Questions concerning life on Mars have driven intensive studies of the Red Planet for decades, including focused investigation of possible organic molecules in recent Mars exploration. NASA’s Mars Science Laboratory, Curiosity, reported various organic materials including sulfur and/or chlorinebearing hydrocarbons from roughly 3.5-billion-year-old mudstones in Gale Crater. Chlorine-bearing methane was also found from an earlier exploration by the Viking landers, which was considered to be of Martian origin. These previous investigations suggested the existence of organic matter in the near-surface system on Mars. However, little is known about the origin, distribution, preservation, and evolution of such organics, as well as their possible relationship with Martian biological activity. Along with robotic exploration, complementary knowledge has been obtained from detailed geochemical investigations of Martian meteorites (meteorites believed to have derived from the Martian surface). Allan Hills 84001, a unique early/middle Noachian (the Noachian Period on Mars is considered to have lasted from about 4100 to 3900 million years ago, coincident with the Late Heavy Bombardment) igneous rock, contains fine-grained assemblages of secondary carbonate minerals. Previous studies reported the presence of organic carbon components in Allan Hills Meteorite carbonates, which are either Martian or terrestrial contaminants. Because Allan Hills Meteorite carbonates are considered to have precipitated via a low-temperature aqueous alteration at 4.04–3.90 billion years ago in a Martian near-surface environment, their organic records, if any, should reflect the geochemical conditions at Noachian Mars. In situ study of their chemical speciation, including nitrogen, hydrogen, oxygen, and sulphur, will help further understanding of this environment.
In a paper published in the journal Nature Communications on 24 April 2020, Mizuho Koike of the Department of Solar System Sciences at the Japan Aerospace Exploration Agency, Ryoichi Nakada of the Kochi Institute for Core Sample Research at the Japan Agency for Marine-Earth Science and Technology, Iori Kajitani, also of the Department of Solar System Sciences at the Japan Aerospace Exploration Agency, and of the Department of Earth and Planetary Science at the University of Tokyo, Tomohiro Usui, again of the Department of Solar System Sciences at the Japan Aerospace Exploration Agency, and of the Earth-Life Science Institute at the Tokyo Institute of Technology, Yusuke Tamenori of the Spectroscopy and Imaging Division at the Japan Synchrotron Radiation Research Institute, Haruna Sugahara, once again of the Department of Solar System Sciences at the Japan Aerospace Exploration Agency, and Atsuko Kobayashi, also of the Earth-Life Science Institute at the Tokyo Institute of Technology, and of the Division of Geological and Planetary Sciences at the California Institute of Technology, present the results of a study of nitrogen-bearing organic molecules from the Allan Hills Meteorite carbonates.
Nitrogen is an essential element for all life on Earth, as it is necessary for protein, DNA, RNA, and other vital materials. Nitrogen is also a useful geochemical tracer to reveal the coevolution of the planetary atmospheres, hydrospheres, lithospheres, and biospheres. Previous studies of Martian nitrogen-chemical and isotopic signatures were done by bulk-rock destructive analyses. However, in situ analysis of nitrogen chemical speciation has not been achieved due to technical difficulties. Koike et al. demonstrated for the first time the presence of trapped nitrogen-bearing organic compounds from micrometer-scale in situ analysis of nitrogen K-edge micro X-ray absorption near-edge structure (in X-ray absorption spectroscopy, the K-edge is a sudden increase in x-ray absorption occurring when the energy of the X-rays is just above the binding energy of the innermost electron shell of the atoms interacting with the photons) on the 4-billion-year-old Allan Hills Meteorite carbonates. Several carbonate grains were peeled off from a rock fragment of Allan Hills 84001fragment 248 (allocated to the researchers from NASA's Johnson Space Center in Texas, US), using silver double-sided sticky tape, which allowed Koike et al. to investigate the interiors of the individual carbonate grains. Similarly, a silicate grain in the same rock fragment was collected to serve as a background control Nitrogen K-edge micro X-ray absorption near-edge structure measurements were conducted along with various nitrogen-bearing reference compounds at the SPring-8 synchrotron facility im Hyogo Prefecture, Japan. The X-ray absorption near-edge structure spectra indicate the presence of indigenous nitrogen-bearing organics, which may have been trapped in Allan Hills Meteorite carbonates on Noachian Mars and preserved since then.
