Gale Crater Research Articles

Insights into Martian bedform migration: Results from Gale, Jezero and Pasteur craters

AbstractAn attempt is made in this study to advance the understanding of the sand movement on Mars by studying the bedform migration at Gale, Jezero and Pasteur craters. The study on the grain size distribution at Gale Crater using Curiosity rover (MAHLI and APXS) observations reveals that the grains with smaller diameters (~50–150 μ) are more prone to migration and vice‐versa, which gives an idea of the necessary requirements that initiate bedform migration. The chemical analysis of the surface materials at the Gale crater revealed elevated concentrations of P2O5, SO3, Cl and Zn in soil compared to sand and active transportation processes for sand but not soil. The comprehensive chemical makeup of the Martian soil (inactive bedforms) and sand (active bedforms) is characterized by its basaltic nature, with enriched volatile elements such as sulphur, chlorine and zinc, and the presence of minerals like plagioclase, pyroxene and olivine due to the cohesive nature of inactive bedforms. Physical weathering and wind flow velocity play a pivotal role in the formation of different sedimentary bodies, impacting grain size distribution and mineralogy. The effect of dust‐lifting on surface features is studied by analysing Perseverance‐MEDA observations at the Jezero crater to understand the short‐term changes in the bedform. These events are found to involve the redistribution of only a small amount of materials and, thereby, changing surface features on Mars over a short period. To detect the bedform migration in the Pasteur crater, several HiRISE images acquired over different time intervals were used. The changes in the ripple crest (~0.29–1.18 m/Earth year) and dune slip face suggest new grain flow events. In the Pasteur crater, extensive changes in sand deposits near the dunes signify a widespread bedform migration. The stronger north‐westerly and north‐easterly winds dominate these changes. Thus, the bedform migration in the three tropical craters exhibits significant variability driven by localized aeolian processes. This variability is crucial for understanding Mars' geological history, current surface dynamics and eventually, helps in planning future missions.

Constraining the Duration and Ages of Stratigraphic Unconformities on Mars Using Exhumed Craters

AbstractCrater counting is a widely applied methodology for dating large areas of planetary surfaces, but is difficult to apply the method to constrain the durations of stratigraphic unconformities. Unconformities with exhumed craters are thought to indicate long hiatuses that can only be indirectly dated through stratigraphic relationships with other surfaces with uniform exposure ages. On Mars, sedimentary deposits with prominent unconformities with exhumed craters are found in layered deposits in the Arabia Terra region as well as Gale crater within Mount Sharp. In this work, we present a Linear Crater Counting methodology and apply it to constrain these unconformities observed in Arabia Terra and in Mount Sharp. The method applies a linear sampling domain correction to conventional two‐dimensional crater size frequency distributions and Bayesian Poisson process statistics in order to constrain the likely durations of these unconformities. We found that unconformities in Arabia Terra were on the order of 0.1–1 Gyr in length and that the unconformity preserved at Mount Sharp is at least 0.2 Gyr in length given estimates of the ages of the host craters. Hiatuses of these lengths constrain the age of the overlying deposits to be Late Hesperian or Amazonian in age. Two utility plots are also provided, along with the derivation, for researchers to apply this method to dating arbitrary geologic contacts on Mars and to adapt it to other bodies.

MARS CHRONOLOGY DERIVED FROM CRATERS HETEROGENEITY AT GALE CRATER

Craters sample into diachronic surfaces and different depths on Mars and other planetary surfaces. These surfaces are affected by cosmonuclide radiation that offers one of the most reliable age anchors to date. Craters also accumulate, evolve and display an universal characteristic that can be measured at any scale, regardless of the geologic unit they sample, in the form of the heterogeneity parameter (Capitan, 2021). Here we use an age equation, which is based on the measurements of craters diameter, depths and area they occupy, to derive the ages of deposits that are sampled by medium-scale craters (meters to few hundred meters in diameter). We show that units sampled by the deepest craters near MSL exploration area are formed during the early stages of Gale crater formation before 2870 Ma. In contrast, units sampled by shallower craters were formed during the stages that correspond to the time of sediment recycling and lithification periods, near 2129 Ma to present. Given the heterogeneity of initial formation conditions of craters of diverse diameters and their different depths of sampling, our proposed synchronous ages with ground-truth ages has the potential to redefine the paradigm of using the impact crater morphometry as a tool to date the planetary surfaces.

