•Addressing the open question on high ClO4 content and high ClO4/Cl ratio found on Mars.•In situ sensing of free radicals generated by electrostatic discharge (ESD) in a Mars chamber.•Detection of ...NaClO3, NaClO4, and Na2CO3 formed in NaCl through redox plasma chemistry.•Evidences of 1000 times oxidation power of ESD-electron over UVC-photon in the experimental setting.•Scaling to Mars conditions suggests an important contribution of plasma chemistry occurred in Martian dust events in perchlorate formation.
We report experimental evidences to support a new formation mechanism, multiphase redox plasma chemistry, for perchlorate on Mars observed during the Phoenix mission, whose high concentrations and high ClO4/Cl ratio cannot be fully interpreted by photochemistry. This chemical reaction occurs between Cl-bearing minerals on the Mars surface and free radicals generated by electrostatic discharge (ESD) during Mars dust events (dust storms, dust devils, and grain saltation).
We conducted simulated ESD experiments in a Mars chamber with pure CO2, CO2 + H2O(g), and Mars Simulate Gas Mixture at Martian atmospheric pressure. We directly observed (1) the instantaneous generation of atmospheric free radicals CO+2, CO+, OI, HIII, HII, OH, ArI, N2, and N+2 in normal glow discharge (NGD), detected by in situ plasma emission spectroscopy, and O3 by UV and Mid-IR spectroscopy; (2) the fast transformation of NaCl to NaClO3 and NaClO4 detected by laser Raman spectroscopy, with oxychlorine enrichment at the sample surfaces confirmed by ion chromatography. Through two sets of experimental comparison, we found that the oxidation power of ESD-electron is three orders of magnitude higher than that of UVC-photon. We scaled our experimental results to the modeled ESD in Mars dust events and Mars surface UV radiation level, and concluded that plasma chemistry occurred during Mars dust events can be an additional important formation mechanism for the large amounts of perchlorates observed during various missions to Mars.
To investigate the origin of fine-grained rims around chondrules (FGRs), we compared presolar grain abundances, elemental compositions and mineralogies in fine-grained interstitial matrix material ...and individual FGRs in the primitive CO3.0 chondrites Allan Hills A77307, LaPaz Icefield 031117 and Dominion Range 08006. The observation of similar overall O-anomalous (∼155ppm) and C-anomalous grain abundances (∼40ppm) in all three CO3.0 chondrites suggests that they all accreted from a nebular reservoir with similar presolar grain abundances. The presence of presolar silicate grains in FGRs combined with the observation of similar estimated porosity between interstitial matrix regions and FGRs in LAP 031117 and ALHA77307, as well as the identification of a composite FGR (a small rimmed chondrule within a larger chondrule rim) in ALHA77307, all provide evidence for a formation of FGRs by accretion of dust grains onto freely-floating chondrules in the solar nebula before their aggregation into their parent body asteroids. Our study also shows systematically lower abundances of presolar silicate grains in the FGRs than in the matrix regions of CO3 chondrites, while the abundances of SiC grains are the same in all areas, within errors. This trend differs from CR2 chondrites in which the presolar silicate abundances are higher in the FGRs than in the matrix, but similar to each other within 2σ errors. This observation combined with the identification of localized (micrometer-scaled) aqueous alteration in a FGR of LAP 031117 suggests that the lower abundance of presolar silicates in FGRs reflects pre-accretionary aqueous alteration of the fine-grained material in the FGRs. This pre-accretionary alteration could be due to either hydration and heating of freely floating rimmed chondrules in icy regions of the solar nebula or melted water ice associated with 26Al-related heating inside precursor planetesimals, followed by aggregation of FGRs into the CO chondrite parent-body.
