Equifinality of Venus-like CO$_2$ Atmospheres

TL;DR

Venus's massive CO₂ atmosphere can result from multiple evolutionary pathways, making it an ambiguous indicator of past habitability, as shown by climate-weathering models and volcanic degassing scenarios.

Contribution

This study demonstrates that Venus-like CO₂ atmospheres are an equifinal outcome, arising from various geological and volcanic processes, challenging the use of atmospheric composition as a sole indicator of habitability.

Findings

Venus could have stored ~20 bar CO₂ as crustal carbonates during a habitable phase.

Stagnant-lid models limit outgassing to ~25 bar CO₂ due to mantle volatile depletion.

Multiple processes, including magmatic enrichment and recycling, can produce Venus-like 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.