Optical and secondary electron images of Allan Hills Meteorite carbonates. (a) Rock fragment 248 of ALH 84001. The whole size is about 1.5 cm. White squares (Crb-1,2 and Crb-3,4) indicate the locations of orange-colored carbonate patches used for Koike et al.'s X-ray absorption near-edge structure spectra measurements. A silicate grain was collected from the visibly carbonate-free area, shown by a white arrow in this image (Opx-1). Enlarged images of the carbonate patches (b) and the carbonatefree silicates (c). d The carbonate grains on a silver double-sided sticky tape. Secondary electron images of the ALH carbonate (e) and silicate (f) grains. These images were taken after the Focused Ion Beam-Scanning Electron Microscope processing to remove surface contaminants. Koike et al. (2020).
Detection of nitrogen-bearing organics in Allan Hills Meteorite carbonates. Nitrogen X-ray absorption near-edge structure spectra of Allan Hills Meteorite carbonates present two prominent absorption peaks at 398.9 and 399.9 eV (electronvolts) with a broader absorption peak around 408 eV. There are additional smaller peak(s) between 400.7 and 402 eV. Such spectral shapes do not match the X-ray absorption near-edge structure spectra of molecular nitrogen, sodium nitrate, or ammonium chloride, suggesting that contributions from the inorganic nitrogen-bearing species are insignificant. On the other hand, the first two peaks at 398.9 and 399.9 eV are consistent with the absorptions of organic imino and nitrile groups, respectively. Pyridinic nitrogen-heterocyclic groups also have a similar energy range. The third peak(s) between 400.7 and 402 eV may correspond to pyrrolic nitrogen-heterocyclic, amide, and/or amino groups. It is uncertain whether all of these organic groups are intrinsic to Allan Hills Meteorite carbonates or if some of them are due to X-ray beam damage during the X-ray absorption near-edge structure measurements. Considering that the imino and/or nitrile signatures were also observed in our amino acid references, some of the imino and/or nitrile features may come from the X-ray-induced decomposition of the intrinsic compounds. Consequently, Koike et al.'s X-ray absorption near-edge structure spectra indicate that Allan Hills Meteorite carbonates contain a variety of the nitrogen-bearing organic components, whereas contributions from the inorganic N (e.g., nitrogen gas, nitrate or ammonium salt) are negligible. Plausible organic groups are the imino, nitrile, nitrogen-heterocyclic, amide, and/or amino groups.
Nitrogen K-edge X-ray absorption near-edge structure spectra. (a) A whole spectral image. The upper three spectra (Crb-1 to Crb-3) are Allan Hills Meteorite carbonates, the others are the selected reference compounds. (b) An enlarged area of the energy between 397 and 403.5 eV. Significant absorption peaks for gaseous nitrogen (N₂, 400.8 eV; magenta), Sodium Nitrate (NaNO₃, 401.4 and 405.2 eV; green), and the organic compounds (398–402.5 eV; light blue) are highlighted in both images. Major nitrogen-bearing groups of imino, nitrile, N-heterocyclic, and amino groups in this energy range are plotted. Koike et al. (2020).
When analysing extraterrestrial organic materials, terrestrial contamination is always a serious concern. In Koike et al.'s study, the possibility of laboratory contamination was minimised by conducting experiments carefully in a class 100 clean laboratory. However, contamination may have occurred from Antarctic ice before the collection of this meteorite. According to the stepwise heating analyses and the laser desorption mass spectrometry analyses for carbon isotopic ratios of ALH 84001, some portions of its non-carbonate carbon might be of Martian origin, whereas the other parts could be terrestrial contaminants. The presence of amino acids in Allan Hills Meteorite carbonates was reported through the bulk destructive analysis of high-performance liquid chromatography, most of which were regarded as Antarctic contaminants. At this moment, it is difficult to extract the Martian indigenous components from the mixtures with terrestrial contaminants. However, Koike et al.'s in situ K-edge micro X-ray absorption near-edge structure analyses reduce the risk and degree of contamination by focusing on the apparently fresh interior of the carbonate grains.
