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The high perchlorate (ClO4−) to chloride (Cl−) ratios observed at the Phoenix landing site, northern polar region of Mars, have been puzzling since detection. However, a lack of understanding of perchlorate-chloride-water systems under cryogenic conditions makes it difficult to assess ClO4−/Cl− ratios during deliquescence-related processes. Here we quantitatively evaluate ClO4−/Cl− fractionation in deliquescence-induced brines of magnesium- and calcium-perchlorate-chloride salt mixtures under subzero conditions, by measuring solubility data and constructing temperature-dependent thermodynamic models. We find that under specific relative humidity (RH) and temperature (T) conditions, deliquescence of perchlorate-chloride mixtures may form brines with fractionated ClO4−/Cl− signatures. Appropriate RH-T, water-limited conditions, and aeolian processes are required to produce and preserve the elevated ClO4−/Cl− signatures in soils. Under the present climate, the north polar region can support ClO4−/Cl− fractionation and potentially enrich perchlorate for longer periods on global Mars. This highlights the uniqueness of Mars’ arctic environment and its implications for modern habitability.

Technology advancement in modern planetary exploration has extended extraterrestrial geological science by sending rovers on challenging, possibly risky but highly scientific-valuable ventures, such as ascending towards the crater walls, traversing steep slopes of sand dunes and travelling through the lava tube. On the 41st lunar day, the first lunar farside prospector, the Yutu-2 rover, carried out an exciting expedition towards a scientifically interesting fin-shaped rock for spectral investigation, taking a high risk of wheel skidding and lateral slippage along a narrow and uneven passage. The rover successfully achieved new findings by extending its locomotion margin on perilous peaks, while its safety was maintained on the basis of ingenious exploration strategies and digital twin-based performance analysis. Surface morphology analysis of the fin-shaped rock indicates that it has suffered certain degrees of impacts. The further in situ spectral investigations suggest that the target rock is composed of Fe/Mg-rich low-Ca pyroxene, thus inferred to belongs to the Zhinyu crater ejecta, rather than those of the Finsen crater. Engineering locomotive data of the rover was used for comprehensive lunar regolith property identification, presenting the first shear parameter range of the farside regolith and an initial estimation on its lateral property in the extraterrestrial environment. The estimated internal friction angle is within 21.5°-42.0° and the associated cohesion is 520-3154 Pa, which suggests that the lunar regolith at Chang’E-4 site had similar shear characteristics to samples measured by direct shear approach on the Apollo 12 mission, but relatively larger cohesion than the counterpart investigated on most of nearside lunar missions. This study demonstrates a paradigm of the in-depth integration of science and technology in space exploration, where planetary science is enabled by engineering support, and new demands of scientific exploration in turn generate motivation for the improvement of technology.

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 (Fa10) has been subjected to extensive studies, SWSAEs of Fe-rich (Fa# > 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 (Fa29, Fa50, Fa71, and Fa100) 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 Fa100 compared to those hundreds of nanometers thick in Fa29, Fa50, and Fa71. With increasing irradiation energy levels and Fa# values, nanophase iron (np-Fe0) particles increase in size but decrease in quantity. The np-Fe0 formed via in situ decomposition are ubiquitously present, while those formed via vapor deposition are primarily found in Fa29 but rarely in Fa# ⩾ 50. The size fraction of intermediate (10–40 nm) and large (40–60 nm) np-Fe0 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-Fe0 formed via space weathering may cause darkening-reddening and potentially brightening-reddening spectral effects on the Martian moons.

The Mars’ climate is cold and dry in the most recent epoch, and liquid water activities are considered extremely limited. Previous orbital data only show sporadic hydrous minerals in the northern lowlands of Mars excavated by large impacts. Using the short-wave infrared spectral data obtained by the Zhurong rover of China’s Tianwen-1 mission, which landed in southern Utopia Planitia on Mars, we identify hydrated sulfate/silica materials on the Amazonian terrain at the landing site. These hydrated minerals are associated with bright-toned rocks, interpreted to be duricrust developed locally. The lithified duricrusts suggest that formation with substantial liquid water originates by either groundwater rising or subsurface ice melting. In situ evidence for aqueous activities identified at Zhurong’s landing site indicates a more active Amazonian hydrosphere for Mars than previously thought.

