Primordial argon isotope fractionation in the atmosphere of Mars measured by the SAM instrument on Curiosity and implications for atmospheric loss.

[1] The quadrupole mass spectrometer of the Sample Analysis at Mars (SAM) instrument on Curiosity rover has made the first high-precision measurement of the nonradiogenic argon isotope ratio in the atmosphere of Mars. The resulting value of 36Ar/38Ar = 4.2 ± 0.1 is highly significant for it provides excellent evidence that "Mars" meteorites are indeed of Martian origin, and it points to a significant loss of argon of at least 50% and perhaps as high as 85-95% from the atmosphere of Mars in the past 4 billion years. Taken together with the isotopic fractionations in N, C, H, and O measured by SAM, these results imply a substantial loss of atmosphere from Mars in the posthydrodynamic escape phase.

Application of TMAH thermochemolysis to the detection of nucleobases: Application to the MOMA and SAM space experiment.

Thermochemolysis of seven nucleobases-adenine, thymine, uracil, cytosine, guanine, xanthine, and hypoxanthine-in tetramethylammonium hydroxide (TMAH) was studied individually by pyrolysis gas chromatography mass spectrometry in the frame of the Mars surface exploration. The analyses were performed under conditions relevant to the Sample Analysis at Mars (SAM) instrument of the Mars Curiosity Rover and the Mars Organic Molecule Analyzer (MOMA) instrument of the ExoMars Rover. The thermochemolysis products of each nucleobase were identified and the reaction mechanisms studied. The thermochemolysis temperature was optimized and the limit of detection and quantification of each nucleobase were also investigated. Results indicate that 600°C is the optimal thermochemolysis temperature for all seven nucleobases. The methylated products trimethyl-adenine, 1, 3-dimethyl-thymine, 1, 3-dimethyl-uracil, trimethyl-cytosine, 1, 3, 7-trimethyl-xanthine (caffeine), and dimethyl-hypoxanthine, respectively, are the most stable forms of adenine, thymine, uracil, cytosine, guanine, and xanthine, and hypoxanthine in TMAH solutions. The limits of detection for adenine, thymine, and uracil were 0.075 nmol; the limits of detection for guanine, cytosine, and hypoxanthine were higher, at 0.40, 0.55, and 0.75 nmol, respectively. These experiments allowed to well constrain the analytical capabilities of the thermochemolysis experiments that will be performed on Mars to detect nucleobases.

Oxidants at the Surface of Mars: A Review in Light of Recent Exploration Results.

In 1976, the Viking landers carried out the most comprehensive search for organics and microbial life in the martian regolith. Their results indicate that Mars' surface is lifeless and, surprisingly, depleted in organics at part-per-billion levels. Several biology experiments on the Viking landers gave controversial results that have since been explained by the presence of oxidizing agents on the surface of Mars. These oxidants may degrade abiotic or biological organics, resulting in their nondetection in the regolith. As several exploration missions currently focus on the detection of organics on Mars (or will do so in the near future), knowledge of the oxidative state of the surface is fundamental. It will allow for determination of the capability of organics to survive on a geological timescale, the most favorable places to seek them, and the best methods to process the samples collected at the surface. With this aim, we review the main oxidants assumed to be present on Mars, their possible formation pathways, and those laboratory studies in which their reactivity with organics under Mars-like conditions has been evaluated. Among the oxidants assumed to be present on Mars, only four have been detected so far: perchlorate ions (ClO4-) in salts, hydrogen peroxide (H2O2) in the atmosphere, and clays and metal oxides composing surface minerals. Clays have been suggested as catalysts for the oxidation of organics but are treated as oxidants in the following to keep the structure of this article straightforward. This work provides an insight into the oxidizing potential of the surface of Mars and an estimate of the stability of organic matter in an oxidizing environment. Key Words: Mars surface-Astrobiology-Oxidant-Chemical reactions. Astrobiology 16, 977-996.

Mars atmosphere. The imprint of atmospheric evolution in the D/H of Hesperian clay minerals on Mars.

