Abstract The Mars 2020 Perseverance rover mission spent ∼250 sols investigating the Otis Peak formation (OPfm), a sequence of rocks containing the youngest recorded interval of sediment deposition via aqueous activity on Jezero crater’s western fan. This study uses orbiter and rover observations to interpret the formation and emplacement history of the OPfm. Orbiter images reveal a series of superimposed fan‐shaped to elongate crosscutting deposits on the western fan's upper surface that highlights a complex, prolonged depositional history. Perseverance and Ingenuity observations reveal that the OPfm comprises: planar bedded and cross‐stratified sandstones and pebble conglomerates, which are the main lobe‐ and ridge‐forming units, a boulder conglomerate that overlies the sandstone to pebble conglomerate, and a massive mafic unit whose boulders armor the western fan. We interpret the sandstone to be pebble conglomerate facies and the boulder conglomerate to have formed via fluvial deposition in an alluvial fan emanating from Neretva Vallis. The boulders armoring the fan have previously been interpreted as detrital clasts transported into the crater via Neretva Vallis, but we consider an alternative hypothesis that the majority may represent the degraded remnants of a younger igneous unit emplaced in situ within the crater. This study provides a depositional model for a fluvial system capable of transporting boulders on Mars and new insights into the extraordinary preservation of the Jezero western fan.
Abstract Impact cratering is a geologic process that can fracture crustal materials and change their physical properties. On the Moon, the cumulative effects of impact cratering have given rise to a surficial fine‐grained impact gardened regolith that is 10s of meters thick, along with deeper layers of thick impact ejecta and in situ fractured crust. Collectively, the thick ejecta deposits and underlying fractured crust are referred to as the megaregolith, but its physical properties and thickness remain poorly understood. We use high resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission to investigate how density and porosity vary with depth below the surface. We employ localized multitaper spectral analyses with a Bayesian inversion approach that uses Markov Chain Monte Carlo sampling of density profiles comprised of constant density layers. Analyses are restricted to the polar regions where the quality of the gravity models is the best. Our results reveal a distinct two‐layer density structure that is characterized by an upper megaregolith layer that is about 1–2 km thick with porosities of ∼15%–20% and a less porous underlying layer with porosities of 7%–13%. These two layers are interpreted as being composed of allochthonous impact ejecta and autochthonous in situ fractured crust, respectively. The depth of the lower layer extends to at least 11 km beyond which our inversion approach loses sensitivity to density. We show that these levels of porosity reduce the bulk seismic velocities and thermal conductivity of the crust.
Abstract The Hyperion package is a three‐dimensional Monte‐Carlo radiative transfer approach to modeling dust ejecta. Because it has proven to be a powerful tool for simulating optically thick environments with complicated structure, we are applying the routines to observations of the Double Asteroid Redirection Test (DART) impact on asteroid Dimorphos. Our analysis focuses on the LICIACube LUKE red‐filter image acquired 175 s after impact, when the near‐surface plume remained dense and not strongly structured. The model incorporates the updated Dimorphos shape model, spacecraft and solar geometry derived, and size‐dependent scattering properties of realistic dust particles. We adopt an axisymmetric conical ejecta geometry and iteratively adjust the particle size distribution (PSD) and vertical number density profile to reproduce observed brightness profiles extracted from the LUKE image. We find that a power‐law PSD with slope and a number density peak located at approximately 120 m above the surface best reproduces the brightness contrast between the illuminated surface, the bright ejecta plume, and the adjacent dark band produced by shadowing within the optically thick material. The resulting best‐fit model implies a total ejecta mass of kg within 500 m of the impact site.
Abstract The surface of Mercury contains elevated amounts of sulfur of up to 4 wt.%. Volcanic degassing of S from the interior of the planet could have initiated sulfidation reactions between the degassed S and surface materials in the planet's past, leading to the observed surface S enrichment. Mercury's highly reducing conditions cause nominally lithophile elements, such as Mg and Ca, to become chalcophile and to form sulfides such as oldhamite (CaS) and niningerite (MgS) when reacting with S. Reactions of silicate minerals, serving as an analog for Mercury's surface, and gaseous S under highly reducing conditions (at the graphite‐CO buffer) have been investigated before, but the underlying mechanisms and kinetics of the sulfidation reaction are not well understood. In this study, olivine and diopside were exposed to temperatures between 800 and 1,300°C and experimental durations between 1 hr and 4 weeks in the presence of reduced S gas. Results show that the reaction in both minerals is initially interface‐controlled, but mineralogical changes in the substrates cause a shift toward a transport‐controlled kinetic regime. Moreover, the results are a combination of products from the primary sulfidation reactions as well as secondary processes, such as reduction in olivine and chemical re‐equilibration in diopside. These findings highlight potential processes that formed oldhamite and niningerite as well as silicate byproducts, both in enstatite chondrites and on the surface of Mercury. This is especially relevant in the light of upcoming observations by the ESA/JAXA BepiColombo mission arriving at Mercury in late 2026.
Abstract The Viking missions landed two spacecraft on Mars each equipped with a pyrolysis‐gas chromatograph‐mass spectrometer system capable of detecting signs of life. Although working as intended, detections of chloromethane and dichloromethane were initially attributed to terrestrial contamination. Decades later, the Phoenix Mars Lander identified perchlorate salts in the Martian regolith that are known to catalyze the oxidation and chlorination reactions of organics. Subsequent analyses by the Sample Analysis at Mars (SAM) instrument suite aboard the Curiosity rover have since confirmed a variety of syngenetic chlorinated hydrocarbons in Gale crater samples. However, the nature of the parent organic matter (exogenous, abiotic endogenous, or biological) remains unresolved. Here we evaluate the Murchison (CM2) carbonaceous chondrite as a proxy for organic matter delivered to the surface of Mars by meteoritic and interplanetary dust particle infall over geologic timescales. We characterize the generation of chlorinated hydrocarbons during the pyrolysis of Murchison in the presence of 1 and 5 wt.% calcium perchlorate. Experiments produced a suite of C 1 ‐C 7 chlorinated hydrocarbons dominated by aromatic species, including compounds detected by both Viking and SAM. Increasing the perchlorate content from 1 to 5 wt.% did not amplify the relative abundances or diversity of chlorinated products but rather preferentially oxidized parent organics. Results suggest that organics detected at Gale crater represent a fraction of molecules liberated from a macromolecular source that is compositionally consistent with meteoritic organic matter. Unambiguously resolving the origin and diversity of Martian organic matter may ultimately require the coordinated laboratory analysis of samples returned to Earth.
