The isotopic composition of organic carbon deposited on Mars could trace potential sources of its native organics, revealing the processes of the Martian carbon cycle. Over the past nine years (August 2012-July 2021), the Mars Science Laboratory (MSL) - Curiosity rover has conducted long-term explorations of the river and lake sediment system near Gale Crater, collecting and analyzing more than 30 borehole samples taken from different lithological units in the hundreds of meters of Gale Crater (Figure 1), representing the complex evolutionary history of the region and providing an excellent opportunity for carbon isotope research.
Figure 1 Geological background information of the samples in this study. (A) A histogram of the formation marked with each MSL drilling location. (B) A Moray_Firth Mastcam mosaic (mcam14053) taken on Mars Day 2685 shows the foothills of greenheugh pediment near the EB borehole. (C) Hu borehole in the Glasgow section of the Murray Formation located at the foot of Green Hill. (D) HF drill hole in Murray grey mudstone in the jura section of the upper VRR. (E) Namib dunes (belonging to the Bagold dunes) where GB samples were collected. (F) Location of Yellowknife Bay, where CB drilling is drilled in the muddy rocks of the Sheepbed section of the Bradbury Group
In order to constrain the source of carbon in Martian sedimentary rocks, Dr. Christopher House of Pennsylvania State University and his collaborators performed carbon isotope (δ13C) analysis of CH4 produced by pyrolysis in 24 samples of Gale Crater by tunable laser spectrometer (TLS) during the precipitation gas analysis (EGA) mission, combined with the ISO2 sulfur isotope composition of SO2 sulfur produced by the lysis of these samples published by previous publications (δ34S, determined by quadrupole mass spectrometer), Three possible models of carbon sources are proposed. The study was published in PNAS.
The results show that the CH4 produced by the cleavage has a wide range of δ13C values (-133‰~+22‰), of which the CH4 with an extreme loss of 13C (that is, the δ13C value is very negative) corresponds to the SO2 of the lower δ34S value in most cases (Figure 2). Interestingly, in 10 losses 13C (δ13C2).
The current major end-element carbon pools of Mars include atmospheric CO2 and carbon from ignifying sources such as volcanic eruptions. The former has a significantly positive δ13C value (+46±4‰) due to the massive dispersion of volatiles in the atmosphere (Webster et al., 2013), while the latter δ13C value is relatively negative (-20±4‰) (Steele et al., 2012). In contrast, some of the samples reported in this article have an unusually large 13C drawdown. The authors considered the possible contribution of mtBSTFA (N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide, δ13C = -35‰) background in sample analysis instruments to the lysis of CH4, and even this could not explain those extremely negative δ13C values.
On Earth, the researchers' classic explanation for the high loss of 13C in the ancient surface environment is the process by which methane trophic microorganisms convert methane, which has already lost 13C, into biomass. For example, total organic matter δ13C values as low as -60‰ were found in the archean Tumbiana formation (Eigenbrode and Freeman, 2006). Based on the observed release of large amounts of methane plumes from the subsurface of Mars and the understanding of the way microbes oxidize methane on Earth, researchers have suggested that microbial methane trophic patterns may be a metabolic mode on recent and ancient Mars (House et al., 2011). This process can also explain the presence of reduced sulfur compounds in many samples with a relatively negative δ13C value, that is, the coexistence of microbial organic matter and sulfides with a loss of 13C can be achieved through anaerobic oxidation of methane with sulfates as electron receptors. The anaerobic oxidation of methane on Earth causes δ13C to be minus 30‰ (House et al., 2009). If methane microbial metabolism on Mars produces the same isotope fractionation, the methane consumed by the microorganisms needs to have a delta13C value of -40 to -100‰ to explain the 13C loss observed here. This requires the inorganic CO2 supplied to the microbial system to have the same carbon isotope composition (-20‰) as The Martian magma carbon, rather than the 13C highly enriched value (+46‰) observed in the Martian atmosphere today. For this model to be effective, one of two scenarios is that 13C of depleted carbon has been deposited before the Martian atmospheric carbon has undergone a large amount of loss, or that the Martian subsurface CO2 reservoir is isolated from the atmosphere. The paleosurface studied in this paper is later in the Gala Crater sedimentary strata and does not support the first case. The second case is difficult to assess because our knowledge of the hydrodynamics of the Martian subsurface is very limited. All in all, the explanation for this surface methane vegetative organism has been sufficiently questioned due to the lack of sedimentary evidence of microbial surface activity and the need to avoid being affected by the 13C-enriched Martian atmosphere.
