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arxiv: 2604.16846 · v1 · submitted 2026-04-18 · 🌌 astro-ph.EP

Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus

Haskelle T. White-Gianella , Joshua Krissansen-Totton This is my paper

Pith reviewed 2026-05-10 07:10 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords arid planetscarbon cyclesilicate weatheringplanetary habitabilityVenusexoplanetssurface water inventoryvolcanic outgassing
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The pith

Terrestrial planets require an initial surface water inventory of 20-50 percent of Earth's oceans to maintain a balanced carbon cycle and temperate conditions over 4.5 billion years.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that arid terrestrial planets lack enough surface water for silicate weathering to balance volcanic carbon dioxide outgassing. Without this balance, atmospheric CO2 rises and surface temperatures become uninhabitable over geological timescales. A coupled model of the geologic carbon cycle identifies a threshold of roughly 20-50 percent of Earth's ocean mass as the minimum water needed to sustain the weathering thermostat. This threshold applies to M-dwarf planets and early solar system bodies and suggests that limited water could have driven Venus from a temperate state to its current conditions. Even planets in the habitable zone may lose long-term habitability if too dry.

Core claim

Using a coupled geologic carbon cycle model, arid planets with surface water inventories below approximately 20-50 percent of Earth's ocean mass cannot maintain silicate weathering fluxes high enough to balance volcanic CO2 degassing, resulting in a runaway increase in atmospheric CO2 and a transition to uninhabitable states over 4.5 Gyr.

What carries the argument

The silicate weathering flux, which scales with available surface water to regulate atmospheric CO2 against volcanic outgassing.

If this is right

  • Arid planets enter a regime where weathering cannot keep up with volcanic degassing, causing atmospheric CO2 to increase.
  • Venus could have shifted from temperate to uninhabitable conditions due to insufficient early surface water destabilizing its carbon cycle.
  • Planets in the habitable zone can still lose habitability over time if their surface water is too limited.
  • Arid terrestrial exoplanets are less likely to remain habitable on long timescales.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Surface water inventory acts as an independent filter for long-term habitability beyond orbital distance from the star.
  • Atmospheric models for exoplanets should incorporate water-dependent weathering to better predict CO2 levels on dry worlds.
  • Biosignature searches may be less productive on arid candidates because carbon cycle failure raises the chance of uninhabitable states.

Load-bearing premise

Silicate weathering flux scales directly with surface water inventory in a manner that can be extrapolated from Earth-like conditions without other limiting factors such as land area or temperature feedbacks.

What would settle it

A calculation or observation showing that silicate weathering rates on a planet with less than 20 percent of Earth's ocean water can still balance volcanic CO2 outgassing and maintain temperate temperatures over billions of years.

read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript presents a numerical model coupling volcanic CO2 outgassing and silicate weathering for terrestrial planets with limited surface water. It concludes that an initial water inventory of at least ~20-50% of Earth's ocean mass is necessary to balance the carbon cycle and maintain temperate surface temperatures over 4.5 Gyr, with applications to arid exoplanets and the evolution of Venus.

Significance. Should the central result prove robust, it would offer a valuable quantitative criterion for assessing long-term habitability on water-scarce terrestrial worlds, particularly those orbiting M-dwarfs. This could refine target prioritization for atmospheric characterization missions by identifying planets unlikely to sustain habitable conditions due to carbon cycle failure, even within the traditional habitable zone.

major comments (2)
  1. [Coupled carbon cycle model] The weathering rate is assumed to scale monotonically with liquid water inventory. However, the model lacks an explicit dependence on exposed land area or hypsometric effects, which become dominant when water inventory falls below the level needed to cover low-lying topography. This assumption is load-bearing for the reported 20-50% threshold, as a faster drop in reactive surface area would shift the runaway boundary to higher water masses.
  2. [Numerical results and parameter choices] The abstract and results report outcomes from the integration but provide no validation of the model against Earth's present-day carbon fluxes (with V_water = 1 Earth ocean), no sensitivity analysis on the free parameters (weathering scaling and outgassing rate), and no discussion of how post-hoc choices influence the threshold. These omissions undermine confidence in the quantitative claim.
minor comments (2)
  1. [Abstract] The range '~20-50%' is presented without specifying the exact criteria (e.g., time to imbalance or CO2 level) used to define the boundary; a brief clarification would improve precision.
  2. [Implications for Venus] The Venus-like instellation exploration is intriguing but would benefit from a direct comparison to existing Venus carbon cycle models in the literature.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thoughtful and constructive comments on our manuscript. We have addressed each major comment below and will incorporate revisions to strengthen the presentation and robustness of our results.