Evaporitic–lacustrine mudstone laminites and prodelta mudlobes – continuity and change in the Hartmann's Valley interval, Murray formation, Gale crater, Mars

ABSTRACTThe sedimentary fill of Gale crater on Mars is dominated by lacustrine strata that are the focus of investigations into surface processes, climate change and habitability early in the planet's history. This study focuses on the lower part of the explored portion of the Murray formation on Aeolis Mons (Mount Sharp) in the Pahrump Hills and Hartmann's Valley field areas. At Pahrump Hills, sedimentary features and geochemical attributes of exposed mudstones suggest accumulation in an underfilled lake basin in which lake levels, shorelines and salinities fluctuated greatly at various temporal scales. The immediately superjacent stratigraphic interval, designated by the rover team as the Hartmann's Valley member, has previously been interpreted as an aeolian‐deposited sandstone interval. This fresh examination of all available data, however, suggests that the Hartmann's Valley member is strongly dominated by the same mudstone facies that compose most of the underlying Pahrump Hills member. The one novel facies element encountered in the Hartmann's Valley member are undulose bedded to convex bodies of mudstone that are oriented roughly north–south, suggestive of sediment supply from a northern fluvial source. Mudstones analogous in scale and character to these undulose mudstones occur on Earth in multiple modern and ancient lacustrine successions, and are related to deposition of prodelta mud lobes. The mudstones of the Hartmann's Valley member record a continuation of the underfilled evaporitive lake system proposed previously for the underlying Pahrump Hills member but with somewhat more variable environmental conditions. Hartmann's Valley mudstones contain more abundant matrix CaSO4, suggesting more severe evaporitic conditions, whereas the interbedded undulose bedded mudstones signify intermittent freshening and buildout of mud‐dominated deltas into the lake. The textural similarities between Pahrump Hills and Hartmann's Valley strata suggest that the Gale lacustrine system operated at a stable depositional mode with similar environmental parameters during both stratigraphic intervals.

Radiation-Driven Destruction of Thiophene and Methyl-Substituted Thiophenes.

Thiophene and two derivatives (2-methylthiophene and 3-methylthiophene) have been detected on the surface of Mars with the Sample Analysis at Mars instrument suite onboard NASA's Curiosity rover. Thiophene could serve as a secondary chemical biosignature since the secondary biosynthesis of thiophene is considered an important production pathway. However, it is critical to understand the abiotic formation and destruction of thiophene and its derivatives since these pathways could affect the molecules' stabilities on planetary surfaces over geological timescales. Here, we present the radiolytic destruction kinetics of thiophene, 2-methylthiophene, and 3-methylthiophene as single-component ices and when diluted in water ice at low temperatures. Using infrared spectroscopy, we determined the destruction rate constants and extrapolated our radiolytic half-lives to the surface of Mars, assuming the measured and modeled surface dose rates. We found that our rate constants strongly depend on temperature and presence of water ice. Based on our determined radiolytic half-life for thiophene under conditions most similar to those of thiophene groups in Martian macromolecules, we expect thiophene to be stable on the surface for significantly longer than the Martian surface exposure age of sites in Gale crater where thiophenes have been detected.

