine (CH 3 NH 2), methane (CH 4), and ammonia (NH 3).

Degradation of Adenine on the Martian Surface in the Presence of Perchlorates and Ionizing Radiation: A Reflectron Time-of-flight Mass Spectrometric Study

“…By knowing the volume of the solution added onto the substrate, the average density and area of the solid sample and the average sample thicknesses could also be calculated. The samples were prepared by utilizing the method established in our previous work (Góbi et al 2016a). Briefly, pure adenine (for the neat adenine samples) or adenine with Mg(ClO 4 ) 2 •6H 2 O in a 1:1 molar ratio (for the adenine-Mg (ClO 4 ) 2 •6H 2 O 1:1 mixture samples) were dissolved in distilled water (H 2 O), then 0.250-0.390 ml of these solutions (Table 2) was placed onto the surface of the silver substrates.…”

“…A later work also found an alternative pathway resulting in chlorine dioxide (ClO 2 ) aside from oxygen, which may further accelerate the decomposition of organics as it is an even more proficient oxidant (Góbi et al 2016b). It has also been previously shown that two parallel decay mechanisms coexist when amino acids are irradiated in the presence of magnesium perchlorate hexahydrate (Mg(ClO 4 ) 2 •6H 2 O): the radiolytic decomposition of the organic molecule by the energetic electrons and the oxidation of the molecule and its irradiation products by oxygen formed upon the irradiation of neighboring perchlorate units (Góbi et al 2016a). It is important to note that the most stable form of magnesium perchlorate under Martian conditions is hexahydrate (Chevrier et al 2009;Toner et al 2014).…”

IN SITU DETECTION OF CHLORINE DIOXIDE (ClO2) IN THE RADIOLYSIS OF PERCHLORATES AND IMPLICATIONS FOR THE STABILITY OF ORGANICS ON MARS

“…This finding may suggest that they can also oxidize sensitive molecules upon irradiation of energetic particles, which contributes to the lack of organics on the Martian surface. Laboratory experiments have provided compelling evidence that even complex organic molecules such as amino acids like glycine (H 2 NCH 2 COOH, Góbi et al 2016) can be oxidized efficiently in the presence of oxidants such as perchlorates ( -ClO 4 ) exposed to ionizing radiation in the form of ultraviolet (UV) photons and energetic galactic cosmic-ray (GCR) particles in the Martian soil (Pavlov et al 2012). Although their overall energy flux is four orders of magnitude less than the energy flux of the solar photons (Pavlov et al 2012), GCRs are the most relevant agent in the destruction of the organics in the deeper layers of the Martian soil since UV photons are effectively absorbed within the first few microns of the soil (Muñoz-Caro et al 2006).…”

Catalytic/Protective Properties of Martian Minerals and Implications for Possible Origin of Life on Mars

“…This observation can be explained if the oxychlorine in Gale Crater is poorly crystalline, or its concentration is below the 1 wt% detection limit of CheMin, which means that the concentration of oxychlorine species may vary across the planet. Unfortunately, only a few studies have been reported in the literature about the effect of irradiation on the stability of organic molecules in the presence of perchlorates [136,137], while many studies have tried to reproduce the pyrolysis results of the Viking, Phoenix, and MSL missions (see Lasne et al [125], and references therein). Hydrogen peroxide has been detected so far only in the atmosphere of Mars, at levels ranging from 18 to a maximum of 40 part per billion (ppb), with seasonal variations depending on the abundance of atmospheric water vapor and water ice clouds [140][141][142].…”