Equifinality of Venus-like CO$_2$ Atmospheres

Harrison Nicholls; Oliver Shorttle; Tereza Constantinou

arxiv: 2604.25810 · v1 · submitted 2026-04-28 · 🌌 astro-ph.EP

Equifinality of Venus-like CO₂ Atmospheres

Tereza Constantinou , Oliver Shorttle , Harrison Nicholls This is my paper

Pith reviewed 2026-05-07 14:25 UTC · model grok-4.3

classification 🌌 astro-ph.EP

keywords Venus atmosphereCO2planetary evolutionoutgassinghabitabilitycarbon cyclestagnant lidequifinality

0 comments

The pith

A massive CO2 atmosphere on Venus can arise through primary outgassing, secondary degassing, or carbonate remobilisation, making it an equifinal state rather than evidence of a specific past.

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

The authors test if Venus's current thick CO2 atmosphere uniquely indicates that the planet once had a temperate climate with liquid water before transitioning to a runaway greenhouse. They model three possible ways carbon could have moved from the interior to the atmosphere: early release during magma ocean solidification, ongoing volcanic activity without plate tectonics, and release of carbon previously locked in the crust after the climate changed. Calculations show each route can produce roughly 20 to 100 bar of CO2 under plausible conditions, so the atmosphere alone does not diagnose one history over the others. This matters because it affects how we read atmospheric data from Venus and similar exoplanets to infer their past climates and potential for life.

Core claim

Using a coupled climate-weathering framework, a past habitable Venus could have stored about 20 bar of CO2 as crustal carbonates that would be released over tens of millions of years upon climate destabilization. Secondary degassing from a MORB-like mantle reaches about 25 bar but can reach Venus levels with carbon enrichment, higher extrusion rates, or carbon recycling. Primary outgassing can exceed 100 bar though retention after escape is uncertain. A Venus-like CO2 atmosphere is therefore an equifinal outcome and does not uniquely diagnose a temperate past.

What carries the argument

Three carbon release pathways—primary magma-ocean outgassing, secondary volcanic degassing in stagnant-lid regime, and remobilisation of crustal carbonates after runaway greenhouse transition—each evaluated with a coupled climate-weathering model to show they can produce comparable atmospheric inventories.

If this is right

A past habitable Venus would imply a substantial crustal carbonate reservoir that gets released upon climate collapse over tens of Myr.
Stagnant-lid secondary outgassing can build massive CO2 atmospheres if the mantle is carbon-enriched or if melt delivery to the surface is high.
Primary outgassing during formation can supply over 100 bar of CO2, though the amount retained after early atmospheric escape remains uncertain.
Venus's observed atmosphere is consistent with multiple evolutionary scenarios and therefore cannot alone confirm or refute a temperate phase.

Where Pith is reading between the lines

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

Observations of crustal mineralogy or mantle composition on Venus could distinguish which pathway dominated if they show signatures of past carbonate storage or volatile depletion.
Similar modeling applied to exoplanets with thick CO2 atmospheres might reveal that atmospheric mass alone is insufficient to infer past surface conditions without geological context.
Testing the sensitivity to mantle composition assumptions could show how common equifinal massive atmospheres are among rocky planets.

Load-bearing premise

The secondary degassing calculations rely on a MORB-like mantle that depletes its volatiles over time and on the climate-weathering framework correctly forecasting about 20 bar of crustal carbonate storage during habitable conditions.

What would settle it

Measurement of Venus's surface showing either no ancient carbonates or a mantle volatile content that does not match progressive depletion would undermine the ability of the secondary and remobilisation routes to explain the current atmosphere.

Figures

Figures reproduced from arXiv: 2604.25810 by Harrison Nicholls, Oliver Shorttle, Tereza Constantinou.

**Figure 1.** Figure 1: The Earth’s carbon reservoirs in the atmosphere and conti view at source ↗

**Figure 2.** Figure 2: Final atmospheric partial pressures after 4 Gyr of volcanic degassing. view at source ↗

**Figure 3.** Figure 3: Atmospheric composition as a function of mantle CO view at source ↗

**Figure 4.** Figure 4: Sensitivity of atmospheric composition to the volcanic extrusive view at source ↗

**Figure 5.** Figure 5: Silicate weathering sink as a function of surface temperature view at source ↗

**Figure 6.** Figure 6: Crustal carbonate stability and post-runaway CO view at source ↗

**Figure 7.** Figure 7: Primary atmospheres resulting from outgassing of early magma oceans under varying initial conditions view at source ↗

