Lipid molecules are organic compounds, insoluble in water, and based on carbon-carbon chains that form an integral part of biological cell membranes. As such, lipids are ubiquitous in life on Earth, which is why they are considered useful biomarkers for life detection in terrestrial environments. These molecules display effective membrane-forming properties even under geochemically hostile conditions that challenge most of microbial life, which grants lipids a universal biomarker character suitable for life detection beyond Earth, where a putative biological membrane would also be required. What discriminates lipids from nucleic acids or proteins is their capacity to retain diagnostic information about their biological source in their recalcitrant hydrocarbon skeletons for thousands of millions of years, which is indispensable in the field of astrobiology given the time span that the geological ages of planetary bodies encompass. This work gathers studies that have employed lipid biomarker approaches for paleoenvironmental surveys and life detection purposes in terrestrial environments with extreme conditions: hydrothermal, hyperarid, hypersaline, and highly acidic, among others; all of which are analogous to current or past conditions on Mars. Although some of the compounds discussed in this review may be abiotically synthesized, we focus on those with a biological origin, namely lipid biomarkers. Therefore, along with appropriate complementary techniques such as bulk and compound-specific stable carbon isotope analysis, this work recapitulates and reevaluates the potential of lipid biomarkers as an additional, powerful tool to interrogate whether there is life on Mars, or if there ever was.

An Overview of Lipid Biomarkers in Terrestrial Extreme Environments with Relevance for Mars Exploration

Chlorine has important roles in the Earth's systems. In different forms, it helps balance the charge and osmotic potential of cells, provides energy for microorganisms, mobilizes metals in geologic fluids, alters the salinity of waters, and degrades atmospheric ozone. Despite this importance, there has not been a comprehensive summary of chlorine's geobiology. Here, we unite different areas of recent research to describe a biogeochemical cycle for chlorine. Chlorine enters the biosphere through volcanism and weathering of rocks and is sequestered by subduction and the formation of evaporite sediments from inland seas. In the biosphere, chlorine is converted between solid, dissolved, and gaseous states and in oxidation states ranging from −1 to +7, with the soluble, reduced chloride ion as its most common form. Living organisms and chemical reactions change chlorine's form through oxidation and reduction and the addition and removal of chlorine from organic molecules. Chlorine can be transported through the atmosphere, and the highest oxidation states of chlorine are produced by reactions between sunlight and trace chlorine gases. Partial oxidation of chlorine occurs across the biosphere and creates reactive chlorine species that contribute to the oxidative stress experienced by living cells. A unified view of this chlorine cycle demonstrates connections between chlorine biology, chemistry, and geology that affect life on the Earth.

The biogeochemical cycling of chlorine

Space weathering on airless bodies produces metallic iron (Fe0) particles in the rims of mineral grains, which affect visible and near-infrared spectra and complicate the identification of surface materials. The Chang’e-5 mission provides an opportunity to couple information gained from its returned samples with in situ observations and orbital monitoring to gain insight on the details of space weathering on extremely Fe-rich basalts. By putting together all these data, we could extract a soil maturity index (Is/FeO) at the Chang’e-5 landing site of ~66 ± 3.2, indicative of a formation age for the Xu Guangqi crater, whose ejecta dominate the site, of 240–300 Myr ago. In addition, abundant large Fe0 particles were found in the sample, indicating that both the inherited Fe0 particles from late-stage mare basalts and the dense clustering of oversaturated Fe0 in extremely FeO-rich (>17 wt%) basalts contribute to observed Fe0 abundances. We suggest that space weathering of Fe-richer basalt generates Fe0 particles with a larger grain size and faster production rate. 

/pdf/mature-lunar-soils-from-fe-rich-and-young-mare-basalts-in-16flog01.pdf

Mature lunar soils from Fe-rich and young mare basalts in the Chang’e-5 regolith samples

Secondary fluorescence (SF) is known to be a potential source of error in electron probe microanalysis (EPMA) when analyzing for a trace or minor element near a phase boundary. This often overlooked effect leads to a concentration enhancement whenever the neighboring phase contains a high concentration of the analyzed element. Here we show that SF may also lead to a concentration decrease, which can be mistakenly interpreted as a depletion. To examine this issue, we compare Ni profiles measured on well-characterized, homogeneous olivine [(Mg,Fe)2SiO4] grains embedded in basaltic glass, with semi-analytical calculations and numerical simulations of SF across phase boundaries. We find that the Ni content consistently decreases with decreasing distance to the interface or grain radius, deviating from the expected concentration by ∼2-5% at 10 μm from the interface. This decrease is explained by the lower bremsstrahlung fluorescence emitted from the sample as compared to that emitted from the standard. The analytical error due to boundary fluorescence affecting other elements of petrologic importance in olivine is discussed.