The deuterium-to-hydrogen (D/H) ratio in strongly bound water or hydroxyl groups in ancient martian clays retains the imprint of the water of formation of these minerals. Curiosity's Sample Analysis at Mars (SAM) experiment measured thermally evolved water and hydrogen gas released between 550° and 950°C from samples of Hesperian-era Gale crater smectite to determine this isotope ratio. The D/H value is 3.0 (±0.2) times the ratio in standard mean ocean water. The D/H ratio in this ~3-billion-year-old mudstone, which is half that of the present martian atmosphere but substantially higher than that expected in very early Mars, indicates an extended history of hydrogen escape and desiccation of the planet.

Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars.

The Sample Analysis at Mars (SAM) instrument on board the Mars Science Laboratory Curiosity rover is designed to conduct inorganic and organic chemical analyses of the atmosphere and the surface regolith and rocks to help evaluate the past and present habitability potential of Mars at Gale Crater. Central to this task is the development of an inventory of any organic molecules present to elucidate processes associated with their origin, diagenesis, concentration, and long-term preservation. This will guide the future search for biosignatures. Here we report the definitive identification of chlorobenzene (150-300 parts per billion by weight (ppbw)) and C2 to C4 dichloroalkanes (up to 70 ppbw) with the SAM gas chromatograph mass spectrometer (GCMS) and detection of chlorobenzene in the direct evolved gas analysis (EGA) mode, in multiple portions of the fines from the Cumberland drill hole in the Sheepbed mudstone at Yellowknife Bay. When combined with GCMS and EGA data from multiple scooped and drilled samples, blank runs, and supporting laboratory analog studies, the elevated levels of chlorobenzene and the dichloroalkanes cannot be solely explained by instrument background sources known to be present in SAM. We conclude that these chlorinated hydrocarbons are the reaction products of Martian chlorine and organic carbon derived from Martian sources (e.g., igneous, hydrothermal, atmospheric, or biological) or exogenous sources such as meteorites, comets, or interplanetary dust particles. KEY POINTS: First in situ evidence of nonterrestrial organics in Martian surface sediments Chlorinated hydrocarbons identified in the Sheepbed mudstone by SAM Organics preserved in sample exposed to ionizing radiation and oxidative condition.

Analysis of complex mixtures recovered from space missions statistical approach to the study of Titan atmosphere analogues (tholins).

To study Titan, the largest moon of Saturn, laboratory simulation experiments have been performed to obtain analogues of Titan's aerosols (named tholins) using different energy sources. Tholins, which have been demonstrated to represent aerosols in Titan's haze layers, are a complex mixture, resulting from the chemical evolution of several hydrocarbons and nitriles. Their chromatographic analysis yields complex chromatograms, which require the use of mathematical procedures to extract from them all the information they contain. Two different chemometric approaches (the Fourier analysis approach and the statistical model of peak overlapping) have been successfully applied to pyrolysis-GC-MS chromatogram of a tholin sample. Fundamental information on the mixture's chemical composition (number of components, m) and on the separation system performance (separation efficiency, sigma) can be easily estimated: the excellent correspondence between the data calculated by the two independent procedures proves the reliability of the statistical approaches in characterizing a tholin chromatogram. Moreover, the plot of autocorrelation function contains, in a simplified form, all the information on the retention pattern: retention recursivities can be easily singled out and related to specific molecular structure variations. Therefore, the autocorrelation function (ACF) plot constitutes a simplified fingerprint of the pyrolysis products of tholins, which can be used as a powerful tool to characterize a tholin sample.

Role of trace metal ions in chemical evolution. The case of free-radical reactions.

We have studied the effect of iron in the free-radical oligomerization of hydrogen cyanide and acetic acid, and found that iron(II) and iron(III) readily reduces or oxidizes free radicals, respectively. The transient species produced by these reactions do not induce a chain oligomerization process and, therefore, they protect the solute molecules from degradation. Analysis of the available kinetic data for the reactions of a variety of transition metal ions with free radicals indicate that transition metal ions behave similarly to iron. Since Fe, Zn and Mo are essential to all living organisms, and there seems to be no apparent difference in chemical reactivity among transition metal ions towards free radicals, we suggest that these metal ions probably protected the biomolecules from degradation induced by free-radical reactions in the later stages of chemical evolution.