Detection limits of X-ray absorption near-edge structure measurements are typically in the order of tenss to a few parts per million. Although Koike et al.'s study does not focus on quantitative analyses, Allan Hills Meteorite carbonates should contain nitrogen-bearing organics above such a level. In contrast, concentrations of the organic materials in Antarctic surface ice are much lower, with the possible exception of locally enhanced areas in water gaps between ice crystals. Assuming sub-parts-per-billion-levels for the organic concentration in Antarctic ice meltwater co-existing with ALH 84001, intense incorporation of the organic matter into the interior of Allan Hills Meteorite carbonates at higher than 1000 times would be required for the X-ray absorption near-edge structure detection (e.g., parts-per-million-level). Such a concentration is difficult to achieve via simple adsorption because partition coefficients between minerals and organics in the coexisting liquids are far below 1000. Moreover, the apparent absence of nitrate in Koike et al.'s Allan Hills Meteorite carbonates spectra seems to be incompatible with the contributions of Antarctic ice water or Martian oxidized nitrogen. Therefore, the detected nitrogen-bearing organics in Allan Hills Meteorite carbonates are most likely of Martian origin.
If the organics in Allan Hills Meteorite carbonates are of Martian origin, they should have survived in the Martian near-surface system from Noachian period. The 4 billion year ago aqueous fluids, from which Allan Hills Meteorite carbonates precipitated, must have provided non-destructive environments for organic matter (i.e., moderate pH, Hartree energy, ultraviolet radiation, and cosmic-ray irradiation). Strong oxidants, such as chlorine oxides and nitrogen oxides, which are known to degrade organics and oxidise the Martian surface, have been reported in both current Martian regolith and in Amazonian-aged (less than three billion years old) young Martian meteorites. In contrast, Koike et al.'s Allan Hills Meteorite carbonates do not show the X-ray absorption near-edge structure features of nitrate. Previous studies of Allan Hills Meteorite carbonates propose less oxidising conditions for the 4 billion years ago Martian near-surface fluid (pH in the range 6–9, and Hartree energy in the −0.25–0 V range). Under such an Hartree energy-pH range, the co-existing nitrogen could be in the form of dissolved gaseous nitrogen and/or ammonium ions, consistent with the lack of the nitrate signatures. It is inferred that past Mars had the less oxidising surface environments compared with today, at least locally and temporally. The organic matter was able to survive in the 4 billion year ago fluids, which were trapped and preserved in the hydrous alteration minerals (i.e., Allan Hills Meteorite carbonates) over long geological times. An experimental study demonstrates that amino acids can survive for tens of million years under the current Martian UV level, if they are embedded in the shallow regolith. Energetic particles from galactic cosmic rays can penetrate down to 1–2m below on the present Martian surface and would degrade organic molecules. However, theoretical study demonstrated that simple organic molecules (about 100 atomic mass units) can survive at about 10 cm depth and complex organic molecules also have possibility to survive in much deeper subsurface for long periods. The host rock of ALH 84001 is likely to have resided in the subterranean system, indicating that its organic compounds have been protected from the severe ultraviolet radiation and cosmic-ray irradiation for billions of years.
ALH 84001 has a complicated history of formation and metamorphism. Its igneous crystallisation age was dated at about 4.09 billion years ago using lutetium–hafnium chronology (the isotope lutetium¹⁷⁶ decays to hafnium¹⁷⁶ at a predictable rate, with a half life of 37.1 billion years, enabling this system to be used for dating minerals which would not have contained hafnium at their time of formation), whereas its samarium–neodymium and rubidium–strontium ages indicate crystallisation at about billion years ago, possibly due to counterclockwise rotation of the radiometric systems during later metamorphism. The potasium-argon and argon-argon systems recorded a slightly younger age at about 3.9–4.1 billion years, associated with severe impact reheating. Allan Hills Meteorite carbonates formed at the coincident timing. Their uranoum-lead and rubidium-strontium chronologies revealed the crystallisation age at 4.04–3.90 billion years.
