Chemical weathering over hundreds of millions of years of greenhouse conditions on Mars

The current Martian surface is dry, cold, and oxidized, yet ancient crustal regions exhibit features (channel networks, dry lakes) indicative of a warmer, wetter past. Understanding the mechanisms behind this past climate warming is a major challenge. While the exact mechanisms remain debated, most plausible explanations involve strong greenhouse warming from a thicker early atmosphere rich in reducing gases like H₂ and CH₄, alongside CO₂. Key uncertainties include the nature of climate change—was it a single major warming event, or were there multiple cycles?—and the spatial extent of warming (global versus regional variations). A complex model linking redox cycles with climate cycles has been proposed, suggesting oscillations between warm, reducing and colder, oxidizing atmospheric states. Hydrated alteration minerals, formed through water-rock interaction, offer clues. On Earth, weathering profiles record modern and past weathering processes, tracing changes driven by atmospheric acids and oxidants. Mobile elements (Ca, Mg, Na, K, Mn) are leached, while immobile elements (Ti, Al, Zr) remain, creating distinct soil profiles. This concept has been applied to Mars, where the observed compositional stratigraphy (upper Al clay-rich layer over a lower Fe/Mg smectite layer) is interpreted as paleo-weathering profiles. These profiles are not only evidence of water-rock interaction but also indicators of redox conditions; under reducing conditions, soluble ferrous Fe is leached. Recent work demonstrating Fe mobility in these profiles supports past climate warming under reducing conditions. The southern Martian highlands contain numerous exposures of these weathering deposits, prompting several key research questions: Were these weathering events local or global? Are multiple events preserved in the same stratigraphy, or does each deposit reflect a single event? What are the ages of the exposures, and do they represent the same or different climate episodes? Can the age, distribution, and style of the deposits constrain the climate warming mechanism?

Previous research has identified and characterized numerous occurrences of compositional stratigraphy on Mars, primarily focusing on specific regions like Mawrth Vallis, Eridania, Valles Marineris, and Nili Fossae. These studies used a variety of remote sensing data (OMEGA, CRISM, HiRISE) to identify and map the distribution of hydrated minerals, primarily focusing on the identification of phyllosilicates and their associated mineralogical associations. Many of these studies suggested a warmer and wetter early Mars, with some proposing multiple climate change events. However, a comprehensive global analysis incorporating a larger dataset and integrating various dating techniques was lacking. This study builds upon this existing foundation, incorporating 54 new sites and re-examining previously identified locations to establish a more holistic understanding.

This study conducted a global analysis of 203 exposures of compositional stratigraphy (Al/Si materials and Fe/Mg smectites) on Mars, including 54 new detections. Data sources included: * **Visible Imagery:** Context Camera (CTX) at 6 m/pixel for geological context; High-Resolution Imaging Science Experiment (HiRISE) at 0.25 m/pixel for detailed geomorphological characteristics; and Thermal Emission Imaging System (THEMIS) at 100 m/pixel for thermophysical properties. * **Hyperspectral Imagery:** Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data (0.4–3.9 µm, 18 m/pixel) to investigate mineralogy. Raw CRISM data were processed for viewing geometry calibration and atmospheric correction using CRISM Analysis Toolkit v7.4. Spectral parameters were calculated to map minerals (Al-OH, Fe/Mg-OH, H₂O). * **Topography:** Blended MOLA/HRSC DEMs (200 m/pixel) and high-resolution CTX and HiRISE DEMs from Ames Stereo Pipeline (ASP). The researchers compiled a catalog of weathering sequences, integrating existing literature and new findings. They used CRISM data and spectral parameter maps to confirm the presence of Fe/Mg smectites and Al clays and HiRISE color images to analyze color differences within clay-rich deposits. The geological context, including age, elevation, and location were carefully analyzed. Crater counting, using CTX mosaic images and Crater Tools extension in ArcMap, was used to estimate the ages of geological units. The model ages were determined using Ivanov's production function and Hartmann and Neukum's chronology function. Stratigraphic relations were used for additional age constraints. Terrestrial analogs (John Day Formation, Deccan Volcanic Province, Murrin Murrin profile) were considered to contextualize the findings, alongside estimates of mineral dissolution rates and clay formation rates, as well as other geomorphological features (fluvial fans, valley networks, lakes). Various climate warming mechanisms were also considered in the discussion.

