Jeffrey Johnson

Mars is primarily composed of mafic mineral assemblages and their alteration products, but small, scattered rocks strewn across the landscape offer clues to greater petrological diversity. While traversing the Jezero crater rim, the Perseverance rover encountered several plagioclase‐rich light‐toned float rocks. SuperCam identified the distinctive signature of corundum (α‐Al ₂ O ₃ ) in these rocks using time‐resolved luminescence spectroscopy. Two strong peaks (692.7 and 694.1 nm) with millisecond lifetimes, and additional supporting lines, are consistent with Cr ³⁺ substitution for Al ³⁺ in corundum. Corundum forms in Al‐rich, Si‐depleted environments through magmatic or metamorphic processes. Given the rocks' small size, association with plagioclase, and location on the crater rim, we interpret the most plausible formation scenario to be impact induced metamorphism at the interface of a felsic and a mafic/ultramafic member with the likely action of fluids at some stage, although other possibilities are not excluded.

During Sols 63–65 of Mars 2020, Mastcam‐Z multispectral (442–1022 nm) images were acquired at Van Zyl Overlook (VZO), spanning phase angles from ∼0° to 150°, to model photometric parameters for seven surface units. These units included rocks, soils, regolith, and rover tracks near the Jezero landing site. Observations at five wavelengths were collected as stereo mosaics under varying illumination geometries and converted to radiance and reflectance using calibration targets. Stereo disparity maps were generated to derive topography and associated illumination and viewing geometries, including incidence, emission, and phase angles. Regions of interest were identified for seven unit types: “blue” soils, dusty “red” soils, regolith, dust‐poor “blue” rocks, dusty “red” rocks, intermediate‐toned rocks, and rover tracks. Hapke models using both 1‐term and 2‐term Henyey–Greenstein single‐scattering phase‐functions were fit to data to estimate single‐scattering albedo ( w ), macroscopic roughness (), and scattering parameters (ξ, b, c ). Blue rocks exhibited relatively low w and strong backscattering, similar to early‐mission Mars Science Laboratory results, while red and intermediate rocks displayed higher w , weaker backscattering, and more forward‐scattering trends. Blue soil and rover tracks had w spectra consistent with more ferrous material. Rover tracks showed unexpected backscattering relative to previous observations, but also the lowest and h values, consistent with compaction from wheel interaction. These results highlight both similarities and key differences in scattering behavior among Mars sites and motivate Hapke modeling of additional Mars 2020 and Mars Science Laboratory data sets to refine interpretations of Martian surface scattering properties and improve terrain characterization.

The basal strata of the orbitally defined Layered Sulfate unit (LSu) in Gale crater, Mars, marks the end of a major mineralogical transition from clay‐to sulfate‐bearing stratigraphy. This has previously been attributed to a period of significant aridification of Gale's climate. Here, we present ground‐based observations by the Curiosity rover of the sedimentary facies and architecture of the basal strata of the LSu, and reconstruct the past depositional environments. The succession predominantly comprises four members of the Mirador formation: the Contigo, Catrimani, Amapari, and Chenapau members. The Contigo member records the migration of simple aeolian dunes. The Catrimani and Chenapau members record a facies characteristic of wind‐ripple migration in aeolian sand sheets. Periodic damp sand sheet deposition is inferred through candidate adhesion structures and evaporite crystals. Evidence for periodic surface water occurs as lacustrine deposits (the Amapari member) and an embedded fluvial lens. Near‐surface water likely restricted the availability of loose sediment, suppressing dune formation and promoting the accumulation of a thick sandsheet succession that, at ∼60 m, exceeds known terrestrial equivalents. Deposition under these conditions would have required a continuously rising water table, but the rate of sediment accumulation was likely high enough for a predominantly aeolian environment. Contrary to earlier aridification models for the LSu, this basal succession likely records an overall aeolian environment controlled by a rising water table in Gale crater during the Hesperian.