read the original abstract

While Earth locks much of its carbon in its crust as carbonates, Venus retains a comparable carbon inventory almost entirely in its atmosphere as CO$_2$. On Earth, the geological carbon cycle that has produced this vast crustal carbonate inventory is regulated by biology, liquid water, and plate tectonics, which together have stabilised climate over geological timescales. Venus presently lacks all these processes. We test whether Venus's massive CO$_2$ atmosphere is diagnostic of a specific evolutionary pathway by quantifying three routes: primary magma-ocean outgassing, secondary volcanic degassing in a stagnant-lid regime, and remobilisation of crustal carbonates after climate destabilisation. Using a coupled climate--weathering framework, we find that a past habitable Venus could have stored $\sim$20 bar of CO$_2$ as crustal carbonates. Following transition to runaway conditions, crustal heating releases this reservoir over tens of Myr. In stagnant-lid secondary-degassing models with a MORB-like mantle, outgassing reaches only $\sim$25 bar CO$_2$, limited by progressive mantle volatile depletion. However, Venus-like inventories can be achieved through: (i) magmatic carbon enrichment, (ii) increased magmatic delivery to the surface (high extrusion or melt production), and (iii) the recycling of undegassed carbon back into the planet's interior. Primary magma-ocean outgassing can generate $>10^2$ bar CO$_2$, but the retained fraction after early escape remains uncertain. Ultimately, a Venus-like massive CO$_2$ atmosphere is an equifinal outcome and does not uniquely diagnose a temperate past.

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.

Desk Editor's Note private letter to a colleague

Venus CO2 levels can arise from multiple routes, but the 20-bar and 25-bar numbers rest on particular model choices that lack reported sensitivity checks.

read the letter

Venus's thick CO2 atmosphere does not require a single evolutionary story. The paper walks through three routes—primary magma-ocean outgassing, secondary degassing under stagnant-lid conditions, and later remobilization of crustal carbonates—and shows each can reach Venus-like inventories. That equifinality point is the core message, and the authors back it with specific estimates rather than hand-waving. They couple a climate-weathering model to degassing calculations and arrive at roughly 20 bar stored in carbonates under habitable conditions, about 25 bar from MORB-like secondary outgassing before mantle depletion caps it, and well over 100 bar from early primary outgassing (with escape losses left open). They also flag three adjustments—magmatic enrichment, higher extrusion, or recycling—that can push secondary degassing higher. This synthesis of the pathways is the useful new piece; it turns prior carbon-cycle ideas into a direct comparison for Venus and similar exoplanets. The framing is clear and the logic follows from the models they cite. The main limitation is that the headline numbers depend on the chosen inputs. The 20-bar crustal storage comes from their weathering framework under temperate conditions, and the 25-bar secondary limit follows from assuming progressive MORB-like depletion and extrusion rates. The abstract and summary give no error bars, no sweeps over weathering kinetics or initial mantle C/H2O, and no cross-checks against independent Venus mantle constraints. If those parameters shift by a factor of a few, the boundaries between routes move. The primary-outgassing retention fraction is also left uncertain. Readers working on solar-system evolution or CO2-rich exoplanet atmospheres will find the comparison helpful for avoiding over-interpretation. The work shows honest engagement with the literature and a coherent argument on its own terms, so it merits a full referee review to test the model details and see how robust the equifinality claim stays under variation.

Referee Report

2 major / 2 minor

Summary. The manuscript models three pathways to a Venus-like CO2 inventory (>90 bar): primary magma-ocean outgassing, secondary volcanic degassing under stagnant-lid conditions with a MORB-like mantle, and remobilization of ~20 bar of crustal carbonate after a habitable-to-runaway transition. A coupled climate-weathering framework is used to show that habitable conditions can sequester ~20 bar in carbonates, which are later released by crustal heating; secondary degassing is limited to ~25 bar by progressive mantle depletion but can reach Venus-like levels via magmatic enrichment, higher extrusion rates, or recycling. The central conclusion is that a massive CO2 atmosphere is equifinal and does not uniquely diagnose a temperate past.