Highly enriched carbon and oxygen isotopes in carbonate-derived CO2 at Gale crater, Mars

Carbonate minerals are of particular interest in paleoenvironmental research as they are an integral part of the carbon and water cycles, both of which are relevant to habitability. Given that these cycles are less constrained on Mars than they are on Earth, the identification of carbonates has been a point of emphasis for rover missions. Here, we present carbon (δ13C) and oxygen (δ18O) isotope data from four carbonates encountered by the Curiosity rover within the Gale crater. The carbon isotope values range from 72 ± 2‰ to 110 ± 3‰ Vienna Pee Dee Belemnite while the oxygen isotope values span from 59 ± 4‰ to 91 ± 4‰ Vienna Standard Mean Ocean Water (1 SE uncertainties). Notably, these values are isotopically heavy (13C- and 18O-enriched) relative to nearly every other Martian material. The extreme isotopic difference between the carbonates and other carbon- and oxygen-rich reservoirs on Mars cannot be reconciled by standard equilibrium carbonate-CO2 fractionation, thus requiring an alternative process during or prior to carbonate formation. This paper explores two processes capable of contributing to the isotopic enrichments: 1) evaporative-driven Rayleigh distillation and 2) kinetic isotope effects related to cryogenic precipitation. In isolation, each process cannot reproduce the observed carbonate isotope values; however, a combination of these processes represents the most likely source for the extreme isotopic enrichments.

Evaluating possible glacial modification in the Martian tropics near Gale crater using a new U-shaped valley metric

Growing evidence supports the case for glacial processes within the equatorial regions of Mars. However, few examples of modeling or numerical analysis exist to support this hypothesis. Here we apply an automated method on valleys near Gale crater to extract morphometric data and analyze formative processes responsible for their current expression. The V-index utilized is a new robust method that is an alternative to traditional valley parabolic curve fits. This approach more easily characterizes irregular valley cross sections and thus aids in distinguishing between glacial and nonglacial forms. We applied this method along with standard power law and quadratic curve fits on valley cross sections along a 2300 km extent of the Martian tropics near Gale crater. Both V-index and power law b fit values suggest that the majority of the valleys are U-shaped and possibly created through glacial erosive processes. Further, V-index and power law b values plotted against elevation show a positive trend with higher V-index (glaciated) values associated with higher elevation. This suggests that an equilibrium-line altitude (ELA) may have existed at the time of valley formation. The timing of the formation of these U-shaped valleys is poorly defined. However, our initial crater statistics conducted within the study area suggest a young ∼1.05 (±0.25) Ga relative ages of the valley floors. Equatorial glaciers that produced these and other related glacial forms may be as old as the Hesperian to Amazonian transition (∼3 Ga) but more intriguingly may be associated with Amazonian equatorward migration of the Martian cryosphere during more recent periods of high obliquity oscillations.

Morphological, hydrogeochemical and sedimentological analysis of hypersaline Sambhar Lake, India: An analog to understand evaporitic paleolake basins on Mars

In the recent decade of astrobiological exploration of the Martian surface, there has been a shift from identifying habitable environments to finding markers indicative of biological activity. It requires a prior understanding of the physical and geochemical environment of the setting to decipher whether the conditions were conducive. Generally, quiescent surroundings of lacustrine basins are considered one of the best targets for the preservation of any biological signatures. However, due to logistical limitations, the geochemical information available is mostly restricted to small areas on the surficial level (or in the subsurface in case of layered deposits or impact craters) where sufficient satellite coverage is available and, in some areas, where rovers/landers have been deployed. In this context, terrestrial lacustrine basins offer valuable insights into the environment required for the formation of the minerals observed on the Martian surface.This study was carried out within Sambhar Lake located in the arid/semiarid climatic zone within the Thar desert. It is a hypersaline playa that has undergone several cycles of desiccation and re-filling, sharing its climate-controlled history with that of several paleolakes on Mars. We conducted physicochemical analysis of the samples collected from the lake and its surrounding area and compared our results with samples from the Curiosity rover (at Gale crater) and to those of the studies carried out in basalt-rich parent settings of Iceland. Our results suggest that Sambhar Lake is a Na-Cl type brine with climate-driven hydrology. The shallow cores and rock samples indicated that the area is rich in evaporites. We propose that even the sites with different parent material may be crucial in understanding the geological evolution of paleolakes on Mars and that Sambhar is a great example to study tectono-geomorphic evolution and the climate-induced transition of a lacustrine basin to a playa. Additionally, the lake is also desirable to study extremophiles and their adaptation to changing environmental variables for future planetary missions, including but not limited to, Mars.