Figure 2 EGA TLS CH4δ13CVPDB value compared with EGA SO2δ34SVCDT value. The error bar is 1SE. As a reference, the plot is divided by a dotted line over the origin, and the gray line shows a weighted linear fit (y = [8 ± 3]x – [59 ± 12], mean standard weight deviation (MSWD) = 7). In most sample analyses of Gale Crater, when the precipitated gas CH4 has a largely negative δ13C value, the precipitated gas SO2 also loses 34S
To explain this unusual isotopic signature of carbon on Mars, Dr. House et al. broke with conventional thinking and proposed three possible sources of carbon.
Cosmic Dust Clouds: The Molecular Cloud Hypothesis states that the solar system passes through a huge galactic molecular cloud every 100 million years. 1% of this interstellar molecular cloud is made up of dust. Evidence from the Allende meteorite suggests that the delta13C value of interstellar dust can be as low as -260‰ (Ash et al., 1988). This could provide a highly depleted 13C carbonaceous material to the Surface of Mars. Similarly, these particles may also be associated with sulfides with a relatively negative delta34S. The flux of interstellar dust particles is very low and is usually diluted by carbon from other sources. But molecular clouds cause the Martian globe to cool sharply, leading to glacial motion, while interstellar dust can be deposited on the surface of glaciers, protected from the carbon typically deposited on Mars. After the glacier melts, this interstellar dust is retained on the surface of the glacier landform, forming a layer of deposits containing light carbon material. The existing sedimentary evidence does not rule out the possibility of glacial processes. Overall, this explanation is possible, but additional research is needed to determine the δ34S value and the relative amount and properties of carbon in cosmic dust.
Abiotic reduction of carbon dioxide: Possible ways to produce organic carbon through abiotic processes on Mars include electrochemical reduction (Steele et al., 2018) or photochemical reduction (Franz et al., 2020). In both processes, CO2 is converted into organic matter, which theoretically causes a loss of 13C in the product. Moreover, for electrochemical reduction, sulfides can be used as reducing minerals to drive the occurrence of this carbon sequestration reaction, and likewise, sulfides are also catalytic minerals that promote photochemical reduction of CO2, which can explain the coexistence of 13C loss and reducing sulfur. However, abiotic reduction reactions can only produce ~50‰ carbon isotope fractionation, which cannot yet explain those very negative δ13C values. However, the degree of fractionation of carbon isotopes produced by the photochemical reduction of CO2 on the surface of minerals is unknown, so research work is needed to fully evaluate whether abiotic reduction can lead to extreme loss of carbon isotope values of 13C.
Photolysis of CH4/CO and SO2: There are several different photochemical reaction pathways that can produce large 13C losses. CH4 is a known component of the Martian atmosphere, although its concentration varies. In a relatively dry and hypoxic atmosphere, CH4 is capable of producing organic aerosols and particulate matter by photolysis, a process that can lead to negative carbon isotopes. If SO2 and CH4 undergo a photochemical reaction to produce sulfur-containing organic molecules, this model can explain the coexistence of reduced sulfur at 34S loss and 13C loss. However, previous studies have shown that the photodegradation of CH4 results in fractionation of carbon isotopes of less than 15‰ (Nair et al., 2005), and even assuming that CH4 has an isotopic composition of the interior of Mars (-20‰) cannot explain the negative anomalies of carbon isotopes observed at Gale Crater. In view of this, Dr. House et al. proposed a model that includes a series of reactions, namely the introduction of CO2 abiotic reduction in the serpentine petrochemical process, or microbial methane-producing action. CO2 (δ13C = -20‰) first undergoes abiotic reduction into CH4, and then a photochemical reaction occurs, so that the resulting organic material may have a carbon isotope composition of -85‰, but it is not enough to explain all the observed δ13C values. If CH4 in the atmosphere is generated by microbial methanogenesis, and these CH4s are photolysis again, a carbon isotope composition with a strong loss of 13C can be obtained. But before this explanation is accepted, at least new observations are needed to determine that the Martian CH4 plume originated from microbial action.
In addition, the photochemical reduction of CO2 to formaldehyde (CH2O) may also produce 13C loss of organic matter, because the photochemical division between CO2 and CO will cause 13C to enter CO less often. This reaction requires the participation of ultraviolet light, especially vacuum ultraviolet light (VUV) with a wavelength of about 100 nm. In fact, coolysis occurring at the top of the Martian atmosphere results in fractional fractionation of carbon up to 600‰ (Hu et al., 2015). If a Martian photochemical reaction produces 13C of loss of CO in the past atmosphere, then generates formaldehyde or other organic compounds through photolysis reactions, and then deposits into the paleosurface exposed by Gale Crater, the isotopic results reported here could result. However, the organic material produced by the CO2 photolysis reaction is difficult to deposit because the reaction yield is low and the photochemical stability of the formaldehyde produced is poor. However, photochemically produced methane may react rapidly with SO2 to form hydroxymethylsulfonate, which in turn prevents it from being photolyzed into CO. After larger volcanic eruptions, there will be higher concentrations of CO, SO2, and H2 in the atmosphere, at which point the deposition of photochemical organic products (which may include formaldehyde, hydroxymethylsulfonate, carbonyl sulfur, and thioformaldehyde) will reach maximum. Because SO2 has a long photochemical equilibrium lifetime (about 1 Martian year) and a blocking effect on formaldehyde photodegradation, the likelihood of such photochemical reactions increases. The 13C loss of organic matter from the reaction added to the fog or glacial ice on the surface of Mars can also act as a concentration, causing them to be preserved locally. Due to the lack of VUV reaction experiments at different temperatures, and the need to investigate the detailed characteristics of the Martian CO spectrum and the carbon isotope effects of photochemical processes, it is not yet possible to draw definitive conclusions.