Significance. If the quantitative thresholds hold, the result is significant for Venus evolution and exoplanet interpretation: it demonstrates that multiple independent geological pathways converge on the observed inventory, weakening the inference that the current atmosphere requires a specific (habitable) history. The coupled climate-weathering treatment and explicit enumeration of three routes constitute a clear strength.

major comments (2)

[§3] §3 (secondary-degassing calculations): the ~25 bar upper limit is produced by the chosen MORB-like mantle volatile content, depletion rate, and extrusion assumptions; the abstract states that Venus-like inventories require magmatic enrichment or recycling, but no sensitivity tests to initial mantle C/H2O, melt fraction, or extrusion rate are reported, making the claim that standard secondary degassing cannot reach the observed inventory load-bearing yet unquantified.
[§4] §4 (coupled climate-weathering framework): the ~20 bar crustal carbonate storage under habitable conditions depends on the specific weathering kinetics and climate parameters; the abstract presents this as a model output without error bars, sensitivity to initial mantle C/H2O or temperature, or cross-checks against independent Venus mantle constraints, which directly supports the remobilization route and thus the equifinality conclusion.

minor comments (2)

[Abstract] Abstract: the retained fraction after early escape in the primary-outgassing route is stated as uncertain; a brief quantitative range or reference to the escape model would improve clarity.
Notation: the distinction between total outgassed CO2 and the retained atmospheric inventory is not always explicit in the route summaries; consistent use of symbols or a summary table would reduce ambiguity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive review and for recognizing the significance of demonstrating equifinality in Venus-like CO2 atmospheres. We address each major comment point by point below, agreeing where the manuscript can be strengthened through additional analyses.

read point-by-point responses

Referee: [§3] §3 (secondary-degassing calculations): the ~25 bar upper limit is produced by the chosen MORB-like mantle volatile content, depletion rate, and extrusion assumptions; the abstract states that Venus-like inventories require magmatic enrichment or recycling, but no sensitivity tests to initial mantle C/H2O, melt fraction, or extrusion rate are reported, making the claim that standard secondary degassing cannot reach the observed inventory load-bearing yet unquantified.

Authors: We agree that the ~25 bar upper limit in our baseline stagnant-lid model is specific to the MORB-like mantle volatile content, depletion rate, and extrusion assumptions adopted in §3. The manuscript already states that Venus-like inventories (>90 bar) require magmatic carbon enrichment, higher extrusion rates, or recycling of undegassed carbon, as noted in both the abstract and the discussion of secondary degassing. To make this claim more robust and quantified, we will add a new subsection (or appendix) in the revised manuscript that presents sensitivity tests varying initial mantle C/H2O content, melt fraction, and extrusion rate. These tests will delineate the parameter ranges under which secondary degassing can approach or exceed the observed Venus inventory, directly addressing the load-bearing aspect of the standard-case limitation. revision: yes
Referee: [§4] §4 (coupled climate-weathering framework): the ~20 bar crustal carbonate storage under habitable conditions depends on the specific weathering kinetics and climate parameters; the abstract presents this as a model output without error bars, sensitivity to initial mantle C/H2O or temperature, or cross-checks against independent Venus mantle constraints, which directly supports the remobilization route and thus the equifinality conclusion.

Authors: The ~20 bar crustal carbonate storage is indeed an output of our specific coupled climate-weathering model under the assumed habitable conditions and weathering kinetics described in §4. We acknowledge that the absence of error bars, sensitivity tests, and cross-checks makes this result less generalizable. In the revised manuscript we will add error bars derived from parameter uncertainties, include sensitivity tests to weathering kinetics, initial mantle C/H2O, and surface temperature, and expand the discussion to incorporate cross-checks against independent Venus mantle constraints available in the literature (e.g., from geophysical models and meteoritic evidence). These additions will better substantiate the remobilization pathway and reinforce the equifinality conclusion. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper's central equifinality claim is reached by running a coupled climate-weathering model to obtain ~20 bar crustal carbonate storage under habitable conditions and stagnant-lid secondary-degassing models (MORB-like mantle) to obtain a ~25 bar limit set by progressive volatile depletion. These are presented as computed outputs under explicitly stated assumptions rather than tautological redefinitions or fitted parameters renamed as predictions. Alternative pathways (magmatic enrichment, higher extrusion/melt production, recycling) are explored as independent routes to Venus-like inventories, with primary outgassing noted as capable of >100 bar but with uncertain retention. No equations, self-citations, or uniqueness theorems in the abstract reduce any load-bearing step to its own inputs by construction; the derivation remains self-contained model exploration.