Soil diversity at Jezero crater and Comparison to Gale crater, Mars

The martian soil is of particular interest as it can help us understand the different processes that have occurred on Mars by studying the chemistry and mineralogy of its constituents as a function of grain size. The fine-grained martian soil is thought to be homogeneous across the planet and thus to represent a global component. In this study we report on the soil targets analysed by the SuperCam instrument aboard the Perseverance rover, which is currently exploring Jezero crater. A total of 343 targets were analysed. Their grain size distribution confirms the sparsity of 250–900 Âμm particles in the martian soil, although both smaller and larger grains are present. We found that the local components, due to erosion of the local bedrock, are present not only in the very coarse grains or larger gravels of the soil, but also in the very fine ones (¡250 Âμm). We detected some very coarse grains enriched in olivine, pyroxene and carbonate in both the crater floor and the delta front locations, whereas phyllosilicate-rich grains have been encountered only in the delta front. We have compared the Jezero fine-grained soil targets with those of Gale crater using ChemCam data. We found that those at Jezero show no evidence of Mg sulfates, in contrast to the observation at Gale. In addition, the fine-grained soil at Jezero is more hydrated than that at Gale, probably due to its higher specific surface area.

Gypsum on Mars: A Detailed View at Gale Crater

Gypsum is a common mineral at Gale crater on Mars, currently being explored by the Mars Science Laboratory (MSL) rover, Curiosity. In this paper, we summarize the associations of gypsum with other sulfate minerals (bassanite, anhydrite, jarosite, starkeyite, and kieserite) from the lowest levels of the crater’s northern moat zone (Aeolis Palus) up through ~0.8 km of the stratigraphic section in the lower slopes of the sedimentary mound developed around the central peak, Aeolis Mons (informally, Mount Sharp). The analysis is based on results from the CheMin X-ray diffraction instrument on Curiosity, supplemented with information from the rover’s versatile instrument suite. Gypsum does not occur with the same frequency as less hydrous Ca-sulfates, likely, in most cases, because of its dehydration to bassanite and possibly to anhydrite. All three of these Ca-sulfate phases often occur together and, along with other sulfates, in mixed assemblages that are evidence of limited equilibration on a cold, dry planet. In almost all samples, at least one of the Ca-sulfate minerals is present, except for a very limited interval where jarosite is the major sulfate mineral, with the implication of more acidic groundwater at a much later time in Gale crater’s history. Although observations from orbit reveal a sulfate-rich surface, currently active dark basaltic dunes at Gale crater have only small amounts of a single sulfate mineral, anhydrite. Gale crater has provided the most complete mineralogical analysis of a site on Mars so far, but the data in hand show that Gale crater mineralogy is not a blueprint with planet-wide application. The concurrent study of Jezero crater by the Mars 2020 mission and comparisons to what is believed to be the most extensive deposit of gypsum on Mars, in the dune fields at the north polar ice cap, show significant diversity. Unraveling the stories of gypsum and other sulfates on Mars is just beginning.

An Orbital Comparison of a Late Mantling Unit on Aeolis Mons With Other Erosion‐Resistant Strata Explored by MSL in Gale Crater, Mars

AbstractPrevious investigations along Curiosity's traverse in Gale crater have explored the relationship between orbital and in situ observations. This work aims to better understand the geologic environment of units only observable from orbit and compare them to the properties of units examined by Curiosity. Here, we map an erosion‐resistant dark‐toned mantling unit that overlies the modern topography of Aeolis Mons (informally known as Mt. Sharp) and compare this unit to two other previously mapped dark‐toned resistant units, the marker band and the mound skirting unit (MSU), that have been inferred to represent different geologic environments (lacustrine and aeolian, respectively). Visible to short wave infrared spectra from the Compact Reconnaissance Imaging Spectrometer for Mars and visual images from the High Resolution Imaging Science Experiment and Context Cameras aboard the Mars Reconnaissance Orbiter are used for this comparison. Spectral data suggest a mafic composition with minor alteration, although the composition varies more with location around Mt. Sharp rather than between units. Morphologically, the mantling unit has strong similarities to the marker band based on their consistent low‐albedo, erosion‐resistance, and smooth appearance, contrasting with the highly variable surface texture of the MSU. We hypothesize that all three units had a similar sediment source but experienced aqueous alteration at different times: early ubiquitous cementation in a surface aqueous environment in the mantling unit and marker band versus patchy late diagenesis in the MSU. If true, these results suggest that water activity continued within the Gale crater long after the erosion of Mt. Sharp.