Over 3.5 years of exploration in Jezero Crater, the Perseverance rover has explored several geological units of diverse origins and natures, performing multi‐technique remote analyses of the chemistry and mineralogy of rocks with the SuperCam instrument suite. Three of these units are dominated by mafic to ultramafic rocks: the igneous rocks of Séítah (olivine cumulate on the crater floor), the sedimentary rocks of the Upper Fan (Western delta) and the Margin Unit, of likely igneous origin. Despite their diverse natures, these three different units present similar mineralogical assemblages with: (a) primary igneous minerals (olivine, pyroxene, Cr‐rich Ti‐Fe oxides), (b) Fe‐Mg carbonate, (c) hydrated/hydroxylated silica, and (d) phyllosilicates. The abundance of carbonate is variable, and we estimate it around 3–9 wt.% and up to 6–16 wt.% carbonate mineral in the Upper Fan and Margin Unit, respectively. We propose that most of these carbonates formed through in situ carbonation of mafic/ultramafic material, whether these rocks were emplaced through igneous or sedimentary processes. The distribution of carbonate with elevation in the Upper Fan and Margin Unit suggests a contribution of the lacustrine activity to the carbonate process, possibly enhanced by hydrothermal activity. These in situ observations may be extrapolated to other carbonates‐bearing rocks on Mars and would make the amount of carbon potentially stored in Martian ultramafic rocks overall significant. This would suggest that carbonation of ultramafic rocks might have played a key role in pumping CO ₂ from and therefore in cooling the Martian atmosphere.

NASA's Curiosity rover is exploring a 5 km tall sedimentary mound that is hypothesized to record the transition from a warm and wet (phyllosilicate‐rich) to a cold and drier (sulfate‐rich) Mars. Evidence of magnesium sulfate‐bearing rock has shown that Curiosity has crossed through this phyllosilicate‐sulfate transition. Recently, Curiosity arrived at the Amapari Marker Band, a darker, indurated unit that can be traced laterally for tens of kilometers in orbiter images. Here, Curiosity found evidence for a very broad lake, and bedforms interpreted as wave‐ripple laminated sedimentary rock that likely was deposited in shallow water in the explored location, before becoming a deeper lake. These rocks are enriched in Fe, Mn, and Zn which has major implications for groundwater paleohydrology in Gale crater. Three formation hypotheses are considered: concretion formation during early diagenetic alteration of shallow lake sediments, laterization or leaching of the sediments, and addition of Fe, Mn, and Zn by a mildly acidic and reducing groundwater interacting with a redox and/or pH front in a stratified lake. The preferred interpretation of the metal enrichments within the Amapari Marker band sedimentary rocks is that they formed in a shallow water environment at a redox and/or pH front within the ripple unit, which drove precipitation and concentration of metals. If the enrichments are due to groundwater alteration, these processes could link subsurface and surface environments. Water and the presence of high amounts of redox sensitive elements and other metals are favorable indicators for habitability.

During its exploration of Neretva Vallis in Jezero crater, NASA's Mars 2020 Perseverance rover examined Mount Washburn, a set of light‐toned sedimentary mounds that exhibited a diverse array of float rocks. Mastcam‐Z visible/near‐infrared (VISIR) multispectral imaging revealed five spectral types dominated by low‐calcium pyroxene‐bearing rocks, although olivine‐bearing and ferric materials were also identified. SuperCam VISIR point spectroscopy of eight targets noted four spectral groups, including those with low‐Ca pyroxene, hydrated alteration products, and phyllosilicates. SuperCam laser‐induced breakdown spectroscopy data confirmed a wide compositional array among the targets. Overall, these data spanned much of the spectral and compositional range in rocks observed by Perseverance through Sol 1348, with no major outliers, although some rock textures were unique. The rock diversity at Mount Washburn is among the most extensive observed in situ at a single site on Mars, pointing to a multifaceted geological history. Bedrock beneath Mount Washburn exhibited high silica and iron content, closely resembling materials from the nearby Bright Angel sedimentary units and suggesting a possible stratigraphic link. Some geological models of Neretva Vallis are consistent with the idea that Bright Angel‐like materials may have formed the foundation of Mount Washburn, with subsequent deposition of cobbles and boulders as late‐stage fluvial events. These findings suggest that similar rocks—and possibly their source regions—may be encountered during future exploration of the Jezero crater rim and intracrater plains. Such materials would likely sample the olivine/carbonate‐bearing regional units and/or ancient Noachian basement, reflecting the rich geological complexity of the Neretva Vallis watershed.