Experimental simulation of early Martian volcanic lightning.

A mixture of possible Martian volcanic gases were reproduced and irradiated by a high-energy infrared laser to reproduce the effects of lightning on the production of prebiotic molecules. The analysis of products were performed by a gas chromatograph interfaced in parallel with a FTIR-detector and a quadrupole mass spectrometer equipped with an electron impact and chemical ionization modes. The main products identified were hydrocarbons and an uncharacterized yellow film deposit. Preliminary results indicate the presence of hydrogen cyanide among the resultant compounds.

Corona discharge of Titan's troposphere.

The atmosphere of Titan is constantly bombarded by galactic cosmic rays and Saturnian magnetospheric electrons causing the formation of free electrons and primary ions, which are then stabilized by ion cluster formation and charging of aerosols. These charged particles accumulate in drops in cloud regions of the troposphere. Their abundance can substantially increase by friction, fragmentation or collisions during convective activity. Charge separation occurs with help of convection and gravitational settling leading to development of electric fields within the cloud and between the cloud and the ground. Neutralization of these charge particles leads to corona discharges which are characterized by low current densities. These electric discharges could induce a number of chemical reactions in the troposphere and hence it is of interest to explore such effects. We have therefore, experimentally studied the corona discharge of a simulated Titan's atmosphere (10% methane and 2% argon in nitrogen) at 500 Torr and 298 K by GC-FTIR-MS techniques. The main products have been identified as hydrocarbons (ethane, ethyne, ethene, propane, propene + propyne, cyclopropane, butane, 2-methylpropane, 2-methylpropene, n-butene, 2-butene, 2,2-dimethylpropane, 2-methylbutane, 2-methylbutene, n-pentane, 2,2-dimethylbutane, 2-methylpentane, 3-methylpentane, n-hexane, 2,2-dimethylhexane, 2,2-dimethylpentane, 2,2,3-trimethylbutane, 2,3-dimethylpentane and n-heptane), nitriles (hydrogen cyanide, cyanogen, ethanenitrile, propanenitrile, 2-methylpropanenitrile and butanenitrile) and an uncharacterized film deposit. We present their trends of formation as a function of discharge time in an ample interval and have derived their initial yields of formation. These results clearly demonstrate that a complex organic chemistry can be initiated by corona processes in the lower atmosphere. Although photochemistry and charged particle chemistry occurring in the stratosphere can account for many of the observed hydrocarbon species in Titan, the predicted abundance of ethene is to low by a factor of 10 to 40. While some ethene will be produced by charged-particle chemistry, its production by corona processes and subsequent diffusion into the stratosphere appears to be an adequate source. Because little UV penetrates to the lower atmosphere to destroy the molecules formed there, the corona-produced species may be long-lived and contribute significantly to the composition of the lower atmosphere and surface.

Chemical studies on the possible existence of life on Mars.

Although there is no direct evidence yet for the existence of life on Mars, it is reasonable to conclude that the emergence of life on Earth, which appears to have been controlled by universal laws of physics and chemistry, may have been repeated elsewhere in the universe. The dual approach of synthesis and analysis in our experimental studies has provided ample evidence in support of this hypothesis.

Transport of extraterrestrial biomolecules to the Earth: problem of thermal stability.

The idea of extraterrestrial delivery of organic matter to the early Earth is especially attractive at present and is strongly supported by the detection of a large variety of organic compounds, including amino acids and nucleobases, in carbonaceous chondrites. Whether these compounds can be delivered by other space bodies is unclear and depends primarily on capability of the biomolecules to survive high temperatures during atmospheric deceleration and impacts to the terrestrial surface. In the present study we estimated survivability of simple amino acids (alpha-aminoisobutyric acid, L-alanine, L-valine and L-leucine), purines (adenine and guanine) and pyrimidines (uracil and cytosine) under rapid heating to temperatures of 400 to 1000 degrees C under N2 or CO2 atmosphere. We have found that most of the compounds studied cannot survive the temperatures substantially higher than 700 degrees C; however at 500-600 degrees C, the recovery can be at a per cent level (or even 10%-level for adenine, uracil, alanine, and valine). Implications of the data for extraterrestrial delivery of the biomolecules are discussed.