Figure 3 Three possible scenarios for isotopic carbon loss sources observed by SAM TLS. The blue section shows that methane, a biological source from the interior of Mars, can lead to the deposition of 13C loss of organic matter after photolysis. Although the deposition of 13C loss organic matter may also be caused by methane vegetative organisms, there is no evidence of the existence of any methane vegetative microbial seats on the paleosurface. The orange section shows that photochemical reactions (UVs) are capable of producing a variety of atmospheric products, some of which are deposited in the form of relatively unstable organic matter. Under Martian conditions, it is still unknown whether the process by which CO2 is photochemically reduced to CO produces large isotope fractionation. The gray part shows that if our solar system passes through a giant molecular cloud (GMC), the 13C loss of organic matter will enter the Martian atmosphere
Taken together, the authors' explanations consistent with isotopic values and geological evidence include cosmic dust deposition as Mars passes through giant molecular clouds, photolysis of biomethane, and photoretrination of CO2 (Figure 3). All three of these scenarios are unusual, unlike the processes that normally occur on Earth. With the current level of knowledge, it is difficult to determine which scenario best portrays the events that took place on Mars billions of years ago. The Curiosity rover will once again traverse the greenheugh pediment slope near Gale Crater in the next Martian annual, providing a good opportunity to sample the surface again and reveal the chemistry of the landscape and any organic carbon associated with it. Similarly, the link between the observed isotope anomaly and the paleoerosion surface is instructive for nasa's Perseverance rover research team to identify target samples that will provide additional evidence for insight into the Martian processes that lead to fractionation of this isotope.
This paper proposes several reasonable hypotheses for interpreting the Martian carbon cycle, and also points out the direction for studying whether life processes are involved in the Martian carbon cycle. At the same time, scientists' understanding of atmospheric chemistry and related processes is based on the Earth, and the author can abandon the preference of "Earth knowledge" and really try to understand the surface process of Mars, which is worth learning from in our planetary scientific research.
Key References (Swipe up and down to view)
Ash R D, Arden J W, Grady M M, et al. An interstellar dust component rich in 12C [J]. Nature, 1988, 336: 228–230.
Eigenbrode J L, Freeman K H. Late Archean rise of aerobic microbial ecosystems [J]. Proceedings of the National Academy of Sciences, 2006, 103: 15759–15764.
Franz H B, Mahaffy P R, Webster C R, et al. Indigenous and exogenous organics and surface–atmosphere cycling inferred from carbon and oxygen isotopes at Gale crater [J]. Nature Astronomy, 2020, 4: 526–532.
House C H, Beal E J, Orphan V J. The apparent involvement of ANMEs in mineral dependent methane oxidation, as an analog for possible Martian methanotrophy [J]. Life (Basel), 2011, 1: 19–33.
House C H, Orphan V J, Turk K A, et al. Extensive carbon isotopic heterogeneity among methane seep microbiota [J]. Environmental Microbiology, 2009, 11: 2207–2215.
House C H, Wong G M, Webster C R, et al. Depleted carbon isotope compositions observed at Gale crater, Mars[J]. Proceedings of the National Academy of Sciences, 2022, 119(4): e2115651119.
Hu R, Kass D M, Ehlmann B L, et al. Tracing the fate of carbon and the atmospheric evolution of Mars [J]. Nature Communications, 2015, 6: 10003.
Nair H, Summers M E, Miller C E, et al. Isotopic fractionation of methane in the martian atmosphere [J]. Icarus, 2005, 175: 32–35.
Steele A, McCubbin F M, Fries M, et al. A reduced organic carbon component in Martian basalts [J]. Science, 2012, 337: 212–215.
Steele A, Benning L G, Wirth R, et al. Organic synthesis on Mars by electrochemical reduction of CO2 [J]. Science Advances, 2018, 4: eaat5118.
Webster C R, Mahaffy P R, Flesch G J, et al. Isotope ratios of H, C, and O in CO2 and H2O of the Martian atmosphere [J]. Science, 2013, 341: 260–263.
Written by: Wang Xu/New Generation
Editor: Chen Feifei
Proofreader: Wan Peng