Abstract
The chemical compositions of relatively young mare lava flows have implications for the late volcanism on the Moon. Here we report the composition of soil along the rim of a 450-m diameter ...fresh crater at the Chang′e-3 (CE-3) landing site, investigated by the Yutu rover with
in situ
APXS (Active Particle-induced X-ray Spectrometer) and VNIS (Visible and Near-infrared Imaging Spectrometer) measurements. Results indicate that this region's composition differs from other mare sample-return sites and is a new type of mare basalt not previously sampled, but consistent with remote sensing. The CE-3 regolith derived from olivine-normative basaltic rocks with high FeO/(FeO+MgO). Deconvolution of the VNIS data indicates abundant high-Ca ferropyroxene (augite and pigeonite) plus Fe-rich olivine. We infer from the regolith composition that the basaltic source rocks formed during late-stage magma-ocean differentiation when dense ferropyroxene-ilmenite cumulates sank and mixed with deeper, relatively ferroan olivine and orthopyroxene in a hybridized mantle source.
Given the volume of spectral data required for providing accurate compositional information and thereby insight in mineralogy and petrology from laser-induced breakdown spectroscopy (LIBS) ...measurements, fast data processing tools are a must. This is particularly true during the tactical operations of rover-based planetary exploration missions such as the Mars Science Laboratory rover, Curiosity, which will carry a remote LIBS spectrometer in its science payload. We have developed: an automated fast pre-processing sequence of algorithms for converting a series of LIBS spectra (typically 125) recorded from a single target into a reliable SNR-enhanced spectrum; a dedicated routine to quantify its spectral features; and a set of calibration curves using standard hydrous and multi-cation sulfates. These calibration curves allow deriving the elemental compositions and the degrees of hydration of various hydrous sulfates, one of the two major types of secondary minerals found on Mars. Our quantitative tools are built upon calibration-curve modeling, through the correlation of the elemental concentrations and the peak areas of the atomic emission lines observed in the LIBS spectra of standard samples. At present, we can derive the elemental concentrations of K, Na, Ca, Mg, Fe, Al, S, O, and H in sulfates, as well as the hydration degrees of Ca- and Mg-sulfates, from LIBS spectra obtained in both Earth atmosphere and Mars atmospheric conditions in a Planetary Environment and Analysis Chamber (PEACh). In addition, structural information can be potentially obtained for various Fe-sulfates.
► Routines for LIBS spectral data fast automated processing. ► Identification of elements and determination of the elemental composition. ► Calibration curves for sulfate samples in Earth and Mars atmospheric conditions. ► Fe curves probably related to the crystalline structure of Fe-sulfates. ► Extraction of degree of hydration in hydrous Mg-, Ca-, and Fe-sulfates.
Heterogeneous electrochemistry induced by Martian dust activity is an important type of atmosphere‐surface interaction that affects geochemical processes at the Martian surface and in the Martian ...atmosphere. We have experimentally demonstrated that heterogeneous electrochemistry stimulated by mid‐strength dust events can decompose common chloride salts, which is accompanied by the release of chlorine atoms into the atmosphere and the generation of (per)chlorates (chlorates and perchlorates) and carbonates. In this study, we present quantitative analyses on the above products from 26 heterogeneous electrochemical experiments on chloride salts. Based on these quantifications, our calculation indicates that such atmosphere‐surface interaction during a portion of Amazonian period could accumulate the observed abundance of (per)chlorates, carbonates, and HCl by landed and orbital missions, and thus can be considered as a major driving force of the global chlorine‐cycle on Mars. This study emphasizes the importance of measuring the electrical properties of dust activity on Mars.
Plain Language Summary
Frictional electrification is a common process in our solar system, with Martian dust activities known to be a powerful source of electrical charge buildup. Furthermore, the thin atmosphere on Mars makes the breakdown of accumulated electrical fields, in form of electrostatic discharge (ESD), much easier to occur (a hundred times easier than on Earth). ESD generates a huge amount of energetic electrons that collide with Martian atmospheric molecules and generate free radicals. These free radicals react with the Martian chlorides to generate new species. This study found the yields of (per)chlorates, carbonates, and chlorine from the ESD process, with the strength matching mid‐strength Martian dust activity, are at per thousand or percent levels (normalized to the starting chlorides). Based on these results, it is possible to calculate the total yields of those species produced from known chloride sources on Mars by global dust storms during defined durations in the Amazonian period. It was found that the contributions of Mars dust activity can account for the abundances of (per)chlorates, carbonates, and chlorine observed by past and current Mars missions. This study supports that Martian atmosphere‐surface interaction in dust events is a major driving force for the global chlorine‐cycle on Mars.
