Perchlorate formation on Mars through surface radiolysis-initiated atmospheric chemistry: A potential mechanism

Emergent ribozyme behaviors in oxychlorine brines indicate a unique niche for molecular evolution on Mars

Mars is a particularly attractive candidate among known astronomical objects to potentially host life. Results from space exploration missions have provided insights into Martian geochemistry that indicate oxychlorine species, particularly perchlorate, are ubiquitous features of the Martian geochemical landscape. Perchlorate presents potential obstacles for known forms of life due to its toxicity. However, it can also provide potential benefits, such as producing brines by deliquescence, like those thought to exist on present-day Mars. Here we show perchlorate brines support folding and catalysis of functional RNAs, while inactivating representative protein enzymes. Additionally, we show perchlorate and other oxychlorine species enable ribozyme functions, including homeostasis-like regulatory behavior and ribozyme-catalyzed chlorination of organic molecules. We suggest nucleic acids are uniquely well-suited to hypersaline Martian environments. Furthermore, Martian near- or subsurface oxychlorine brines, and brines found in potential lifeforms, could provide a unique niche for biomolecular evolution.

Intercropping on Mars: A promising system to optimise fresh food production in future martian colonies

Future colonists on Mars will need to produce fresh food locally to acquire key nutrients lost in food dehydration, the primary technique for sending food to space. In this study we aimed to test the viability and prospect of applying an intercropping system as a method for soil-based food production in Martian colonies. This novel approach to Martian agriculture adds valuable insight into how we can optimise resource use and enhance colony self-sustainability, since Martian colonies will operate under very limited space, energy, and Earth supplies. A likely early Martian agricultural setting was simulated using small pots, a controlled greenhouse environment, and species compliant with space mission requirements. Pea (Pisum sativum), carrot (Daucus carota) and tomato (Solanum lycopersicum) were grown in three soil types ("MMS-1" Mars regolith simulant, potting soil and sand), planted either mixed (intercropping) or separate (monocropping). Rhizobia bacteria (Rhizobium leguminosarum) were added as the pea symbiont for Nitrogen-fixing. Plant performance was measured as above-ground biomass (g), yield (g), harvest index (%), and Nitrogen/Phosphorus/Potassium content in yield (g/kg). The overall intercropping system performance was calculated as total relative yield (RYT). Intercropping had clear effects on plant performance in Mars regolith, being beneficial for tomato but mostly detrimental for pea and carrot, ultimately giving an overall yield disadvantage compared to monocropping (RYT = 0.93). This effect likely resulted from the observed absence of Rhizobia nodulation in Mars regolith, negating Nitrogen-fixation and preventing intercropped plants from leveraging their complementarity. Adverse regolith conditions-high pH, elevated compactness and nutrient deficiencies-presumably restricted Rhizobia survival/nodulation. In sand, where more favourable soil conditions promoted effective nodulation, intercropping significantly outperformed monocropping (RYT = 1.32). Given this, we suggest that with simple regolith improvements, enhancing conditions for nodulation, intercropping shows promise as a method for optimising food production in Martian colonies. Specific regolith ameliorations are proposed for future research.

Permeation of photochemically-generated gaseous chlorine dioxide on Mars as a significant factor in destroying subsurface organic compounds

It has been shown that ultraviolet (UV) irradiation is responsible for the destruction of organic compounds on the surface of Mars. When combined with the photochemically-driven production of oxychlorines (ClOx) it can generate highly reactive species that can alter or destroy organic compounds. However, it has been assumed that since UV only penetrates the top few millimeters of the martian regolith, reactive ClOx oxidants are only produced on the surface. Of all the oxychlorine intermediates produced, gaseous chlorine dioxide [ClO2(g)] is of particular interest, being a highly reactive gas with the ability to oxidize organic compounds. Here we report on a set of experiments under Mars ambient conditions showing the production and permeation of ClO2(g) and its reaction with alanine as a test compound. Contrary to the accepted paradigm that UV irradiation on Mars only interacts with a thin layer of surface regolith, our results show that photochemically-generated ClO2(g) can permeate below the surface, depositing ClOx species (mainly Cl- and ClO 3 - ) and destroying organic compounds. With varying levels of humidity and abundant chloride and oxychlorines on Mars, our findings show that permeation of ClO2(g) must be considered as a significant contributing factor in altering, fragmenting, or potentially destroying buried organic compounds on Mars.

