Venus as a baseline in the search for life

Abstract

The search for life beyond Earth ultimately depends on our ability to distinguish living worlds from the multitude of forms lifeless ones take. While exoplanet observations can now reveal biosignature gases, chemical species associated with life, their presence is yet to evidence inhabitation. Every signal from a distant world is caught in ambiguity, the line between unconstrained chemistry, physics, or geology, and biology hopelessly blurred. Resolving this ambiguity requires first understanding what it means to be lifeless. This thesis argues that the search for life must begin by defining the abiotic baseline against which biology can be recognised. For Earth-like planets, this means determining what a truly lifeless Earth would look like, so that any deviation from that chemical, physical, and geological expectation can be identified as exceptional. Rather than beginning with distant, poorly understood exoplanets, this dissertation turns to the best natural laboratory available: Venus. Nearly identical to Earth in size, composition, and solar insolation, Venus is our nearest planetary twin — yet is presently inhospitable to life. Understanding how two such similar planets diverged so completely provides a control experiment for defining the limits of lifelessness, and for developing the diagnostic tools needed to recognise life elsewhere. To serve as this baseline, the processes that shape Venus’s present day state need to be completely defined. Given the physical similarities between Earth and Venus, a central uncertainty in Venus science is whether the planet was ever Earth-like, temperate and ocean- bearing, or whether it formed hot and dry, never passing through a habitable state. This work addresses this century-long debate by reading the planet’s history through its atmosphere. Using photochemical models to quantify the destruction rates of key atmospheric species, I infer the composition of volcanic gases required to sustain the modern atmosphere. The inferred volcanic fluxes are extremely dry, implying a desiccated mantle and ruling out a past with oceans on the planet’s surface. These results favour a planet that was never liquid-water habitable. Even if Venus never hosted oceans, the origin of its massive CO2 greenhouse atmosphere remains unresolved. Was it inherited from primary magma-ocean outgassing, built gradually on a perpetually hot world, or partly supplied by remobilisation of crustal carbonates after climate collapse? Earth stores most of its carbon in crustal rocks, cycled by plate tectonics and liquid water; Venus, lacking both, holds a comparable carbon mass almost entirely as atmospheric CO2. By modelling these pathways, I find that carbon release from a former carbonated crust is unlikely to explain the modern atmosphere on its own. Secondary stagnant-lid outgassing with Earth-like mantle geochemistry is likewise limited unless carbon is strongly enriched, magmatic delivery to the surface is efficient, or volatile recycling is sustained. Primary magma-ocean outgassing remains a viable contributor, but the fraction ultimately retained is uncertain. Taken together, a Venus-like CO2 atmosphere is an equifinal outcome and does not uniquely diagnose a temperate past. With the planet’s evolution constrained, the analysis expands to abiotic processes capable of producing life-like signals. Lightning is a source of atmospheric disequilibrium; an atmospheric property often associated with biological activity. Using nitric oxide as a tracer for lightning, I estimate the contribution of electrical activity to atmospheric disequilibrium. The results indicate that Venusian lightning must be several times more energetic than Earth’s, demonstrating that even in the absence of biology, substantial chemical disequilibrium can emerge. This finding strengthens the broader argument: understanding the baseline of lifeless processes is essential to recognising when they are exceeded. Ultimately, these results converge into a new, powerful paradigm for astrobiology: com- parative biosignatures. By empirically defining the full expression of lifeless worlds within a system, we gain the abiotic baseline against which to detect true biological anomalies. This principle is evidenced in our own Solar System, where the anomalously high O2 and depleted CO2 in Earth’s atmosphere stand in contrast to the abiotic baseline set by Venus and Mars. Thus, the pathway to identifying life elsewhere begins not with inhabited worlds, but with comparison to their lifeless counterparts. By defining the full expression of a dead terrestrial world through the lens of Venus, this work provides the essential tools to one day confidently recognise a living one.