Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus

Abstract: Arid terrestrial exoplanets are potentially abundant and are thus interesting targets in the search for life. In particular, M-dwarf planets such as those in the TRAPPIST-1 system may possess limited surface water, whereas early solar system terrestrials may have had small surface water inventories postmagma ocean solidification. On modern Earth, there is enough surface water for a balanced geologic carbon cycle, meaning silicate weathering balances the volcanic outgassing of CO2. However, on arid planets, there may not be enough surface water for this silicate weathering thermostat to maintain habitable conditions. Here, we show that arid planets enter a regime where weathering cannot keep up with volcanic degassing of CO2. Using a coupled model of the geologic carbon cycle, we find that terrestrial Earth-like planets require an initial surface water inventory of at least ~20-50% of Earth's ocean mass to maintain a balanced geologic carbon cycle and temperate surface temperature over 4.5 Gyr of evolution. Arid planets with less than ~20-50% of Earth's oceans cannot maintain high silicate weathering fluxes, potentially causing a runaway increase in atmospheric CO2. In addition, we explore Venus-like instellations and find that limited surface water could have destabilized Venus's carbon cycle, triggering a transition from temperate to uninhabitable. Even if a planet resides in the habitable zone of its star, if arid, it may transition to an uninhabitable state due to an unbalanced carbon cycle. More broadly, arid terrestrial exoplanets are less likely to remain habitable on long timescales, and may thus be poor candidates for biosignature searches.

• The paper demonstrates that arid rocky planets with less than 20–50% of Earth’s ocean water fail to sustain balanced carbonate-silicate weathering, triggering runaway CO2 accumulation.
• It employs a coupled atmosphere-interior box model with Monte Carlo parameter sampling to robustly quantify the impact of water inventory on long-term climate stability.
• The study links Venus’s current CO2-rich state to insufficient early water, offering refined empirical constraints on exoplanet habitability.

Habitability and Carbon Cycle Imbalance on Arid Rocky Planets

Overview

This work systematically analyzes the long-term evolution of planetary habitability for arid, Earth-sized rocky planets, focusing on the coupling between surface water inventory and the global geologic carbon cycle. Through a physically-based coupled atmosphere-interior box model with Monte Carlo parameter sampling, the study demonstrates that planets with initial surface water masses less than approximately 20–50% of Earth's modern ocean are unable to maintain a balanced carbonate-silicate weathering regime over gigayear (Gyr) timescales. Instead, they enter a regime in which volcanic outgassing of CO$_2$ outpaces silicate weathering, leading to runaway atmospheric CO$_2$ accumulation, elevated surface temperatures, and rapid water loss even within the classical stellar habitable zone (HZ). By applying these results to terrestrial exoplanets and to the case of Venus, the work delineates a generalized constraint on long-term planetary habitability that is not captured by the traditional radiative boundaries of the HZ.

Geologic Carbon Cycle Modeling Innovations

The study improves on earlier parameterizations of planetary carbon cycling in several key respects:

• Precipitation-limited Weathering: The weathering model is based on the Maher-Chamberlain (MAC) formulation, incorporating dependencies on atmospheric pCO$_2$, surface temperature, land fraction, and runoff, with an explicit analytic limit on precipitation set by wind-driven evaporation. For arid planets, precipitation and hence silicate weathering fluxes decrease nonlinearly with declining ocean fraction.
• Deep Water Cycling Parameterization: Multiple end-member scenarios for water transport from surface to interior (ingassing) are modeled, including mass-dependent, surface area-dependent, and no-ingassing cases, encompassing uncertainty in planetary interior-lithosphere exchange.
• Monte Carlo Sampling: Uncertainties in initial water and carbon budgets, albedo, outgassing history, weathering kinetics, and the deep water cycle are propagated via large ensembles, quantifying the probability distribution of long-term climate outcomes.
• Comparison With Classical Models: The study demonstrates that classical WHAK-type (pCO$_2$ and T dependent only) weathering models fail to capture the critical precipitation/runoff limitation and therefore overpredict climate stability for arid planets.

Results: Thresholds for Carbon Cycle Stability

The results reveal that Earth-like planets with less than 20–50% of an Earth ocean of surface water systematically undergo a breakdown of the negative feedback loop that stabilizes climate by CO$_2$ drawdown:

• At low water inventories, precipitation and runoff severely limit silicate weathering, resulting in an outgassing-dominated carbon budget.
• CO$_2$ accumulates, surface temperatures rise, and water is partitioned increasingly into the atmosphere, further reducing precipitation/runoff in a positive feedback loop.
• A hard nonlinear transition is observed: above the threshold, stable, temperate climates persist over Gyr timescales; below, rapid transition to uninhabitable, CO$_2$-dominated greenhouse states occur.

Model outcomes are robust across choices of deep water cycle parameterization, interior water capacity, and assumptions regarding weathering kinetics, except that mass-dependent ingassing can slightly reduce the critical water threshold compared to surface-area-dependent models.

Application to Venus

When applied to Venus, the model supports a scenario in which early moderate water inventories (≲20–50% of an Earth ocean) were insufficient to maintain balanced weathering, even in the presence of potentially stabilizing high albedo cloud decks. In this framework, Venus’s transition from potential clementity to its present day hot, CO$_2$-rich state could have been triggered not by transient outgassing fluxes or increasing solar luminosity alone, but by its inability to offset degassing with silicate weathering due to aridity. The result is that, unless Venus formed with a water inventory comparable to or greater than Earth, its present climate is a predictable outcome of carbon cycle imbalance on an arid world.

Implications for Exoplanet Habitability

The findings impose a new empirical constraint for exoplanet habitability assessments: the classical HZ is not sufficient for long-term temperate climates unless a substantial fraction of an Earth ocean of water is present. This water threshold is directly testable by planned and proposed facilities (e.g., HWO, JWST, reflected light mapping of land/ocean fractions), and suggests that arid exoplanets, particularly in M-dwarf systems or those expected to have desiccated during the pre-main-sequence phase, are poor candidates for long-term biosignature searches. Conversely, planets with evidence for extensive surface water are the most promising venues for enduring habitability.

Limitations and Future Directions

The study employs a 1D globally averaged framework. Key limitations acknowledged include:

• Omission of spatial/seasonal effects, which GCMs suggest could further reduce the efficiency of weathering by cold trapping water or confining precipitation to unweatherable reservoirs.
• Fixed assumptions for N$_2$ background pressure (which affects cold trapping and escape fluxes) and neglect of O$_2$ photochemistry.
• Simplified representations of albedo and no explicit cloud microphysics.

Further work should employ spatially resolved GCMs coupled to carbon-water cycling to examine the intersection of geography, climate, and geochemical feedbacks. The impact of land distribution and hydrologic isolation, as well as varying tectonic regimes and interior volatile cycling, remains important for refining habitable thresholds.

Conclusion

This work establishes that the long-term climatic and habitability prospects of terrestrial planets hinge crucially on the initial size of their surface water reservoirs, with a critical lower bound of 20–50% of Earth’s ocean mass required for sustained balance of the geologic carbon cycle. Arid planets below this threshold are expected to evolve toward uninhabitable CO$_2$0 super-greenhouse states regardless of their position in the HZ or the presence of clouds with high planetary albedo. The framework provides a predictive basis for interpreting future remote sensing of exoplanet land/ocean fractions and offers a new perspective on the divergent climate evolution of Earth and Venus.

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Citation: "Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus" (2604.16846).