Organic matter is the material basis for the production of life, and all living organisms contain organic compounds. Water-rock interactions can form organic matter, enrich mineral species, and affect the habitability of planets. The probe found complex organics in ancient rocks on the martian surface and methane in the modern Martian atmosphere. It is unclear what processes produce these organics and methane. The Martian meteorite Allan Hills 84001 (ALL84001) is about 4.09 billion years old (Lapen et al., 2010) and was formed during the Noahian period of Mars, one of the oldest Martian rocks and a window into the early planetary processes of Mars. Early planetary processes recorded in martian rocks can also occur on Earth. ALH84001 Meteorites consist mainly of orthophodoite (~95%) and a small amount of carbonate (~5%). Carbonates may be related to fluid activity on Mars 3.9 billion years ago (water-rock reactions) (Borg et al., 1999). Organic carbon and nitrogen-containing organic compounds have also been found in the ALH84001 meteorite (Koike et al., 2020). Hypotheses about the origin and formation mechanisms of these organics include impact correlation, magmatic genesis, hydrothermal processes, biological causes, and contamination of Earth matter.
In response to the controversy over the origin and formation mechanism of organic matter on Mars, Steele, a scholar at the Carnegie Institute of Science in the United States, and collaborators sampled the serpentine-talc and carbonate in the ALL84001 Martian meteorite, prepared three ultra-thin samples, and carried out nanostructure and carbon-hydrogen isotope analysis by transmission electron microscopy, synchrotron radiation spectroscopy and nanoion probes, which defined the composition and distribution of organic matter (Figure 1).
Fig. 1 FIB slices (A) and analysis results (B-K) of serpentine petrochemical area. (B-C) High-angle annular dark field (HAADF) image showing that the orthophopod fissures are filled with fibrous minerals; (D) MgO-SiO2-FeO ternary diagram showing the composition of serpentine in different regions; (E) High-resolution image of the microcrystalline in Figure B region 1, indicating that it is a flake silicate, and its Fourier rapid transformation diagram is shown in F; (G-H) TEM image of region 3 in Fig. C and its diffraction pattern; (I) Spectral peaks of carbon were obtained in different regions (scanning transmission X-ray microscopy analysis), indicating the presence of aromatic organic carbon (peak 284.9 eV) and organic oxygen functional groups (including carbonyl, 286.5 eV, carboxyl, 288.5 eV and inorganic carbonate, 290.4 eV), and the peaks indicated by asterisks may be caused by aliphatic or amide-based organic group functional groups (spectrum 3); (J) Abundance map of 12C isotopes in regions B and C (carbo-hydrogen isotopes are nanoSIMS analysis results); Hydrogen isotope distribution map of regions B and C showing that the carbon-rich region is relatively more enriched with heavy hydrogen (δD value of 790 ± 140 ‰), higher than the Earth sample, but consistent with the Martian crustal water (δD value of 700-2700 ‰) (Steele et al., 2022)
The results of the study are used to explore the early water-rock reaction process of Martian rocks and their genesis links with organic matter, which has certain implications for the source, formation mechanism and habitability of organic matter on Mars.
(1) The source of organic matter. The mineral combinations at the orbidopyrene edges of the ALH84001 meteorite (amorphous silica, talc, magnetite, and Fe-Mg-Ca-rich carbonates) are similar to rocks on Earth that underwent serpentine and carbonation alteration, suggesting that a similar alteration process occurred on early Mars. However, a smaller proportion of carbonate, amorphous silicate, and other silicate phases formed by alteration suggests that the water-rock reactions of Martian rocks are not very active. Importantly, what exactly is the geological event that caused the alteration of Martian rocks? Previous studies have shown that ARAH84001 samples may undergo multi-stage fluid interpretation. Analogous to the formation of serpentine rocks on Earth, the alteration process is controlled by a variety of elements such as rock composition, temperature, pressure, pH, SiO2 activity, anion and cation concentration, redox conditions, and water activity and CO2 escape. Therefore, the fluid composition, formation conditions, the exact formation time and mechanism of talc, carbonate and organic matter in the ALH84001 sample are currently difficult to define. However, the close symbiosis of altered minerals with organics suggests that serpentination and carbonate alteration are the sources of organic matter in the ALH84001 sample (Figure 1). Combined with the formation era of carbonation alteration (39-4 billion years, Borg et al., 1999), Steele et al. inferred that the serpentine petrochemical and carbonation of orychopyroxene may have formed the organic carbon of early Mars.
(2) The formation mechanism of organic matter. Serpentine petrochemicalization is an abiotic organic synthesis mechanism. Basalt rocks react with fluids to produce serpentine, magnetite and hydrogen, which reduces CO2 to methane, CO and organic matter such as formic acid and formaldehyde. CO and H can also be further reacted to form alkanes and other organic molecules, including nitrogen-containing organics. The alh84001 sample of organic matter coexisted with magnetite in the mineral combination of serpentine and carbonate (Figure 1), indicating that serpentinization and carbonation are important processes for the formation of organic compounds. It is important to note that no organic matter was detected in the fissures of undeveloped serpentine-slip petrochemicals and carbonateization. It can be inferred that the organic matter in the ALH84001 meteorite sample is closely related to the water-rock reaction.
(3) An indication of Mars habitability. The above findings indicate that serpentination and carbonation reactions have occurred on Mars, which coincide with the serpentine region observed by Martian satellites and are consistent with the study of the Nakhla Martian meteorite. The formation of organic matter in ALH84001 rocks is similar to the hydro-rock reaction process of Earth rocks, serpentine and carbonate are formed by co2-containing fluids to magnesium iron rocks, and aromatic, aliphatic, carbonyl, carboxyl and carbonate substances are produced from CO2-containing fluids. The correlation between organic matter and serpentine-carbonate minerals suggests that these organic matter are abiotic. Interestingly, the ARAH84001 rock, formed 4 billion years ago, has similar organic characteristics to the Tissint meteorite formed over 600 million years. This suggests that Mars has had synthesis reactions of organic matter of abiotic genesis for most of its history. On Earth, the hydro-rock reactions of ultra-magnesiumite dominate the synthesis of abiotic organic matter, the formation of methane, and mineral diversity. On Mars, this reaction may be closely related to its habitability and could explain the source of methane in the atmosphere.
(Acknowledgements: Thanks to Gu Lixin's senior engineer for perfecting the analysis methods and research conclusions, Dr. Yuan Fanglin conducted a text proofreading.) ）
Key references
Borg L E, Connelly J N, Nyquist L E, et al. The age of the carbonates in Martian meteorite ALH84001[J]. Science, 1999, 286(5437): 90-94.
Lapen T J, Righter M, Brandon A D, et al. A younger age for ALH84001 and its geochemical link to shergottite sources in Mars[J]. Science, 2010, 328(5976): 347-351.
Koike M, Nakada R, Kajitani I, et al. In-situ preservation of nitrogen-bearing organics in Noachian Martian carbonates[J]. Nature Communications, 2020, 11(1): 1-7.
Steele A, Benning L G, Wirth R, et al. Organic synthesis associated with serpentinization and carbonation on early Mars[J]. Science, 2022, 375(6577): 172-177.
Written by: Mao Yajing/Mining Room
Editor: Chen Feifei
Proofreader: Wan Peng