Emily L. Cardarelli

Magnesite (MgCO ₃ ) is a magnesium (Mg) carbonate mineral that records the aqueous environmental conditions of its formation. On Earth, magnesite forms in metamorphic, diagenetic or pedogenic environments, and distinguishing between these environments is critical for understanding the fluid chemistry during magnesite precipitation. Mg carbonates have been observed across the Nili Fossae region on Mars and in Jezero crater by orbital spectroscopy as well as in situ by the Perseverance rover. Rover acquired core samples with Mg carbonates may provide constraints on the chemical conditions of the ancient aqueous environments of Jezero crater, and may also be an important target for the preservation of potential biosignatures. This work explores magnesite phases found in Vertisols of the Kunwarara Mine, Australia, as a potential analog environment for magnesite on Mars. We document the principal microtextures, mineralogical context, and elemental compositions. We investigate the processes involved in the formation and diagenesis of magnesite nodules and a magnecrete. Kunwarara hosted magnesite shows complex textural relationships at the outcrop scale, and these relationships extend down to the nanoscale in samples that were collected along a depth profile. By characterizing textural and chemical variations in magnesite at different scales, this work reveals a continuum between diagenetic and pedogenic magnesites. It illustrates that diagenetic reactions produce magnesite from ascending Mg ²⁺ ‐rich groundwater interacting with detrital phases; groundwater interaction with descending meteoric solutions result in the conversion of magnesite into authigenic dolomite. Overall, this work shows how the superposition of textures and elemental compositions permits reconstruction of pedogenic processes leading to magnesite authigenesis.

Using a Raman spectrometer onboard the Perseverance rover, we report the heterogeneous distribution of organic carbon within mudstones located in an ancient river valley on Mars. Measurements of two mudstones show hundreds of organic detections, making this the most robust organic detection in Jezero crater thus far, and, to our knowledge, the only detection of macromolecular carbon on a natural rock surface on Mars. Spectra of the interior of one rock reveal an association of organics with secondary carbonate and sulfate minerals, whereas another rock exhibits an association of organics with primary silicate-dominated matrix. Although in situ Raman analyses cannot determine whether these organics denote abiotic or biotic sources, the organic association with both depositional and diagenetic minerals and the detection of organics on the martian surface suggests that the organics observed ubiquitously at the Bright Angel outcrop may be resistant to radiation and oxidation or have been relatively recently exposed.

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.

The surface of Mars once hosted flowing liquid water and a warmer climate than today, with past water-rock interactions recorded by the carbonate deposits found on its surface. Here, we analyze the depositional setting of the Margin unit, a major magnesium-carbonate deposit near the fluvial inlet to Jezero crater using ground penetrating radar data collected by the Mars 2020 Perseverance rover Radar Imager for Mars Subsurface Experiment (RIMFAX) instrument. We report soundings from more than 35 m belowground, ~1.75 times deeper than other Jezero geologic units explored to date. We identify numerous subsurface features and submeter to hundred-meter scale layering across an ~6.1-km rover traverse. We infer that subsurface reflectors are consistent with buried fluvial features and deltaic foresets, which have experienced multiple erosional-depositional episodes. This work illuminates a well-preserved paleolandscape wherein a deltaic environment developed prior to the formation of the Jezero Western Delta, as early as the Noachian (~4.2 to 3.7 billion years ago).

Oxychlorines (i.e., perchlorates (ClO ₄ ⁻ ) and chlorates (ClO ₃ ⁻ )) have been detected by several landed missions on Mars at various locations. These missions have provided crucial information about the geographic distribution and abundances of oxychlorines on Mars but have not definitively identified the cation and anion type of in situ, solid oxychlorines. By speciating and precisely locating oxychlorines in Martian rocks, we may be able to better interpret the aqueous history of the rocks, understand the chlorine cycle on Mars, understand the chlorine isotope systematics on Mars, identify the potential for liquid brines on the surface, and advance in situ resource utilization activities for future robotic or landed missions. The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument on the Mars 2020 Perseverance rover may have the ability to identify oxychlorine species (i.e., cation and anion) in solid surface targets due to their characteristic Raman and fluorescence bands. Additionally, the location and distribution of oxychlorines within rocks can be determined using the Autofocus and Context Imager (ACI) or Wide Angle Topographic Sensor for Operations and eNgineering (WATSON) camera, a subsystem of SHERLOC that provides high‐resolution, close‐up images of targets analyzed by the SHERLOC Deep‐Ultraviolet Raman spectrometer. The overarching goal of this work is to test SHERLOC's ability to identify and differentiate oxychlorine species in synthetic pure and natural mixed samples using a laboratory analog to the SHERLOC Raman and fluorescence spectrometer, identify instrumental limitations, and to further constrain potential detections made within Jezero crater, Mars.