Key Points
Heterogeneous electrochemistry induced by Mars dust activity can decompose chloride, form (per)chlorate, carbonate, and release chlorine
This experimental study simulated mid‐strength Mars dust events and revealed the high yields of (per)chlorates, carbonates, and chlorine
A calculation based on the results supports Martian dust activity as the major driving force for the global Cl‐cycle in Amazonian period
We investigated the petrologic, geochemical, and spectral parameters that relate to the type and degree of aqueous alteration in nine CM chondrites and one CI (Ivuna) carbonaceous chondrite. Our ...underlying hypothesis is that the position and shape of the 3 μm band is diagnostic of phyllosilicate mineralogy. We measured reflectance spectra of the chondrites under dry conditions (elevated temperatures) and vacuum (10−8 to 10−7 torr) to minimize adsorbed water and mimic the space environment, for subsequent comparison with reflectance spectra of asteroids. We have identified three spectral CM groups in addition to Ivuna. “Group 1,” the least altered group as determined from various alteration indices, is characterized by 3 μm band centers at longer wavelengths, and is consistent with cronstedtite (Fe‐serpentine). “Group 3,” the most altered group, is characterized by 3 μm band centers at shorter wavelengths and is consistent with antigorite (serpentine). “Group 2” is an intermediate group between group 1 and 3. Ivuna exhibits a unique spectrum that is distinct from the CM meteorites and is consistent with lizardite and chrysotile (serpentine). The petrologic and geochemical parameters, which were determined using electron microprobe analyses and microscopic observations, are found to be consistent with the three spectral groups. These results indicate that the distinct parent body aqueous alteration environments experienced by these carbonaceous chondrites can be distinguished using reflectance spectroscopy. High‐quality ground‐based telescopic observations of Main Belt asteroids can be expected to reveal not just whether an asteroid is hydrated, but also details of the alteration state.
The dominant feature of the olivine Raman spectrum is a doublet that occurs in the spectral region of 815–825
cm
−1 (DB1) and 838–857
cm
−1 (DB2). These features arise from coupled symmetric and ...asymmetric stretching vibrational modes of the constituent SiO
4 tetrahedra. The frequencies of both peaks show monotonic shifts following cation substitution between forsterite and fayalite. We present a calibration for extracting olivine Fo contents (Fo
=
Mg/(Mg
+
Fe) molar ratio; Fo
0–100) from the peak positions of this doublet, permitting estimates of chemical composition from Raman spectra (acquired in the laboratory or field) as well as providing information on crystal structure (distinction of polymorphs). Eight samples spanning the compositional range from forsterite to fayalite were used to develop the calibration equations for the DB1 and DB2 peaks individually and together. The data indicate that the DB1 peak is more reliable for calculating the compositions of Fe-rich olivine but that the DB2 peak is better for magnesian compositions. The two-peak calibration overcomes the limitations of the single-peak calibrations and is capable of calculating olivine compositions to within ±10
Fo units.