Microbes from Brine Systems with Fluctuating Salinity Can Thrive under Simulated Martian Chemical Conditions

The waters that were present on early Mars may have been habitable. Characterising environments analogous to these waters and investigating the viability of their microbes under simulated martian chemical conditions is key to developing hypotheses on this habitability and potential biosignature formation. In this study, we examined the viability of microbes from the Anderton Brine Springs (United Kingdom) under simulated martian chemistries designed to simulate the chemical conditions of water that may have existed during the Hesperian. Associated changes in the fluid chemistries were also tested using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The tested Hesperian fluid chemistries were shown to be habitable, supporting the growth of all of the Anderton Brine Spring isolates. However, inter and intra-generic variation was observed both in the ability of the isolates to tolerate more concentrated fluids and in their impact on the fluid chemistry. Therefore, whilst this study shows microbes from fluctuating brines can survive and grow in simulated martian water chemistry, further investigations are required to further define the potential habitability under past martian conditions.

Perchlorate contamination of tea leaves and a corresponding probabilistic dietary risk assessment using Monte Carlo simulation

Perchlorate is known as a thyroid disrupter. Its contamination in various tea samples was monitored, and 286 samples belonging to four types of tea leaves were analysed. The detection rate of perchlorate in tea was 99.3%. The mean concentration in different tea types decreased in order from green tea, oolong tea, white tea to black tea. A probabilistic approach was performed to evaluate the dietary exposure of perchlorate for six different subpopulations. The daily intakes (EDIs) for consumers over the age of 41 were higher than that of other subpopulations. The hazard quotient for six groups was lower than 1 even at the extreme percentile (P99). It indicates that the risk of dietary exposure to perchlorate from tea consumption for Fujian people is acceptable without considering other foodstuffs. However, the high occurrence of perchlorate in tea samples suggested that the actual source of this contaminant should be further investigated.

Perchlorate and Agriculture on Mars

Perchlorate (ClO4−) is globally enriched in Martian regolith at levels commonly toxic to plants. Consequently, perchlorate in Martian regolith presents an obstacle to developing agriculture on Mars. Here, we assess the effect of perchlorate at different concentrations on plant growth and germination, as well as metal release in a simulated Gusev Crater regolith and generic potting soil. The presence of perchlorate was uniformly detrimental to plant growth regardless of growing medium. Plants in potting soil were able to germinate in 1 wt.% perchlorate; however, these plants showed restricted growth and decreased leaf area and biomass. Some plants were able to germinate in regolith simulant without perchlorate; however, they showed reduced growth. In Martian regolith simulant, the presence of perchlorate prevented germination across all plant treatments. Soil column flow-through experiments of perchlorate-containing Martian regolith simulant and potting soil were unable to completely remove perchlorate despite its high solubility. Additionally, perchlorate present in the simulant increased metal/phosphorous release, which may also affect plant growth and biochemistry. Our results support that perchlorate may modify metal availability to such an extent that, even with the successful removal of perchlorate, Martian regolith may continue to be toxic to plant life. Overall, our study demonstrates that the presence of perchlorate in Martian regolith provides a significant challenge in its use as an agricultural substrate and that further steps, such as restricted metal availability and nutrient enrichment, are necessary to make it a viable growing substrate.

Transient HCl in the atmosphere of Mars

A major quest in Mars' exploration has been the hunt for atmospheric gases, potentially unveiling ongoing activity of geophysical or biological origin. Here, we report the first detection of a halogen gas, HCl, which could, in theory, originate from contemporary volcanic degassing or chlorine released from gas-solid reactions. Our detections made at ~3.2 to 3.8 μm with the Atmospheric Chemistry Suite and confirmed with Nadir and Occultation for Mars Discovery instruments onboard the ExoMars Trace Gas Orbiter, reveal widely distributed HCl in the 1- to 4-ppbv range, 20 times greater than previously reported upper limits. HCl increased during the 2018 global dust storm and declined soon after its end, pointing to the exchange between the dust and the atmosphere. Understanding the origin and variability of HCl shall constitute a major advance in our appraisal of martian geo- and photochemistry.