Mars is currently in the spotlight of solar system exploration and planetary science study. The scientific questions of Mars are closely integrated with big enigmas of the solar system, highlighting the centrality of Mars in understanding the formation and evolution of our solar system. After nearly six decades of exploration, Mars is easily the most studied planet in the solar system besides Earth, with a profusion of research across space environment, atmosphere, surface-subsurface compositions, topography and structure, impact history, glaciers and cryosphere, climate change, and internal structure. Martian meteorite and laboratory simulation studies (experimental and numerical) are also developing rapidly. Key discoveries in the past 20 years include evidence of past and current aqueous activity, geological environment diversity, modern geological processes, methane emissions and preserved organics, atmospheric composition and evolution, current and recent climate change, gravity fields and surface radiation environments, etc. Such scientific achievements, underpinned by peer-reviewed research goals developed by the planetary community, in turn shape future goals and targets. We review how the Mars exploration goals and targets (e.g., life, climate, geology, preparation for human exploration) have evolved in the past 20 years, and show priorities and focii for future international Mars exploration. For example, the Mars exploration strategy evolved thematically from “follow the water” to “understand Mars as a system”, “understand the long-term evolution of habitability on Mars”, and “exploration by humans on Mars”. In the next 10−20 years, Mars exploration will further characterize the internal structure of Mars, start a new era of life detection, and return samples from Mars and its satellites. China’s first Mars mission will contribute to the international Mars science community with an orbiter and a rover exploring the northern lowlands with advanced payloads. New findings such as the structure of the Martian critical zone (i.e., vertical zone of interaction between the atmosphere and crust with immediate relevance for habitability), local and global characteristics of the residual crustal magnetism, volcanic-geothermal evolution and the cyclicity of glaciations will clarify the still poorly known provenance and evolution of the topographic dichotomy. Hypotheses such as the formation of the global dichotomy by an oblique impact or an ancient ocean-sustaining climate may be tested and constrained. Internationally planned sample-return from Mars and its satellites, combined with study of Martian meteorites, experimental and numerical simulation of Martian processes, and study of terrestrial analog sites and samples will build absolute geochronology of Mars and constrain the timing and duration of critical events, such as cessation of the global magnetic field, quiescence of volcanism across the Noachian-Hesperian temporal boundary, transition to a single-plate planet, aqueous and sedimentary processes, and global climate change.

Widely distributed perchlorate on the Martian surface and over three orders of magnitude variation in bromine abundances in surface samples are difficult to explain solely by chloride and bromide aqueous geochemistry. New experiments show that photochemical oxidation (ultraviolet wavelength 254 nm) of chloride- and bromide-bearing evaporative brines in the presence of silica beads produces substantial perchlorate (ClO − 4 ), chlorate (ClO − 3 ), and bromate (BrO − 3 ) under conditions relevant to Mars. Neutral to alkaline aqueous environments result in the dominance of chlorate over perchlorate. Preferential atmospheric recycling of Br over Cl causes variable Br/Cl ratios, consistent with numerous in-situ measurements of Cl and Br abundances on Mars. Bromate reacts with organics more readily than chlorate or perchlorate, and its presence in subsurface brines could challenge habitability in the Martian subsurface.

