Equifinality of Venus-like CO$_2$ Atmospheres

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.

• The paper demonstrates that Venus-like 92 bar CO2 atmospheres can form via primary magma ocean degassing, prolonged volcanic outgassing, or crustal decarbonation.
• It uses coupled models of interior evolution, atmospheric outgassing, and weathering to detail the mass balance and kinetics driving CO2 accumulation.
• The results imply that high atmospheric CO2 pressures alone do not diagnose planetary history, necessitating multi-proxy analyses for exoplanet habitability.

Equifinality of Venus-like CO$_2$ Atmospheres

Introduction and Scientific Context

This paper investigates the equifinality problem in planetary atmospheric evolution: multiple geological and geochemical pathways can yield comparable present-day atmospheric CO$_2$ inventories, notably exemplified by Venus. Despite its Earth-like bulk composition and proximity, Venus possesses a 92 bar CO$_2$ atmosphere, in stark contrast to Earth's thin, N$_2$-dominated envelope with most carbon sequestered in crustal carbonates. The authors probe whether a massive CO$_2$ atmosphere, as on Venus, uniquely diagnoses a planet’s evolutionary trajectory, or can arise from a variety of geochemical histories.

Key evolutionary pathways for generating a Venus-like CO$_2$ atmosphere are considered:

1. Primary degassing during a magma ocean phase
2. Secondary degassing via sustained volcanic outgassing under a stagnant-lid regime
3. Crustal decarbonation following climatic runaway after a temperate, habitable episode

By coupling interior evolution, outgassing, and climate-weathering models, the authors seek to disambiguate the relative contributions of these mechanisms and their observational signatures.

Carbon Reservoirs and Planetary Evolution

The starting point for understanding Venus's current $p_{\textrm{CO}_2}$ is the partitioning of carbon among the atmosphere, crust, and mantle through geologic time.

Figure 1: The evolution of atmospheric and crustal carbon reservoirs on Earth; the dashed line shows Venus's present-day atmospheric CO$_2$ for comparison.

On Earth, the vast majority of carbon resides in the crust as carbonates, emplaced episodically via silicate weathering and tectonic cycling. In contrast, Venus's modern carbon reservoir is nearly entirely atmospheric. The absence of plate tectonics, running water, and biological mediation on Venus restricts the carbon cycle to outgassing and possible crustal storage, with minimal recycling.

The authors highlight that while large CO$_2$ inventories, as inferred for Venus, are suggestive of non-Earth-like carbon cycling, their presence alone is insufficient to uniquely reconstruct planetary histories due to equifinality across possible evolutionary regimes.

Evaluating Secondary Volcanic Outgassing

A central result of the study is a comprehensive mass-balance and kinetic model assessing whether sustained secondary degassing, under various mantle conditions and tectonic modes, can account for a Venus-like CO$_2$ atmosphere.

Figure 2: Atmospheric pressures after 4 Gyr of degassing for various mantle volatile inventories, with and without efficient volatile recycling.

The findings indicate that:

• For a planetary mantle with Earth-like carbon and water content (50–450 ppm CO$_2$0), stagnant-lid settings without volatile recycling fail to accumulate more than $_2$125 bar CO$_2$2 even under strongly oxidizing conditions.
• Only with volatile recycling—analogous to efficient subduction or high crustal turnover—can pressures approach or surpass Venus’s observed value.
• Achieving modern Venus’s 92 bar CO$_2$3 through degassing alone requires a mantle strongly enriched in carbon ($_2$41450 ppm CO$_2$5), an enrichment well above values modeled for Earth or predicted after magma ocean degassing.

The authors further elucidate that melt delivery efficiency is a dominant control: high extrusive fractions or elevated total melt production rates can amplify atmospheric build-up, but Venus’s geological and geodynamic context disfavors such rates as a norm.

Figure 3: Sensitivity of final atmospheric pressure to volcanism style, specifically the extrusive-to-intrusive magma ratio.

Constraints from a Hypothetical Habitable Past

Assuming an early temperate Venus with liquid water and an active silicate weathering cycle, the authors quantify the maximum crustal carbonate reservoir emplaced before climate destabilization.

Figure 4: Maximum silicate weathering sink as a function of surface temperature on early Venus, with constraints from solar insolation and greenhouse thresholds.

Under even the most favorable conditions, steady-state weathering can sequester at most $_2$620 bar CO$_2$7 into the crust, a limit set by radiative-convective equilibrium and the efficiency of hydrologically mediated weathering.

Upon a transition to uninhabitable surface conditions, thermal destabilization (decarbonation) of mantle and crustal carbonates releases this stored CO$_2$8 back into the atmosphere on timescales (17–78 Myr) short compared to Venus’s post-habitable epoch.

Figure 5: Crustal carbonate stability field, release rate, and total liberated CO$_2$9 as a function of habitable episode duration and resurfacing rate.

This upper limit ($_2$020 bar) is significantly less than Venus’s observed CO$_2$1 inventory, implying that even a fully decarbonated crust post-habitability cannot account for the present-day atmosphere without significant contributions from alternative sources.

The Role of Primary Magma Ocean Degassing

The authors incorporate results from coupled magma ocean models showing that primary outgassing during the solidification of a global magma ocean can, under a wide range of redox states and starting volatile budgets, generate $_2$2 bar CO$_2$3 atmospheres.

Figure 6: Surface pressure and atmospheric composition after magma ocean outgassing as a function of key interior parameters.

Retention of such a massive primary atmosphere, however, is highly sensitive to XUV-driven escape, impact erosion, and partitioning between surface reservoirs during early evolution. The true surviving fraction is unconstrained but potentially substantial.

Implications for Exoplanetary Diagnostics and the Habitability Demarcation

The equifinality demonstrated in the paper has critical implications for exoplanet science, especially with the proliferation of atmospheric observations of short-period rocky planets.

The central assertion is that atmospheric CO$_2$4 inventory, by itself, is non-diagnostic for reconstructing planetary tectonic or climatic history. High CO$_2$5 surface pressures can result from:

• Magma ocean retention (primary degassing)
• Prolonged secondary degassing under certain tectonic and geochemical regimes
• Catastrophic crustal decarbonation post-habitability (but only up to a point)

Thus, inferring habitable pasts or identifying "dead Earth" analogs among exoplanets requires constraints beyond bulk atmospheric composition, necessitating tracers sensitive to time-dependent processes (e.g., isotopic ratios, non-CO$_2$6 species, surface mineralogy).

Conclusion

This study rigorously demonstrates that Venus-like CO$_2$7 atmospheres are an equifinal outcome produced by disparate geological and climatic pathways. A dense CO$_2$8 atmosphere is compatible with a wide variety of planetary evolutionary tracks; uniquely diagnosing a planet’s history requires a multi-proxy approach accounting for interior composition, tectonic style, volcanic flux, and weathering processes. The results delimit the value of CO$_2$9 as an exoplanetary habitability diagnostic and guide future observational strategies targeting more discriminating tracers of planetary evolution.