Capacity of Chlorate to Oxidize Ferrous Iron: Implications for Iron Oxide Formation on Mars

Chlorate is an important Cl-bearing species and a strong potential Fe(II) oxidant on Mars. Since the amount of oxychlorine species (perchlorate and chlorate) detected on Mars is limited (<~1 wt.%), the effectiveness of chlorate to produce iron oxides depends heavily on its oxidizing capacity. Decomposition of chlorate or intermediates produced during its reduction, before reaction with Fe(II) would decrease its effective capacity as an oxidant. We thus evaluated the capacity of chlorate to produce Fe(III) minerals in Mars-relevant fluids, via oxidation of dissolved Fe(II). Each chlorate ion can oxidize 6 Fe(II) ions under all conditions investigated. Mass balance demonstrated that 1 wt.% chlorate (as ClO3−) could produce approximately 6 to 12 wt.% Fe(III) or mixed valent mineral products, with the amount varying with the formula of the precipitating phase. The mineral products are primarily determined by the fluid type (chloride- or sulfate-rich), the solution pH, and the rate of Fe(II) oxidation. The pH at the time of initial mineral nucleation and the amount of residual dissolved Fe(II) in the system exert important additional controls on the final mineralogy. Subsequent diagenetic transformation of these phases would yield 5.7 wt.% hematite per wt.% of chlorate reacted, providing a quantitative constraint on the capacity of chlorate to generate iron oxides on Mars.

Occurrence and cycling of trace elements in ultramafic soils and their impacts on human health: A critical review

The transformation of trace metals (TMs) in natural environmental systems has created significant concerns in recent decades. Ultramafic environments lead to potential risks to the agricultural products and, subsequently, to human health. This unique review presents geochemistry of ultramafic soils, TM fractionation (i.e. sequential and single extraction techniques), TM uptake and accumulation mechanisms of ultramafic flora, and ultramafic-associated health risks to human and agricultural crops. Ultramafic soils contain high levels of TMs (i.e. Cr, Ni, Mn, and Co) and have a low Ca:Mg ratio together with deficiencies in essential macronutrients required for the growth of crops. Even though a higher portion of TMs bind with the residual fraction of ultramafic soils, environmental changes (i.e. natural or anthropogenic) may increase the levels of TMs in the bioavailable or extractable fractions of ultramafic soils. Extremophile plants that have evolved to thrive in ultramafic soils present clear examples of evolutionary adaptations to TM resistance. The release of TMs into water sources and accumulation in food crops in and around ultramafic localities increases health risks for humans. Therefore, more focused investigations need to be implemented to understand the mechanisms related to the mobility and bioavailability of TMs in different ultramafic environments. Research gaps and directions for future studies are also discussed in this review. Lastly, we consider the importance of characterizing terrestrial ultramafic soil and its effect on crop plants in the context of multi-decadal plans by NASA and other space agencies to establish human colonies on Mars.

Production of nitrates and perchlorates by laser ablation of sodium chloride in simulated Martian atmospheres. Implications for their formation by electric discharges in dust devils