read point-by-point responses

  1. Referee: [Coupled carbon cycle model] The weathering rate is assumed to scale monotonically with liquid water inventory. However, the model lacks an explicit dependence on exposed land area or hypsometric effects, which become dominant when water inventory falls below the level needed to cover low-lying topography. This assumption is load-bearing for the reported 20-50% threshold, as a faster drop in reactive surface area would shift the runaway boundary to higher water masses.

    Authors: We agree that the model does not explicitly include hypsometric effects or a reduction in exposed reactive surface area at low water inventories. Our scaling of weathering rate with liquid water inventory is motivated by the assumption that water availability is the dominant control on silicate weathering for arid planets, with land area remaining sufficient for reaction as long as liquid water is present. However, we recognize this is a simplification that could influence the precise threshold value. In the revised manuscript, we will add a discussion of this limitation and include supplementary simulations in which weathering is further reduced by an assumed fractional land exposure that decreases with water volume. These tests will quantify the sensitivity of the 20-50% threshold to such effects. revision: yes

  2. Referee: [Numerical results and parameter choices] The abstract and results report outcomes from the integration but provide no validation of the model against Earth's present-day carbon fluxes (with V_water = 1 Earth ocean), no sensitivity analysis on the free parameters (weathering scaling and outgassing rate), and no discussion of how post-hoc choices influence the threshold. These omissions undermine confidence in the quantitative claim.

    Authors: We thank the referee for highlighting this gap in presentation. The model parameters were selected to reproduce Earth's present-day carbon cycle balance at one Earth ocean water inventory, but we acknowledge that explicit validation, sensitivity tests, and discussion of parameter influence were not included. In the revised manuscript, we will add a dedicated subsection in the methods describing the calibration to Earth's observed outgassing and weathering fluxes. We will also include a sensitivity analysis varying the weathering scaling exponent and outgassing rate over reasonable ranges, with results shown in a new table or figure. This will demonstrate the robustness of the reported threshold and clarify the role of parameter choices. revision: yes

Circularity Check

0 steps flagged

No significant circularity; threshold emerges from forward integration of standard carbon-cycle ODEs

full rationale

The central result is obtained by numerically integrating a coupled outgassing–weathering system forward in time for 4.5 Gyr and identifying the minimum initial water inventory that keeps atmospheric CO2 from runaway growth. The weathering term is written as a monotonic function of liquid-water inventory whose functional form is taken from Earth-calibrated runoff or regolith models; this is an explicit modeling choice, not a self-definition or a fitted parameter that is then renamed as a prediction. No load-bearing self-citation, uniqueness theorem, or ansatz-smuggling step is required for the threshold to appear. The output is therefore an emergent dynamical property of the chosen equations rather than a tautological restatement of the inputs.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The central claim rests on a numerical model whose weathering efficiency and volcanic outgassing rates are parameterized; these parameters are not derived from first principles within the paper and are expected to be tuned to Earth observations.

free parameters (2)
  • weathering rate scaling with water inventory
    Functional dependence of silicate weathering flux on surface water volume is a tunable relationship in the coupled model.
  • volcanic CO2 outgassing rate
    Baseline degassing flux is an input that must be chosen or fitted to produce Earth-like steady states.
axioms (2)
  • domain assumption Silicate weathering is the dominant long-term sink for atmospheric CO2 on terrestrial planets
    Invoked to close the carbon cycle budget in the model.
  • domain assumption Surface temperature and water availability control weathering rates monotonically
    Used to link low water inventories to reduced CO2 drawdown.

pith-pipeline@v0.9.0 · 5601 in / 1523 out tokens · 57712 ms · 2026-05-10T07:10:31.907718+00:00 · methodology

discussion (0)

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Reference graph

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