Expanded Insights into Martian Mineralogy: Updated Analysis of Gale Crater’s Mineral Composition via CheMin Crystal Chemical Investigations

This study presents mineral composition estimates of rock and sediment samples analyzed with the CheMin X-ray diffraction instrument on board the NASA Mars Science Laboratory rover, Curiosity, in Gale crater, Mars. Mineral composition is estimated using crystal-chemically derived algorithms applied to X-ray diffraction data, specifically unit-cell parameters. The mineral groups characterized include those found in major abundance by the CheMin instrument (i.e., feldspar, olivine, pyroxene, and spinel oxide). In addition to estimating the composition of the major mineral phases observed in Gale crater, we place their compositions in a stratigraphic context and provide a comparison to that of martian meteorites. This work provides expanded insights into the mineralogy and chemistry of the martian surface.

Variable and Large Losses of Diagnostic Biomarkers After Simulated Cosmic Radiation Exposure in Clay- and Carbonate-Rich Mars Analog Samples.

Mars has been exposed to ionizing radiation for several billion years, and as part of the search for life on the Red Planet, it is crucial to understand the impact of radiation on biosignature preservation. Several NASA and ESA missions are looking for evidence of ancient life in samples collected at depths shallow enough that they have been impacted by galactic cosmic rays (GCRs). In this study, we exposed a diverse set of Mars analog samples to 0.9 Megagray (MGy) of gamma radiation to mimic 15 million years of exposure on the Martian surface. We measured no significant impact of GCRs on the total organic carbon (TOC) and bulk stable C isotopes in samples with initial TOC concentration > 0.1 wt. %; however, diagnostic molecular biosignatures presented a wide range of degradation that didn't correlate to factors like mineralogy, TOC, water content, and surface area. Exposure dating suggests that the surface of Gale crater has been irradiated at more than five times our dose, yet using this relatively low dose and "best-case scenario" geologically recalcitrant biomarkers, large and variable losses were nevertheless evident. Our results empasize the importance of selecting sampling sites at depth or recently exposed at the Martian surface.

Fe-rich X-ray amorphous material records past climate and persistence of water on Mars

X-ray amorphous material comprises 15-73 wt.% of sedimentary rocks and eolian sediments in Gale crater. This material is variably siliceous and iron rich but aluminum poor. The presence of volatiles is consistent with the existence of incipient weathering products. To better understand the implications of this material for past aqueous conditions on Mars, here we investigate X-ray amorphous material formation and longevity within terrestrial iron rich soils with varying ages and environmental conditions using bulk and selective dissolution methods, X-ray diffraction, and transmission electron microscopy. Results indicate that in situ aqueous alteration is required to concentrate iron into clay-size fraction material. Cooler climates promote the formation and persistence of X-ray amorphous material whereas warmer climates promote the formation of crystalline secondary phases. Iron rich X-ray amorphous material formation and persistence on Mars are therefore consistent with past cool and relatively wet environments followed by long-term cold and dry conditions.

The Geochemical and Mineralogical Signature of Glaciovolcanism Near Þórisjökull, Iceland, and Its Implications for Glaciovolcanism on Mars

