RESUMO
The martian surface has been continuously exposed to galactic cosmic radiation. Since organic compounds are degraded by ionizing radiation, knowledge of their decay constants is fundamental to predicting their stability on the martian surface. In this study, we report the radiolysis constant for the destruction of soil organic compounds at a starting concentration of â¼2011 µg C/gsoil from the Mojave Desert. The soils were exposed to gamma irradiation with absorbed doses of up to 19 MGy at room temperature, representing â¼250 million years of exposure to galactic cosmic rays. The destruction of total soil organic carbon and the formation of gases were investigated by a sequential on-line analytical array coupled to gas chromatography-mass spectrometry. Soil inorganic and organic carbon were degraded exponentially with a radiolysis constant 0.3 MGy-1(30%) producing mostly carbon dioxide (93.2%), carbon monoxide (6.2%), and methane (0.6%). Using the dose rate measured by the Radiation Assessment Detector on board the Curiosity rover, we make predictions on the survival of organic compounds in the cold martian subsurface. It is estimated that soil organic compounds with initial concentrations as those found today at the Mojave Desert would have been destroyed to levels <1 ppb at 0.1 m in depth in â¼2000 Myr. Pristine organic compounds are expected to be present at a depth of â¼1.5 m. These results are relevant for the search of organic compounds in past, present, and future missions to Mars. In particular, we predict that the upcoming ExoMars will encounter pristine organic compounds at this depth.
Assuntos
Radiação Cósmica , Marte , Carbono , Meio Ambiente Extraterreno , Raios gama , Metano , SoloRESUMO
Nitrates and perchlorates are present both on Earth and Mars. In the Martian environment perchlorates dominate over nitrates whereas on Earth is contrariwise. This implies that the mechanisms responsible for their formation are different for both planets. The chemical elements required for their formation are nitrogen and chlorine, which are present in the atmosphere and surface, respectively. Dust in the Martian atmosphere causes atmospheric perturbations that lead to the development of dust-devils and sandstorms. Dust devils contain both chemical elements simultaneously, and normally generate high electric fields that can trigger the formation of electric discharges. Here we present laboratory experiments of this phenomenon using laser ablation of a sodium chloride (NaCl) plate in two different simulated atmospheres: (1) 96% CO2, 2% N2 and 2% Ar; and (2) 66% CO2, 33% N2 and 1% Ar. The dust that condensed and accumulated on the walls of the reactor was analyzed by different analytical techniques that included Fourier transform infrared spectroscopy, visible spectroscopy using azo dyes, thermogravimetry/simultaneous thermal analyses coupled to mass spectrometry, powder X-ray diffraction, and ion chromatography. The main components of the ablated dust corresponded to NaCl ≥ 91.5%, sodium nitrate (NaNO3 = 1.6-6.0%), and sodium perchlorate (NaClO4 â¼ 0.2-0.3%). It is interesting to note that these salts formed in a dry process that is relevant to Mars today. A thermochemical model was used to understand the chemical steps that led to the formation of these salts in the gas phase. The NaNO3NaClO4 (wt/wt) ratio of this process was estimated to vary from 5.0 to 30.0; this ratio is too high compared to that found on Mars (NO3-ClO4- (wt/wt)) from 0.004 to 0.13). This implies that gaseous NaCl was not efficiently oxidized to perchlorate by the electric discharge process. We propose instead that gaseous metal chlorides (e.g., MgCl2, NaCl, CaCl2, KCl) were supplied to the atmosphere by the volatilization of chloride minerals present in the dust by electric discharges generated in dust devils and were subsequently oxidized to perchlorate by photochemical processes. Further work is required to assess the relative contribution of this possible source.
Assuntos
Atmosfera/química , Marte , Nitratos/química , Percloratos/química , Poeira , Meio Ambiente Extraterreno , Simulação de Ambiente Espacial , Eletricidade EstáticaRESUMO
A key goal for astrobiology is the search for evidence of life on Mars. Because liquid water is a fundamental environmental requirement for life, the recent set of missions to Mars have focused on a strategy known as "follow the water." Since life is made of organic molecules, a logical next step is "follow the organics." However, organics are expected to be present at very low levels on Mars, which would make their detection challenging. Viking was unable to detect organics at parts per billion (ppb), but the effective upper limit could be higher due to the low efficiency of the thermal volatilization (TV) step in releasing organics. Due to its ease of use, TV is still the method selected for current and future NASA and ESA missions. Here, we show that when organics are present in the soil at levels above 1500 parts per million (ppm), there are several characteristic organic fragments detected by TV-mass spectrometry; however, when the levels are below <150 ppm, TV oxidizes them, and no organic fragments are released. Instead, nitric oxide (NO) is produced and can be used to determine quantitatively the organic content if the C/N ratio is determined. Any atmospheric NO sorbed or mineral nitrogen (e.g., nitrates) present in the soil would release NO by TV at distinctive temperature regimes that would not overlap with the organic nitrogen source. Therefore, we suggest that monitoring NO provides the best chance for Phoenix and other future Mars missions to detect nitrogen-containing organics in the soil or ice.
Assuntos
Exobiologia , Marte , Espectrometria de Massas/métodos , Compostos Orgânicos/análise , Solo/análise , Oxirredução , Voo Espacial/instrumentação , Temperatura , VolatilizaçãoRESUMO
The failure of Viking Lander thermal volatilization (TV) (without or with thermal degradation)-gas chromatography (GC)-MS experiments to detect organics suggests chemical rather than biological interpretations for the reactivity of the martian soil. Here, we report that TV-GC-MS may be blind to low levels of organics on Mars. A comparison between TV-GC-MS and total organics has been conducted for a variety of Mars analog soils. In the Antarctic Dry Valleys and the Atacama and Libyan Deserts we find 10-90 mug of refractory or graphitic carbon per gram of soil, which would have been undetectable by the Viking TV-GC-MS. In iron-containing soils (jarosites from Rio Tinto and Panoche Valley) and the Mars simulant (palogonite), oxidation of the organic material to carbon dioxide (CO(2)) by iron oxides and/or their salts drastically attenuates the detection of organics. The release of 50-700 ppm of CO(2) by TV-GC-MS in the Viking analysis may indicate that an oxidation of organic material took place. Therefore, the martian surface could have several orders of magnitude more organics than the stated Viking detection limit. Because of the simplicity of sample handling, TV-GC-MS is still considered the standard method for organic detection on future Mars missions. We suggest that the design of future organic instruments for Mars should include other methods to be able to detect extinct and/or extant life.
Assuntos
Marte , Compostos Orgânicos/análise , Compostos Orgânicos/química , Solo/análise , Carbono/química , Dióxido de Carbono/química , Catálise , Cromatografia Gasosa , Espectrometria de Massas , Oxirredução , Temperatura , Termodinâmica , VolatilizaçãoRESUMO
The Viking missions showed the martian soil to be lifeless and depleted in organic material and indicated the presence of one or more reactive oxidants. Here we report the presence of Mars-like soils in the extreme arid region of the Atacama Desert. Samples from this region had organic species only at trace levels and extremely low levels of culturable bacteria. Two samples from the extreme arid region were tested for DNA and none was recovered. Incubation experiments, patterned after the Viking labeled-release experiment but with separate biological and nonbiological isomers, show active decomposition of organic species in these soils by nonbiological processes.