The climatic history of Venus constitutes one of the most enduring enigmas of modern planetology. For decades, scientists have oscillated between two radically opposed visions of his past : that of a planet having sheltered oceans of liquid water for billions of years, like the primitive Earth, and that of a world condemned from its formation to perpetual aridity.
A new study published In Nature Astronomy provides some answers to this fundamental question. By studying the rates of destruction and renewal of different chemical species in the current atmosphere of Venus, researchers were able to go back in time to reconstruct the water history of the planet.
The Venusian interior: primordial aridity
The study reveals a specific feature of the internal structure of Venus: extreme dryness of his coatthe intermediate layer located between the crust, the outermost part, and the core, the innermost part. Analyzes of current volcanic emanations demonstrate a water content not exceeding 6% of the molecules emitted, a rate drastically lower than that of terrestrial magmas under similar conditions.
This particular composition cannot be explained by a simple gradual evolution: it testifies to an early and profound desiccation, which occurred during the primitive magma ocean phase of the planet. Venus may have experienced a sudden cataclysmic event that occurred very early in its history, while the planet was still forming. This event resulted in massive and irreversible water lossgiving Venus its current desert appearance. Solar winds, galloping greenhouse effect or giant impact with a massive celestial body, several hypotheses are possible.
The deuterium/hydrogen ratio of the Venusian atmosphere, 150 times higher than that of Earth, had long been interpreted as irrefutable proof of ancient, vanished oceans.
However, the new calculations demonstrate that such a ratio can be explained by a constant supply balance, where atmospheric losses are compensated by continued volcanic outgassing from an inherently dry mantle. This fundamental discovery completely calls into question all of our hypotheses on the early evolution of Venus.
Venus: a furnace from the beginning
As explained previously, the scientific community has until now found itself divided between two evolutionary paradigms. The first, largely influenced by our understanding of Earth history, envisioned a temperate early Venus with liquid oceans for several billion years. This vision was based in particular on general circulation models (simplified mathematical representation of the climate system of a planet) demonstrating the theoretical possibility of such a state, maintained by a complex cloud system forming on the dayside of the planet.
The second scenario, more radical, postulated a perpetually torrid planet, whose initial conditions would have prevented any condensation of water on the surface. New geochemical data therefore strongly corroborate this second hypothesis. The composition of current volcanic gases suggests that the planet has retained the scars of a particularly hot primitive phase, during which the water vapor atmosphere would have persisted long enough to allow complete dehydration of the planetary mantle.
The data collected now allows to definitively rule out the hypothesis of primitive oceans on Venus. The arid nature of the current Venusian mantle can only be explained by a particular primitive history: that of a planet having no never experienced the condensation of liquid water on its surface. Upcoming space missions to Venus, such as VERITAS in 2031 and DAVINCI+ in the 2030s, should help us better understand the geological and climatic processes that have shaped this hellish planet.
- Venus has never known liquid oceans; its mantle has been dry since its beginnings.
- The water losses were compensated by volcanic degassing, thus confirming an early drought.
- Venus’ initial conditions prevented any water condensation, making the planet steamy from the moment it formed.