After the formation of Allan Hills Meteorite carbonates, some of them suffered additional impact(s), being fractured and possibly reheated. Magnesite-rich rims, typical characters for Allan Hills Meteorite carbonates, have been attributed either to the result of heat decomposition from earlier iron-rich carbonates or to the result of low-temperature fluid variation during precipitation. In the former case, the reheating temperature would have had to be uniformly around 500 °C, which would have degraded considerable part of organic compounds. However, clumped oxygen and carbon isotopes in the carbonate yield the crystallizsation temperature of about 18°C, which would have been reset to higher values even by brief heating to temperatures above 450°C. Such heating would also have erased the fine-scale geochemical zoning and would have remagnetised the rock. According to high-resolution magnetic studies ALH 84001 passes a palaeomagnetic conglomerate test, demonstrating that the interior was never above 40°C from the time of Allan Hills Meteorite carbonate formation on Mars to the meteorite’s arrival to Earth. Hence, Koike et al.'s identification of the intact-nitrogen-bearing organic compounds trapped within Allan Hills Meteorite carbonates supports this latter interpretation.
Possible origins for the Martian organics are in situ syntheses and/or meteoritical supplies. As atmospheric dinitrogen (nitrogen gas) is chemically inert due to the high activation energy of its N≡N bond, fixation of nitrogeninto accessible forms is required to produce the nitrogen-bearing compounds. Oxidised nitrogen (e.g., nitrate and nitrite) may be produced on Mars through thermal shocks by previous volcanic lightning, meteoritical impacts, cosmic-ray and solar X-ray irradiations. Meanwhile, reduced nitrogen (e.g. ammonia and hydrogen cyanide) has not been identified on Mars, mainly because of their instability on the present Martian surface environments. However, several abiotic paths for the reduction of nitrogen are proposed for the Hadean Earth, such as the reduction of nitrous oxides by metallic iron and/or aqueous solutions, as well as the photochemical reduction of atmospheric nitrogen gas. Similar processes could have occurred on early Mars. The less oxidising conditions of the Noachian period, as discussed before, seem to be suitable for ammonium ions. Moreover, the redox state of the Martian mantle is highly reducing with the free oxygen level at the iron-wustite buffer or slightly higher, where nitrogen should be present as ammonia. The reduced nitrogen may have been partly supplied from the Martian interior through early volcanic events. Ammonia is a highly reactive and important starting chemical for producing more complex nitrogen-bearing molecules. Allan Hills Meteorite carbonates may have recorded a part of the nitrogen-bearing organic/inorganic chemical evolutions process of Noachian Mars.
Possible history for the Martian nitrogen-bearing organics. Nitrogen-bearing organic matter was either synthesized locally or delivered meteoritically on the early Mars. The former case requires an abiotic reduction of nitrogen (e.g. nitrogen gas or nitrous oxides to nitrate) to start the ammonia-related chemical reactions. The latter case is also possible if adequate amounts of nitrogen-bearing components were supplied. This organic matter survived in the 4 billion year ago Martian (near)surface fluids and was trapped into Allan Hills Meteorite carbonates during precipitation. The carbonates resided in the subterranean system, preserving the organic components over long geological times. Koike et al. (2020).
Meteoritic supply of organic compounds is also a plausible interpretation. Carbonaceous chondrites are known to contain a variety of insoluble and soluble nitrogen-bearing organic groups including amino acids, amines, amides, nitrogen-heterocyclic compounds, and macromolecular organic matter (e.g., polycyclic aromatic hydrocarbons) with sub-parts per million to 100s of parts per million levels. Interplanetary dust particles and comets contain various components as well. Previous detection of the organic matter in a much younger Martian meteorite (i.e., Tissint; with a crystallisation age of about 570 million years) may reflect such an extra-Martian origin. At 4 billion years ago, however, the higher impact flux of meteoritical materials should have been supplied larger quantities of organic material onto Earth and possibly onto Mars, although their influx rates are not determined quantitatively.
Whatever the origin, the presence of the organic and reduced nitrogen on early/middle Noachian Mars indicates the importance of Martian nitrogen cycle. If considerable amounts and variations of organic matter were produced and/or delivered and preserved at the Martian near-surface system over geological time scales, these compounds have a chance to evolve into more complicated forms. It is expected that additional hidden records of the Martian nitrogen cycle will be acquired by future investigations, including a sample return mission from the Martian Moons (Martian Moons eXploration), Mars Sample Return missions, and exploration of the Martian subsurface, as well as further advanced studies of Martian meteorites.
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