AbstractCandidate glaciovolcanic landforms have been identified across Mars, suggesting that volcano‐ice interactions may have been relatively widespread in areas that once contained extensive surface and near‐surface ice deposits. To better constrain the detection of glaciovolcanism in Mars' geological record, this study has investigated and characterized the petrology, geochemistry, and mineralogy of three intraglacial volcanoes and an interglacial volcano in the Þórisjökull area of southwest Iceland. Our results show that glaciovolcanism creates abundant, variably altered hyaloclastite and hyalotuff that is sufficiently geochemically and mineralogically distinctive from subaerially erupted lava for identification using instruments available on Mars rovers and landers. Due to the lower gravity and atmospheric pressure at the surface of Mars, hyaloclastite and hyalotuff are also more likely to form in greater abundance in Martian glaciovolcanoes. Our results support that magmatism following deglaciation likely triggers decompression melting of the shallow mantle beneath Iceland, creating systematic changes in geochemistry and mineralogy. Glaciation can also suppress magmatism at its peak, encouraging the formation of shallow fractionated magma chambers. As such, it is possible for the crustal loading of an ice cap to enhance igneous diversity on a planet without plate tectonism, creating glass‐rich, altered, and mineralogically diverse deposits such as those discovered in Gale crater by the Curiosity rover. However, as the eroded products of glaciovolcanism are similar to those formed through hydrovolcanism, the presence of a glaciovolcanic landform at the source is required to confirm whether volcano‐ice interactions occurred at the sediment source.

Geological context and significance of the clay-sulfate transition region in Mount Sharp, Gale crater, Mars: An integrated assessment based on orbiter and rover data

Geological context and significance of the clay-sulfate transition region in Mount Sharp, Gale crater, Mars: An integrated assessment based on orbiter and rover data

Phosphates on Mars and Their Importance as Igneous, Aqueous, and Astrobiological Indicators

This paper reviews the phosphate phases in meteorites and those measured by landed spacecraft, what they reveal about past igneous and aqueous conditions on Mars, and important implications for potential prebiotic chemistry, past habitability, and potential biosignatures that could be detected in samples returned from Mars. A review of the 378 martian meteorites as of 2023 indicate that of the two most common phosphate minerals in Mars meteorites, merrillite and apatites, the apatite composition is largely F- and Cl-rich, with shergottites containing more OH. The phosphate concentrations examined across multiple missions show a relatively narrow range of phosphate, with higher concentrations observed in the Mount Sharp Group in Gale crater and Wishstone at Gusev crater and lower concentrations observed at Jezero crater floor and Jezero fan. Possible secondary phosphates detected on Mars, including Fe phosphates at Jezero crater and Gusev crater and Ca- and Al-bearing secondary phosphates, temperatures of formation of secondary phases and their dissolution rates and solubilities are reviewed and summarized. Despite this wealth of information about phosphates on Mars, due to their fine scale and relatively low concentrations, Mars Sample Return is needed to better understand phosphate and its implications for the igneous, aqueous, and astrobiological history of Mars.

Quantifying the Potential for Nitrate-Dependent Iron Oxidation on Early Mars: Implications for the Interpretation of Gale Crater Organics.

Geological evidence and atmospheric and climate models suggest habitable conditions occurred on early Mars, including in a lake in Gale crater. Instruments aboard the Curiosity rover measured organic compounds of unknown provenance in sedimentary mudstones at Gale crater. Additionally, Curiosity measured nitrates in Gale crater sediments, which suggests that nitrate-dependent Fe2+ oxidation (NDFO) may have been a viable metabolism for putative martian life. Here, we perform the first quantitative assessment of an NDFO community that could have existed in an ancient Gale crater lake and quantify the long-term preservation of biological necromass in lakebed mudstones. We find that an NDFO community would have the capacity to produce cell concentrations of up to 106 cells mL-1, which is comparable to microbes in Earth's oceans. However, only a concentration of <104 cells mL-1, due to organisms that inefficiently consume less than 10% of precipitating nitrate, would be consistent with the abundance of organics found at Gale. We also find that meteoritic sources of organics would likely be insufficient as a sole source for the Gale crater organics, which would require a separate source, such as abiotic hydrothermal or atmospheric production or possibly biological production from a slowly turning over chemotrophic community.

