Michael Tice

The prevailing view that Mars’ crust is predominantly basaltic is based on remote sensing, mainly Amazonian-age Martian meteorites, and rover analyses that until recently were limited to Hesperian terrains, collectively suggesting a cooling planet with a thickening lithosphere and declining interior H₂O over time. In contrast, Noachian-aged (~4.1–3.7 Ga) outcrops examined by the Mars 2020 Perseverance rover along the Jezero crater rim have exposed a far broader suite of ancient lithologies, including ultramafics, felsites, and impactites. Critical to their interpretation are elemental analyses by Planetary Instrument for X-ray Lithochemistry (PIXL), a micro-X-Ray Fluorescence (XRF) spectrometer, which maps chemical, mineralogical, and textural information (spot size ~120 µm) on rock surfaces. The inner crater rim is a faulted mixture of coarsely crystalline and finer grained rocks. The former (grain size >2 mm) include low-Ca pyroxene (LCP)-bearing intrusive rocks, including feldspar-rich norite (Cat Arm Reservoir, sol 1361) and anorthosite (Knapsack Pass, sol 1309); the latter (grain size <2 mm) include olivine+carbonate (Eremita Mesa, sol 1261) and olivine+LCP (Ranger Falls with Al-chromite, sol 1317, and Rio Chiquito with plagioclase feldspar, sol 1335) rocks. Outside the crater, Perseverance encountered mega-breccia that was likely emplaced by the Isidis Basin-forming impact. Outcrop-scale blocks in the mega-breccia include variably serpentinized dunite (e.g., Stigbreen, sol 1634, Mg#=~76) and the melt-bearing impact breccial Aitkenodden (sol 1658), which includes evolved igneous lithologies, even a 3 mm granitic clast with quartz, alkali feldspar-and accessory zircon and apatite. The regional olivine carbonate unit was examined at several locations, including the Coopers Head outcrop (sol 1506), which is dunite with lower Mg# (~65) and larger (up to 7 mm) relict olivine than in the mega-breccia. The likely youngest unit in the crater rim region is spherule-bearing basaltic deposit that is rich in S and Cl and interpreted as impact ejecta (Hare Bay, sol 1480). These observations show that Noachian crust was diverse and dynamic, and likely fueled by high heat flow with multiple inputs of high-degree mantle melts and crystallization and churned and redistributed by impacts.

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Fossilized tufted microbial mats in the 3.22 Ga Moodies Group, South Africa, record one of the oldest known microbial communities colonizing a sandy coastal-marine system. These mats are morphologically unique among known Paleoarchean microbial structures, yet little is known about their constituent microorganisms or how they interacted with surface sedimentation during growth. To investigate the formation of their tufted topographic relief, modern tuft-forming filamentous cyanobacteria were grown on similar medium- to coarse-grained quartz sand and the resulting morphologies compared with those of the fossil examples. Our results reveal near identical grain-sorting patterns between the fossil and laboratory-grown mats. Specifically, modern tufts formed through a dynamic communal swarming process, in which hundreds to thousands of filaments moved in response to day-night cycles, their motion likely coordinated by physical touch. Tuft-forming cultures also exerted force on each other and on the surrounding substrate by rolling surface sand grains into piles beneath each tuft, thereby constructing regularly spaced coarse-grained piles separated by finer-grained lags. Collectively, these findings likely demonstrate an early communal behavioral adaptation to a photosynthetic lifestyle, revealing the great antiquity of coordinated bacterial motile behavior and sensing. Recognition of such behaviorally produced sedimentary textures may aid the interpretation of biosignatures both on Earth and in comparable extraterrestrial environments.

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Investigating sedimentary carbonate phases in situ is a primary objective of Mars exploration. Such mineral phases commonly precipitate from surficial waters or near-surface diagenetic fluids, and therefore their presence can constrain past aqueous conditions, have the potential to preserve biosignatures, and may be a mineralogical record of the ancient CO2-rich atmosphere. Jezero crater, the field site for NASA’s Mars 2020 Perseverance rover, has experienced multiple periods of aqueous activity and has crater-rim-margin carbonate mineralization evident in orbital spectral data. Here, we report on two new carbonate-bearing sedimentary rocks identified in data collected by the Perseverance rover in the lower portion of the Jezero western sedimentary fan. This study integrates multiple image datasets at these previously unrecognized carbonate localities, Rockytop and Jenkins Gap, to investigate their depositional setting. Differences in their sedimentary attributes and geologic context indicate distinct conditions of carbonate formation that are likely linked to the former lake in Jezero crater. We find that the Rockytop outcrop, comprised of alternating fine gravel and medium sandstone couplets, has carbonate present in detrital grains as well as in the matrix. We interpret these couplets as event beds formed via sedimentary gravity flows. In contrast, the second site has carbonate-encrusted gravels in an unsorted conglomerate that are texturally similar to carbonate cementation in near-shore lacustrine settings (‘tufas’). Both carbonate rocks were apparently deposited during the lacustrine period in Jezero crater, but may not be contemporaneous. We also present an alternate model with the sediment in upper Rockytop deposited in an alluvial/fluvial setting with ambiguous timing.

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