National Aeronautics and Space Administration (NASA)′s Opportunity rover landed at Meridiani Planum on Mars in 2004, and discovered the first sedimentary record on Mars called Burns formation. The detection of magnesium- and iron- sulfates, jarosite (KFe3(SO4)2(OH)6), and hematite (Fe2O3) in the outcrop of Burns formation indicates that aqueous activities have been once present in this region. The Alpha Particle X-ray Spectrometer (APXS) onboard the rover detected enrichment of elemental Br in rock and soil samples which varied by three orders of magnitude, and primarily controlled the variations of Br/Cl ratios in these samples. Although aqueous related processes have been suggested to explain the enrichment of Br, the speciation of Br and the mechanisms for Br variations are poorly constrained. Jarosite has been reported to be able to preferentially incorporate Br− over Cl− during precipitation, and result in Br enrichment and Br/Cl fractionation. However, the partitioning behavior of Br− and Cl− during jarosite formation and how the incorporation of halogens would influence properties of jarosite are not well known. In this work, we synthesized a series of halogen bearing jarosite with Br− and Cl− concentration gradients by oxidation of ferrous sulfate at room temperature. After synthesis, we used X-ray diffraction (XRD), Raman spectrometer, infrared spectrometer (IR), scanning electron microscope (SEM) to analyze structure information and surface morphology of jarosite solids. Anion and cation concentrations in solution and solid were analyzed by ion chromatography (IC), atomic absorption spectrophotometer (AAS) and X-ray fluorescence (XRF). Our work shows that Br and Cl contents in the synthesized jarosite are positively correlating with its initial concentrations in solutions. Starting at the same concentrations in solution, incorporation of Br− in jarosite is about two orders of magnitude higher than that of Cl−. Coexisting Br− can significantly interfere with Cl− partitioning into jarosite. The distribution coefficients of Br− and Cl− are negatively correlated with their initial concentrations in solutions, and in the Br− and Cl− coexisting setting, the distribution coefficients of Cl− decrease as the initial Br− concentration increase in the solution. Therefore, during precipitation of jarosite, Br− prefers to participate into jarosite while Cl− prefers to remain in solution. Incorporation of halide anions into jarosite directly affected the range of hydroxyl and water in Raman spectra, indicating that Br− and Cl− substitute for OH position in jarosite without changing its fundamental structure. We calculated that in our experiment settings, Br/Cl ratios in jarosite are about two orders of magnitude higher than that of initial solutions. Therefore, if jarosite precipitates from a brine containing Br− and Cl−, it can enrich Br over Cl and bearing significant higher Br/Cl ratio signature comparing to the initial brine. For sedimentary outcrop composed of substantial amount of jarosite, Br/Cl ratios may not be controlled solely by evaporation and precipitation of halide evaporites, but might also be influenced by precipitation and dissolution of halogen bearing jarosite. The aqueous stability of halogen bearing jarosite and the possible release of halide anions during jarosite dissolution require further evaluation.

Benjamin Fernando and colleagues report on the international cooperation involved InSight's attempt to gather seismic data from the arrival at Mars of China's Zhurong rover.

Anticipated human missions to Mars require a methodical understanding of the unconsolidated bulk sediment that mantles its surface, given its role as an accessible resource for water and as a probable substrate for food production. However, classifying martian sediment as soil has been pursued in an ad hoc fashion, despite emerging evidence from in situ missions for current and paleo-pedological processes. Here we find that in situ sediment at Gusev, Meridiani and Gale are consistent with pedogenesis related to comminuted basalts mixing with older phyllosilicates – perhaps of pluvial origin – and sulfates. Furthermore, a notable presence of hydrated amorphous phases indicates significant chemical weathering that mirrors pedogenesis at extreme environments on Earth. Effects of radiation and reactive oxygen species are also reminiscent of such soils at Atacama and Mojave deserts. Some related phases, like perchlorates and Fe-sulfates, may sustain brine-driven weathering in modern martian soils. Meanwhile, chemical diversity across in situ and regional soils suggests many different soil types and processes. But the two main soil classification systems – the World Reference Base for Soil Resources (WRB) and the U.S. Soil Taxonomy – only inadequately account for such variability. While WRB provides more process insight, it needs refinement to represent variability of martian soils even at the first level of categorical detail. That will provide a necessary reference for future missions when identifying optimal pedological protocols to systematically survey martian soil. Updating Earth-based soil classification systems for this purpose will also advance soil taxonomy as a research field.

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.

