A coupled model of episodic warming, oxidation and geochemical transitions on early Mars (2103.06736v1)

Abstract: Reconciling the geology of Mars with models of atmospheric evolution remains a major challenge. Martian geology is characterized by past evidence for episodic surface liquid water, and geochemistry indicating a slow and intermittent transition from wetter to drier and more oxidizing surface conditions. Here we present a new model incorporating randomized injection of reducing greenhouse gases and oxidation due to hydrogen escape, to investigate the conditions responsible for these diverse observations. We find that Mars could have transitioned repeatedly from reducing (H2-rich) to oxidizing (O2-rich) atmospheric conditions in its early history. Our model predicts a generally cold early Mars, with mean annual temperatures below 240 K. If peak reducing gas release rates and background CO2 levels are high enough, it nonetheless exhibits episodic warm intervals sufficient to degrade crater walls, form valley networks and create other fluvial/lacustrine features. Our model also predicts transient buildup of atmospheric O2, which can help explain the occurrence of oxidized mineral species such as manganese oxides at Gale Crater. We suggest that the apparent Noachian--Hesperian transition from phyllosilicate deposition to sulfate deposition around 3.5 billion years ago can be explained as a combined outcome of increasing planetary oxidation, decreasing groundwater availability and a waning bolide impactor flux, which dramatically slowed the remobilization and thermochemical destruction of surface sulfates. Ultimately, rapid and repeated variations in Mars' early climate and surface chemistry would have presented both challenges and opportunities for any emergent microbial life.

• The paper presents a coupled atmospheric-climate model proposing that episodic greenhouse gas releases and oxidation processes caused frequent transitions between reducing and oxidizing conditions on early Mars, reconciling evidence of liquid water with a generally cold climate.
• The model successfully predicts rapid shifts in atmospheric state and surface temperature, showing that episodic warming events could generate sufficient fluvial activity to form observed valley networks and crater degradation, matching geological features for hydrogen input rates between 4e3 and 1e5 mol/s.
• The findings suggest the Noachian–Hesperian geochemical transition and formation of deposits like manganese oxides could result from increasing oxidation, while fluctuating conditions imply challenges for prolonged surface habitability and caution against interpreting atmospheric oxygen alone as a biosignature on exoplanets.

A Coupled Model of Episodic Warming, Oxidation, and Geochemical Transitions on Early Mars

The paper presents a comprehensive model of the atmospheric and climate evolution of early Mars, seeking to reconcile the seemingly contradictory geological and geochemical records that suggest periods of surface liquid water despite a cold climate. The authors propose a coupled atmospheric evolution and climate model that incorporates episodic release of reducing greenhouse gases and oxidation processes driven by hydrogen escape to space. Their model predicts that early Mars underwent numerous transitions between reducing (\ce{H2}-rich) and oxidizing (\ce{O2}-rich) atmospheric conditions. These transitions, combined with a generally cold mean annual temperature below 240 K, are proposed to account for the observed geological features indicative of past liquid water activity.

Key Model Features and Predictions

The model stochastically forces the release of reducing gases, simulating events such as meteorite impacts and volcanic activity. This stochastic approach effectively captures the episodic nature of atmospheric transitions. Calculations demonstrate rapid and frequent shifts in both the atmospheric redox state and surface temperature, with warmer conditions followed by oxidizing intervals. These alternating conditions provide an explanation for fluvial features and the geochemical record without requiring sustained warm, wet conditions, which are inconsistent with other geological observations.

Strong numerical results underscore the model's ability to match observed valley network formations and fluvial erosion over significant ranges in hydrogen input rates, with values estimated between $4 \times 10^3$ mol/s and $1 \times 10^5$ mol/s. The authors note that durations of warming sufficient to cause fluvial activity could be concentrated into episodic bursts, accounting for the geological evidence of valley networks and crater degradation within realistic volcanic and impactor flux limits.

Implications on Mars' Sedimentary Geochemistry

The model's predictions extend to the sedimentary geochemistry of Mars. It suggests that the Noachian–Hesperian transition from phyllosilicate to sulfate deposition around 3.5 billion years ago could be attributed to increasing planetary oxidation and declining groundwater availability. The model posits that the formation of manganese oxide deposits, particularly at sites like Gale Crater, could occur during oxidizing intervals, potentially facilitated by short-term heating events such as bolide impacts.

Atmospheric and Climate Evolution: Broader Implications

The findings present potential implications for understanding the broader planetary evolution, particularly the conditions required for prebiotic chemistry and potential life viability on early Mars. The redox fluctuations coupled with episodic warming would have posed challenges for prolonged surface habitability, indicating that life, if it arose, may have persisted in niches during warmer, reducing intervals or subsurface environments.

Moreover, the paper offers insight into the criteria used in exoplanetary research, cautioning that oxygen detection alone, as a biomarker gas, could be misleading—signaling abiotic processes under particular planetary conditions. This challenges existing narratives about the development of atmospheric \ce{O2} as a biosignature.

Conclusions and Future Considerations

The model provides a plausible framework for reconciling the geological and geochemical evidence of Mars’ early history, advancing the hypothesis of a dynamically evolving atmospheric and climatic environment. Future research is expected to build upon this framework, incorporating more refined constraints about early Mars conditions and exploring additional warming effects of minor species within the atmosphere. Enhanced understanding of Mars' complex chemical environments will continue to inform astrobiological prospects and comparative planetology studies.

By providing a robust mechanistic explanation for the early Martian environment, this research lays the groundwork for further exploration of Mars' habitability and extends a cautionary note on the interpretation of exoplanetary atmospheres.