A Climate That Came and Went: Limit Cycles and the Changing Weather of Ancient Mars
Early Mars shows clear signs that liquid water once sculpted its surface, from sprawling valley networks to inland deltas, yet its faint young Sun should have left it frozen solid. In this paper, Haqq-Misra proposes a possible resolution to this long-standing puzzle: early Mars may have cycled between long cold phases and short-lived warm intervals, driven by the carbonate–silicate cycle in combination with additional greenhouse warming. The author highlights evidence of precipitation-fed channels, possible oceans, and aqueous minerals, then frames the question: how could Mars have ever been warm enough for liquid water? Past ideas include thick CO₂ atmospheres, warming from impacts, or brief seasonal melting, but Haqq-Misra emphasizes another possibility, climate limit cycles, in which volcanic outgassing slowly builds greenhouse gases until melting occurs, followed by rapid cooling.
Model Description
To explore this idea, the study uses a latitudinal energy balance model (EBM) that tracks temperature across different latitudes over time. The model includes the balance of incoming sunlight, infrared cooling, heat transport between latitudes, and temperature-dependent albedo. Crucially, it incorporates an active carbonate–silicate cycle, which controls long-term CO₂ levels through volcanic outgassing and weathering. Additional greenhouse warming, possibly from H₂, CH₄, or mixtures of gases, is represented by a single adjustable term (Fₐdd). Haqq-Misra intentionally chooses parameters that make limit cycles more likely, allowing the study to test whether Mars could have undergone such oscillations.
Limit Cycles on Earth and Mars
The paper next explains how limit cycles operate on planets near the outer edge of the habitable zone. Cold climates suppress weathering, allowing CO₂ to accumulate until greenhouse warming triggers a rapid deglaciation event. Once warm, enhanced weathering draws CO₂ back down, returning the planet to a glacial state. On early Earth, such cycles could theoretically occur under low solar input, but the focus here is on Mars. Model results show that CO₂ alone cannot warm early Mars, as even high CO₂ levels remain below freezing due to Rayleigh scattering. With at least ~40 W/m² of additional greenhouse forcing, however, warm periods become possible. The calculations also reveal that Mars’ obliquity and surface geography strongly influence whether limit cycles occur and how long they last, with certain land–ocean distributions more prone to rapid freeze–thaw cycles.
A Climate History of Mars
Haqq-Misra then simulates a schematic, time-forward climate history from 4.3 to 1.8 billion years ago. The model incorporates a gradually brightening Sun and Mars’ obliquity varying between ~0° and 60°, based on long-term dynamical studies. Using a constant additional greenhouse forcing (Fₐdd = 65 W/m²), the model produces repeated temperature spikes, warm intervals long enough to carve valley networks and deltas. These warm periods appear when obliquity is low, aligning with the estimated ages of valley networks (4.1–3.5 Ga) and fan/delta formations (3.3–3.0 Ga). After volcanism declines around 1.8 Ga, CO₂ input drops, the planet cools irreversibly, and it eventually enters a CO₂ condensation regime consistent with present-day Mars. The author stresses that this is not a definitive reconstruction, but a plausible demonstration that episodic limit cycling could match the geologic record.
Conclusion
Haqq-Misra concludes that explaining fluvial features on Mars likely requires multiple warming mechanisms, and limit cycles offer one compelling piece of that puzzle. Additional greenhouse gases, obliquity variations, volcanic outgassing, and evolving surface geography all may have shaped Mars’ episodic warm periods. The hypothesis can be tested further as Mars missions refine the timeline and magnitude of ancient runoff, constrain volcanic activity, and evaluate how global past climates may have shifted. Rather than a long-lasting warm climate, early Mars may have been shaped by intermittent bursts of warmth interrupting long frozen eras, providing conditions just sufficient to form the features observed today.
Source: Haqq-Misra