Olivine‐ and carbonate‐bearing rocks are key exploration targets on Mars because of the information they can provide about magma evolution and source characteristics, implications for surface water availability and climate, and their biosignature preservation potential. The NASA Mars 2020 Perseverance rover has explored olivine‐ and carbonate‐bearing rocks in the Séítah formation and the Margin unit within Jezero crater, thought to be part of the largest exposures of such rocks on Mars. This study uses Mastcam‐Z red‐green‐blue (RGB) and multispectral (442–1,022 nm) data to assess and compare the Séítah formation and the Margin unit, constrain their potential origins, and determine whether they might share a genetic relationship. Mastcam‐Z data shows that the Séítah formation is a series of layered, olivine‐dominated crystalline igneous rocks, and that the Issole member is more altered and/or more evolved than the Bastide and Caille members, consistent with data from other Perseverance instruments. In the more carbonate‐bearing Margin unit, pervasive alteration complicates the interpretation of its original emplacement mechanism. The unit may be consistent with an igneous deposit(s) that has experienced varying amounts of aqueous alteration and, in the East Margin, sedimentary reworking, possibly related to a past lake in Jezero. Margin unit rocks found at higher elevations on the inner rim are most similar to the Séítah formation and may represent a relatively unaltered component of the Margin unit. The extent to which local versus regional processes have formed olivine and carbonate rocks in this area of Mars remains an important area of study.

Investigating the stability of hydrated minerals is integral for examining the preservation of rocks for potential Mars Sample Return and has major implications for models that use rover‐based observations to quantify Mars' global water budget. The Mars 2020 Perseverance rover produces abrasion patches to investigate fresh rock surfaces at Jezero crater, Mars. However, due to operational constraints, the full analysis process typically takes several martian days (sols), and freshly exposed hydrated minerals may dehydrate upon atmospheric exposure between abrasion patch creation and their analyses. To assess the potential for short‐term dehydration, the SuperCam instrument conducted the first in situ rover‐based dehydration experiment on rock exposures of the “Bright Angel formation.” The SuperCam and SHERLOC rover instruments indicated that the primary mineral hydration phases were Fe‐hydroxides, Ca‐sulfates such as bassanite (mixed with anhydrite), with possible minor contributions from non‐interlayer‐water phyllosilicates (e.g., hydroxyl‐bearing only). The experiment involved a four‐sol sequence of observations on the Steamboat Mountain abrasion patch, beginning just 22 min after abrasion. Dehydration was assessed by tracking changes in the 1.93 μm H ₂ O absorption feature, which is sensitive to structural, absorbed, and adsorbed water. No significant changes in hydration were observed over the 93 hr, suggesting that the exposed minerals were already in a low hydration state and/or exhibit high stability under current martian surface conditions. These findings imply bulk rocks with low hydration and high stability minerals may not dehydrate upon exposure to the modern martian atmosphere on short time scales, consistent with predictions from laboratory simulations of Mars‐like environments.

Aluminum-rich clay minerals are detected across the ancient surface of Mars and record intervals of intense alteration by liquid water. On Earth, these clay minerals can form from hydrothermal alteration or rainfall-driven chemical weathering over thousands to millions of years, but how they formed on Mars remains a mystery. The Perseverance rover discovered light-toned, cobble-sized, aluminum-rich (30-45 wt% Al₂O₃) “float” rocks (rock fragments), with some exhibiting spectral signatures of kaolinite, an aluminum-rich clay mineral. These rocks now enable an investigation into the ancient kaolinite-bearing terrains of Mars. To interpret their formation, we use data from the SuperCam and Mastcam-Z instruments onboard the rover to compare the chemistry and reflectance spectra of the float rocks with deeply weathered paleosols and hydrothermal kaolin deposits from Earth’s geological record. Aluminum and titanium enrichments coupled with depletion of iron and magnesium are unlike hydrothermal deposits and instead comparable to bleached horizons of paleosols that formed under high rainfall during past greenhouse climates on Earth. These rocks therefore likely represent some of the wettest intervals of Mars’ history.