The behaviors of bromide, chloride, and phosphate were studied experimentally under previously proposed Martian diagenetic conditions, involving jarosite (KFe3(OH)6(SO4)2), goethite (α-FeOOH), and hematite (α-Fe2O3). Experiments evaluated (1) the behavior of Cl−/Br− with and without aqueous phosphate during oxidation of Fe2+ to Fe3+, (2) the stability of halogen-bearing jarosite, and (3) the uptake of Cl−, Br−, H2PO4−, and SO42− by halogen-free-hematite, -goethite, and -jarosite through adsorption. Our results demonstrate that when precipitated from a solution, in which Cl− is higher than Br−, jarosite preferentially incorporated at least an order of magnitude more Br− than Cl−. Such enrichment of Br− over Cl− in the solids compared to initial solutions suggests that jarosite could be a host for elevated Br on the Martian surface, and the fluids from which jarosite forms could be depleted in Br− with respect to Cl−. Moreover, the incorporation of halogens in jarosite would affect its stability during aqueous alteration, and the dissolution rates of four types of jarosite at both 25°C and 70°C were in the same order: Br,Cl bearing > Br only > halogen free > Cl only. In addition, competitive adsorption of Cl−, Br−, SO42−, and H2PO4− on halogen-free-hematite, -goethite, and -jarosite demonstrates that in a sulfate-dominant aqueous system, Cl−, Br−, and H2PO4− could not compete with SO42−. This observation suggests that the adsorption may not result in an enrichment of phosphate or halogens in Fe oxides in a sulfate-dominant aqueous system like Meridiani Planum, consistent with the absence of significant correlations of Cl and P with nanoparticle Fe oxides found in Martian soils.

Detailed statistical examination of Cl, Br, and S distributions, in martian soil profiles at Gusev Crater and Meridiani Planum, indicates decreasing Br abundance and weakening Br–S association towards the surface. All three elements decrease towards the surface in the order Cl

The behavior of Ni(II), Zn(II) and Cr(III) during the melanterite (FeSO4·7H2O) to hematite (α-Fe2O3) oxidative transformations involving evolution pathways via jarosite ((H3O,K)Fe3(OH)6(SO4)2), schwertmannite (Fe8O8(OH)6(SO4)) and goethite (α-FeOOH) were investigated in an acidic saturated MgSO4 matrix. Results provide important clues about how elevated levels of trace elements are incorporated into the secondary Fe mineralogy assemblages found on Mars and the mechanism for formation of hematitic concretions at Meridiani Planum on Mars. Our results demonstrate that starting at the same concentrations in the initial solution, final amounts of Ni, Zn and Cr in hematite via different pathways are very different. In Path 1 (melanterite → jarosite → hematite), partitioning of Ni, Zn and Cr into jarosite and hematite (formed through dissolution of jarosite) is most likely in the order: Cr > Zn > Ni. In Path 2 (melanterite → schwertmannite → goethite → hematite), schwertmannite and goethite exhibited strong affinities for divalent Ni and Zn. During such a pathway, Ni should accumulate more than Zn by at least a factor of two, and partitioning of Ni, Zn and Cr to the hematite is most likely in the order: Cr > Ni > Zn. Therefore, our results suggest that the high Ni and moderate Zn distribution pattern observed in Meridiani hematitic spherule-bearing samples can be explained best by the schwertmannite–goethite to hematite pathway (Path 2), without need for an additional high Ni source in this region. Although the lack of goethite at Meridiani renders it uncertain if goethite ever served as a precursor to facilitate hematite formation, dehydration of nano-crystalline goethite is thermodynamically favored and cannot be ruled out. On the other hand, if hematitic concretions were formed by dissolution of jarosite (Path 1), then much higher initial Ni/Zn ratios than 1 in initial diagenetic fluids may be necessary to explain the elevated levels of Ni in the spherules. Cr(III), when starting in equal amounts as Ni and Zn in the solution, accumulated at least two orders of magnitude more than Ni and Zn in all Fe(III)-phases. Given that Cr concentrations are of the same order of magnitude as Ni and Zn in most rocks and soils measured by the rover Opportunity, Cr concentrations should be much lower than Ni and Zn in the initial diagenetic fluids. Mobility of Cr was greatly limited by precipitation of Fe(III)-phases, due to substitution for Fe(III) and sequestration in the solids.

Electrostatic discharge experiments under simulated martian atmospheric conditions indicate that atmospheric CO2 has been sequestered into carbonate by the Mars dust activities during the Amazonia era.