We report laboratory experimental results that support a brine-related hypothesis for the recurring slope lineae (RSL) on Mars in which the subsurface Cl-salts, i.e., hydrous chlorides and ...oxychlorine salts (HyCOS) are the potential source materials. Our experiments revealed that within the observed RSL temperature window TRSL (250–300 K), the deliquescence of HyCOS could occur in relative humidity ranges (RH ≥ 22%–46%) much lower than those for hydrous (Mg, Fe)-sulfates (RH ≥ 75%–96%). In addition, we demonstrated that the RH values kept by common HyCOS and hydrous sulfates in enclosures have a general trend as RHsulfates > RHperchlorates > RHchlorides (with same type of cation) in wide T range. It means that the required RH range for a Cl-bearing salt to deliquescence can be satisfied by a co-existing salt of different type, e.g., in the subsurface layers of mixed salts on Mars. Furthermore, we found a strong temperature dependence of the deliquescence rates for all tested HyCOS, e.g., a duration of 1–5 sols for all HyCOS at the high end (300 K) of TRSL, and of 20–70 sols for all tested HyCOS (except NaClO4·H2O) at the low end (250 K) of TRSL, which is consistent with the observed seasonal behavior of RSL on Mars. From a mass-balance point of view, the currently observed evidences on Mars do not support a fully-brine-wetted track model, thus we suggest a brine-triggered granular-flow model for the most RSL. Considering the recurrence of RSL in consecutive martian years, our experimental results support the rehydration of remnant HyCOS layers during a martian cold season through H2O vapor-to-salt direct interaction. We found that the evidences of HyCOS rehydration under Mars relevant P-T-RH conditions are detectable in a few minutes by in situ Raman spectroscopy. This rehydration would facilitate the recharge of H2O back into subsurface HyCOS, which could serve as the source material to trigger RSL in a subsequent warm season. The major limiting factor for this rehydration is the H2O supply, i.e., the H2O vapor density carried by current Mars atmospheric circulation and the diffusion rate of H2O vapor into the salt-rich subsurface in a cold season. In a worst-case scenario, these H2O supplies can support a maximum increase of hydration degrees of two for totally dehydrated HyCOS, whereas the full rehydration of subsurface HyCOS layers can be easily reached during a >30° obliquity period that has H2O vapor density 10× to 20× times the value of current obliquity. Overall, our results imply the existence of a large amount of Cl-bearing salts in the subsurface at RSL sites.
•The deliquescence of Cl-bearing salts requires mid-to-low RH that is easier to achieve than sulfates on Mars.•Mixed salts in subsurface can provide suitable RH at TRSL to facilitate the deliquescence of Cl-bearing salts at T > Te.•The deliquescence rates of Cl-bearing salts as function of T support the observed seasonal behavior of RSL.•The ultrahigh rehydration rate of Cl-bearing salts does not limit the H2O recharge, but the H2O supply on current Mars does.•This study supports a Cl-bearing-brine-triggered granular-flow for most of RSL and the annually partial recharge of H2O.
Dehydration/rehydration experiments were conducted on pure Mg‐sulfates and mixtures of epsomite with Ca‐sulfates, Fe‐sulfates, Fe‐oxide, and Fe‐hydroxide. The goal was to investigate the stabilities ...and phase transition pathways of Mg‐sulfate hydrates, under temperature and relative humidity conditions relevant to Mars, as a function of starting structure and coexisting species. Two pathways were found to form Mg‐sulfate monohydrates between 5°C and 50°C through dehydration of epsomite or hexahydrite. Two polymorphs of Mg‐sulfate monohydrates were characterized in this study. It is important to distinguish among these phases on Mars because they have different formation conditions that have the potential to provide additional information on surface and subsurface geologic processes. We found that Mg‐sulfates with moderate hydration states (especially starkeyite and amorphous Mg‐sulfates) can be very stable under current Martian surface conditions. On the basis of NIR spectral features, these phases are good candidates for polyhydrated sulfates identified on Mars by OMEGA and CRISM; thus, they may contribute to the high hydrogen concentrations found by the neutron spectrometer on the orbiting Odyssey spacecraft. Our experiments indicate that the maximum number of water molecules per SO4 held by the amorphous Mg‐sulfate structure is three. In addition, the amorphization rate of Mg‐sulfates is strongly dependent on temperature. The low temperature (approximately −80°C) in the early morning hours during the Martian diurnal cycle would slow the dehydration rate, which would favor the stability of starkeyite over amorphous Mg‐sulfates and would lead to a low abundance of the latter.
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