A quantitative assay of biologically important compounds in simulated primitive Earth experiments.

A CH4-N2-H2OV gas mixture was subjected to a high voltage (20 kV), high frequency (0.3 MHz) electric discharge. The energy input in the electric discharge was varied from 0.016 to 3.048 MJ mol-1. The chemical yields (G), expressed as the number of molecules formed or destroyed per 100 eV of energy input were calculated for several products. The G values calculated at the lowest energy input were (-CH4) = 6.48; (-N2) = 2.51; (C2H2) = 1.16; (HCN) = 0.215; (CH3CHO) = 0.115; (CH3CH2CHO) = 0.00161; (CH3(CH2)2CHO) = 0.0165; ((CH2CO2H)2) = 0.0000339; (CH4 --> Solid material) = 0.196; (N2 --> Solid material) = 0.00355. This is the first report in prebiotic studies in which the G values of various products in electric discharge experiments are determined. This type of study is needed in order to get a better insight into the relative role of electric discharges on the primitive Earth.

Possible contribution of different energy sources to the production of organics in Titan's atmosphere.

A quantitative comparison of the products arising from the irradiation of a Titan's simulated atmosphere is presented. The energy sources used represent some of the main events that occur in the satellite's atmosphere. All of the compounds identified are classified in the hydrocarbon and nitrile chemical families. Almost all of the detected compounds in Titan's atmosphere are produced by one or more energy sources. The compounds with the highest energy yields include the C2 hydrocarbons, methanonitrile and ethanonitrile. The possibility of using some of the produced organics as tracer compounds during the Huygens descend to identify the leading energy form in the different atmospheric levels remains open.

Chemical and optical behaviour of tholins, laboratory analogues of Titan aerosols.

Since 1997, after having identified for the first time C4N2 (the only molecule detected on Titan and undetected in the laboratory at this date) in a simulated atmosphere of Titan, our group intended to determine several properties (including optical behavior) of laboratory analogues of Titan's tholins. This article summarizes the results obtained in the frame of that program (observation by microscopy, solubility in hydrocarbons and nitriles, chemical composition, and optical behavior in the 200-900nm range), and finally investigates the following items: what are the key questions still remaining?; how to answer them?

Radiolysis of aqueous formaldehyde relevant to cometary environments.

The radiation chemistry of aqueous solutions of formaldehyde was studied in order to obtain an insight into the possible role of ionizing radiation on cometary environments. Aqueous solutions of 1.0 mol dm-3 formaldehyde were exposed to gamma-radiation in the dose range from 0.01 to 1200 kGy at 298 K. The radiation chemical yield of decomposition of formaldehyde was determined to be: G(-CH2(OH)2)-26.3 +/- 1.2. The high radiation chemical yield of decomposition was explained by a chain reaction initiated by the radical CH(OH)2 with formaldehyde. Computer fitting of the experimental data gives k(CH(OH)2 + CH2(OH)2)- 8.0xl0(1) dm3 mol-1 s-1. In the computer treatment of experimental findings we used 54 equations to consider the radiolysis of water and 11 reactions for the radiolysis of aqueous formaldehyde. Based on previous estimates of the total dose of ionizing radiation that comets have accumulated over 4.6 billion years, we predict a radiation damage-depth curve of formaldehyde in comet nuclei.

Radiation-induced syntheses in cometary simulated models.

The behavior of an aqueous-dominant multicomponent cometary model is examined at high doses of ionizing radiation. The system is composed of a water mixture of HCN (0.2 mol dm-3), CH3CN (0.04 mol dm-3), C2H5CN (0.02 mol dm-3), CH3OH (0.12 mol dm-3) and HCO2H (0.01 mol dm-3. It was exposed to gamma rays at doses up to 18.5 MGy. The chemical kinetic database used in the computer treatment of experimental data consists of 79 reactions. A complex mixture of products has been synthesized: gases, amino acids, carboxylic acids and polymeric material. The results suggest that the pristine material in cometary nuclei may have been chemically altered by the action of cosmic rays and embedded radionuclides.

