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

ARTICLE https://doi.org/10.1038/s43247-022-00602-7 OPEN Chemical weathering over hundreds of millions of years of greenhouse conditions on Mars 1234567890():,; Binlong Ye 1 & Joseph R. Michalski 1✉ Chemical weathering profiles on Mars which consist of an upper Al clay-rich, Fe-poor layer and lower Fe/Mg clay-rich layer are believed to have formed due to precipitation-driven top down leaching process in an ancient, reducing greenhouse climate. Here we use remote sensing imagery and spectroscopy coupled with topographic data and crater chronology to explore the geological characteristics, stratigraphy and relative age of >200 weathering profiles across the southern highlands of Mars. We find that nearly all exposures show a similar, single stratigraphic relationship of Al/Si materials over Fe/Mg clays rather than multiple, interbedded mineralogical transitions. This suggests either one single climate warming event or, perhaps more likely, chemical resetting of weathering horizons during multiple events. While the time required to form a typical martian weathering profile may have been only ∼106−107 years, the profiles occur in deposits dating from the Early Noachian into the Hesperian and suggest that chemical weathering may have occurred over a large range of geologic time, with a peak around 3.7–3.8 billion years ago. 1 Department of Earth Sciences, University of Hong Kong, Hong Kong, China. ✉email: COMMUNICATIONS EARTH & ENVIRONMENT | (2022)3:266 | https://doi.org/10.1038/s43247-022-00602-7 | www.nature.com/commsenv 1 ARTICLE T COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-022-00602-7 he surface of Mars is desiccated, cold, and oxidized today. However, ancient parts of the crust contain hundreds of channel networks and dry lakes that must have formed in a warmer and wetter climate of the modern environment1,2. Theoretical modeling of the conditions and mechanisms of past climate warming has presented a major challenge to scientists. Though the enigma of past climate change on Mars is far from answered, several pieces of the puzzle are now better resolved. Most plausible explanations for a wetter ancient climate involve strong greenhouse warming of a thicker early atmosphere3–5 driven by reducing greenhouse gas, such as H2 and CH4 mixed with other gases, including CO24,6–9. Geological uncertainties surrounding ancient climate include the question of whether climate changed in one major warming event5,10,11 or whether multiple or many cycles of episodic warming and cooling occurred in the Noachian. Another question is whether the climate was truly warm globally or if significant regional variations might have occurred. A complex model was put forward to link redox cycles with climate cycles on Mars, posing the question of whether Mars oscillated between a warm reducing and colder oxidizing atmospheric state9. Further clues to ancient climate include hydrated alteration minerals that formed by a reaction between the silicate crust with surface water, atmosphere, aerosols, and possibly snow or ice. On Earth, weathering profiles, or the chemical-mineralogical-textural alteration fronts affecting rocks and particulate matter at the surface trace modern chemical weathering and paleo-weathering. During weathering, chemical changes caused by top-down leaching processes are driven by the acids and oxidants in the atmosphere-lithosphere interface. Ca, Mg, Na, K, and Mn are considered as mobile elements, while Ti, Al, and Zr are essentially immobile. The loss of mobile elements and retention of immobile elements leads to significant differences in soil profiles12,13. This concept has been applied to Mars, where exposures of compositional stratigraphy, consisting of an upper Al clay-rich layer and a lower Fe/Mg smectite layer, have been interpreted as paleoweathering profiles14–17. Paleo-weathering profiles are not only evidence for water–rock interaction, but also serve as an indicator of redox conditions. Under reducing conditions, Fe occurs in its soluble ferrous form and the process of chemical weathering usually leaches Fe2+ 12. Tracking of Fe-mobility is therefore a tool to understand both climate and redox state on early Mars at the same time. Recent work showing evidence of Fe-mobility in weathering profiles suggests past climate warming occurred under reducing conditions18. The southern highlands of Mars contain many exposures of candidate weathering deposits where Fe/Mg smectites are overlain by Al/Si materials. Most of them have not been studied in considerable detail, and questions regarding the climatic evolution of Mars remain: (1) Are these weathering events local or global? (2) Do we see multiple events preserved in the same stratigraphy, or does each deposit record only one event? (3) What are the ages of the different exposures? Could they plausibly all represent the same geologic/climate episode, or are they indicative of different climate excursions at different times? (4) Could the age, distribution, and style of the chemical weathering deposits/relationships constrain the climate warming mechanism? Here, we carried out a global analysis of 203 exposures of compositional stratigraphy, consisting of Al/Si materials and Fe/ Mg smectites, in order to present a comprehensive picture of evidence for paleo-weathering and implications for climate. Building upon multiple prior studies14,15,19–41, we analyzed 54 additional deposits, and revisited previously reported sites in order to provide several additional key observations about global weathering, constraining climate, and redox evolution recorded in these critical ancient geological deposits. 2 Results Geological characteristics of compositional stratigraphy. Over 203 compositional deposits, consisting of Al clays and Fe/Mg smectites, across the surface of Mars were evaluated, including 54 new detections in this study (Fig. 1). Most of them were found in Mawrth Vallis, Eridania northern basin, Valles Marineris, Nili Fossae, Simois colles/Gorgonum chaos, Noachis Terra and Hellas Basin (Fig. 1a). The latitude distribution of these sequences is mainly within the range from 40 °S to 30 °N, although it may be affected by the modern polar process that have physically obliterated or spectrally obfuscated the deposits. This latitude distribution of weathering sequences is similar to the distribution of other relevant geologic features such as valley networks and open basin lakes42–44 (Supplementary Fig. 1). Latitude-dependence of compositional stratigraphy could be consistent with a “tropical” control on precipitation patterns. They show considerable variation in elevation from −3000 to 6000 m (Fig. 1b). Nearly 88% of these exposures occur in Noachian terrain units45. Specifically, 65 locations occur in the early Noachian units (Early Noachian highland unit and Early Noachian highland massif unit), 85 in the middle Noachian units (Middle Noachian highland unit and Middle Noachian highland massi (...truncated)