Axiom & Free-Parameter Ledger

3 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard planetary geology models whose key inputs are chosen to represent Venus scenarios; no new physical entities are introduced.

free parameters (3)

MORB-like mantle volatile content
Sets the upper limit of ~25 bar for secondary degassing via progressive depletion.
Crustal carbonate storage under habitable conditions
Produces the ~20 bar reservoir available for later remobilization.
Magmatic carbon enrichment and extrusion rate
Required to reach Venus-like inventories in the stagnant-lid case.

axioms (2)

domain assumption Coupled climate-weathering framework accurately predicts carbonate formation and thermal release
Invoked for the remobilization pathway and the 20 bar storage estimate.
domain assumption Stagnant-lid regime with no plate tectonics and MORB-like mantle composition
Defines the secondary-degassing pathway and its volatile-depletion limit.

pith-pipeline@v0.9.0 · 5594 in / 1518 out tokens · 59583 ms · 2026-05-07T14:25:41.709526+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

9 extracted references · 9 canonical work pages

[1]

CaCO3 +MgSiO 3 +SiO 2 ← − →CaMgSi2O6 +CO 2 65.1 63.3 61.3 58.7

work page
[2]

SiO2 +MgCO 3 ← − →MgSiO3 +CO 2 82.8 81.4 79.4 76.9

work page
[3]

CaCO3 +SiO 2 ← − →CaSiO3 +CO 2 86.5 84.9 83.0 80.6

work page
[4]

CaMg(CO3)2 +SiO 2 ← − →CaCO3 +MgSiO 3 +CO 2 87.5 86.3 84.4 82.1

work page
[5]

MgSiO3 +MgCO 3 ← − →Mg2SiO4 +CO 2 89.0 87.1 85.0 82.6

work page
[6]

CaMg(CO3)2 +MgSiO 3 ← − →CaCO3 +Mg 2SiO4 +CO 2 93.7 92.0 90.0 87.8

work page
[7]

MgCO3 ← − →MgO+CO2 116.8 115.7 114.3 110.5

work page
[8]

CaMg(CO3)2 ← − →CaCO3 +MgO+CO 2 120.4 119.2 117.6 115.7

work page
[9]

Thermal-front and CO2 arrival time as a function of crustal source depth, for the post-runaway decarbonation model.The orange curve shows the thermal-front arrival time

CaCO3 ← − →CaO+CO2 176.0 174.6 173.4 171.9 Figure A2. Thermal-front and CO2 arrival time as a function of crustal source depth, for the post-runaway decarbonation model.The orange curve shows the thermal-front arrival time. The blue dashed and dotted curves show the total CO2 arrival time for the continental crust (Manning & Ingebritsen 1999) and stagnant...

work page 1999

[1] [1]

CaCO3 +MgSiO 3 +SiO 2 ← − →CaMgSi2O6 +CO 2 65.1 63.3 61.3 58.7

work page

[2] [2]

SiO2 +MgCO 3 ← − →MgSiO3 +CO 2 82.8 81.4 79.4 76.9

work page

[3] [3]

CaCO3 +SiO 2 ← − →CaSiO3 +CO 2 86.5 84.9 83.0 80.6

work page

[4] [4]

CaMg(CO3)2 +SiO 2 ← − →CaCO3 +MgSiO 3 +CO 2 87.5 86.3 84.4 82.1

work page

[5] [5]

MgSiO3 +MgCO 3 ← − →Mg2SiO4 +CO 2 89.0 87.1 85.0 82.6

work page

[6] [6]

CaMg(CO3)2 +MgSiO 3 ← − →CaCO3 +Mg 2SiO4 +CO 2 93.7 92.0 90.0 87.8

work page

[7] [7]

MgCO3 ← − →MgO+CO2 116.8 115.7 114.3 110.5

work page

[8] [8]

CaMg(CO3)2 ← − →CaCO3 +MgO+CO 2 120.4 119.2 117.6 115.7

work page

[9] [9]

Thermal-front and CO2 arrival time as a function of crustal source depth, for the post-runaway decarbonation model.The orange curve shows the thermal-front arrival time

CaCO3 ← − →CaO+CO2 176.0 174.6 173.4 171.9 Figure A2. Thermal-front and CO2 arrival time as a function of crustal source depth, for the post-runaway decarbonation model.The orange curve shows the thermal-front arrival time. The blue dashed and dotted curves show the total CO2 arrival time for the continental crust (Manning & Ingebritsen 1999) and stagnant...

work page 1999