The gamma-irradiation of aqueous hydrogen cyanide in the presence of ferrocyanide or ferricyanide: implications to prebiotic chemistry.

The gamma-irradiation of 0.1 M, O2-free, aqueous HCN was studied in the presence of ferrocyanide or ferricyanide in the concentration range 10(-3) - 10(-5) M. This study was carried out in order to get an insight into the possible role that cyanocomplexes of iron may have played in promoting prebiotic syntheses via the free-radical oligomerization of HCN. It was found that ferrocyanide or ferricyanide have no effect on the irradiation of 0.1 M HCN solutions at concentrations < or = 10(-4) M. At high concentrations, 10(-3) M, they lead to a marked decrease in the conversion of HCN. There was no significant difference due to the oxidation state of iron used, particularly at high doses > or = 100 kGy.

Production of nitrogen oxides by lightning and coronae discharges in simulated early Earth, Venus and Mars environments.

We present measurements for the production of nitrogen oxides (NO and N2O) in CO2-N2 mixtures that simulate different stages of the evolution of the atmospheres of the Earth, Venus and Mars. The nitrogen fixation rates by two different types of electrical discharges, namely lightning and coronae, were studied over a wide range in CO2 and N2 mixing ratios. Nitric oxide (NO) is formed with a maximum energy yield estimated to be ~1.3 x 10(16) molecule J-1 at 80% CO2 and ~1.3 x 10(14) molecule J-1 at 50% CO2 for lightning and coronae discharges, respectively. Nitrous oxide (N2O) is only formed by coronae discharge with a maximum energy yield estimated to be ~1.2 x 10(13) molecule J-1 at 50% CO2. The pronounced difference in NO production in lightning and coronae discharges and the lack of formation of N2O in lightning indicate that the physics and chemistry involved in nitrogen fixation differs substantially in these two forms of electric energy.

Production of hydrocarbons and nitriles by electrical processes in Titan's atmosphere.

Although lightning has not been observed in Titan's atmosphere, the presence of methane rain in the troposphere suggests the possibility of electrical activity in the form of corona and/or lightning discharges. Here we examine the chemical effects of these electrical processes on a Titan simulated atmosphere composed of CH4 in N2 at various mixing ratios. Corona discharges were simulated in two different experimental arrays. For the detection of reactive intermediates we used a mass spectrometer to study the main positive ions arising by bombarding low-energy electrons from a hot filament into low-pressure methane. The final stable products, generated by applying a high voltage in a coaxial reactor with either positive or negative polarity, were separated and detected by gas chromatography-Fourier transform infrared spectroscopy and electron impact mass spectrometry (GC-FTIR-MS). Lightning discharges were simulated by a hot and dense plasma generated by a Nd-YAG laser and the final products were separated and detected by GC-FTIR-MS. Corona discharges produce linear and branched hydrocarbons as well as nitriles whereas lightning discharges generate mainly unsaturated hydrocarbons and nitriles. Lightning discharges are about 2 orders of magnitude more efficient in product formation than corona discharges.

PM3, AM1, MINDO3 semi-empirical IR spectra simulations for some nitriles of interest for Titan's chemistry.

A set of the semi-empirical methods (PM3, AM1, MNDO and MINDO3) has been tested to find the best auxiliary tool for the identification of nitriles by gas chromatography/Fourier transform IR spectroscopy/mass spectrometry, considering five nitriles of interest for Titan's chemistry as test compounds: acetonitrile, acrylonitrile, cyanoacetylene, 2-butynenitrile and dicyanoacetylene. Of the four semi-empirical methods, MNDO can be considered as the most advantageous auxiliary tool for the gas chromatography/Fourier transform IR spectroscopy/mass spectrometry (GC/FTIR/MS) identification of nitriles of interest for Titan's atmospheric chemistry, since (1) the simulated IR spectra best match the experimental (in some cases AM1 gives comparable results); (2) it provides the best linearity between the calculated and experimental frequencies (correlation coefficient of 0.990); a scaling factor of 0.90 can be applied to afford better correspondence between the calculated and experimental wavenumbers. At the same time, none of the methods is able to predict infrared intensities and a spectral intensity pattern.