The analysis of 203 compositional deposits revealed the following: * **Geographic Distribution:** Most deposits were found in the southern highlands (Mawrth Vallis, Eridania, Valles Marineris, Nili Fossae, etc.), mainly between 40°S and 30°N latitude, consistent with the distribution of valley networks and lakes. A significant portion (88%) occurred in Noachian terrain units, spanning the Early to Late Noachian and even into the Hesperian. * **Geological Context:** The deposits occurred in diverse geological settings (crater floors, rims, walls, plains, valleys), with observational biases favoring detection in high-slope, low-dust areas. The true extent is likely much greater than currently observed. * **Mineralogical Characteristics:** HiRISE color images revealed a gradual, not sharp, transition between upper Al-rich (blue-toned) and lower Fe/Mg-rich (red/brown) units, suggesting Fe loss under reducing conditions. This Fe mobility was widespread in the southern highlands. * **Stratigraphic Relationships:** Remarkably, 201 out of 203 (99%) weathering horizons showed a single stratigraphic relationship (Al-rich layer over Fe/Mg-rich layer). Only two exceptions suggest multiple pedogenic episodes. This consistent pattern supports either a single major climate transition or, alternatively, a model of chemical resetting during multiple events, with the latest event overwriting older signatures. * **Age Constraints:** Crater counting and stratigraphic relationships revealed weathering profiles ranging from ~3.97 Ga (oldest, Thaumasia Planum) to ~3.18 Ga (youngest, Orson Welles crater), indicating that significant chemical weathering occurred throughout the Noachian and possibly into the Late Hesperian. The time to form a typical profile (10⁶-10⁷ years) represents a small fraction of this time span. * **Terrestrial Analogs:** Comparison with terrestrial analogs (John Day Formation, Deccan Traps, Murrin Murrin) helped to understand the rates and timescales of clay mineral formation and the factors affecting these processes. * **Mineralogical Diversity:** The diverse mineralogical assemblages (Al-rich deposits, hydrated silica, jarosite, alunite, carbonate, allophane/imogolite) within the weathering sequences indicate varying aqueous environments through time and space.

The predominantly single mineralogical transition observed in the weathering profiles is initially consistent with a single major climate transition event. However, the broad age range (~700-800 My) and the evidence for Fe mobility under reducing conditions suggest a scenario where multiple climate warming events occurred, but the uppermost layers were consistently reset by subsequent events due to rapid Fe loss. The thickness of the weathering profiles and estimated clay abundance provide only coarse time constraints, influenced by factors like weathering reaction propagation limits, physical erosion, and host rock properties (permeability, porosity). The susceptibility of volcanic ash and impactites to rapid alteration is also a significant factor. Sedimentary features in many sections further complicate this picture, with multiple events of transport, burial, and exhumation. The observed association of weathering profiles with features such as degraded impact craters, valley networks, and lakes suggests a complex interplay of these geological processes during the Noachian period. While the formation of individual weathering profiles was relatively rapid (millions of years), the presence of these profiles across a wide age range points to a prolonged period of aqueous activity spanning hundreds of millions of years.

This study provides strong evidence for widespread, precipitation-driven chemical weathering on early Mars, active across a significant portion of the Noachian and possibly extending into the Hesperian. The consistent stratigraphic relationship observed in most weathering profiles, coupled with the age range of these features, strongly suggests a model in which multiple climate warming events occurred, but subsequent events chemically reset the uppermost horizons due to enhanced Fe mobility under reducing conditions. This integrated geological record reflects the long-term impacts of reduced greenhouse gas-driven warming, shaping the Martian surface over an extended period of geological time. Future research should focus on improved age dating techniques and higher-resolution mineralogical analysis to better constrain the duration and frequency of these warming events.

The study acknowledges several limitations: observational biases in remote sensing data may lead to underestimation of the extent of weathering deposits; crater counting methods have inherent uncertainties, particularly for smaller craters in older terrains; and the exact timing and duration of individual weathering events are difficult to pinpoint. However, the large dataset used in this study and the integration of multiple lines of evidence allow for robust inferences concerning the overall timescale and spatial extent of ancient chemical weathering on Mars.