The Chemistry and Mineralogy (CheMin) X-ray Diffractometer on the MSL Curiosity Rover: A Decade of Mineralogy from Gale Crater, Mars

For more than a decade, the CheMin X-ray diffraction instrument on the Mars Science Laboratory rover, Curiosity, has been returning definitive and quantitative mineralogical and mineral–chemistry data from ~3.5-billion-year-old (Ga) sediments in Gale crater, Mars. To date, 40 drilled rock samples and three scooped soil samples have been analyzed during the rover’s 30+ km transit. These samples document the mineralogy of over 800 m of flat-lying fluvial, lacustrine, and aeolian sedimentary rocks that comprise the lower strata of the central mound of Gale crater (Aeolis Mons, informally known as Mt. Sharp) and the surrounding plains (Aeolis Palus, informally known as the Bradbury Rise). The principal mineralogy of the sedimentary rocks is of basaltic composition, with evidence of post-depositional diagenetic overprinting. The rocks in many cases preserve much of their primary mineralogy and sedimentary features, suggesting that they were never strongly heated or deformed. Using aeolian soil composition as a proxy for the composition of the deposited and lithified sediment, it appears that, in many cases, the diagenetic changes observed are principally isochemical. Exceptions to this trend include secondary nodules, calcium sulfate veining, and rare Si-rich alteration halos. A surprising and yet poorly understood observation is that nearly all of the ~3.5 Ga sedimentary rocks analyzed to date contain 15–70 wt.% of X-ray amorphous material. Overall, this &gt;800 m section of sedimentary rock explored in lower Mt. Sharp documents a perennial shallow lake environment grading upward into alternating lacustrine/fluvial and aeolian environments, many of which would have been habitable to microbial life.

Solfataric Alteration at the South Sulfur Bank, Kilauea, Hawaii, as a Mechanism for Formation of Sulfates, Phyllosilicates, and Silica on Mars

Abstract Solfataric alteration at the South Sulfur Bank of the former Kilauea caldera produced opal, Mg- and Fe-rich smectites, gypsum, and jarosite through silica replacement of pyroclastic Keanakako’i ash and leaching of basaltic lavas. This site on the island of Hawaii serves as an analog for formation of several minerals found in altered deposits on Mars. Two distinct alteration environments were characterized in this study including a light-toned, high-silica, friable outcrop adjacent to the vents, and a bedded outcrop containing alternating orange/tan layers composed of smectite, gypsum, jarosite, hydrated silica, and poorly crystalline ferric oxide phases. This banded unit likely represents deposition of pyroclastic material with variations in chemistry over time that was subsequently altered via moderate hydrothermal and pedogenic processes and leaching of basaltic caprock to enhance the Si, Al, Mg, Fe, and Ca in the altered layers. In the light-toned, friable materials closest to the vents along the base of the outcrop glassy fragments were extensively altered to opal-A plus anatase. Lab measurements of samples returned from the field were conducted to replicate recent instruments at Mars and provide further characterization of the samples. These include elemental analyses, sample texture, XRD, SEM, VNIR/mid-IR reflectance spectroscopy, TIR emittance spectroscopy, and Mössbauer spectroscopy. Variations in the chemistry and mineralogy of these samples are consistent with alteration through hydrothermal processes as well as brines that may have formed through rain interacting with sulfuric fumes. Silica is present in all altered samples and the friable pyroclastic ash material with the strongest alteration contains up to 80 wt.% SiO2. Sulfate mineralization occurred at the South Sulfur Bank through fumarolic action from vents and likely included solfataric alteration from sulfuric gases and steam, as well as oxidation of sulfides in the basaltic caprock. Gypsum and jarosite are typically present in different layers of the altered wall, likely because they require different cations and pH regimes. The presence of both jarosite and gypsum in some samples implies high sulfate concentrations and the availability of both Ca2+ and Fe3+ cations in a brine percolating through the altered ash. Pedogenic conditions are more consistent with the observed Mg-smectites and gypsum in the tan layers, while jarosite and nontronite likely formed under more acidic conditions in the darker orange layers. Assemblages of smectite, Ca-sulfates, and jarosite similar to the banded orange/tan unit in our study are observed on Mars at Gale crater, Noctis Labyrinthus, and Mawrth Vallis, while high-silica outcrops have been identified in parts of Gusev crater, Gale crater, and Nili Patera on Mars.