Nitrates and perchlorates are present both on Earth and Mars. In the Martian environment perchlorates dominate over nitrates whereas on Earth is contrariwise. This implies that the mechanisms responsible for their formation are different for both planets. The chemical elements required for their formation are nitrogen and chlorine, which are present in the atmosphere and surface, respectively. Dust in the Martian atmosphere causes atmospheric perturbations that lead to the development of dust-devils and sandstorms. Dust devils contain both chemical elements simultaneously, and normally generate high electric fields that can trigger the formation of electric discharges. Here we present laboratory experiments of this phenomenon using laser ablation of a sodium chloride (NaCl) plate in two different simulated atmospheres: (1) 96% CO₂, 2% N₂ and 2% Ar; and (2) 66% CO₂, 33% N₂ and 1% Ar. The dust that condensed and accumulated on the walls of the reactor was analyzed by different analytical techniques that included Fourier transform infrared spectroscopy, visible spectroscopy using azo dyes, thermogravimetry/simultaneous thermal analyses coupled to mass spectrometry, powder X-ray diffraction, and ion chromatography. The main components of the ablated dust corresponded to NaCl ≥ 91.5%, sodium nitrate (NaNO₃ = 1.6-6.0%), and sodium perchlorate (NaClO₄ ∼ 0.2-0.3%). It is interesting to note that these salts formed in a dry process that is relevant to Mars today. A thermochemical model was used to understand the chemical steps that led to the formation of these salts in the gas phase. The NaNO₃NaClO₄ (wt/wt) ratio of this process was estimated to vary from 5.0 to 30.0; this ratio is too high compared to that found on Mars (NO₃^-ClO₄^- (wt/wt)) from 0.004 to 0.13). This implies that gaseous NaCl was not efficiently oxidized to perchlorate by the electric discharge process. We propose instead that gaseous metal chlorides (e.g., MgCl₂, NaCl, CaCl₂, KCl) were supplied to the atmosphere by the volatilization of chloride minerals present in the dust by electric discharges generated in dust devils and were subsequently oxidized to perchlorate by photochemical processes. Further work is required to assess the relative contribution of this possible source.

Clustering of HClO4 with Brønsted (H2SO4, HClO4, HNO3) and Lewis acids BX3 (X = H, F, Cl, Br, OH): a DFT study

Interaction of HClO₄ with Lewis and Brønsted acids leads to a variety of clusters exhibiting a wide range of acidity.

Dalangtan Saline Playa in a Hyperarid Region on Tibet Plateau: II. Preservation of Salts with High Hydration Degrees in Subsurface

Based on a field expedition to the Dalangtan (DLT) saline playa located in a hyperarid region (Qaidam Basin) on the Tibet Plateau and follow-up investigations, we report the mineralogy and geochemistry of the salt layers in two vertical stratigraphic cross sections in the DLT playa. Na-, Ca-, Mg-, KCaMg-sulfates; Na-, K-, KMg-chlorides; mixed (K, Mg)-chloride-sulfate; and chlorate and perchlorate were identified in the collected samples. This mineral assemblage represents the last-stage precipitation products from Na-K-Mg-Ca-Cl-SO₄ brine and the oxychlorine formation from photochemistry reaction similar to other hyperarid regions on Earth. The spatial distributions of these salts in both stratigraphic cross sections suggest very limited brine volumes during the precipitation episodes in the Holocene era. More importantly, sulfates and chlorides with a high degree of hydrations were found preserved within the subsurface salt-rich layers of DLT saline playa, where the environmental conditions at the surface are controlled by the hyperaridity in the Qaidam Basin on the Tibet Plateau. Our findings suggest a very different temperature and relative humidity environment maintained by the hydrous salts in a subsurface salty layer, where the climatic conditions at surface have very little or no influence. This observation bears some similarities with four observations on Mars, which implies not only a large humidity reservoir in midlatitude and equatorial regions on Mars but also habitability potential that warrants further investigation.

Chemistry, mineralogy, and grain properties at Namib and High dunes, Bagnold dune field, Gale crater, Mars: A synthesis of Curiosity rover observations

The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine to medium sized (~45-500 μm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust-covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt-sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising >90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si enriched relative to other soils at Gale crater, and H₂O, S, and Cl are lower relative to all previously measured Martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse-sieved fraction of Bagnold sands, corroborated by visible/near-infrared spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in Martian soils: (1) amorphous components in the sand-sized fraction (represented by Bagnold) that are Si-enriched, hydroxylated alteration products and/or H₂O- or OH-bearing impact or volcanic glasses and (2) amorphous components in the fine fraction (<40 μm; represented by Rocknest and other bright soils) that are Fe, S, and Cl enriched with low Si and adsorbed and structural H₂O.