Element Depletion Due to Missing Boundary Fluorescence in Electron Probe Microanalysis: The Case of Ni in Olivine.

Sulfate-rich sedimentary rocks explored by the Opportunity rover during its 14-year surface mission at Meridiani Planum provide an invaluable window into the thousands of sulfate deposits detected on Mars via remote sensing. Existing models explaining the formation of martian sulfates can be generally described as either bottom-up, groundwater-driven playa settings or top-down icy chemical weathering environments. Here, we propose a hybrid model involving both bottom-up and top-down processes driven by freeze-thaw cycles. Freezing leads to cryo-concentration of acidic fluids from precipitations at the surface, facilitating rapid chemical weathering despite low temperatures. Cryosuction causes the upward migration of vadose water and even groundwater with dissolved ions, resulting in the accumulation of ions in near-surface environments. Evaporation precipitates salts, but leaching separates chlorides from sulfates during the thawing period. Freeze-thaw cycles, therefore, can enrich sulfates at the surface. While freeze-thaw is more commonly understood as a mechanism of physical weathering, we suggest that it is a fundamental aspect of chemical weathering on Mars.

Freeze-thaw cycles drove chemical weathering and enriched sulfates in the Burns formation at Meridiani, Mars

Preferential Formation of Chlorate over Perchlorate on Mars Controlled by Iron Mineralogy

Context. Olivine responds to space weathering in the fastest and most profound way, which results in significant space weathering spectral alteration effects (SWSAEs) on airless silicate bodies. Although Mg-rich olivine (Fa 10 ) has been subjected to extensive studies, SWSAEs of Fe-rich (Fa# &gt; 20) or Fa-dominant (Fa# ⩾ 50) olivine are still poorly understood. Aims. We aim to systematically characterize the space weathering effects and the associated spectral alterations of Fe-rich olivine on the surface of Phobos and the Moon. Methods . We conducted nanosecond pulsed laser irradiation experiments on a set of synthetic Fe-rich olivine (Fa 29 , Fa 50 , Fa 71 , and Fa 100 ) with energy levels simulated for Phobos and the Moon and analyzed the irradiated olivine for microscopic characteristics and near-infrared (NIR) and Raman spectroscopy. Results . Micron-level thick alteration layers are found in Fa 100 compared to those hundreds of nanometers thick in Fa 29 , Fa 50 , and Fa 71 . With increasing irradiation energy levels and Fa# values, nanophase iron (np-Fe 0 ) particles increase in size but decrease in quantity. The np-Fe 0 formed via in situ decomposition are ubiquitously present, while those formed via vapor deposition are primarily found in Fa 29 but rarely in Fa# ⩾ 50. The size fraction of intermediate (10–40 nm) and large (40–60 nm) np-Fe 0 proportionally increases with Fa# values. The NIR spectra of weathered olivine show darkening over reddening in most cases, but Fa100 under the most irradiated condition shows brightening-reddening spectral effects. The Raman spectra of weathered olivine show a reduction in intensity without peak shifts. Conclusions . The Fa# values of olivine are a more critical factor in controlling the SWSAEs on Phobos than those on the Moon. If Phobos and Deimos contain substantial Fe-rich or Fa-dominant olivine, similar to Mars, thick alteration rims and large np-Fe 0 formed via space weathering may cause darkening-reddening and potentially brightening-reddening spectral effects on the Martian moons. 

https://www.aanda.org/articles/aa/pdf/2023/04/aa45453-22.pdf

Space weathering effects and potential spectral alteration on Phobos and the Moon: Clues from the Fe content of olivine