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.

The Planetary Instrument for X‐ray Lithochemistry (PIXL) onboard the Perseverance rover has characterized the composition of Martian regolith at a scale of hundreds of microns using micro‐focus X‐ray fluorescence spectroscopy. PIXL data reveal a diverse population of regolith grains with distinct spectral, chemical, and crystallographic properties, through which we identified the mineralogy of individual regolith components. Olivine, phyllosilicate, carbonate‐bearing phases, Cr‐Ti‐spinel, plagioclase, and Fe‐sulfate are all inferred from the data, and potential local and regional bedrock sources are proposed for each. PIXL also inspected dust and soil components, which were found to be geochemically similar to analogous components characterized elsewhere by preceding missions. Unlike other sites, regolith on the western fan front of Jezero crater contains fewer sulfates, but is highly enriched in Cl (up to 2.0 ± 0.5 wt. %), which likely includes chlorides, chlorates, and/or perchlorates. PIXL also finds evidence of hydration in the regolith, potentially carried by salts, as interstitial water, and/or in adsorbed phases. PIXL's observations of diverse amorphous and crystalline components, multiple styles of aqueous alteration, and enrichments of brine‐forming salts provide exciting new justification for the return of a Martian regolith sample to Earth for further study.

During the NASA Perseverance rover's exploration of the Jezero crater floor, coatings were commonly observed on rocks. These features may record past water‐rock‐atmosphere interactions on the crater floor, and understanding their origin is important for constraining the timing of potential water activity and habitability at Jezero. Here, we characterize the morphologic, chemical, and spectral properties of the crater floor rock coatings using color images, visible/near‐infrared reflectance spectra, and chemical data from the Mastcam‐Z and SuperCam instruments. We show that coatings are common and compositionally similar across the crater floor, and consistent with a mixture of dust, fine regolith, sulfates, and ferric oxides indurated as a result of one or more episodes of widespread surface alteration. All coatings exhibit a similar smooth homogenous surface with variable thickness, color, and spatial extent on rocks, likely reflecting variable oxidation and erosional expressions related to formation and/or exposure age. Coatings unconformably overlie eroded natural rock surfaces, suggesting relatively late deposition that may represent one of the last alteration episodes on the Jezero crater floor. While more common at Jezero, these coatings may be consistent with rock coatings previously observed in situ at other landing sites and may be related to duricrust formation, suggesting a global alteration process on Mars that is not unique to Jezero. The Perseverance rover likely sampled these rock coatings on the crater floor and the results from this study could provide important context for future investigations by the Mars Sample Return mission aimed at constraining the geologic and alteration history of Jezero crater.

The Perseverance rover has explored and sampled igneous and sedimentary rocks within Jezero Crater to characterize early Martian geological processes and habitability and search for potential biosignatures^1-7. Upon entering Neretva Vallis, on Jezero Crater's western edge⁸, Perseverance investigated distinctive mudstone and conglomerate outcrops of the Bright Angel formation. Here we report a detailed geological, petrographic and geochemical survey of these rocks and show that organic-carbon-bearing mudstones in the Bright Angel formation contain submillimetre-scale nodules and millimetre-scale reaction fronts enriched in ferrous iron phosphate and sulfide minerals, likely vivianite and greigite, respectively. This organic carbon appears to have participated in post-depositional redox reactions that produced the observed iron-phosphate and iron-sulfide minerals. Geological context and petrography indicate that these reactions occurred at low temperatures. Within this context, we review the various pathways by which redox reactions that involve organic matter can produce the observed suite of iron-, sulfur- and phosphorus-bearing minerals in laboratory and natural environments on Earth. Ultimately, we conclude that analysis of the core sample collected from this unit using high-sensitivity instrumentation on Earth will enable the measurements required to determine the origin of the minerals, organics and textures it contains.