No abstract

On the Hadean-Early Archean Earth, the first islands must have provided hot and dry environments for abiotically formed organic molecules. The heat sources, mainly volcanism and meteorite impacts, were also available on Mars during the Noachian period. In recent work simulating this scenario, we have shown that neat glycine forms a black, sparingly water-soluble polymer ("thermomelanoid") when dry-heated at 200 °C under pure nitrogen. The present study explores whether relevant minerals and mineral mixtures can change this thermal behavior. Most experiments were conducted at 200 or 250 °C for 2 or 7 days. The mineral matrices used were phyllosilicates (Ca-montmorillonites SAz-1 and STx-1, Na-montmorillonite SAz-1-Na, nontronite NAu-1, kaolinite KGa-1), salts (NaCl, NaCl-KCl, CaCl, artificial sea salt, gypsum, magnesite), picritic basalt, and three Martian regolith simulants (P-MRS, S-MRS, JSC Mars-1A). The main analytical method employed was high-performance liquid chromatography (HPLC). Glycine intercalated in SAz-1 and SAz-1-Na was well protected against thermomelanoid formation and sublimation at 200 °C: after 2 days, 95 and 79 %, respectively, had either survived unaltered or been transformed into the cyclic dipeptide (DKP) and linear peptides up to Gly. The glycine survival rate followed the order SAz-1 > SAz-1-Na > STx-1 ≈ NAu-1 > KGa-1. Very good protection was also provided by artificial sea salt (84 % unaltered glycine after 200 °C for 7 days). P-MRS promoted the condensation up to Gly, consistent with its high phyllosilicate content. The remaining matrices were less effective in preserving glycine as such or as peptides.

Prebiotic chemical evolution on the early Earth may have been driven in part by fluctuating environments, for example wet‒dry and temperature cycling in volcanic rock pools. Here, we describe the setup, operating principle and test applications of a newly developed ‘wet‒dry apparatus’ (WDA) designed to simulate such fluctuating environments. The WDA allows adjusting the duration of the cycles, the temperature during the wet and dry phases and the organic and mineral components, which are all key parameters in wet–dry simulations. The WDA's most important features, however, are (i) that it is automated, which means that long-time experiments are possible without the need for an operator and (ii) that the virtual absence of free oxygen in the early Earth's atmosphere at ground level can be simulated. Rigorously oxygen-free conditions were achieved by passing 99.999% nitrogen gas through two alkaline pyrogallol solutions in series, prior to entering the WDA. We used three chemical systems to test the WDA: (i) the amino acid glycine in the presence and absence of clay minerals, (ii) linoleic acid (an oxygen-sensitive amphiphile) with and without the mineral olivine, and (iii) alkaline pyrogallol solution. We observed that clay minerals greatly accelerated the decomposition of glycine under wet‒dry conditions. Glycine peptides were formed as minor products. In the course of the glycine experiments, we developed a reliable gas chromatographic method to quantify the cyclic dipeptide 2,5-diketopiperazine. The decomposition of linoleic acid in wet‒dry cycles was promoted by both air and olivine. And finally, the extremely oxygen-sensitive pyrogallol solution was used as a colour indicator for residual oxygen in the WDA. The simulation facility in our laboratory currently consists of eight identical WDAs and a surrounding infrastructure. It can be made available to others who wish to perform cyclic wet–dry experiments.

Metal complexes of porphyrins and porphyrin-type compounds are ubiquitous in all three domains of life, with hemes and chlorophylls being the best-known examples. Their diagenetic transformation products are found as geoporphyrins, in which the characteristic porphyrin core structure is retained and which can be up to 1.1 billion years old. Because of this, and their relative ease of detection, metalloporphyrins appear attractive as chemical biosignatures in the search for extraterrestrial life. In this study, we investigated the stability of solid chlorido(2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(III) [FeCl(oep)], which served as a model for heme-like molecules and iron geoporphyrins. [FeCl(oep)] was exposed to a variety of astrobiologically relevant extreme conditions, namely: aqueous acids and bases, oxidants, heat, and radiation. Key results are: (1) the [Fe(oep)] + core is stable over the pH range 0.0–13.5 even at 80°C; (2) the oxidizing power follows the order ClO − > H 2 O 2 > ClO 3 − > HNO 3 > ClO 4 − ; (3) in an inert atmosphere, the iron porphyrin is thermally stable to near 250°C; (4) at high temperatures, carbon dioxide gas is not inert but acts as an oxidant, forming carbon monoxide; (5) a decomposition layer is formed on ultraviolet irradiation and protects the [FeCl(oep)] underneath; (6) an NaCl/NaHCO 3 salt mixture has a protective effect against X-rays; and (7) no such effect is observed when [FeCl(oep)] is exposed to iron ion particle radiation. The relevance to potential iron porphyrin biosignatures on Mars, Europa, and Enceladus is discussed.