Abstract Fluorine (F) and chlorine (Cl) are important volatiles in olivine and its high-pressure polymorphs, which would significantly affect olivine phase transition, melting temperature, and physical property of the mantle. F and Cl concentrations in olivine can be detected by electron probe microanalysis (EPMA). However, the analytical accuracy and precision can be impeded by severe peak overlaps, low peak intensities of traditional analytical crystals, and secondary fluorescence effects. In this study, we constructed an optimized analytical method with high accuracy and precision to analyze trace F and Cl in olivine. Key parameters of analytical crystals, beam conditions, peak overlaps, and secondary fluorescence effects were discussed. Variations in the levels of the analyzed trace elements fall within ± 10%. The detection limits (3σ) for F and Cl are lowered to 30 ppm and 5 ppm, respectively. This method can provide precise F and Cl analysis for natural olivine samples and help to provide significant information on its formation process. 

/pdf/high-precision-measurement-of-trace-f-and-cl-in-olivine-by-jkm7xv3r.pdf

High precision measurement of trace F and Cl in olivine by electron probe microanalysis

Understanding crust–atmosphere interactions on Venus is fundamentally important to interpretations of Venus’ surface spectroscopic data. Olivine, in basaltic crust, is oxidized under a heated CO2 atmosphere. However, the oxidation rates, product assemblages and spectral characteristics of olivine samples with different Fa# values remain largely unclear. Herein, we investigated the oxidation of olivine with different Fa# values (Fa09, Fa29 and Fa71) under CO2 atmosphere at 470 °C and 900 °C and characterized the oxidation products (both microscopically and macroscopically), conversion rates and VNIR spectra. The results showed that the oxidation of olivine produced magnesioferrite, magnetite, laihunite, hematite and maghemite at 470 °C and hematite, magnetite, magnesioferrite and amorphous SiO2 at 900 °C. Both high temperature and high Fa# values accelerated the oxidation rates. The production of oxide coatings on olivine grains (74 μm in size) was estimated to be completed within tens to hundreds of years at 470 °C in natural settings, with even shorter periods under higher temperatures. Thus, CO2 oxidation would quickly eliminate olivine spectral characteristics, and spectral parameters at 850 and 1020 nm, as well as other relevant spectral windows (considering shifts induced by the elevated temperature), could be used to trace olivine oxidation processes. This work presented a case study connecting microscopic features to spectral characteristics for Venus’ surface–atmosphere interactions. Further studies considering more realistic Venus’ surface–atmosphere conditions will be essential to better interpret the measured spectroscopic data and determine the origins of the high emissivity detected on elevated terrain on Venus.

/pdf/high-temperature-oxidation-of-magnesium-and-iron-rich-10lycdzy.pdf

High-Temperature Oxidation of Magnesium- and Iron-Rich Olivine under a CO2 Atmosphere: Implications for Venus

The dissolution rates of jarosite can constrain the duration of aqueous activities on Mars. To date, few studies have considered the influences of formation pathways and anion substitutions on jarosite dissolution rates. Here, we investigated how the formation pathways (Fe(II)‐oxidation and Fe(III)‐forced hydrolysis) and incorporation of bromide influence the dissolution rates of K‐jarosite under eight aqueous conditions combining T (277 K, 298 K, and 323 K) and αw (0.35, 0.75 and 1), except for 277 K−0.35αw. The results show that jarosite dissolution rates are primarily influenced by aqueous T‐αw conditions. Formation pathways and Br contents are secondary factors and only become notable under low T (277 K) and low αw (0.35) conditions. Taking the jarosite formation pathways and Br incorporation into account, the maximum lifetime of jarosite may be slightly longer than that of the halogen‐free counterparts formed via Fe(III)‐forced hydrolysis. Jarosite of the Burns Formation (Meridiani Planum) and the Pahrump Hills member (Gale Crater) are likely formed via Fe(II)‐oxidation and halogen‐bearing. Their estimated field lifetime (∼150 μm–1 mm particles) in low‐T groundwater may last for hundreds of thousand years to a few million years. Jarosite in the Vera Rubin Ridge would share a similar lifespan if low‐T solutions account for jarosite formation and subsequent interactions; otherwise, interactions with hydrothermal fluids (∼100°C) would substantially shorten the jarosite lifetime. We conclude that Martian jarosite may survive continuous aqueous interactions for up to a few million years, indicating an extended duration of aqueous environments than previously thought.

Effects of Formation Pathways and Bromide Incorporation on Jarosite Dissolution Rates: Implications for Mars

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